NMR spectra of monosubstituted alkanes I. N-propyl derivatives

JOURNAL

OF MAGNETIC

RESONANCE

6,243-255

(1972)

NMR Spectraof MonosubstitutedAlkanes I. N-Propyl Derivatives G.SCHRUMPF Organisch-Chemisches Institut der Universitiit, Giiitingen, Germany Received May 6,197l The general features of the NMR spectra of n-propyl groups have been studied using subspectral methods. The conditions for obtaining the complete set of NMR data for this spin system from the spectrum are discussed and demonstrated by complete numerical analysis of the spectra of several n-propyl derivatives. The results, in particular, the coupling constants, are interpreted in terms of the electronegativity of the substituent. INTRODUCTION

It is well known that the magnitude of chemical shifts and spin-spin coupling constants depends on the kind of substituent attached to the molecular fragment concerned. In a large molecule this influence is a function of the distance between the substituent and the nuclei studied. Therefore, the NMR spectra of aliphatic hydrocarbons and their mono-derivatives are well suited for studying the mechanism and the extent to which substituent effects propagate along a chain of saturated carbon atoms. The main obstacle to a study of this kind is the great number of vicinal protons in this class of molecules leading to very complex NMR spectra, which at increasing chain length are soon unamenable to complete analysis. At present, precise NMR data of mono-substituted paraffins are available only for methyl and ethyl compounds (1). In this investigation, the NMR spectra of a number of simple mono-substituted n-propyl derivatives have been completely analyzed. The NMR spectra of several n-propyl derivatives were studied by Cavanaugh and Dailey ten years ago (2). Their analysis was based on two simplifying assumptions: (I) In a trial and error fashion, several theoretical spectra were calculated until one was considered similar enough to the experimental spectrum. Starting from a guessed set of parameters, they did not calculate the exact eigenfunctions and energies, but only approximate values using perturbation theory carried to higher order. (II) A further simplification was the treatment of the seven propyl protons as an A*B,C, spin system, for which only three coupling constants are needed for a complete description. Although it is true that rapid internal rotation makes the methyl protons magnetically equivalent, this does not necessarily hold for the two pairs of methylene protons. Therefore, the spin system actually is of the AA’BB’C, type. The analysis of the NMR spectra of n-propyl groups (Fig. 1) is expected to give Jac and & and in addition two generally different geminal coupling constants JaA, and 6 1972 by Academic

Press, Inc.

243

244

SCHRUMPF

JBBt and also the vicinal couplings J AB and JAB,. That the coupling constants derived from an A,B,C, analysis are reported to reproduce the main features of the experimental 60MHz spectrum within an error of 0.2-0.3 Hz may be explained by considering the relative insensitivity of the strong spectral lines to differences in JAATand JBBfand also JAB and JAB’. Furthermore, a number of lines of small intensity were not taken into account in the previous analysis. In the following the general structure of the AA’BB’C, spectra of the n-propyl moiety is studied. It is shown what conditions must be fulfilled in order to be able to extract all the NMR parameters from the spectrum. GENERAL STRUCTURE OF THE SPECTRA

The spin system of a n-propyl group is completely described by 3 chemical shifts vA, vg, and vc and 6 coupling constants J AB, J AB I, JAC> JBC, J AA , and JBB*(Fig. 1). The spin Hamiltonian is extensively factorized by using the symmetry properties of the spin assembly. (I) Magnetic equivalence of the methyl protons simplifies the secular problem and leads to two separate subsystems, which will subsequently be designated as Q for

i------JAG-----I

JAB’ FIG. 1. Coupling constantsin n-propyl derivatives.

quadruplet and D for doublet spin states of the methyl protons according to composite particle theory notation. (II) Additional symmetry with respect to the two protons of each of the two methylene groups permits further splitting into a symmetric and an antisymmetric subsystem for each complex particle. (III) Using coupling constants within the range expected for this type of molecules and approximate chemical shifts obtained by inspection of the experimental spectra at 100 MHz, it can be shown that the overall structure of the present AA’BB’C, spin systems is not very different from that calculated on the basis of the AA’BB’X3 approximation. The Hamiltonian of the AA’BB’X, spin system is further factorized according to the total spin of the X, composite particle leading to the 1Zterm systems listed in Table 1. In the following discussion it must be kept in mind, however, that we used the

NMR

SPECTRA

OF N-PROPYL

245

DERIVATIVES

AA’BB’X, approximation only to recognize the general pattern of the spectra and to give names to individual lines and line groups of the AA’BB’C, spin system, which are strictly applicable only in the limit of infinite chemical shift difference between methyl and methylene group protons. TABLE SUBSPECTRA’

1

OF A 12-PROPYL GROUT APPROXIMATION

States

Symmetric

Quadruplets

(uu’bb’):‘* Qc (au bb’): ‘* Qc (uu’bb’);“* Qc (au’ 66’);“‘* Q, (au’ bb’): ‘* DC (au’ bb’);‘12 DC

Doublets

IN THE AA’BB’X3

Antisymmetric (uu’bb’):” (a~‘b6’):‘~ (~u’bb’);“~ (~~‘66’);~‘~ (uu’bb’):‘* (au’ bb’);“’

Qc Qc Qc Q, DC DC

a Using composite particle notation rigorously the subspectra should be designated as Da DA, DB DB, Qc etc. The notation used here shall be considered a shorthand in case of simultaneous application of total spin and symmetry factorisation.

The aa’bb’ subspectra of the two methylene groups in n-propyl derivatives are similar to those of unsymmetrically 1,Zdisubstituted ethanes which have been extensively discussed previously (3). The spectral features of those spin systems are expected to appear similarly in the n-propyl spectra. This should be valid with regard to both the conditions rendering a complete spectral analysis possible and the method to be used for assigning the experimental lines. Therefore, those characteristics of AA’BB’ spectra which are of importance in the present investigation will be shortly summed up in the following. Introducing the symmetry parameters K= JAAt + JBBr,

M= JAA* - JBB*,

N z JAB + JAB’,

L = JAB -JABS,

the characteristic spectral features of unsymmetrically originate from the particular relation K>

1,2-disubstituted

ethanes

N>L,M.

Figure 2 shows the B part of an AA’BB’ spectrum of this type. Lines l-8 correspond to transitions between symmetric levels and lines 9-12 to those between antisymmetric energy levels. It is common practice to call lines 5-8 the Kquartet, and lines 9-12 the M quartet (3). Although a value of N may be derived from the position of lines 1-4, which are always observable, determination of K, L, and A4 is limited by several conditions in spectra of this type.

246

SCHRUMPF

(1) An accurate value of K can be found only if line 7 can be observed. For 1K 1 z+L and \KJ G dv,, the intensity of line 7 goes to zero. (2) L may be determined from lines 9-12. There are two restrictions to this. For L 4 K, M the spectrum is independent of L. In this case the intensities of lines 9 and 12 approach zero, and lines 10 and 11 become degenerate, a typical feature of A2B, spectra for which L = 0. In borderline cases a rough estimate of L may be obtained from the

Af

spectrum

FIG. 2. B part of an AA’BB’

spectrum calculated with N = 13 Hz, L = 3 Hz, K = -24 Hz, M = 2 Hz.

“2Dc

I FIG. 3. B part of the AA’BB’C, spectrum of ethyl butyrate with the individual The calculation was carried out to higher order.

subspectra separated.

position of lines 10 and 11 alone. For particular combinations of K, L ,and dv,, an approximate value of L can be calculated from the unusual frequencies and intensities of lines 3.4, and 7’ (3).

NMR

SPECTRA

OF N-PROPYL

DERIVATIVES

247

(3) A precise determination of A4 depends on finding the outer lines 9 and 12 of the M quartet in the experimental spectrum. Again, in principle, a less accurate estimate of M is obtained from the precise position of the central lines 10 and 11. The appearance of an AA’BB’ spectrum can be shown to be independent of the sign of L and M relative to N, but it depends on the relative sign of K and N. Although, in principle, all four coupling constants and their relative signs can be extracted from an AA’BB’ spectrum, neither JAAf and JBBrnor JABand JAB,can be assigned to a particular pair of protons. In the X-approximation with respect to the methyl protons this ambiguity is expected to hold also for the spectra of n-propyl groups. Figure 3 shows the symmetric (solid) and the antisymmetric (dotted) B lines of the individual Q and D states of an AA’BB’C, spin system calculated using NMR parameters obtained in this study for ethyl butyrate (cf. Fig. 1, X = COOC,H,). It is apparent from Fig. 3 that the first-order approximation AA’BB’X, with respect to the methyl protons introduced for the sake of naming line groups in the following discussion is not a very good description of these spectra as judged, for example, by the missing degeneracy of the doublet and quartet spectra aa’bb”12 DC and aa’bb’li2 Qc and also aa’bb’-‘I2 D, and aa’bb’-‘i2 Qc. An important consequence of the higher-order interaction between methyl and adjacent methylene group protons is the dependence of the spectrum on the sign of M, in contrast to simple AA’BB’ cases. The lines 10 and 11 of an AA’BB’ spectrum collapse for L = 0, i.e., the case of magnetically equivalent protons, while transitions 9 and 12 are forbidden. The higher-order perturbation of the antisymmetric spectrum by the methyl group protons results in a splitting of the lines 10 and 11 of all subspectra in the case L = 0. The magnitude of this splitting depends on the m, value of the composite particle corresponding to the methyl group protons. For constant L the dependence of the M quartet of three typical subspectra on M is illustrated in Fig. 4. As in AA’BB’ spectra, when the absolute value of M is large compared to L, the M quartets of the AA’BB’C, spectrum approach those for L = 0, but with the line pairs 10 and 11 still split. As A4 decreases from positive values toward zero, the central A4 lines of most subspectra move apart. However, as negative M values change toward zero the central lines approach each other. In either case, the intensity decreases. At the same time transitions 9 and 12 increase in intensity and approach 10 and 11, respectively. In the subspectrum aa’bbf3j2 Qc, the central lines cross each other at A4 = -1.5 Hz. The intensity of the outer lines 9 and 12 is larger for a negative value than for the same positive value of M. For A4 = 0, simple AA’BB’ spectra show just two lines of high intensity representing the pairs 9, 10 and 11, 12. Because of higher-order interaction the behavior in n-propyl spectra is different. For example, the four transitions 9-12 of the subspectrum aa’bb’3’2 Qc d o not degenerate pairwise at M = 0, but remain split into four single lines with different intensities. The quantitative dependence of the M quartet on M is different for the individual subspectra. In most of them, lines 10 and 11 move away from their common center for increasing negative values and approach each other for increasing positive A4 values. The behavior is opposite to this in the subspectra aa’bb’-3’2 Qc and aa’bb’-‘J2 D,. For each subspectrum there is a value of M at which the central lines of the M quartet collapse to one line. These crossing points are listed in Table 2. Their position is a rough measure for the sensitivity of a given subspectrum to changes in M.

248

SCHRUMPF

AA’BB’C, aa’bb’

AABB’C,

‘2Q,

aa’bb’

b

a 11

II 10

IO

9 M=-4 i

12

\

\

\

/ / \

“2DC

\

1 \\

/’

; \\

\

I \\ \1 \

./1

i /

I’

! :

I\Ii 4 it II /’

/’

’ I /

‘,

\ \

M=-2

I /

\ 1 \ \

M=-1

I ‘\ \

L i’

1

1

M=l

/

\

/ / /

I

/

’

‘4

0

‘\

\

\

,

I

IHz FIG.

L=1.4

4a.

M-1 I \

:

M=2

? \

/

-

M--l

M=O

/

/

/’

I \ ,I ‘I

M=O

M--2

I I

I \

\

M-3

\ -

M--3

/

/

\

/I

i

: N=-& I

/I I

>’ ‘,

/

\

/’

i’

/

M=-3

Ij 83

I

I l’i i’

,I ,’

‘f \

1 Hz FIG.

L-1.4

4b.

In order to study the dependence of all six M quartets in the B part of the spectrum as a function of A4 more generally, one ought to use the explicit expressions for the matrix elements of the spin Hamiltonian and solve for the corresponding eigenvalues. Although it is possible in AA’BB’ spectra to give analytical expressions for the eigenvalues of the antisymmetric states and, from these, for the frequencies of the A4 quartet, the antisymmetric doublet states of the AA’BB’C, system give rise to 4 x 4 matrices. Moreover, the dimension of the largest matrix for the antisymmetric quadruplet states is of the order 6. In the two latter casesthe eigenvalues cannot be given in closed form. Therefore, an analytical study of the M dependence of the antisymmetric spectrum is beyond the scope of this investigation. It is important to realize that with the knowledge of the sign of A4 one is able to say which geminal couplingJAA, or&. is the greater one. On the other hand, because of the nature of the perturbation of the AA’BB’ fragment by the methyl group, the ambiguity

NMR

SPECTRA

OF IV-PROPYL

DERIVATIVES

249

AA’BB’C, aa’bb’

-bQ,

C 10

11

1

12

\

\

1

9 M--3

:

I

’

\

/ I/

\ \

\

\

\

/

\

,

,

\

M=-I

I I

/

I

/

I; I II II ‘\/’ F

M=l I ’\

\

:

M=2

\ I \

,’ :

-

M=O

1

Ii / 1’

I

/’

1 x ,\I \

/

//

i/ \

I

M=-2

If \I

I

/

L 1

M=3 \

\\

,

M=4

L=1.4

IHz FIG.

\\ \

4~.

FIG. 4. Variation of the A4 quartet with the value of M for three typical subspectra of the B part of the ethyl butyrate spectrum.

of the assignment of JAB and JAB’ remains, i.e., the sign of L relative to N still does not influence the spectrum of n-propyl derivatives. RESULTS

The spectra of the compounds investigated at 100 MHz may be divided into two groups. In the first group the difference between the chemical shifts of the two methylene group protons dv AB is large, and the spectra are completely reproduced on the basis of an A,B& analysis. In these pseudo A2B,C, systems, only a value for N can be found. In the second group of compounds Av,, is considerably smaller and for some compounds L and M have been determined, in one compound Mis so large with respect to L that the spectrum is independent of L, and neither M nor L can be determined. In no example studied here was the transition 7 unambiguously assigned. Therefore,

SCHRUMPF

250

TABLE

2

CROSSING POINTS OF THE CENTRAL LINES OF THE M QUARTET IN THE SIJBSPECTRA OF THE B PART OF THE ETHYL BUTYRATE SPECTRUM

value of the crossing point G-W

M

Subspectrum a~‘bb’~‘* Qc aa’bb”” Qc uu’bb’-“* Qc aa’ bb’- 312 au’ bb”” Df aa’bb’-“2

D c

-1.5 -1.0 -3.5 +2..5 -2.5 +3.0

at best, only the difference between geminal coupling constants, but never the couplings themselves, was obtained. All n-propyl halides and the di-n-propyl ether yield spectra of the type A,B,C,. Three coupling constants are sufficient to describe the spectrum. These are the vicinal and the long-range coupling constant to the methyl group protons and the value of l/2 N, representing an average coupling between the protons of the two methylene groups. Individual transitions needed for analysis are most accurately measured in the region of the B proton spectrum, because overlapping of lines is not as frequent here as elsewhere. The vicinal couplings may be determined using only three lines-one line at each wing of the B part of the spectrum and one line of the doublet spectrum which is readily recognized by its relatively large intensity. The long-range coupling in these pseudo A2BZC3 spectra can be found from the splitting of the degenerate TAS, D, and TA S, Qc lines in the methyl region and its sign relative to the vicinal coupling constant from the contours of the extreme TA TBDC and TAT, Qc bands (4). The results of the calculation for n-propyl derivatives of the pseudo A2BZC) type using about 30-70 lines are presented in Table 3. In a second group, four n-propyl derivatives have been investigated which have the common feature that the substituent atom attached to the n-propyl group is carbon. CH,CH,CH2X

X = CHO, COCH,, COOC2Hs, CN

Their internal methylene group shift is approximately half that of the halides. The analysis of the spectra was started by assuming magnetic equivalence and, as before, taking the two extreme quadruplet and one doublet line of the B part in order to fit JBc and N. The parameters obtained in this way gave theoretical spectra which only for butyronitrile (X = CN) matched the experimental spectrum line by line within experimental error. Most of the lines of the other three compounds are also reproduced by these theoretical spectra, but in some regions of the B part of the spectrum deviations occurred which are several times the experimental uncertainty. An illustration of the refined analysis is that for ethyl butyrate (X = COOC,H,). The B part of its spectrum is shown in Fig. 5. The deviations between the experimental and

NMR

SPECTRA

OF N-PROPYL

251

DERIVATIVES

TABLE 3 NMR PARAMETERSOF II-PROPYL DERIVATIVES CH&HtCH2X” X VA w4 vc J”’AB

0

Cl

Br

I

-395.63 -570.08 -633.78 6.45

-384.76 -553.74 -629.49 6.60 -0.06 7.35 0.034

-397.27 -546.67 -630.20 6.67 -0.05 7.29 0.014

-415.85 -549.08 -633.03 6.80 -0.05 7.25 0.005

J AC

0.0

JBC

7.43 0.020

RMS

a All values are given in Hz, chemical shifts are measured at 100 MHz relative to internal benzene. The accuracy of the chemical shifts is 0.05 Hz, that of the coupling constants 0.03 Hz.

the theoretical A,B,C, spectrum are apparent in the region of the second group of lines (Fig. 6) which is especially well suited for analysis. Seven strong absorption lines including the shoulder 23 are distinguishable. These are to be assigned to six symmetric lines and one M quartet. The experimental lines 25 and 26 are the transitions 3 and 4 (cf. Fig. 2) of the subspectrum aa’bb”/* DC, while line 22 is the degenerate pair 3 and 4 of the subspectrum aa’bb”/* Qc. The other four

FIG. 5. B part of the 100 MHz spectrum of neat ethyl butyrate. Arrows indicate the position of the outer M lines 9 and 12.

lines belong to the subspectrum aa’bb13!* Qo, lines 23 and 27 representing transitions 5 and 8 of its K quartet. The frequencies of these five experimental lines are accurately reproduced by the A2B2C3 type analysis. Only the remaining lines 21 and 24, i.e. transitions 10 and 11 of the M quartet, are not matched. Both experimental lines are

252

SCHRUMPF

shifted from the theoretical positions of the A,B,C, calculation towards each other by 0.20 and 0.23 Hz, respectively. From Fig. 4 it is evident that a displacement in this direction only occurs for negative M values, i.e., if the geminal coupling constant between the A protons is more negative than that between the B protons. There is still an ambiguity as to two values M, and M,--one at either side of the crossing point-corresponding to a given separation between lines 10 and 11 (cf. Fig. 4). There are two additional criteria differentiating between those cases: (I) If the outer M

25 231 26

FIG. 6. Secondgroup of lines in the B part of the spectrum of neat ethyl butyrate (cf. Fig. 5).

lines 9 and 12 can be observed, their position and intensity will give a unique value of M. (II) As the M values of the crossing points are different for each subspectrum, there are several pairs M, and M2, M, and M,, Mi and M4, and so on, corresponding to the observed separations of any line pair such as 10 and 11. But there is only one value Mi giving a satisfactory match for all pairs simultaneously. In this context it is particularly important that the crossing points of the subspectra aa’bb’-3/2 Qc and aa’bb’-‘/* DC occur for positive M values. Since the dependence of the M quartet on the value of L is also different for each subspectrum, the identification of several pairs 10 and 11 in the experimental spectrum is important, because it permits a determination of M and L for this type of N-propyl group spectrum. The L and M value determined from the intense lines 10 and 11 alone yield theoretical spectra having lines 9 and 12 near those positions where weak lines are observed in the experimental spectra, e.g., lines 201 and 202. The calculated parameters of those compounds where the substituent atom attached to the n-propyl group is carbon are presented in Table 4. DISCUSSION

The chemical shift data obtained in the present study differ from those given in Ref. (2) by several tenths of a ppm, our values being exclusively displaced towards higher field. These differences are due to the fact that Cavanaugh and Dailey used benzene as an external standard, while internal referencing was used in this investigation. On the other hand, the internal chemical shifts are approximately the same to within 1 or 2 Hz.

NMR

SPECTRA

OF N-PROPYL

TABLE 4 NMR PARAMETERS OF n-PROPYL DERIVATIVES

54 YEI

2. + J,,w

JAB-JAW

CHJCH2CH2X’

CHOb

COCHsc

COOC2H5

-497.01 -570.40 -639.10 14.41 0.96

-494.80 -578.06 -641.15 14.50 1.66 -0.10 7.41 -3.25 0.029

-508.20 -569.04 -637.55 14.68 0.92 -0.12 1.43 -2.01 0.022

X

J AC J BC J AA’ - J BB’ RMS

-0.10 7.45 -3.0 0.030

253

DERIVATIVES

CN -507.69 -515.17 -633.98 14.07 -0.05 7.39 0.028

a Seefootnote to Table 3. b Parameters are calculated from the propyl group spectrum decoupled from the CHO proton. The chemical shift and the absolute values of the coupling constants of the CHO proton are obtained from the undecoupled spectrum by first order analysis : vCHo= 237.2 k 0.1 Hz; 3JCH2CHr, = 1.70 z!z0.03 Hz; ?ICH2CH2CH0 = 0.06 f 0.03 Hz. c Propyl group spectrum is perturbed by COCH3 protons to first order. The chemical shift and the absolute value of the coupling constant of the COCH3 protons are: vCHICO= -528.4 5 0.1 Hz; 4JCH3Cr,CH2 = 0.46 zt 0.05 Hz. The small deviations reflect dilution effects caused by the addition of benzene as an internal standard. The average vicinal coupling constants calculated from our complete analysis and those given in Ref. (2) are compared in Table 5. The couplings in the pseudo A2B2C3 cases (entries 1-4) are similar to each other, and the agreement between the two sets of TABLE AVERAGE

VICINAL

COUPLING

5

CONSTANTS

OF WPROPYL

J”’A”

JBC Substituent

This work

1 -o2 Cl 3 Br 41 5H 6 CHO 7 COOC2H5 8 COCH, 9 CN

7.43 7.35 7.29 7.25 7.45 7.43 7.41 7.39

a All values are given in Hz. b Reference (8). c Butyric acid, Ref. (2).

DERIVATIVES

Ref.5) 1.5 7.4 7.3 7.2 7.35b ,Z) 7.7

This work

Ref. (2)

6.45 6.60 6.67 6.80 -

6.3 6.3 6.5 7.0 7.35b

7.34 7.21 7.25 7.03

,E)C 6.7

254

SCHRUMPF

couplings to the methyl group protons is excellent. The full analysis of the AA’BB’C, spectra yield data which are up to 0.37 Hz different from those reported previously. This may be due to the approximations used by Cavanaugh and Dailey in their analysis. The internal chemical shifts of ethyl derivatives have been used to demonstrate the correlation between the electronegativity of the substituent and the proton chemical shift (5). According to the modified empirical Dailey and Shoolery equation (6), substituent electronegativities may be calculated from chemical shifts, which parallel the coupling constants 3J in n-propyl halides and di-n-propyl ether. A larger electronegativity decreases J$$a but increases JBc. In the series of derivatives where the substituent atom bound to the n-propyl group is a carbon atom, these trends are not followed as rigorously, probably because the corresponding substituent electronegativities as calculated from the internal shift of ethyl compounds contain contributions from long-range anisotropic effects of the carbonyl and nitrile group. However, the generally smaller electronegativity of these substituents compared to halogen and oxygen is still apparent in larger values of JaAyB.Judged from electron density considerations alone, the vicinal couplings JBc to the methyl group protons in these cases are unexpectedly large. The values of these coupling constants correspond to those expected for the electronegativity of oxygen. Most probably conformational differences between the first and the second group of compounds are responsible for this anomaly. The overall pattern of the coupling constants found in the present study on n-propyl derivatives supports the idea that strongly electronegative substituents decrease vicinal coupling in H-C-C-H fragments, separated by an even number of bonds, and increase 3J for separations by an odd number of bonds (7). Further work is in progress investigating this idea in greater detail using model compounds with longer carbon chains. EXPERIMENTAL

All compounds except di-n-propyl ether were obtained commercially and purified by gas chromatography. Di-n-propyl ether was prepared by the Williamson method from n-propyl iodide and sodium n-propylate. The spectra were obtained at 100 MHz using a Varian HA 100 in the frequency sweep mode. About 8-10 v/v y0 benzene was added to the neat compounds to serve as an internal lock. The samples were degassed by several freeze-pump-thaw cycles and sealed afterwards under an atmosphere of argon. Line positions in the experimental spectrum were determined with the aid of an electronic counter. The frequency of each peak maximum was measured three times. Measurements were reproducible to an accuracy of 0.02 Hz. The calculations were performed with the aid of a computer programme written by C. W. Haigh, Swansea, taking magnetic equivalence explicitly into account. ACKNOWLEDGMENT The author is obliged to Dr. C. W. Haigh, Swansea, for kindly supplying a copy of this computer programme LAME.

“NMR Darmstadt, 1967.

1. W. BRUEGEL,

2. J. R. CAVANAUGH

REFERENCES Spectra and Chemical Structure”,

AND B.

P.

DAILEY,

J. Chem.

Phys.

34,1094

Vol. 1, Dr. Dietrich (1961).

Steinkopff Verlag,

NMR

3. R. C. HIRST AND D. M. GRANT,

SPECTRA

OF N-PROPYL

DERIVATIVES

J. Chem. Phys. 40,1909 (1964). 4. G. SCHRWWF, to appear. 5. J. R. CAVANAUGH AND B. P. DAILEY, .I. Chem. Phys. 34,1099 (1961). 6. B. P. DAILEY AND J. N. SHOOLERY, J. Amer. Chem. Sot. 77,3977 (1956). 7. A. D. COHEN AND T. SCHAEFER, Mol. Phys. 10,209 (1966). 8. R. C. FERGUSON AND D. W. MARQUARDT, J. Chem. Phys. 41,2087 (1964).

255