Conformational effects in long carbon chains in relation to hydrogen bonding and polarized infrared spectra

CONFORMATIONAL EFFECTS IN LONG CARBON C H A I N S I N R E L A T I O N TO H Y D R O G E N B O N D I N G AND POLARIZED INFRARED SPECTRA JOHN S. SHOWELL

I. INTRODUCTION

The lipids and the related compounds are characterized by extreme structural flexibility as contrasted with a relative rigidity of other classes of natural products such as steroids. This relative structural flexibility is a result of the lower barriers to rotation about the single bond. These single bonds are primarily C - - C type and with the occasional introduction of functionality there may be C - - O or C - - N . However, in each of these cases the barrier to rotation about the single bond is low and usually of a three-fold type. Since the lipids as a class are dominated by their carbon skeleton, the behavior of the carbon skeleton under a variety of structural and physical conditions is relevant for consideration. The gaseous and solid state exhibit the extremities in behavior of the hydrocarbon skeleton. The essentially random distribution of the molecular shapes in the gaseous state is modified by the rotational barriers about the C - - C single bonds, while the solid is characterized by having one or occasionally two types of rigid structural units which are the repeating units in the crystal lattice.

II. THEORY OF ROTATIONALISOMERISM

A. Origin of Barrier It was not originally recognized that there existed a barrier to rotation about the C - - C bond axis in ethane. It was considered that if a molecule in its equilibrium configuration underwent an internal rotation about C - - C single bond, there would be no change in the potential energy. In 1936, Kemp and Pitzer were able to resolve the discrepancy in ethane between the entropy calculated by statistical mechanics and the entropy value derived from the heat of hydrogenation of ethylene, by considering that there existed in ethane a barrier of 3150 cal mole-1. 5z Although the problem had been recognized earlier by 1 Eastern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture. 255

Progress in the Chemistry of Fats and other Lipids other people, their work represented the first and successful treatment of any experimental data to appear in literature. Shortly after this, a paper appeared by Howard who treated the rotation-vibration spectrum of ethane and was able to demonstrate that a barrier of at least 2000 cal mole -1 existed56 The barrier is sinusoidal with three identical minima (see Fig. 1) separated by 120 ~ rotation about the C - - C axis. 46 The general existence of this phenomenon is now widely recognized and has been extensively studied.

'~

f %'~.

~

/

,I

f

!

STAGGERED

ECLIPSED

ENERGY / 0

60

120

DIHEDRAL

180

240

ANGLE,

300

360

(o)

FIG. 1. Potential energy curve for rotation about C--C bond in ethane; eclipsed represents maximum energy form; staggered the minimum energy.

Principle contributions to this work have been due to Pitzer and his school using thermo-chemical and statistical mechanical techniques, 34 and E. B. Wilson, Jr., using the Stark effect in microwave spectroscopy. 108 In principle, the ethane barrier could be studied like any other quantities, by solving the non-relativistic Schroedinger equation. The magnitude of the mathematical and computational difficulties are huge because of the thousands of integrals that must be evaluated. Only recently, with the advent of high-speed digital computers, has it been possible to eliminate the approximations present in the earlier treatments and to study molecules such as ethane which has 18 electrons and 8 nuclei. This problem has been recently attacked by Lipscomb in such a manner that all the mathematical 256

Conformational Effects in Long Carbon Chains approximations used in the evaluations of the necessary integrals have been done to sufficient accuracy to eliminate any rounding-off errors, s0 Work functions and energy values for both the eclipsed and staggered forms of ethane have been calculated using a limited basis set of self-consistent field approximations and using as basis functions a minimum set of Slater orbitals. This ab initio calculation yields a barrier of 3"3 kcal mole -1 which is in good agreement with experimental values. The wave function yielded an electron density map for the eclipsed form (see Fig. 1) which exhibited threefold symmetry in the plane perpendicular to the C - - C bond at its midpoint. In the staggered case (see Fig. 1) the lowest symmetry can only be sixfold. The magnitude of the barrier seemed to be most sensitive to the type of function used to describe the hydrogen atom. This is strongly suggestive that the barrier originates from the interaction of the C - - H bonds. This calculation is important in giving an insight into the origin of the barrier to free rotation about the C - - C bond, but for operational purposes it is much more convenient to consider the barrier from a more empirical viewpoint. The shape of the potential barrier may be described by the mathematical function which stresses the cosine character of the barrier as is indicated in Fig. 1. Mathematically this threefold potential function may be expressed as a series as is given in equation (l). V = 1/2Va (1 -- cos3 ¢) + 1/2 V6 (l - cos6 ¢) + 1/2 Vaz (1 -- cos 12 ¢)

(1)

In this equation, V is the potential energy, expressed as a function of the dihedral angle ¢, and is a summation of a number of terms of which only the first two are ever very important. The Va is the height of the potential for a threefold barrier, while V6 and V12 are those for a sixfold barrier and for a twelvefold barrier, respectively. Experimentally it has been shown that, except in several rather unusual cases, V12 may be neglected and V6 is 1 to 2 per cent of V3, so that it is also usually negligible. However, for certain molecules such as nitromethane the first term is zero and the second term is small but real. A sixfold barrier in nitromethane has been shown to be 0.00603 kcal mole-1. 6z Usually V is greater than 1 kcal mole -1. Qualitatively the types of effects on rotational barriers may be broken up into the following classes: (a) Electrostatic interactions of polar groups; (b) Steric hinderance; and (c) Partial double bond character due to the conjugation. These categories do not explain the origin of the rotational barrier in ethane but they do permit one to describe the influence of substituents upon the barrier which exists in ethane. No systematic effort will be made to discuss these various effects, but reference can be made to Turner, 105 Dauben and Pitzer, a4 Millen, 6z and Wilson. 10s To orient with respect to the magnitude of typical barriers to rotation, the following are taken from the magnitude of typical barriers to rotation, the following are taken from the tabulations of Millen. 62 Barriers in ethane, methylsilane and methylgermane are respectively 2.79, 1.70, and 1.24 kcal mole -1. The effect of atomic radius on the 257

Progress in the Chemistry of Fats and other Lipids barrier is shown by the sequence ethane, methylamine and methanol having values 2.79, 1.98 and 1.07 kcal mole -l, respectively.

B. Effect of Substitution Substitution at the carbon a t o m in ethane destroys the equivalence o f the three potential minima (Fig. 1). The potential energy curve (Fig. 2) has only two minima of equal energy. 1,2-Disubstitution will lead to three unequal

i

I

I

I

I

. . . . . E' DILUTE SOLUTION/ - ~ GAS . . . .

E

it

)re" W Z LU

0

L ! IB

i

w

I

I

I

60 180 240 DIHEDRAL ANGLE, (o) FIG. 2. Potential energy curve for rotation about C--C bond in substituted ethane: --potential curve for gas state; - - - - potential curve for liquid state in which subsidiary minima are lowered, but are still higher than single minimum; - - - potential energy curve for case where subsidiary minima become low energy form.

minima. The existence o f these minima now permits stereoisomerism, although such conformational isomers will have very short half-life and will not be isolable except in exceptional circumstances. However, these conformational isomers m a y be studied by a variety o f techniques such as microwave intensity and frequency measurements, t h e r m o d y n a m i c heat capacity and entropy measurements, electron diffraction studies, dipole m o m e n t and polarization studies, ultrasonic absorption, nuclear magnetic resonance, and lastly by infrared and R a m a n absorption techniques. 65, 88 The results derived from infared and R a m a n spectroscopy will be the primary concern o f this paper. 258

Conformational Effects in Long Carbon Chains Studies on ethane and monodeutero ethane have shown conclusively that the stable conformation of ethane is the staggered and not the eclipsed form (Fig. 1).58 If an acetylenic linkage is interpolated between the two methyl groups as in dimethylacetylene, the rotational barrier is zero, so that completely free rotation of the methyl groups exists about the linear axis of the molecule. 6~ This striking result indicates that the rotation barrier is due to interaction of the C - - H bonds and not from any inherent asymmetry of the C - - C linkage itself; the acetylenic unit is axially symmetric so that the electron density is uniform with respect to the axis of the molecule. If the ethane derivative is 1,2-disubstituted, the potential curve is again illustrated by Fig. 2, in which there are three conformational isomers. One is called trans and has the substituents arranged at 180 ° to each other about the C - - C single bond and there are two equivalent gauche forms. Thus 1,2-disubstituted ethanes, e.g. 1,2-dichloroethane, can exist in two conformational isomers--trans and gauche. Although these conformational isomers have the same type of bonds, they have different symmetry properties and thus different behavior with respect to absorption of infrared radiation. The trans form has a characteristic band at 1231 cm -1 while the gauche form has a band at 1285 cm-1. 81

C. Effect of Environment The measurement of the ratio of these band intensities as function of temperature gives the energy difference between the two conformational isomers. 81 The difference in the energy between the two forms was found to be 1140 cal mole -1 in the gas phase (AE in Fig. 2), 15 while the value determined for liquid phase was 760 cal mole -1 (AEs in Fig. 2). 67 These changes in the energy barriers are a general phenomenon which arise from the electrostatic interactions between the substituent groups. The liquid phase stabilizes the more polar gauche form due to the dielectric constant of the liquid state. In this particular case the gauche conformation has a dipole whereas the trans has none. A more striking example of this is the behavior of 1,1,2,2-tetrachloroethane which has the same type of potential curve as the 1,2 symmetrically disubstituted compound as represented in Fig. 2. However, in this particular case the stabilities of the conformers are completely reversed in passing from gas phase to the liquid phase. In the gas phase, the trans and gauche forms are very nearly equal in energy because of the balancing electrostatic and steric effects; while in the liquid phase the gauche form is the more stable form (AEs' in Fig. 2). 81 In general it has been found that in liquid or in solution the more polar rotational isomer is favoured. 64 This work was extended by Powling and Bernstein sl who developed the equation (2), E -- 1 AE = AE8 + (24 + 1)

/z~ ~ t*~ A3

(2)

which relates AE, the difference in energy of the trans and gauche forms in the 259

Progress in the Chemistry of Fats and other Lipids

gas phase, to AEs, the difference in energy in the solution or liquid phase where is the dielectric constant; ~ is the dipole moment and A is the radius of the solvent (cavity treated as a sphere). N-propyl chloride may be considered a 1,2-disubstituted ethane with methyl and chlorine substituents. This system shows that the gauehe and trans forms have approximately the same energy despite the steric repulsion in the gauche form. The possibility of a stabilizing electrostatic interaction between halogen and methyl groups has received support from a large amount of work due to Sheppard and coworkers. 23 This effect has been further studied by introducing methyl groups. If an allowance is made for the statistical weights, the energy difference between isomers is close to zero for both isobutyl chloride and isobutyl bromide. The infrared spectra of isobutyl halides indicates the existence of three rotational isomers. The operation of repulsive forces alone would destablize the isomer having the halogen situated in the gauche position between two methyl groups. 102

D. Nomenclature The configurational notation for ethane derivatives has been conveniently extended to describe the configuration of molecules (polymethylene derivatives) possessing more than one axis of internal rotation. 65 Beginning with the second

-

-

(T

T)

-

-

--

-

-

( G G ) -

- ( T G )

-

-

-

FIG. 3. Nature of non-bonded interactions in hydrocarbon chain.

carbon atom and passing along the chain, the confguration about each carbon atom may be specified as trans (T) or gauche (G and G') with respect to the 260

Conformational Effects in Long Carbon Chains previous carbon atom (see Fig. 4). G and G' distinguish gauche configurations obtained by rotation of 120 ° in anticlockwise and clockwise directions about the C - - C bond, respectively, from the trans configuration (Fig. 4). There are

R'

H"

R6

R'

'" H

H

(T) trans

H

R'

H

h

~1

(GI

"H

(G') gauche

H =,

.JH

Ro,

..,H

ECLIPSED FIG. 4. Maximum and minimum energy forms of 1,2 disubstituted ethanes.

four rotational isomers with a 5-carbon unit. The isomers are the following: the TT type which has the all trans configuration, the TG type which has one of the carbon bonds with a gauche interaction across it, the GG type which has two carbon bonds with two gauche interactions, (however, the gauche interactions are on the opposite sides of the original plane that was defined by the all trans structure) and a GG' which has the two gauche interactions lying on the same side of the plane defined by the original plane of the TTisomer. The GG' group isomer is unstable because of the large cis interactions and is not observed. However, the other three types for this unit (C5) are observed spectrally. The replacement of the end methylene units (see Fig. 3) by halogen atoms produces a trimethylene dihalide. Each of these types (TT, TG and GG) has a different symmetry, belonging to a different symmetry group, which is reflected in their infrared and Raman absorption spectra. 24

E. Effect on Absorption Spectra The vibrational absorption spectra are governed by certain selection rules which reflect the symmetry of the fundamental modes (or vibrations). The absorption of energy by a molecule and the shifting of the molecule from a lower vibrational state to a higher vibrational state involves the generation of a transition dipole. When electromagnetic radiation is absorbed by a 261

Progress in the Chemistry of Fats and other Lipids molecule (ground state wave function tp,) and it is converted to a state of ~ higher energy (wave function T'), there is an interaction of the oscillating electric dipole of the radiation with an oscillating dipole in the molecule. The principal requirements for the occurrence of this electric dipole interaction are given by quantum mechanics (equations (3) and (4)).

E'-- E

hv

(3)

I oc j" ttt't~Td-r

(4)

Equation (3) is the familiar Planck relationship between the energy levels and the frequency of energy being absorbed: E' -- E is the difference between the energy states represented by T and ~ ' and hv is Planck's constant times the frequency (v) of the absorbed radiation. The relationship between I, the intensity of the absorption band, and the vibrational states, represented by the wave functions ( T and T'), is given by equation (4). In it/~ is an operator corresponding to the vector sum of the three components of the electric dipole that is given by equation (5).

t~

~ e~ x~ + Y~ei yi + ~ e~ z~ i

i

(5)

i

The charge of the ith particle is represented by ei and its coordinates are repre sented by x,, y,, and z~. If equation (5) is substituted into equation (4) then equation (6) is obtained:

I

Ix -k- lu ÷ Iz

(6)

which allows us to see that intensity absorption of the light with its electric dipole oscilla6ng on its x axis will be proportional to Iz and correspondingly Iu for the y axis a n d / z for the z axis.

Izoe f T ' x T d-r, luoe j'T'yT dr, Izoc j'T'zT dr

(7)

This equation will become important when we consider the influence of polarized radiation on absorption spectra of molecules. Equation (4) permits the discussion of the selection rules governing infrared absorption, but is not valid for Raman spectra. If the operator/~ in equation (4) is replaced by polarizability, P, the resulting equation will govern the behavior of Raman spectra.

I oc .~T'PTd-r

(8)

The polarizability, P, is a function of the squares of the individual Cartesian coordinant x, y and z, and their cross products (xy, xz, etc.). A closer look at equation (4) will show that ift~ is zero, the absorption coefficient for the particular band will be zero. More generally, the fundamental vibration will be infraredactive (that is, give rise to an absorption band) if the normal mode o f vibration involves a change in /zx, /zu or tzz, singly or in combination. The corresponding general rule for Raman is as follows. A fundamental transition will be Raman-active (that is, give rise to a Raman shift) if the normal mode involves some change in one or more of the components of the polarizability 262

Conformational Effects in Long Carbon Chains

(that is, one or more of the quadratic functions of the Cartesian coordinates must change). Consideration of both of these rules leads to the so-called exclusion rule which states that in a centro-symmetric molecule no Raman-active vibration can be infrared-active, and no infrared vibration can be Raman-active. Generally, as the symmetry of molecule increases, the infrared and Raman absorption spectra become more exclusive. Conversely, as the symmetry decreases, the bands can be active in both the infrared and Raman spectra.

F. Symmetry Further consideration of infrared spectra can be simplified if we consider the matter of symmetry in some detail. A symmetry operation is a movement of an object such that after the operation has been carried out, every point of the object is coincident with an equivalent point or perhaps the same point for the object in its original orientation. In other words, the position and orientation of the object before and after the symmetry operation is carried out are indistinguishable. These symmetry operations are the reflection of the plane (an, plane perpendicular to, or Cry,including the principal axis of the molecule), center of inversion i, that inverts all atoms through the center of the molecule, and a proper axis, Cn, of rotation which rotates about the principal axis by 360°/n where n is the order of the axis. These are the symmetry operations for a point group, z8 The following point symmetry groups are important for consideration of absorption spectra. They are listed with their Schoenflies symbol followed by the elements of symmetry that they include in the brackets following: CI(E), Cs(E, an), C~(E, i), C2(E, C2), C2h(E, C2, i, crh), C2v(E, C2, Cry), D2n(E, 3C2, 3or, i), Ca(E, C3, C~) and C3v(E, 3C~, 3av). As will be noted from the listing of the symmetry groups the centrosymmetric groups are C~, C2h and D2h so that any molecule in one of these symmetry groups will follow the exclusion rule so that the infrared and Raman will be mutually exclusive. III. BEHAVIOR OF HYDROCARBON SKELETON

A. Polymethylene Halides An extensive study has been made of the infrared spectra of polymethylene halides. 24 The freezing of a liquid results in a simplification of the infrared spectrum of the solid state compared to that of the liquid state. This simplification of the spectra results from only one configuration (or a limited number) being stable in a crystal lattice. 65 This is prima faeie evidence for the existence of conformational isomers in the liquid state. The infrared bands observed in solid state can be correlated with a given conformational isomer and thus use it to analyze the much more complex liquid state. This technique was used in the study of polymethylene halides. 263

Progress in the Chemistry of Fats and other Lipids Earlier consideration showed that trimethylene dihalide had four possible spectrally distinguishable rotational isomers: T-T, T-G, G-G type and the G-G' (energetically unstable). The T - T type has C,zv symmetry, the T-G type, C1 symmetry, and the G-G, C2 symmetry, while the unstable G-G' has C,~ symmetry. The spectra of liquid trimethylene dihalides show five strong C - - X stretching bands. The alkyl halides (CI, Br, I) have gauche isomers with bands at 649, 564, and 503 cm -I respectively, while these of the corresponding trans isomers are at 726, 649, and 593 cm 1. Surprisingly, the spectra of the trimethylene dihalides in the solid state show only bands which correspond very closely to those of the gauche form of the alkyl halides. This indicates that the trimethylene dihalides have the G-G conformation in solid. Two bands are observed in the spectra of the solids, but these correspond to coupled gauche stretching bands. The mean values of these bands are 656, 572, and 515 cm -1 which correspond very closely to the corresponding values for gauche normal alkyl halides. Liquid state shows bands due to the T - T and T-G isomers. The tetramethylene halides (CI, Br) show marked simplification of the spectra upon crystallization. The all trans form T . . . T is apparently the stable form in a solid state, having C2k symmetry as a consequence of its planar carbon skeleton. The halogen atoms assume a gauche configuration with halogen atoms on the opposite sides of the plane of the chain so that the symmetry would be C~. The observed spectra in fact correspond to the lower of the C - - X frequencies observed in the liquid state for the gauche n-alkyl halide. This means then that the structure of the solid state is G . . . G form so that the complete configuration is GTG. In the liquid state, both trans and gauche end groups conformations occur and these coupled with a nonplanar carbon skeleton increase the complexity of the spectra. The pentamethylene dihalides show a similar simplification upon crystallization. However, in this case a different phenomenon is observed. The solid state isomer of pentamethylene dichloride has a T . . . G conformation while the corresponding bromide and iodide have a T . . . T conformation. The former has two bonds at 650 and 724 cm -1 which correspond very closely to the 649 and 726 cm ~ bands of the gauche and trans forms of the alkyl halides. On the other hand, the bromide and iodide show two bands but these bands have an average which corresponds to the trans form so that the dibromide average (637 and 660 cm 1) corresponds closely to 649 cm-1 for the trans form of n-alkyl bromide, while the di-iodide average (583 and 618 cm 1) corresponds closely to 593 cm -1 for the trans form n-alkyl iodide. The doubling of the bands in the all trans forms of the bromide and iodide is due to the mechanical coupling between the terminal halogen atoms which would be maximized in the all trans planar conformation. The decamethylene dihalides (CI, Br) show a comparatively simple solid state spectra indicating only one isomer is present. This all trans conformer has a center of symmetry (i) and the only observed C - - X stretching frequency corresponds to the trans form. The molecule in the crystal must have a T . . . Tconfiguration. These results strongly suggest that the molecule has its long polymethylene chain packed in an all 264

Conformational Effects in Long Carbon Chains

trans manner which has C2n symmetry. These changes reflect the dominating role of the carbon chain, as the chain lengthens, in determining the overall structure in the solid state. After a certain chain length the end groups become essentially isolated from each other in terms of vibrational coupling so that they behave independently in their infrared absorption. With the shorter chains the gauche forms are more important due to the stabilizing effect of the electrostatic interaction of the halogen atom with a neighboring methylene. However, this becomes less an important factor as the chain lengthens. The influence of molecular symmetry on the infrared spectra is seen in the following series. The trans conformer of ethylene dibromide has C2h symmetry and the trans conformer of ethylene chlorobromide has Cs symmetry, while the symmetry of the corresponding gauche conformers have C1 and C2 symmetry, respectively.21 Similarly the trans forms of both propyl chloride and ethylene chlorohydrin have C8 symmetry while the gauche forms have C1 symmetry. Ethylene dichloride in the liquid state has four C--C1 stretching bands at 655, 678, 712 and 750 crrL-1. The 655 and 678 cm -1 bands are due to the gauche form and are a doublet since the two chlorine atoms can couple with each other (average 660 cm-1). The 712 and 750 cm -1 bands are due to similar coupling in the trans form (average 730 cm-~). The solid state spectrum of ethylene dichloride on the other hand shows only two bands at 712 and 750 cm-1; the latter band, however, is Raman active only. This is a consequence of the C2h symmetry of the trans dichloride which causes the fundamental bands to be split in half, one-half being allowed in the infrared and the other half active in Raman. Propyl chloride in the liquid state shows two bands, at 649 and 726 cm -1, while the solid state shows only one band at 726 cm -1. The band in the solid state is due to the trans form, while the 649 cm -1 band corresponds to the gauche form of the n-propyl chloride. These two bands are both active in the infrared and the Raman. Interestingly, ethylene chlorohydrin shows only one band both in the liquid and in the solid state at 649 cm 1, it being active both in the Raman and the infrared. As will be seen, the 649 cm -~ corresponds to gauche form so the ethylene chlorohydrin must exist in a gauche form in both liquid and solid state. 65 The stabilization of the gauche form of ethylene chlorohydrin is due to the intramolecular hydrogen bonding. B. Alkyl Halides As was briefly indicated, in a given molecule the appearance of a band in the infrared and the Raman is dependent on the symmetry characteristics of the fundamental vibration. The relative intensity of the Raman and infrared bands reflects the population of the different isomeric conformations. It depends principally upon the energy difference between them, along with the statistical weights associated with each type. An important factor in determining the energy difference is probably the size of the interacting groups; configurations of 265

Progress in the Chemistry of Fats and other Lipids lowest energy have their largest groups as widely separated as possible. The substitution of the gauche hydrogen in ethyl chloride (C--CI stretch 560 cm- ~) by a CHa-group changes the C - - C I stretching frequency to 648 cm -1, while substitution of the trans hydrogen causes a shift to 672 cm 1. Conversely, neopentyl chloride, which has only one rotational form, shows C - - C I band at 727 cm -1. The removal of a methyl group should produce a gauche isomer which would absorb lower and there should still remain the same high-frequency band. Indeed it is found in isobutyl chloride that there is a band due to the gauche form at 686 cm 1, while the trans is at 729 cm-~. This is strikingly similar to the trans form's stretching frequency in the n-propyl chloride. Further, isopropyl chloride has a band at 612 c m -1, upon substitution with the methyl group to give secondary butyl chloride shows three absorption bands at 609, 629 and 672 cm 1. The former two are due to the two possible gauche forms while the latter is due to a trans structure. 2e Other studies on the C - - X stretching frequencies of the longer chain n-alkyl halides have shown that they occurred essentially like those of the n-propyl halides, for both the gauche and trans conformer21 The n-alkyl bromides have been extensively studied both in the infrared and the Raman for chain lengths going from propyl (N 3) to undecyl ( N = I 1). 110 It was shown that all the normal alkyl bromides showed bands at both 645 and 565 cm -1, characteristic of the trans and gauche conformation at the C - - B r bond, respectively. Since the carbon skeleton itself can assume various conformational structures, the observed constancy of the 645 and 565 cm -1 bands means that all conformers having a T-type C - - C conformation, but which have the trans bromide configuration, absorb close to 645 cm -1. All the G-type C - - C conformers which have the gauche configuration at the C - - B r bond form an envelope at 565 cm -1. Studies of each individual n-alkyl bromide ( N - 3 , 4, 5, 6, 7, 8, 10) showed that the ratio of the intensity of the T-type band envelope to the intensity of the G-type conformer envelope was independent of temperature. This indicates that the energy of both the T- and G-type conformers is essentially the same. The ratio of the intensity of the T-type to the G-type conformers assumed a constant value at N::=7. The rotational isomers' C - - C bonds are designated by small g+ (clockwise) or g - (counterclockwise) for the two gauche conformations and by small t for trans conformation. If the C - - B r conformation is denoted by G, then the conformation at the adjacent C - - C bond can be either G + g+ or G ~ g - . In the former conformer (G" g*) the distance between the terminal carbon and the bromine is identical to that between the first and fifth atoms in cyclohexane, hence it is unstable, while the G ~ g - would have no such interaction so it would be stable. The G + g - g + conformer has the same steric character and distance as the first and sixth carbon in cyclohexane, hence it is an unstable isomer. The evaluation assumed: (1) Pairs of non-bonded carbon atoms that are separated by the same distance as the 1st and 4th carbon atoms of cyclohexane are given the weight of 1/4 (one-fourth corresponds to a ~XE .... 0.8 kcal mole-t). 266

Conformational Effects in Long Carbon Chains (2) Pairs which are separated by a distance corresponding to the 1st and 5th carbon atoms in cyclohexane are given the weight 1/8. (3) Pairs which correspond to the distance between the 1st and 6th carbons of cyclohexane are neglected. Based upon these reasonable assumptions, the trans C--C/gauche C - - C is 1.17 (for N ~ 5 up; but for N equals 3 and 4, that is the propyl and butyl systems, values are slightly smaller). This ratio, trans C--C/gauche C - - C is intermediate between that due to the energy and statistical effects considered separately in determining the conformation of the carbon skeleton. Since the gauche conformation is 0.8 kcal mole -1 higher in energy than the trans conformation (Fig. 4), on this basis alone the trans should be the predominant form. However, the gauche form can occur in two ways, g+ and g-, and since these are energetically identical, this means that a double statistical weight must be given to the gauche form. Thus, if the energies of the gauche and trans forms were equal, the trans C - - C gauche/C--C ratio would be 1/2. The fact that the observed ratio 1.17 favors the trans C---C conformers, means that the energetic factors are structurally more important.

C. Normal Hydrocarbons The electron diffraction studies of Bartell represent the most complete studies to date of the free normal hydrocarbon chains. 10, 16, 17 This technique provides a picture of conformational equilibrium by demonstrating directly the presence of well-defined radial distribution peaks for fairly long non-bonded internuclear distances. The occurrence of these radial peaks rules out immediately the occurrence of random crumpling of the molecules, while the lower area of the radial peaks for longer distances eliminates the possibility that the chains are rigidly extended. The molecules n-butane, n-pentane, n-hexane and n-heptane were studied. The radial distribution curve f(r) represents a one-dimensional spectrum of all internuclear distances occurring in the randomly oriented gas molecules of the specimen, and thus includes all rotational isomers. Figure 3 illustrates the occurrence of hydrogen-hydrogen repulsions that result upon the conversion of the all trans conformation into a one or more gauche conformations. Thus the introduction of one gauche interaction involves introduction of one hydrogen-hydrogen repulsion, whereas the introduction of two gauche conformations adjacent to each other introduces two hydrogen-hydrogen repulsions. The results of the analysis of conformational isomers are tabulated in Table 1 for each of the molecules investigated. The gauche conformation was found to have a higher free energy than the corresponding trans form by 610 cal mole -1. This corresponds to the value of 800 cal mole -1 found by Pitzer through an analysis of the vibrational frequencies and thermodynamic functions of longchain hydrocarbons. 79 A more detailed picture of the interactions and changes in bond angles is given in Fig. 5. In the idealized all trans conformation the angles ~, ~ and 267

Progress in the Chemistry of Fats and other Lipids (Fig. 5) would have values of 109.8 °, 0 ° and 180 ° respectively, while the idealized form o f the gauche structure would have values of 109.8 ~, 0 ~ and 6 0 , respectively. The experimental results indicate that the gauche form is distorted from the ideal picture by having ¢ (gauche) equal 61.0 °, a is increased by 1/10 o f a degree over the corresponding trans f o r m and that there is a distortion o f the trans conformation when a gauche form occurs adjacent to it resulting in ~j

Table 1. Distribution o f conformers in gas phase

Molecule

m

~ Present

ET]ZG

T G

1 2

59.1 40.9

0.73

TT TG GG

1 4 2

38.4 52.7 9-0

1.16

n-Pentane

24"5 33"6 16"8 11"6 11'6 2"0

1.42

n-Hexane

TTT TTG TGT TGG GTG GGG TTTT TTTG TTG T TTGG GTTG TGTG TGGT TGGG GTGG GGGG

15"7 21 '5 21 "5 7'4 7"4 14"8 3'7 2"5 5"1 0"4

1"35

n-Butane

n-Heptane

Isomer

equal to 7 °. The h y d r o g e n - h y d r o g e n repulsion distance is found to be 2 A. 96 A comparison o f the free energy value of 610 cal mole -1 with the liquid state values due to R a m a n A H values that range from 450 to 700 cal mole-l, suggest that, within b r o a d limits o f the error, AS for changes in conformation is not large. These data indicate clearly that individual chains when free to move independently do so, and indeed develop frequent gauche kinks along their length. Thus, as Table 1 shows, n-heptane has only 15.7 per cent o f the all trans structure, T T T T , and indeed has 0.4 per cent o f the all gauche structure, GGGG. A n analysis o f the occurrence o f trans and gauche conformations using the experimental percentage along with their multiplicity values shows that the 268

Conformational Effectsin Long Carbon Chains ratio of the trans to the gauche forms for the molecules listed in Table 1, have the following values: 0.73 for n-C4H10; 1.16 for C5H12; 1.43 for C6H14; and 1.35 for C7H16. Considering n-butane, the ratio of trans isomers to gauche isomers would be 0-5 if the equilibrium were only statistically controlled but the experimental value, 0.73, indicates clearly that the conformations do not have equal energy. In the moderately long stearic acid molecule, for example,

~"~

2~ ..... H

H

FIG. 5. Nature of angle distortions due to gauche conformation.

it would be a rare chain that did not have several gauche conformations in it if the chains were behaving independently as they must in solution. A detailed study has been made of the structure of normal propane, CHa--CH2--CH3, by utilizing the Stark effect in microwave spectroscopy. The study showed that the molecule has a point group symmetry C2v which means that the C Hs- - groups are staggered with respect to the - - C H 2 - - group. The spectra of 1-monodeutero propane, CH2DCH2CHa, could only be interpreted if there were two molecular species present, one having the deuterium between the two hydrogens of the neighboring CH2CH3 and the other species having the deuterium between the hydrogen and the methyl (ratio of the two species being 1 : 2, respectively).59 This important study also indicated that the hydrogens of the two methyl groups showed no significant interaction with each other. The presence of a large interaction would have caused the CHa-groups to be staggered with respect to each which would have meant that the propane molecule would have only Cs symmetry. The replacement of a hydrogen on a methyl group in propane leads to n-butane, CH3CH2CH~CH3, in which

T

269

Progress in the Chemistry of Fats and other Lipids now, however, the hydrogen-hydrogen interaction (of the methyl groups) is replaced by a possible interaction between a hydrogen and a methyl group which has a larger Van der Waals radius. It is for this reason that the n-butane molecule is significant in the treatment of the structure of the long-chain hydrocarbons. It represents the first hydrocarbon in which a non-bonded interaction can significantly determine the structure of the molecule. The two lower energy forms of the n-butane are trans and gauche (Fig. 4, R, R'== CHa). The trans conformation, with the methyl groups farthest removed, represents the lowest energy form, while the gauche form, in which the methyl groups exert a repulsive interaction on each other, represents a higher energy conformer (G and G', forms in this case are identical). As is clear from Fig. 4, the n-butane then represents the first hydrocarbon in which an all trans conformation is possible and this has the carbon atoms lying in a plane. The corresponding gauche form has lost the planar conformation of the carbon chain and now has only C2 symmetry. The Raman spectrum of n-butane has been studied as a function of temperature (147-305°K) and clearly indicates the presence of two rotational isomers, trans and gauche, in the liquid phase. An energy difference of 800 cal mole 1 is found. 99 Further confirmation of the presence of two rotational isomers was made by studying n-butane in the solid and liquid state and comparing it with the gas state spectra. The bands due to the gauche forms disappeared in the solid state. 5 In the gas phase (14°C) the ratio of the trans to gauche isomers is 60 : 40.16 This compares with 19 per cent trans for n-propyl chloride and 73 per cent trans for 1,2-dichloroethane. 65 This behavior in the gas phase, however, is to be contrasted with the behavior in the liquid state where electrostatic interaction between the molecules must be taken into account (as indicated in equation (2)). Since the trans form of 1,2-dichlororethane has no dipole moment ( / * t - 0) because it has a center of symmetry (Czh symmetry), the symmetric C--CI stretch will be infrared inactive. The corresponding gauche isomer, which has no center of symmetry (C,_, symmetry), and as a consequence has a dipole, will have infrared active C--CI stretch. In Fig. 2, kEs' represents the change of energy of the gauche form, the predominent species in dilute solution due to the dielectric effect. n-Pentane can exist in three rotational isomers, the TT, TG and GG. Study of the Raman spectra at 32°C and --72°C has shown that the energy of the gauche form approaches that of the trans form in the liquid state. This arises from the negligibly small dipole moment of the TT form compared with the dipole large moment of the TG form (2.55D). 64 This type of behavior leads to the general conclusion, based on equation (2), that conformational changes which lead to an increase in dipole moment will take place in the liquid state. The extent of this, of course, is balanced by the increase in steric repulsion in the more highly twisted forms. This picture of the hydrocarbon molecule in the liquid state shows that the tendency is to form the various possible conformational isomers by rotation about the C - - C bond and 270

Conformational Effects in Long Carbon Chains is to be contrasted with the behavior of long-chain molecules in the solid state. The chains, even in monolayers, pack in an all trans manner, the characteristic mode of packing of the hydrocarbon in the solid state. Indeed, even in surface films the packing in a monolayer is identical to that observed in the bulk crystal. 10 IV. INFRARED ABSORPTION CHARACTERISTICSOF HYDROCARBON CHAIN

A. Band Progressions

Extensive spectral studies have been made on the longer chain normal paraffins and all this work fairly indicates the presence of more than one conformational isomer. 5, 6, 19, 49, 71, 74, 82, 87, 99 Since each conformational isomer of a normal hydrocarbon has its own type of symmetry and set of vibrational frequencies, and the number of possible isomers increases rapidly with chain length, the detailed analysis of the solution spectra of long-chain molecules is impossible. The Raman and infrared spectra of the polymethylene chain have been extensively studied. The vibrational spectra have been treated by considering the - - C H 2 - - groups to be simple harmonic oscillators which can interact together as coupled harmonic oscillators. Thus, a carbon chain with n-methylene groups would be considered as a linear array of n-coupled harmonic oscillators, each having one degree of vibrational freedom. The end groups are then appended, which are methyl groups, as in the case of the n-hydrocarbons, or are a methyl and a carbomethoxy group in the case of a straight chain methyl ester. As a result of the vibrational coupling between the successive methylene groups, sequences of "bands" are observed which extend over welldefined regions of the spectrum. To facilitate the treatment, the chains are considered to have the all trans conformation so that if the end groups are identical, the compounds of even and odd chain length have C2h or C2v symmetry, respectively. Within each progression of bands, the number increases directly with the length of the polymethylene chain. The relative intensities of consecutive bands in each progression changes in accord with the selection rules for those for even and odd chain length. The peripheral bands usually have maximal or minimal infrared or Raman intensities, depending upon the relevant symmetry. This treatment of the n-paraffins has proved highly successful in correlating the observed spectra with chain length. 8~, 91 This analysis has shown that the asymmetric C H 3 - - and C H 2 - - stretching vibrations at 2967 and 2912-2929 cm -1, respectively, and symmetric C H z - - and C H 2 - - stretching vibrations at 2884 and 2849-2861 cm -1, respectively, are essentially pure methyl and methylene vibrations. The C H 3 - - asymmetric out-of-plane bending vibrations at 1465 cm -1 and the C H 3 - - symmetric H C H angle bending at 1376 cm -1 are essentially independent of chain length. The remaining eight progressions show either monotonic or minimum and/or maximum limiting behavior depending on the manner in which the band progression changes as a function of a chain length. 271

Progress in the Chemistryof Fats and other Lipids These constituted the following classes: CH3 in-plane and H - - C - - H angle bending (1446-1473 cm 0, CHz wagging (1174-1411 cm-1), C--C stretching (885-1132 cm-1), CH3 terminal rocking (835-975 cm-0, C--C--C angle bending (0-535 cm-1), CHz twisting-rocking vibration (1175-1310 cm 1), CH2 rockingtwisting vibration (719-1060 cm--l), and CH2CH2 torsion (0-153 cm~l. s4 The behavior of the rocking-twisting progression with changes in physical state illustrates the effect of orientation on the polymethylene chain vibrations. The striking changes in the nature of spectra that occur in passing from the solid state to the liquid state reflect the change in organization of the chain resulting from rotations about the C--C bonds. Whereas, in the solid state the band progressions are sharply defined, in the liquid state, the occurrence of gauche isomers, as idealized in Fig. 6, causes a change in the symmetry so that

SOLID - T R A N S

v

i

i

=

LIQUID

i

FIG. 6. Conformationalisomerismin hydrocarbonchain.

the band progressions either disappear or shift to nearby frequencies. Thus, the characteristic sharpness of the solid state is replaced by a diffuseness which reflects the multiplicity of the absorption bands due to the various isomeric 272

Conformational Effects in Long Carbon Chains species present in a liquid state. This phenomenon has been studied in detail for the methyl esters by R. N. Jones and his collaborators. 48, 50, 51, 9o The factor of length in paraffinic hydrocarbons has also been studied in detail by Jones who obtained linear relationships between the band intensities and the number of methylene groups. 49

B. Effect of Subcell Packing Elegant use has been made of the limiting 720 cm -1 band of the rockingtwisting normal mode by D. Chapman. ~8 It was shown that the doublet in the 720 cm -1 region of long-chain compounds can be correlated with the orthorhombic sub-cell packing of hydrocarbon chain. This type of sub-cell packing is illustrated in Fig. 11, which shows that there are two types of methylene chains oriented essentially at right angles to each other. This is compared to the sub-cell packing of hexagonal, monoclinic and triclinic types in which the planes of the methylene chains are all parallel, as also shown in Fig. 11 for the triclinic sub-cell. This has been skillfully used to study the structure and polymorphic changes in numerous polymethylene compounds. 3° An application by D. Chapman of the infrared band progressions is provided in the study of anhydrous sodium salts of the n-alkane carboxylic acids. 29 The variation of the spectra with chain length and the influence of temperature on the spectra was studied. The band progression due to the wagging modes at 1200-1350 cm -1 was shown to be a function of the number of methylene groups in a chain, the number being 1/2 n + 1 for even number, and 1/2 (n ÷ 1) ÷ 1 for odd number. The spectrum of sodium palmitate, which shows 8 bands in this region, was studied as a function of temperature. As the temperature approached 100°C the bands became less well-resolved, particularly those at the high frequency end. This is a result of rotational isomerization about the C - - C bond which reduces the average number of methylene groups in the trans conformation to its neighbors. On further heating to 120-130°C the band progression decreased considerably in intensity and completely disappears at 140°C. At this temperature the chain behaves as though it were completely liquid-like in character. This reflects the presence of a large number of rotational isomers which smear out the well-defined band structure since each rotational isomer has its own characteristic absorption series. This transition, which occurs at 160°C below the true melting point of the soap, is consistent with its liquid crystalline appearance. Above this range little change in the spectrum is observed. Consistent with this interpretation was the progressive decrease in the intensity of the singlet band at 720 cm -~ which is characteristic of the all trans conformation of the polymethylene chain. The behavior of the solid state of the a,t~-dicarboxylic acids CO2H(CHz)nCOzH (n = 2 to 16) was studied. 3e The 720 cm -1 band of the dicarboxylic acids and their polyesters (polyethyleneglycol and 1,4-butanediol) indicate they are all trans in the solid state. 35 On melting, the 720 cm -1 band almost disappears for the esters, and 273

Progress in the Chemistry of Fats and other Lipids

disappears completely for the acid. This indicates a wide distribution range of conformers and a very low concentration of the trans species. V. STRUCTURAL EFFECTS ON HYDROCARBON CONFORMATION

A. A l k y l Substituted Normal Hydrocarbons

Considering the n-butane molecule as the basic building block, alkylsubstituted n-butane can be studied to determine the structural effects. Thus studies have been made on 2-methyl- and on 2,3-dimethyl- butane effects by infrared spectroscopic and ultrasonic techniques. TM 100, 111 Two conformational isomers of 2-methylbutane exist, the more stable conformer being present in approximately 90 per cent and being 1600 cal mole -1 more stable. The 2,3dimethylbutane has two forms of approximately equal energy. A particularly elegant study has been made on isobutane using both deuterium and C13 labelling. 60 l-Deutero-2-methylpropane, CH2DCH(CH3)2 exists as two species in a ratio of 1 : 2 with an energy barrier of 3.90 kcal mole 1 separating the two forms. B. Fluorocarbon Chain

The structural similarity between the - - ( C F 2 ) n - - and - - ( C H 2 ) n - - chains provides a measure of importance of Van der Waals repulsions upon the structure. An electron diffraction study of fluorocarbons in the gaseous state gave no direct evidence o f gauche conformations, although they did not exclude a small equilibrium concentration. 11 A study of the solid state shows that in contrast to the paraflinic hydrocarbons, the carbon skeleton does not lie in a plane but it is formed into a slowly twisting helix. 25 The twist is induced by the repulsion between the fluorine atoms on alternate carbon atoms. The H . . . H repulsion (Fig. 3) is replaced by F . . . F repulsion so that in a planar configuration the F . . . F distances would be less than 2.7A. However, since the Van der Waals radius of F is greater than H, the existence of a twist in the fluorocarbon chain would be expected. The present evidence suggests the absence of a twist in the free hydrocarbon chain, but do not allow it to be completely eliminated from consideration. 10

C. Trigonal-Tetrahedral Single Bonds

The rotational barriers that have been discussed so far have been barriers between carbon atoms of a single bond, which have sp z hybridization. The next class of interest is the C - - C linkages between carbons having sp 2 and sp 3 hybridization. This is found in 1-propene in which the methyl carbon has sp 3 hybridization, while those of the double bonds are sp 2. The classic work of Pitzer and his coworkers showed, through an analysis of the thermodynamic 274

Conformational Effects in Long Carbon Chains properties of 1-propene, that the rotational barrier is 1950 cal m o l e - l # a This compares with the barrier of 2900 cal mole -1 for ethane. The rotational barrier in the C - - C single bond in trans 2-butene is 1950 cal mole -1. However, the corresponding rotational barrier in cis-2-butene is only 450 cal mole -1 a surprising decrease of 1500 cal mole -1 as compared to the trans isomer. This anomalous effect is interpreted as being due to the opposing effects of the normal C H a - - C barriers and the steric interactions of the cis methyl groups. The latter effect causes the potential minimum of the ground state to be raised. Although the barrier has not been lowered, the overall effect has been to raise the energy level of the ground state, causing the cis form to have a higher energy content than the corresponding trans. Isobutene, 2-methyl-l-propene, has the barrier raised to 2350 cal mole -1 due to the methyl-methyl interaction which in this case does not lead to an increase in the energy content of the potential minimum.

\

R

\

I

\

..C

H

\

\

H~

/

\

C

\

/c

.__

(a) (b)

\

R = H R = CH 3

\ H \

\

(a,b)

N- H

H -Y

H

H

H

H (c)

(d)

FIG. 7. Conformational isomerism due to double bond: (a) Propene, R = H ; (b) l-Butene, R=CH3; (c) Stable conformer adjacent to multiple bond; (d) Unstable conformer adjacent to multiple bond. A detailed microwave study of 1-propene, using the 3-monodeuteropropene, C H 2 D C H = C H 2 , has shown that there are two isomers; one has the deuterium in the plane of the double bond (Fig. 7c, Y = C H 2 ) and the other is nonplanar 275

Progress in the Chemistry of Fats and other Lipids (Fig. 7d, Y - C H z ) . 43 The former conformation is 1-98 kcal mole -1 more stable. The C - - H bond lying in the nodal plane of the double bond minimizes the nonbonded interaction. Studies of l-alkenes have provided an example of rotational isomerization about the 2--3 bond which is sp2--sp 3. Two bands due to ethylenic twisting vibrations are observed at 552 and 640 cm -1, but upon solidification only the 630 cm -1 band remains. The relative band intensities are temperature independent so that the energy difference between the species is zero. The shape of the 550 cm-1 band suggests that it is due to the planar conformer, while the other is due to the nonplanar alkyl species (Fig. 7b). 42 The microwave spectrum of 1-fluoropropene has revealed the c i s - C H z C H - - C H F molecule to have an abnormally low potential barrier for the methyl group. Thus the barrier height, I/3 (equation (4)) in C H 3 C H - - C H z , 6° trans-CH3CH CHF, 89 and C H 3 C F - CH277 varies from 2.0 to 2.4 kcal mole 1 but V3 is lowered to 1.0 kcal mole -1 in cis-CH3CH CHF. 14 The non-bonded interaction between the fuorine atom and the methyl group hydrogens stabilizes the cis isomer. Replacement of the fluorine atom by chlorine leads to an increase in the Van der Waals repulsion distances from 1.3 A to 1.8 A, respectively, but in spite of this, the internal rotation barrier, 1/3, decreases from 2.2 kcal mole l in trans to 0.620 kcal mole 1 in cis 1-chloropropene. This decrease of the rotational barrier, V3, further supports the existence of a stabilizing non-bonded interaction between the halogens and the methyl hydrogens. The interaction distance in the cis-lchloropropene is less than in cis-l-fluoropropene and since the Van der Waals radius of chlorine is larger, its non-bonded interactions would be expected to be greater in the former. TM 14 A similar effect on the methyl rotational barrier, 1/3, is observed in t r a n s - C H 3 C H ~ C H C N . The barrier of 2.3 kcal mole -1 in the trans isomer is lowered to 1.4 kcal mole -1 in the cis) e, '_,7 The barrier, I/3, observed in cis-2,3-epoxybutane is 1.6 kcal mole -1 which is below the value of 2.5 kcal mole -1 for 1,2-epoxypropane. 83, 97 The C - - C single bond between the sp .~ carbon and the sp ~ carbon of the carbonyl function may also show a barrier to rotation. This behavior is similar to the 1-alkene in which the CH2 group is replaced by a double bonded oxygen (Fig. 7a). Thus, the stable conformation for acetaldehyde, CH3CHO (Fig. 7c, Y=O), is similar to l-propene and the barrier to rotation is 1150 cal mole-t. The replacement of one of the methyl group hydrogens by a methyl group, giving propanaldehyde, produces an isomeric set about the C - - C bond to the carbonyl in which the s-cis form (corresponding to Fig. 7c, Y--0) is 900 cal mole 1 more stable than the gauche form (Fig. 7d, Y--0) in which the trans hydrogen of the methyl group is replaced by a CHz 26. The barrier to rotation about the axis is 2280 cal mole 1. Electron diffraction studies on dimethyl acetaldehyde, isobutylaldehyde, indicate that two isomers exist. The "so-called" trans isomer, present in 90 per cent abundance, has the C - - H bond in the carbonyl oxygen nodal plane, while the "so-called" gauche, present as 10 per cent of the whole, has a methyl which lies in the carbonyl oxygen nodal plane. If the methyl 276

Conformational Effects in Long Carbon Chains groups are tied together as in cyclopropanecarboxyaldehyde, rotational isomerism again occurs. In this case the form which corresponds to trans (Fig. 7c) and the form which corresponds to cis (Fig. 7d) are present in approximately equal quantities and the barrier is in excess of 2 kcal mole-a. 9 D. Pseudo-Single Bonds

The central bond of a 1,3-diene provides an example of a pseudo-double bond (sp3--sp 3) about which hindered rotation and, hence, rotational isomerization

can occur. The model for this system 1,3-butadiene, is predominently in the planar extended configuration, so-called s-trans configuration (with respect to rotation about the central single bond). Since resonance interaction between the double bonds favors coplanarity, rotation by 180 ° around the center bond leads to a second minimum corresponding to the s-cis form. The s-eis isomer is 2-5 kcal mole -1 above the s-trans in energy53 The ultraviolet absorption spectra of a series of mono- and di-substituted 1,3-butiadienes are very similar to 1,3-butadiene so that they too must be predominently in the s-trans form although it does not exclude the presence of small amounts of the s-eis configuration. 68 Detailed studies of 2-chloro- and 2,3-dichloro-butadiene using infrared and Raman spectroscopy indicates the planar s-trans to be the preferred conformation2 °1 However, it was found that the hexachlorobutadiene appeared to exist as the completely nonplanar species resulting from a 90 ° rotation out of the double bond plane, x01 The evidence for the existence of this skew form has been further strengthened by the recent discovery that the highly hindered 2,3-di-t-butyl-l,3-butadiene has its ultraviolet absorption maximum at 185 mt~ with a shoulder at 209 mt~.a°9 The maximum of a typical 1,3-diene is 220 mtz. This above phenomenon is explicable only in terms of the existence of an isolated double bond which can only occur in a molecule with a skew nonplanar structure. The infrared and Raman spectra of 2,4-hexadiene and 1,5-hexadiene indicate they exist in the liquid state as a mixture of rotational isomers; however, in the solid state, the s-trans conformer was the stable isomer. 75 The C - - O single bond linkage of the carboalkoxy group provides another example of hindered rotation about a single bond. The carboalkoxy function can exist as one of two conformational isomers (Fig. 8), the s-trans and the s-cis. Extensive studies have shown the stable form in solution to be the s-eis form (Fig. 8). The s-trans form is observed only where the substituent groups constrain the geometry, as in the 5- or 6-membered lactone rings. 106 Recent detailed studies in the gas phase have shown simple systems such as methyl formate and methyl acetate to exist only in the s-cis planar structure; no evidence has been found for the existence of a s-trans isomer2 °7 Interestingly enough, the monoclinic form of methyl stearate has the carbomethoxy group packed with the s-cis conformation. 2 This, however, is contrasted with the packing observed in the monoclinic form of ethyl stearate. Here the carboethoxy function is packed with the s-trans conformation (Fig. 8). 3 In this structure the C - - O - - C H 2 C H 3 277

Progress in the Chemistry of Fats and other Lipids group is planar, but this plane does not correspond to the plane of the carboxy function, the O - - C O , the two being at an angle of 17 ° with respect to each other. This, and the fact that the structure-temperature factors for the atoms in the

R

\//

0

R

\//

C

0

C

I

I

0

0

a'/

\R'

s-irons

s- cis

FIG. 8. Conformational isomerism in the ester group due to rotational barrier about the C--O single bond. carboalkoxyl region are larger than normal, may reflect the destabilizing nature of the unfavorable s-trans conformation in the crystalline state. VII. CRYSTALLINESTATE

A. Crystal Lattice In general, long-chain compounds in the crystalline state have only one type of molecular conformation although there may be more than one molecule in the unit cell which are related by the symmetry of the crystal. The overall symmetry of the usual organic crystal may be triclinic, or monoclinic, or orthorhombic, but it is possible to describe the chain packing in terms of the local symmetry of the chains. The so-called sub-cell packing may be higher or lower than the overall cell symmetry. Thus, the triclinic and monoclinic sub-cell packing have only one type of molecular conformation as does the orthorhombic, but the latter has two chains related by symmetry at an angle relative to each other (Fig. 11). B. Normal Alcohols An interesting exception to the above statements has been discovered in a study of the crystal structure of the normal chain alcohols, s6 There are at least three low temperature forms/3, 7'1 and 7'2 for the alkanols and an a-form into which they all transform at a transition point which is several degrees below the melting point. In both/3 and a forms the chain axis is very nearly perpendicular to the base, while in both 7' forms the chain axis is at an angle of about 60 ° . The odd alkanols, Cll to C2v, exist in the/3 form as does the even CIZ alkanol. The even alkanols, ClS to C34, have the 7'1 form. The odd alkanols, C2~ and C31 278

Conformational Effects in Long Carbon Chains exhibit both the 13and 73 forms, while the Caa to C37 have the 72 form. 1' s6 The most important difference between the [3 and 71 crystals is that the latter is composed of all trans molecules only, while the former is composed of equal numbers of trans and gauche molecules. In the trans molecule, all the skeletal carbon atoms and the oxygen are coplanar, whereas in the gauche molecule, the oxygen atom is rotated out of the skeletal plane. The interesting point is that the polymorphism in the solid alcohols is related to the rotational isomerism. The differences between the infrared spectra of the [3 and 7 forms are due largely to this rotational isomerism. Thus the complexity in the spectra of the [3 form in the 1150-1400 cm-a region is due to the two overlapping wagging progressions from the trans and gauche molecules. Further, a strong band occurs in the spectra of the [3 crystals in the region of 850-950 cm -1 which is due to the rockingtwisting vibrations in the gauche molecules. 104 Similar differences are observed in the hydroxyl in- and out-of-plane bending vibrations which reflect the differences in the inter- and intra-layer distances. The a structure represents an example of the general phenomenon observed in the long-chain compounds, that of a phase in which long-chain molecules are free to rotate about their hexagonal axis.

C. Normal Hydrocarbons Four distinct crystal structures--hexagonal (all), triclinic (fiT), orthorhombic (rio), and monoclinic (flM)--describe the solid phases of normal paraffins with more than 2 carbon atoms in the chain. 18, 66 The hexagonal structure, the high temperature form, has the carbon chains perpendicular to the end group planes, and the molecules are relatively free to rotate about the long-chain axis. The orthorhombic form, rio, has the chains perpendicular to the end group plane, while the other [3M and fiT forms have the chains inclined to the cell base, and the molecules exhibit no rotation. The odd n-paraffins, (C9-C4z), have [3o as the most stable phase, which undergoes a transition to aH phase which subsequently melts. For the even n-paraffins, (C10-C28), fiT is the stable low temperature structure; it is stable until the melting point for C10 to C20, while from C22 to C26, the fiT transforms to aH and then melts. The stable form for even n from C28 to C36 is [3M which then transforms to the aH and then to the liquid. From Cz6 to C44 the stable form is [30 which transforms to aH then melts. Above C44, flo is the stable phase for both even and odd to the melting point. The aH phase is characterized by rotation of the chain about the chain-axis, so that in this phase the chain can exist in both gauche and trans conformations. This has been verified by a number of techniques.4, 44 The n.m.r, spectra show that with increased temperature the aH structure rapidly approaches the rotational behavior of the liquid state where the chain is conformationally free. Thus, the normal Cz8 hydrocarbon at 99 and 205°K shows no fine structure and the second moment is 26.6 gauss 2, at 228°K the fine structure appears and the second moment decreases smoothly to 279

Progress in the Chemistry of Fats and other Lipids 20.0 gauss e until 326~'K. At about seven degrees prior to the melting point the second moment rapidly decreases toward zero. This is a characteristic behavior of those hydrocarbons which show the aH phase. ~ Recent study of the premelting phenomenon of long-chain fatty acids has revealed the occurrence of the same liquid-like behavior, s. 33 Both the n.m.r, spectra and the characteristic infrared doublet of the orthorhombic sub-cell of the even numbered fatty acids from C10 to Cls indicate the breakdown of the crystalline character and the onset of liquid-like motion in the solid structure several degrees below the melting point. The intensities of the 720 and 727 cm -1 bands of the fatty acids were studied as a function of temperature and were shown to be replaced by a single 720 cm I band just below the melting point. The n.m.r, spectra showed correspondingly that the broad solid state proton signal (large second moment) was gradually replaced by the narrow liquid-like signal as the melting point was approached. Detailed studies by Chapman on glycerides and other longchain compounds have confirmed the general nature of the phenomenon, z° This work stresses the importance of the crystal lattice in general in stabilizing a given conformation (usually all trans). It also indicates that as the packing energy approaches the energy of rotation of a C - - C bond, rotational isomerism becomes important.

D. Symmetry Characteristics of Solid State The arrangement of molecules in a crystal lattice provides a sharp contrast to their behavior in solution since they are arranged in an infinite, repetitive array of a small number of molecular conformations. The triclinic, monoclinic, and orthorhombic crystal systems are the usual ones encountered with organic compounds. The triclinic crystal system has a minimum of no symmetry element and a maximum of a center of symmetry. The monoclinic system has a minimum symmetry of one plane and a maximum symmetry of a twofold rotatory inversion axis, 2. The minimum symmetry of the orthorhombic system is 2 perpendicular twofold axis and the maximum has 3 perpendicular mirror planes of symmetry. Thus, the molecules of a crystal, unlike those of a gas, have a relative fixed orientation in space. If the structure of the crystal is known from X-ray data, the use of polarized radiation in infrared investigations aids enormously in making vibrational assignments. Conversely, if the vibrational assignments of the free molecule are known, it may be possible to use the spectroscopic date to indicate the crystal structure, particularly in hydrogen-containing compounds for which it is difficult to obtain the positions of hydrogen atoms from X-ray data. The crystal lattice must belong to one of the 230 possible space groups. The space group of a crystal is that group (in a mathematical sense) of operations which carries each atom into an identical atom and thus generates the crystal lattice. These symmetry operations include the translations which generate the lattice, the symmetry operations familiar in crystallographic point 280

Conformational Effects in Long Carbon Chains

groups, plus the glide plane and the screw axis; the latter symmetry operations are peculiar to the crystal lattice. The unit cell is the smallest unit in which no atoms are equivalent under simple translations. However, some of them may, in general, be equivalent under any other symmetry operation. The crystal lattice is constructed from the unit cell by the translations which carry any given unit cell into another. Similarly, the complete set of symmetry elements of the crystal is obtained from those of the unit cell by these same translations. Molecules are located within the unit cell, it is the symmetry of the molecules within the unit cell which is important. The type and number of infrared and Raman active bands are determined by (I) the molecular symmetry; (2) the so-called "site" symmetry (describing the symmetry of the molecule within the crystal), and (3) the symmetry of the whole crystal. Since the number of molecules in a unit cell is small, the infrared and Raman molecular modes are split into a few very sharp lines; the actual wide bands that are experimentally found are due to combination lines with lattice vibrations and indicate anharmonic coupling between molecular and lattice modes. Selection rules for these bands will correspond to the internal modes of the molecule obtained by considering the individual molecules in light of the actual local symmetry in the unit cell. 45 This local symmetry will be contained in the crystallographic point group that describes the unit cell. VII. POLARIZED INFRARED RADIATION

A. Technique The infrared absorption spectrum for a single crystal, obtained with plane polarized radiation, will depend upon the orientation of the crystal relative to both the plane of polarization and the direction of incidence of the radiation. 41 Any fundamental mode of a molecule in the crystal which is capable of absorbing radiation must be accompanied by an oscillating electric dipole moment. Thus, since the electric dipole moment is a vector, it will be incapable of absorbing radiation that is polarized in a plane perpendicular to that direction. The structurally significant directions are defined inside the crystal whereas the plane of polarization of the radiation, and its direction of propagation, are defined only outside. To correlate the internal and external directions it is necessary to select certain orientations of the crystals such that the plane of polarized radiation traverses it without undergoing either refraction or a change of polarization. If this is not possible, the plane of polarized radiation will undergo complex refractions and all simple correlation will be lost. These requirements upon the behavior of light inside the medium can be met under certain special conditions. The radiation must be normal to the plane containing two principal axes of the dielectric ellipsoid of the crystal and have the electric vector parallel with one or the other of these principal axes. The orientation of the principal axes in the medium also can depend upon the frequency of light if the 281

Progress in the Chemistry of Fats and other Lipids crystal symmetry is low. Further, for simple interpretation, the axes of the dielectric ellipsoid must coincide with the axes of the conductivity ellipsoid (the latter is a condition for absorbing crystal). Even if this last criterion is met, the relative orientation of the two axes may still be frequency dependent. However, the polarization axes are fixed by symmetry for many forms of crystals and become independent of the frequency of light. In uniaxial crystals of the hexagonal, trigonal and tetragonal systems and for (bi) axial crystals of the orthorhombic system, all distinguishable polarization axes are symmetry fixed along the crystallographic axes. The spectra observed along these axes will assume a fundamental significance, the ones for other orientations being representable as superpositions of them. The crystals of the regular system have no polarized spectra. For the monoclinic system one axis is symmetry fixed along the b axis but the other two are unrestricted within the a c plane. The triclinic crystal system has no restriction upon the axes. 69 B. P o l y m e t h y l e n e C h a i n

A diagramatic representation of plane polarized light is given in Fig. 9. The light ray is polarized in the x z plane and is characterized by an oscillating

X

I

¥

-j

E = EoCOS 2 T=, ( t -

)

H

FIG. 9. Plane polarized light with the electric vector, E, in the xz plane and magnetic vectorH, in yz plane.

electric dipole which lies within this plane. It is this oscillating electric dipole that interacts with the dipoles that are generated by the normal modes that are infrared active. The polymethylene chain, --(CH2)--n, provides a model for a study of the interaction with plane polarized radiation. The polymethylene polymer molecule crystallizes in the monoclinic crystal system with an orthorhombic sub-cell. The five infrared active fundamental vibrational frequencies of linear polymethylene chains are listed in Fig. 10.88, 7o The packing of the polymethylene chains in an all t r a n s manner means that the symmetry of the molecule is such that radiation is either polarized parallel to the axis or perpendicular to the plane of the carbon chain which contains the axis (or vice versa). Fundamental vibrational modes listed in Fig. 10 give also the 282

Conformational Effects in Long Carbon Chains direction of change of dipole generated by this fundamental mode. As will be seen, the mode in Fig. 10 b is along the chain axis whereas those in the other modes are perpendicular to this axis. The polymethylene chain is packed in an

z (a)

~

El

(c)

H

Y

1463,

X- ~ - - ' ' ~ - ' ' ' ; "

1 4 7 3 crn"

(d)

(b}

720,

731 cm - I

(il)

(.L)

•"~---~ ( e )

2851

cm -=

1176 em -I

I (f) 2919 cm -= FiG. 10. Infrared active normal modes of infinite hydrocarbon chain. Wave numbers of components in crystal along with direction of transition moment indicated by double-headed arrow. Diagrams (c), (d), (e) and (f) represent view along chain. Direction of polarization indicated in (d).

orthorhombic sub-cell, Fig. 11, in which there are two polymethylene repeat units whose planes are nearly at right angles to each other. As a consequence of this, those vibrational modes which are parallel to the chain axis could either 283

Progress in the Chemistry of Fats and other Lipids be in- or out-of-phase with those of the other chain. I f they are out-of-phase, the band will be infrared inactive, but if they are in-phase, they will be infrared active so that only one band will be observed. Bands having perpendicular dipole moments do not cancel out, since they are at nearly 90 ° with respect to each other. Both the in- and out-of-phase combinations will produce infrared active bands. These two, however, will be polarized perpendicular to each other.

C = 7.40-7.95

~, I

2 . 4 5 - 2.63

C=

~+0.1991

7.40

-

+ 0.8009

I 4,--0, 4 ",o÷}4

(3

7.95 .~

0.8009

5.40-5.82

A~ '~+ 0.1991

I

D÷if *4 °"

~+0.1991

b

.

4.92- ..5.02 A

ORTHORHOMBIC

÷31¢

b

i

+0,8009

4 . 2 5 - 4.42 ,~

TRICLINIC

SUBCELL

I

I

I

0

I

2

I

~+0.,991 &+ 0.8009

SUBCELL

o

3A

FIG. 11. The orthorhombic and triclinic sub-cells with average dimensions.

Thus these components are observed to be polarized along the a and b crystal axes. The n-hexatriacontane molecule crystallizes in the monoclinic system with an orthorhombic sub-cell. The chains are approximately perpendicular to the basal plane of the cell. Again the same polarization phenomenon is observed in the infrared with the 720 doublet bands. The 720 cm -1 bands polarized along the b axis, while the 730 cm -1 is polarized along the a axis. 54 C. Structural Studies

The crystal structures of long-chain carboxylic acids have been primarily determined using X-ray crystallography. Polarized infrared radiation can be used to provide evidence on the crystal structures and mode of packing. Several examples of the use of this technique will be briefly discussed. 284

Conformational Effects in Long Carbon Chains The C-form o f stearic acid is monoclinic and packs with h y d r o c a r b o n chains tilted at 56 ° with respect to the end g r o u p plane. 98 The h y d r o c a r b o n chain packs with an o r t h o r h o m b i c sub-cell, with the chain axis (roughly) parallel to the

"t. \

\

Q

\\

r~

I

~

lJ

c c_. 2

\\

\

I

II

II

< < Q

G

Fio. 12. A, Schematic ofmonoclinic structure of C-form of stearic acid; B, proposed orthorhombic structure of vaecenic acid; C, orthorhombic sub-cell projected along main axes of hydrocarbon chains. C-axis o f the unit cell. The unit cell o f the C - f o r m o f stearic acid is represented in Fig. 12 a, in which the ac plane is viewed along the b crystallographic axis. A n oriented film o f the C-form o f stearic acid is prepared as a sandwich as

b'

I "-k", 0

FIo. 13. Orientation of electric vector with respect to the sample; *, direction of main axes of the hydrocarbon chains of C-form of stearic acid; c, directi on of the main axes of hydrocarbon chains of vaccenic acid; a, b, a', a", b' and b", directions of electric vector of polarized radiation. indicated in Fig. 13, with the ab plane parallel with the salt plate surface and the chains are oriented with their axes as indicated by an asterisk (*) in Fig. 13. In the monoclinic system, the b axis is arbitrarily chosen as the axis o f symmetry 285 U

Progress in the Chemistry of Fats and other Lipids (indicated in Fig. 12a) so that the all trans carbon skeleton of the chain ties roughly along the c axis. Spectra were recorded with the electric vector (1) along the a axis and the b axis (perpendicular incidence); (2) at ~ 4 5 ° to a while perpendicular to b (a' and a"); (3) at ± 4 5 ° to b while perpendicular to a (b' and b"). The spectra obtained with the electric vector along b' and b" were identical as required by the monoclinic symmetry. The a" orientation is approximately parallel to the chain of the hydrocarbon and a' is approximately perpendicular to this chain. Thus the vibrational modes for the chain are polarized in the ac plane along the chain axis and perpendicular to the chain. 94 In n-hexatriacontane, the orthorhombic sub-cell packing caused the 720 cm -t rocking-twist band to split into a doublet at 720 and 729 cm -1. The polarization of the 720 cm -1 band is along the b axis and 729 cm -1 band along the a axis. The other related band sequences of the hydrocarbon chain show a similar type of polarization. The carboxyl dimer with its eight-membered ring (see Fig. 12a) has its vibrational modes polarized in the ac plane and perpendicular to the b axis. Thus the very strong C - - O stretching band has strong, but nonidentical, components along both the a' and a" directions at 1701 cm -1. Vaccenic acid, t r a n s - l l - o c t a d e c e n o i c acid, has not been studied by single-crystal X-ray crystallography; however, studies on a series of t r a n s - o c t a d e c e n o i c acids, with the double bond position ranging from 6 to 12, have been made using powder diffraction. 61 It was inferred that the acids, having an odd number of methylene groups between the double bond and the carboxyl group, have the axes of the hydrocarbon chain approximately perpendicular to the plane containing the carboxyl groups while those of the even series were tilted. An oriented crystal of vaccenic acid was obtained in a manner similar to that used for stearic acid and studied by following the same procedure. 93 The spectra obtained with the electric vector along a' and a" were identical, as were the spectra observed along the b' and b" directions (Fig. 13). The identity of the spectra indicates that vaccenic acid is packed in the orthorhombic crystal system. A proposed structure is given in Fig. 12b. The doublet at 720 and 729 cm 1 is polarized along the b and a directions, respectively, in a manner similar to n-hexatriacontane, and is consistent with an orthorhombic sub-cell. Interestingly, the characteristic 965 cm ~ band, due to the out-of-plane deformation of the olefinic carbon hydrogen bond, is strongly polarized along the a axis and very weakly along the b axis. This indicates that the planes of all double bonds are approximately parallel to each other. Consistent with this is the polarization of the olefinic stretching band at 3015 cm ~ along the b axis. Further, the carbonyl stretching band is split into a doublet at 1701 and 1714 cm -1, the former being polarized along the a axis and the latter along the b axis. The isomeric t r a n s - 8 - o c t a d e c e n o i c acid showed, in contrast to the transll-isomer which belongs to the orthorhombic system, that it had crystallized in the triclinic crystal system/7 The axes of the dielectric and conductivity elipsoids are not symmetry fixed in the triclinic crystal system as is required for the "simple" interpretation of the spectra obtained with polarized radiation. 286

Conformational Effects in Long Carbon Chains

The characteristic bands at 720 and 1470 cm -1 were found not to be split or polarized in any simple fashion. This indicates the absence of the orthorhombic sub-cell packing for the hydrocarbon chains. The relative polarization of the methylene groups suggest that the major portion of the hydrocarbon chains are parallel. The splitting of the carbonyl stretching and OH out-of-plane bending bands into oppositely polarized sets, suggests two types of carboxyl group dimers. Further, the planes of the carboxyl groups are not parallel to the major portions of the hydrocarbon chain. The olefinic out-of-plane bending frequency is split into a doublet at 959 and 966 cm -1 which are polarized in the same direction. These data suggest that the structure has roughly parallel hydrocarbon chains that are tilted with respect to the layers and are connected to sets of non-parallel carboxyl groups to produce an overall triclinic structure. It is clear from this brief survey that the polarized infrared technique provides a means of studying the molecular organization in a crystal lattice without having to resort to detailed X-ray single-crystal studies. The correspondence between the information obtained for the C-form of stearic acid from polarized infrared and information known from the X-ray crystallographic studies strongly supports this conclusion. However, it must be stressed that care must be taken when the symmetry of the systems is low because of the possible complicated interactions. VIII. HYDROGEN BONDING

A. Definition The concept of the hydrogen bond was introduced in 1920 by W. M. Lattimer and W. H. Rodebush in a classic paper, in which they proposed "that a free pair of electrons on one water molecule might be able to exert sufficient force on a hydrogen held by a pair of electrons on another water molecule to combine the two molecules together". 56 The extraordinary paper opened up an entirely new field of investigation in the subsequent decades. This entire field has been studied and summarized in an elegant book by Pimentel and McClellan who provided the following operational definition of a hydrogen bond: "a hydrogen bond exists between a functional group A - - H or a group of atoms B in the same or a different molecule when (a) there is evidence of bond formation (association or chelation), and (b) there is evidence that this new bond linking A - - H and B specifically involves a hydrogen already bonded to A". 78 The specific concern here is with A - - H , a hydroxyl function, and B, an electron pair associated with a double bond or an electron cloud of an aromatic system and how their interaction modifies the conformation.

B. Saturated Alcohols The introduction of high resolution infrared spectroscopy has permitted 287

Progress in the Chemistry of Fats and other Llplds

the detailed analysis of the band-shape of the hydroxyl vibrational stretching region. Studies of the infrared spectra and the band-shape in the hydroxyl region of a number of alcohols have led to the distinguishing of a variety of the hydroxyl bonds: the free monomer, dimer and higher polymers, m, ~7, 92 The questions as to whether the dimer is linear or cyclic and whether the higher polymeric species are linear or cyclic are under active discussion, m, a7 The absorption frequencies of these species are given in Fig. 14. The conformational

H R 0"~3650

~ 3 6 2 3 cm-I

cm" ~

N ....

0

O~

~ 3 4 9 7 cm- I

I

R

R

R,~3378 cm -I

H

/

o .i:.o

0

/

H - - 0

H

H

1

~3673 em-I

\ R

~ ' ~ 3 4 9 7 c m -I

3 3 7 8 c m -= to 3 6 2 3 c m - '

Fic. 14. Types of hydroxyl bands along with their characteristic absorption frequency.

heterogeneity of the saturated alcohols--ethanol, isopropanol and 2-butanol-was first clearly recognized by Winstein 76 and Schleyer,85 both in this series and in extensive series of structurally related compounds. This conformational heterogeneity was attributed to the occurrence of a restricted rotation about the C - - O bond as represented in Fig. 15 for primary and secondary alcohols. There are two equivalent gauche conformations and the anti conformation in which the OH bond is turned away from the substituent. The free OH band of methanol and of t-butanol are symmetric since the conformers are identical. Recent studies have shown that n-propanol has three partially resolved bands at 3638.5, 3632.5 and 3626.7 cm -1 which have temperature-dependent relative intensities. 5~ There are nine possible conformers (see Fig. 16) due to rotation about both the C - - C and the C - - O single bonds. These nine consist of four pairs of mirror images (see Fig. 16) (d, e), (f, g), (h, i), b and c which are spectroscopically indistinguishable, along with remaining conformer a. Temperature 288

Conformational Effects in Long Carbon Chains

H H

C GAUCHE A,

H

H C C ANTI GAUCHE PRIMARY ALCOHOLS H

Cz

C°H~

C|~

H

Cz

Ci~

CzH

H

H

B, SECONDARY ALCOHOLS FIG. 15. Rotationalisomersof (A) primary,and (B) secondaryalcohols,at angles of potentialminimawithrespectto rotationaboutC--O singlebond.

'

H

'

H H~y

(a)

,O~H

H

;-H" HI

(d,e)

H

(b)

H

Ha C d ~ .

H'

H H~O

H' H

(c)

~z H=w,,?

H~ C ~ 7 ~ / " / ~'H" H HI

HsC ~-~" H" H H'

(f,g)

(h,i)

FIG. 16. Conformational isomers of 1-propanol. 289

Progress in the C h e m i s t r y of Fats a n d other Lipids

studies suggest that the stablest conformer is that which has the all "trans" structure, a, and that it has essentially the same energy as the mirror image pair, b and c. Next conformer in order of decreasing stability is the pair (d, e) and then (f, g), (h, i) being thought to be too unstable to exist. As will be observed, I

I

I

I

I

0

I00

-1

S/

T~ T,. 0

'sV

I._1

4-

50

)t-

/

I,n nO

25

I

0

I

3650

3600

WAVES / CENTIMETER FIG. 17. Infrared hydroxyl b a n d s o f m o n o m e r i c s a t u r a t e d alcohols in CCla. Curves displaced for vertical clarity. Subtract scale cortstant, S, f r o m each curve to get absorptivity: 1-octadecanol, S = 55; 2-octadecartol, S = 25; 9-octadecanol, S = 45; m e t h a n o l , S = -- 30; tertiary butyl alcohol, S = -- 3

290

Conformational Effects in Long Carbon Chains the forms are listed in order of increasing interaction with the methyl group. As the chain length is increased, the shift becomes more profound, as indicated in Fig. 17, for 1-octadecanol, 2-octadecanol and 9-octadecanol. 3n Detailed computer analysis of the 1-octadecanol spectra shows that it can be broken into band envelopes, at 3639 and 3627 cm-1. 37 Thus the changes in the band shapes of the octadecanols in Fig. 17 reflect changes in population of the conformational isomers due to the increased interaction with alkyl groups. The apparent lack of resolution of the higher alkanols into only two bands either reflects inadequate resolution or that the environment of the conformational isomers becomes very similar.

C. Unsaturated Alcohols The introduction of a double bond into an alkanol produces a new phenomenon. It is observed that hydrogen bonds of low strength are formed between hydroxyl groups and the 7r-electron cloud in both the aromatic and olefinic systems.7, 72, 85 Thus it was shown that 2-allylphenol has two bands, one due to the free hydroxyl at 3605 cm -1, and one to an intramolecularly bonded species at 3542 cm-1. 7 It has been observed in the alkenols, CH2=CH(CH2)nCH2OH, that new hydroxyl bands beyond those due to the free hydroxyl conformers, are observed in allyl alcohol, n = 0, and in homallyl alcohol, n = 1, but are not observed for n greater than 2. 72, 85 The hydroxy unsaturated esters have shown a similar phenomenon) 7, 40

D. Homoallylic Alcohols The homoallylic system found in methyl ricinoleate and ricinelaidate (Fig. 18) has been extensively studied in this laboratory. 37 The conformational heterogeneity observed in the 2- and 9-octadecanol (Fig. 17) is complicated by the interaction of the hydroxyl group with the homoallylic double bond. The homoallylic system is diagrammatically represented in Fig. 19, in which the C = C is graphically illustrated by the coparallel pz orbitals to which is connected a - - C H 2 - - C H ( O H ) - - group. As will be seen from the diagram in Fig. 19, the ability of the system to have free rotation about the C - - C - - C - - O - - function is a requisite for the hydroxy group to interact with the ,r-molecular orbitals of the double bonded system. As is clear from Fig. 19, the geometry of the 9,10 bond is maintained by the rr-orbitals, while the 10,11 C - - C bond has a rotational barrier which has at least six-fold symmetry. The 11,12 C - - C and the C - - O bond have a three-fold rotational barrier. In order to study the possible rotational species that permit the OH group to interact with the C = C , a twelve-fold potential barrier was assumed for the 10,11 bond, while the other barriers were assumed to be three-fold. This former assumption is purely ad hoc and is used simply to enable a more complete description of the possible rotomers. With these assumptions the homoallylic system in the open chain methyl ricinoleate 291

Progress in the Chemistry of Fats and other Lipids

and methyl ricinelaidate each have 108 conformational isomers. Twenty-four of these possib|e isomers (or 22 per cent) have the hydrogen atom within 3-0 A of the C10 atom, while there are 10 isomers (9 per cent) in which the distance is less than 3 ~ from the C3 atom. The projection of the bond distances onto I

I

I

u

7,,,

30

._I 0

~E

ILl .J

20

>: I.>

:H~(CHr)sCHOH n-

I

c., .

O U)

/

I0 3650

C=C

X

H 3625

3600

3575

WAVES

I

I

3625

3600

(CHz)rCO2CH-~ I 3575

3550

I CENTIMETER

FIG. 18. Monomeric hydroxyl bands of methyl ricinoleate and methyl ricinelaidate, bzth in CC14.

H FIG. 19. Schematic of homoallylic system showing interaction of OH group with pz-orbitals of the double bond.

292

Conformational Effects in Long Carbon Chains x y plane of 22 isomers of the former class (eleven pair) are plotted in Fig. 20. The nodal plane of the C = C is taken as the x y plane. The

values of z represent the distance i n / k above the nodal plane. The distances of the Cll, C1~, and O and H atoms of the various rotomers are listed for half of the isomers. The other half lie with a negative value of z below the nodal plane. This plot represents the conformational isomers which

t

0

/

//':~

.

/

o...,///

+2

t

I

o-"~>.~,

c-,2

/A,i,,,".,",

÷1

0

01.17

•

--

C-9

C-IO

H

H l+z) -I

I

-2

-I

I

0

I

I

+I

x,

+2_

I

+3

+'~

.~

FIG. 20. Projection onto the plane (xy) of the double bond showing the positions of the key atoms of the conforrnational isomers of homoallylic system. The isomers shown have the H atom of the OH within 3 A. of the C9 or C10 atoms, The numbers listed next to atoms are distances in/~ above xy plane.

have the H atom of the OH within the interacting distance (3 A) of the C = C system. As will be clear from consideration of Fig. 19, the interaction of the eis methylene groups on the double bond (in methyl ricinoleate) will tend to destabilize some of the conformational isomers relative to those of the trans isomers. This will be reflected in a different ratio of conformational isomers. The shape of the hydroxyl envelope (Fig. 18) reflects this difference in the ease of interaction of the cis and trans isomers of the open chain system. If the 11,12 bond is partially frozen by imroducing further conformational requirements, the envelope of the free hydroxyl band should change. This will occur if the homoallylic system is made part of a ring system. As will be seen in Fig. 21, the C5 or cyclopentane system shows no band corresponding to the interaction of the hydroxyl group with the double bond. This is a consequence of the 293

Progress in the Chemistry of Fats and other Lipids unfavourable stereochemistry of the trans substituents on the cyclopentane ring. As a result, only the two characteristic bands of secondary alkanol are seen at 3622 and 3606 cm -1. On the other hand, if the system is part of the C6 ring, the ring geometry is more favorable. In the cyclohexane system, Fig. 21, the 3621 cm -1 band is now only a shoulder while the band at 3585 cm -1 corresponding to the interaction of the hydroxyl group with the double bond is

I,,iJ 0 Z t-I¢0 Z n*" I--

H(~ OH II C-H

HC OH II C-H

I

(CHz)sCH:

I

5576

I

(CHz)4CH: I

5621 I

5585 36122

,OH

H~'C= c~(Hc Hz}sCH3

FIG. 21. Infrared hydroxyl bands of trans-2-(l'-cis-octenyl)-cyclopentanol, trans2-( l'-cis heptenyl)-cyclohexanol and trans-2-(l'-cis-hexenyl)-cycloheptanol, all in CC14. now the most intense. This is a consequence of the favorable geometry of the hydroxyl group relative to the double bond and the inflexibility of the ring. A flexibility resembling that of the open chain system is obtained with the cycloheptane system (Fig. 21) where the hydroxyl envelope would resemble that of the open chain isomer; thus the relative intensities of the band at 3620 and 3574 cm -1 are similar to those for methyl ricinoleate and methyl ricinelaidate. A detailed analysis of the possible conformational isomers was made for the C5, C6 and C7 rings. The C5 ring system has 720 possible isomers of which only 5 per cent have a proton within a distance of less than 3/k from the C = C group. The C6 ring has 72 possible conformational isomers of which 22 per cent are less than 3 ,~, while the C7 ring system has 504 possible isomers of which 14 per cent are less than 3/~. The latter figure is similar to that for the open chain 294

Conformational Effects in Long Carbon Chains system. Interestingly enough, if the double bond is replaced by the triple bond a similar phenomenon is observed. A preliminary computer analysis of the free monomeric hydroxyl groups in the cyclic olefinic alcohols is given in Table 2. The data in Table 2 were obtained by reconstructing the infrared

Table 2. Infrared bands of monomeric OH groups olefinic alcohols

NON-BONDED OH

COMPOUND PEAK

AREAl

HYDROGEN-BONDED OH PEAK

A1/^2

AREA2

OH

~

CH'CR(CH2)sCH3

36221 3606 cm -1

OH I I

~CH'CH(CH2)4CH

3

3621

3,58

3854 cm"1

11.31

3574

19.69 cm"1

5. 500

15.68

1.386

OH

.(~)-e'c"c%>3c~3 I

3620

curves using a computer program which resolved the experimental envelope into the constituent curves of the various conformational species. The ratio of the relative areas of the hydrogen-bonded O H peak to the non-hydrogenbonded O H peak is an approximate measure of the relative number of the conformational isomers in solution. A more complete treatment of the homoallylic system is in preparation. 37

E. Saturated Hydroxy Esters A detailed study was made of the hydroxyl envelope of methyl 12- and 10hydroxyoctadecanoates? 6 The absorbance of the infrared curves was plotted against concentration for a selected number of frequencies and curves extrapolated to zero concentration. In this manner, it was possible to reconstruct an infrared curve for the 10- and 12-hydroxy esters at zero concentration. When the reconstructed curves were compared, it was found, surprisingly, that the curves were different although there was no a priori reason to suspect that merely changing the position of the hydroxy group from the 10- to the 12position would in any way change the character of the hydroxyl envelope. When 295

Progress in the Chemistry of Fats and other Lipids the absorbance curves were compared in a differential manner it was found that there were three peaks, at 3622, 3594, and 3541 cm -1. The former two correspond to the two conformational isomers for a hydroxyl in the middle of a long chain. The third band at 3541 cm 1, which is weak, corresponds to some type of interaction of hydroxyl with some other functionality. The fact that the absorbance curves were not identical suggested that another rotational conformational species is present. The position and relatively low intensity of the band at 3541 cm -1 suggests that it is due to an interaction of the hydroxyl group with a carbomethoxy group. Consideration of the models for such species, Fig. 22, H2 C

C6H13

\/\ H

H2

0 H2 C I

;

/CH~.

o

/

H2 C

c

CH3

\/ c H~

FIG. 22. Intramolecular hydrogen-bonded species formed in methyl 12-hydroxystearate.

suggests that the band at 3541 cm -1 arises from a small concentration of an intramolecularly hydrogen-bonded species. The difference between 10- and 12-hydroxy esters is thought to arise from interactions of methylenes across the ring of the hydrogen-bonded species. In the 10-hydroxy cycle the repulsive interactions are simply too large, whereas in the 12- they occur but do not destabilize these species. This is similar to the so-called transannular interaction observed in the medium ring. On the basis of the observed intensities it is estimated that there is about 1.5 mol ~ of this hydrogen-bonded species in methyl 12-hydroxystearate.

F. Diglycerides A study has been made of hydroxyl bands in the 1,2- and 1,3-diglycerides. 95 296

Conformatienal Effectsin Long Carbon Chains The spectra of the two isomeric diglycerides are shown in Fig. 23. The characteristic bands of both the free primary hydroxy! and the free secondary hydroxyl group have been modified by hydrogen bonding with the neighboring carboalkoxy function. Thus, the 1,2-diglycerides, which have a free primary hydroxy group, show the characteristic bands 3640 and 3624 cm -1 due to the two conformational isomers of primary alkanols (see Fig. 17). The additional band at IOO

%T eO

!

60

" \',/; t ~e~u4

40

r •36£4

ZC

3531

I 3700

o o,oo?. - - - - -

I 3600

1,3DIST. I 3500

O.OIO08M I 3400

cm -~

FIG. 23. Infrared curves 6f fundamental OH stretching region of 1,2 and 1,3-diglycerides.

3531 cm -1 is due to the conformer (or conformers) in which the hydroxy group is intramolecularly hydrogen-bonded with a carboalkoxy function. Conversely, the 1,3-diglycerides which contain a secondary alcohol show tee characteristic bands at 3624 and 3604 cm -1, and show, in addition, a band at 3490 cm -1 due to the hydroxy group which is intramolecularly hydrogen-bonded to the adjacent carboalkoxy function. In summary, the wide gradation in behavior observed for long-chain polymethylene compounds reflects both the nature of the substituents and the state in which they are found: the gaseous, solid and liquid states. The importance of conformation in determining the observed infrared spectra is well illustrated by the changes that are observed in passing from a liquid and/or gaseous state to the solid state. ACKNOWLEDGMENTS

The author wishes to thank his colleagues Drs. C. R. Eddy and H. Susi 297

Progress in the Chemistry of Fats and other Lipids for their patient a n d u n d e r s t a n d i n g guidance. The help of Mr. A. J. M e n n a in the p r e p a r a t i o n of diagrams, a n d the m a n y constructive criticisms of his coworkers are all gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. I I. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

ABRAHAMSSON,S., LARSSON,G., and von SYDOW,E., Acta Cryst. 13, 770 (1960). ALEBY~S., and YON Svoow, E., Acta Cryst. 13, 487 (1960). ALEBY,S., Acta Cryst. 15, 1248 (1962). ANDREW,E. R., J. Chem. Phys. 18, 607 0950). AXFORD,D. W. E., and RANK, D. H., J. Chem. Phys. 17, 430 (1949). AXFORD,D. W. E., and RANK,D. H., J. Chem. Phys. 18, 51 (1950). BAKER,A. W., and SHULGIN,A. T., J. Am. Chem. Soc. 80, 5358 (1958). BARR,M. R., DUNELL,B. A., and GRANT,F. R., Can. J. Chem. 41, 1188 (1963). BARTELL,L. S., CARROLL,B. L., and GUILLORY,J. P., Tetra. Letters, 1964, 705. BARTELL,L. S., and KOHL, D. A., J. Chem. Phys. 39, 3097 (1963). BASTIANSEN,O., and HADLER,E., Acta Chem. Scand. 6, 214 (1952). BEAUDET,R. A., J. Chem. Phys, 38, 2548 (1963). BEAUDET,K. A., J. Chem. Phys. 40, 2705 0964). BEAUDET,R. A., and WILSON,E. B., JR., J. Chem. Phys. 37, 1133 (1962). BERNSTEIN,H. J., J. Chem. Phys. 17, 258 (1949). BONHAM,R. A., and BARTELL,L. S., J. Am. Chem. Soc. 81, 3491 (1959). BONHAM,R. A., BARTELL,L. S., and KOHL,D. A.,J. Am. Chem. Soc. 81, 4765 (1959). BROADHURST,M. G., J. Res. Nat. Bur. Stand. 66A, 241 (1962). BROWN,J. K., SHEPPARD,N., and SIMPSON,D. M., Disc. Faraday Soc. 9, 261 (1950). BROWN,J. K.., and SHEPPARD,N., J. Chem. Phys. 19, 976 (1951). BROWN,J. K., and SHEPPARD,N., Trans. Faraday Soc., 48, 128 (1952). BROWN,J. K., and SHEPPARD,N., Trans. Faraday Soc. 50, 1164 (1954). BROWN, J. K., SHEPPARD,N., and SIMPSON,D. M., Phil. Trans. Roy. Soc. London, A, 247, 35 (1954). BROWN,J. K., and SHEPPARD,N., PFOC.Roy. Soc. 231A, 555 0955). BUNN,C. W., and HOLMES,O. R., Disc. Faraday Soc. 25, 95 0958). BUTCHER,S. S., and WILSON,E. B., JR,, J. Chem. Phys. 40, 1671 (1964). BUTLER,J. N., and McALPINE, R. D., Can. J. Chem. 41, 2487 (1963). CHAPMAN,D., J. Chem. Soc. 1957, 4489. CHAPMAN,D., J. Chem, Soc. 1958, 784. CHAPMAN,D., Chem. Rev. 62, 433 (1962). COaURN,W. C., JR., and GRUNWALD,E., J. Am. Chem. Soc. 80, 1318 (1958). CORISH,P. J., and DAVISON,W. H. T., J. Chem. Soc. 1955, 2431. COTTON,F. A., Chemical Applications of Group Theory, Interscience, New York, 1963, DAUBEN, W. G., add PITZER, K. S., Chap. I. "Conformational Analysis", in M. S. NEWMAN'SSteric Effects in Organic Chemistry, J. Wiley, 1956. DAVISON,W. H. T., and CORISH,P. J., J. Chem. Soc. 1955, 2428. EDDY, C. R., SHOWELL,J. S., and ZELL,T. E., JAOCS, 40, 92 (1963). EDDY, C. R., SI-IOWELL,J. S., unpublished data. ELLIOTT,A., Advances in Spectro. 1,214 (1959). GRANT,R. F., and DUNELL,B. A., Can. J. Chem. 38, 359 (1960). GRUNDY,J., and MORRIS,L. J., Spectrochim. Acta, 20, 695 (1964). HALFORD,R. S., J. Chem. Phys. 14, 8 (1946). HARRAH:L. A., and MAYO,O. W., J. Chem. Phys. 33, 298 (1960). HERSCHBACH,D. R., and KRISHER,L. C., J. Chem. Phys. 28, 728 (1958). HOFEMAN,J. D,, J. Chem. Phys. 20, 541 (1952). HORNIG,D. F., J. Chem. Phys. 16, 1063 (1948). HOWARD,J. B., J. Chem. Phys. 5, 442; 451 (1937). JAHN,A. S., and Susl, H., J. Phys. Chem. 64, 953 (1960). JONES,R. N., McKAY, A. F., and SINCLAIR,R. G., J. Am. Chem. Soc. 74, 2575 (1952). JONES,R. N., Spectrochim. Acta, 9, 235 (1957). 298

Conformational Effects in Long Carbon Chains 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103.

JONES,R. N., Can. J. Chem. 40, 301 (1962). JONES,R. N., and R.IPLEY,R. A., Can. J. Chem. 42, 305 (1964). KEMP,J. D., and PITZER,K. S., J. Chem. Phys. 4, 749 (1936). KILPATRICK,J. E., and PITZER,K. S., J. Res. Nat. Bur. Stand. 37, 163 0946). KR~MM,S., J. Chem. Phys. 22, 567 0954). KRUEGER,P. J., and METTLE,H. D., Can. J. Chem. 42, 347 (1964). LATIMER,W. M., and RODEBUSH,W. H., J. Amer. Chem. Soc. 42, 1419 (1920). LIDDEL,U., and BECKER,E. D., Spectrochim. Acta, 10, 70 (1957). LIDE,D. R., JR., J. Chem. Phys. 29, 1426 (1958). LIDE,D. R., JR., J. Chem. Phys. 33, 1514 (1960). LIDE,D. R., JR., J. Chem. Phys. 33, 1519 0960). LUTTON,E. S., and KOLP, D. G., J. Am. Chem. Soc. 73, 2733 (1951). MILLEN,D. J., Progress in Stereochemistry, 3, 138 (1962). MILLS,I. M., and THOMPSON,H. W., Proc. Roy. Soc. 226A, 306 (1954). MIZUSHIMA,S., and OKAZAKI,H., J. Am. Chem. Soc. 71,3411 (1949). MIZUSHIMA,S., Structure of Molecules andlnternal Rotation, Academic Press, New York, 1954. MNYUKH,YU. V., J. of Phys. and Chem. of Solids, 24, 631 (1963). MORINO,Y., MIZUSHIMA,S., KURATANI,K., and KATAYAMA,M., J. Chem. Phys. 18, 754 (1950). MULLIKEN,R. S., Rev. Mod. Phys. 14, 265 (1942). NEWMAN,R., and HALFORD,R. S., J. Chem. Phys. 18, 1276 0950). NIELSEN,J. R., and HOLLAND,R. F., J. MoL Spectro. 4, 488 (1960). NOVAK,I. I., and SOLV'EV,E. S., Optika i Spektroskopiya, 2, No. 1, 62 (1957). OKI, M., IWAMURA,H., Bull. Chem. Soc. Japan, 32, 567 (1959). PARR, R. G., and MULLIKEN,R. S., J. Chem. Phys. 18, 1338 (1950). PENTIN,YU. A., and TATEVSKII,V. M., Zhur. Fiz. Khim. 31, 1830 (1957). PENTIN,YU. A., PANCHENKO,YU. N., TRESHCHOVA,E. G., and SHARIPOV,Z., Optikai Spektroskpiya, 10, No. 1, 55 (1961). PICCOLINI,R., and WINSTEIN,S., Tetra. Letters, No. 13, 4 (1959). PIERCE,L., and O'R.EILLY,J. M., J. Mol. Spectro. 3, 536 (1959). PIMENTEL,G. C., and MCCLELLAN,A. L., The Hydrogen Bond, W. H. Freeman and Co., San Francisco, 1960. PITZER,K. S., J. Chem. Phys. 8, 711 (1940). PITZER,R.. M., and LIPSCOMB,W. N., J. Chem. Phys. 39, 1995 (1963). POWLING,J., and BERNSTEIN,H. J., J. Am. Chem. Soc. 73, 1815 (1951). RANK,D. H., SHEPPARD,N., and Szgsz, G. J., J. Chem. Phys. 17, 83 (1949). SAGE,M. L., Jr. Chem. Phys. 35, 142 (1961). SCHACHTSCHNEIDER,J. H., and SNYDER,R. G., Spectrochim. Acta, 19, 117 (1963). SCHLEYER,P. VONR.., TRIFAN,D. S., and BACSKAI,R., J. Am. Chem. Soc. 80, 6691 (1958). SETO,T., Memoirs of College of Science, University of Kyoto, Series A, 30, 89 (1962). SHEPPARD,N., and SzAsz, G. J., J. Chem. Phys. 17, 86 (1949). SHEPPARD,N., Advances in Spectroscopy, 1,288 (1959). SIEGEL,S., J. Chem. Phys. 27, 989 (1957). SINCLAIR,R.. G., MCKAY, A. F., and JONES, R. N., J. Am. Chem. Soc. 74, 2570 (1952). SNYDER,R. G., and SCHACHTSCHNEIDER,J. H., Spectrochim. Acta, 19, 85 (1963). SMITH,F. A., and CREITZ,E. C., J. Res. Nat. Bur. Stand. 46, 145 (1951). SusI, H., J. Am. Chem. Soc. 81, 1535 (1959). StrsI, H., and SMITH,A. M., JAOCS, 37, 431 (1960). Sust, H., MORRIS, S. G., ZELL, T. E., and SCOTT,W. E., JAOCS, 40, 329 (1963). SUTULA,C. L., and BARTELL,L. S., J. Phys. Chem. 66, 1010 (1962). SWALEN,J. D., and HERSCrmACH,D. R., J. Chem. Phys. 27, 100 (1957). SYDOW,E. v., Arkivfor Kemi, 9, 231 (1956). SZASZ,G. J., SHEPPARD,N., and RANK, D. H., J. Chem. Phys. 16, 704 (1948). SZASZ,G. J., and SHEPPARD,N., J. Chem. Phys. 17, 93 (1949). SZASZ,G. J., and SHEPPARO,N., Trans. Faraday Soc. 49, 358 (1953). SZASZ,G. J., J. Chem. Phys. 23, 2449 (1955). TANNENBAUM,E., MYERS,R.. J., and GWINN,W. D., J. Chem. Phys. 25, 42 (1956). 299

Progress in the Chemistry of Fats and other Lipids

104. TASUMI, M., SHIMANOUCHI,T., WATANABE,A., and GOTO, R., Spectrochim. Acta, 20, 629 (1964). 105. TURNER, E. E., in Steric Effects in Conjugated Systems, G. W. Gray, Editor, Butterworths, London, 1958. 106. WHELANO G. W., Resonance in Organic Chemistry, pp. 235 if, John Wiley, New York, 1955. 107. WILMHURS'r,J. K., J. Mol. Spectro. 1,201 (1957). 108. WILSON,E. B., JR., Adv. in Chem. Physics, 2, 367 (1959). 109. WYNBERG,H., DEGROOT, A., and DAVIES,D. W., Tetra. Letters, 1963, 1083. 110. YosmNo, T., and BERNSTEIN,H. J., Can. J. Chem. 35, 339 (1957). 111. YOUNG,J. M., and PETRAUSKAS,A. A., J. Chem. Phys. 25, 943 (1956).

300