The dependence of the carbon-chlorine stretching frequency on molecular structure in primary alkyl chlorides

JOURNAL

OF MOLECULAR

SPECTROSCOPY

36, 18-26 (1970)

The Dependence of the Carbon-Chlorine Stretching Frequency on Molecular Structure in Primary Alkyl Chlorides A. B. DEMPSTER~ School of Chemical Sciences,

University

of East

Snylia,

Norwich NOR MC, England

Approximate normal coordinate calculations of ethyl chloride and the trans and gauche isomers of n-propyl chloride have been carried out. From the results it is possible to provide a semiquantitative explanation for the relationship between the v(C-Cl) frequency and the molecular structure in the vicinity of the C-Cl bond; the dependence of the frequency on the length of the alkyl chain is discussed. The explanation may be applied to t,he utC-X) frequency of alkyl halides in general, where X is Cl, Br, or I. INTRODUCTION

The carbon-chlorine stretching frequency in monochlorohydrocarbons occurs as a strong band in both infrared and Raman spectra between 500 and 800 cm-‘. The precise frequency of vibration is closely related to the molecular structure in the neighbourhood of the bond. For some years use has been made of this structural dependence of the v(C-Cl) frequency for studies of internal rotation in small molecules (I-8) and of polyvinyl chloride and its model compounds (9IS). From the infrared spectra of a large number of monochlorohydrocarbons, Shipman et al. (IS) showed that, given the chlorine atom in question is a primary, secondary, or a tertiary chlorine atom and whether it lies tram to a hydrogen or to a carbon atom, it is possible to predict the actual frequency of vibration to within a few wavenumbers. The frequency is found to be virtually independent of the length or conformation of the hydrocarbon chain beyond three carbon atoms from the chlorine atom. For example, the v(C-Cl) frequency of primary 11-alkyl chlorides is close to 7.30 cm-l with the chlorine atom trms to ti carbon atom and close to 650 em-’ when it is hazs to a hydrogen atom. Using vibrating molecular models, Colthup (14) pointed to an explanation for the experimental results in terms of kinetic coupling of the C-Cl motion with the adjacent C-C-C angle bending motion. In their derivation of a force field for secondary chlorides, Opaskar and Krimm (15) successfully assumed that 1Present address: Istituto di Chimica da Vinci 32, Milan0 2Q133. Italy.

Industriale 18

de1 Politecnico,

Piazza

Leonardo

VIBRATIONAL

COUPLING

IN ALKYL

CHLORIDES

19

the changes in frequency with structure arise from differences only in the kinetic energy terms (G matrix). The purpose of this paper is to give a more detailed explanation for thta dependence of the v(C-Cl) frequency on structure. Kormal coordinate calculations on ethyl chloride and trans- and gauche-?I-propyl chloride have been carried out using an approximate force field which, with minor exceptions, was transferred bt%ween all three molecules. Again it is assumed that a satisfactory explanation can be furnished in terms of kinetic coupling rather than in terms of changes in the force field between one molecule and another. Further normal coordinate calculations on models derived from the skeletal atoms of ethyl chloride xd rotational isomers of rl-propyl and n-butyl chloride give some insight into the reason for the relative insensitivity of the v(C-Cl) vibration with chain length beyond a few atoms from the C-Cl bond. CONSII~ERATION

OF KINETIC

COUPLING

INT’OLVING

THE

C-Cl

BOND

One internal coordinate is said to be kinetically coupled to another if there is an off-diagonal term in the G matrix connecting the two coordinates. A nccessary condition for such an off-diagonal term is that the two coordinates contain at least one atom in common. Consider the segment of a primary alkpl chloride s11ow~ in Fig. 1. For cross terms between the C-Cl bond and other stretching coordinates, only neighboring bonds with C1 in common can be effect.ivr. But these terms do not change with internal rotation of the CH&l group around the bond CI-C5, i.e., they are independent of conformation. Cross terms arta :dso possible between the C-Cl coordinate and the angle bending coordinates around both atoms 4 and 5. However, in this case, the magnitude of the cross terms in the G matrix between the CXI coordinate and the angles around the atom C5 (i.e., CA-C5-Hs, C&C&H,, and C&C,&,) will depend on the dihedral anglrs bt+vern the C-Cl bond and the atoms attached to C5 around the bond C,&E,; the terms involving the angles around CJ will not be conformation dependent,. The only other internal coordinates to which the C-Cl bond may brl coupled will br the torsions around the bonds C5-C1U and C,-C5. Now, if there were no off-diagonal elements in the G matrix connrcting the C’XI coordinates with the other internal coordinates (and also no such terms in the F matrix) the normal vibration around 700 cm-’ m-ould be a p(lre C~-Cl bond stretching frequency, i.e., the frequency of the uncoupled oscillator would btl obtained. However, when off-diagonal terms do exist, the normal vibration consists not only of motion of the C-Cl bond but also of the other coordinat’es with which it is coupled; as a consequence, the frequency of tllis normal vibration will be perturbed from that of the uncoupled C-Cl bond stretching vibr:ltion. The actual change in frequency will depend on the magnitude of the cross term between

the C-Cl bond and the other coordinates

l’urthermore,

for any given magnitude

with \vhich it is coupled.

of such an off-diagonal

ttarm, tllv ch:tr+~

20

DEMPSTER

H2 FIG. 1. A segment

H9 of a primary

H alkyl

chloride

in frequency will be greatest for coupling with coordinates whose uncoupled frequencies themselves are near (within, say, 200-300 cm-‘) bhat of the UIIcoupled C-Cl bond stretching frequency. This, of course, will depend on terms in both the G and the F matrices. When the conditions discussed above are applied to the question of the variability of the CXl stretching freyuency with conformation, it is clear that the only coordinates whose couplin g aith the C-Cl coordinate should strongly perturb the v(CX~) frequency, i.e., which are both close in frequency and are COIIformation dependent, are those involving the angles around the carbon atom which is adjacent to the C-Cl bond. For the hydrogen deformation motions of a CH, group only the CH, rocking frequency is below 1000 cm-‘. Consequently, perturbations to the Y(C-Cl) frequency are expected to occur only through mixing of the C-Cl stretching with the CH, rocking or the C-C-C angle bending motions around the adjacent carbon atom. From the distributions of potential energy of the normal vibrations which have been obtained from the normal coordinate calculations of ethyl and fmrrs- and ~a.uc//e-wpropyl chloride it is possible bo give a semiquantitative description of t,his coupling. It should be emphasized that the effects of similar off-diagonal terms in the F matrix have not been considered in this treatment as it was not necessary to introduce such terms into the calculations. Since it appears that only kinetic coupling of the C-Cl stretching motion with angle bending motions around the adjacent carbon atom affects the v(CX1) frequency, it is at once clear why the v(C-Cl) frequencJ7 is relatively insensitive to the structure of the chain beyond the third carbon atom from the chlorine atom. Further perturbation to the v(C-Cl) frequency may occur not through direct coupling of the C-Cl stretching coordinate to other internal coordinates in G but through indirect coupling. Suppose that the C-Cl stretching coordinate is coupled to an internal coordinate A, and that A is coupled to a further internal coordinate B which is not itself coupled with the C-Cl coordinate. As an exrespectively, in ample A and B could be the angles C&(2-C 10 and C5-Cl&11, Fig. 1. Then it is possible to show that any changes in the chain conformation

VIBRATIONAL COUPLING IN ALKYL CHLORIDES

(a 1

(b)

2

3

21

pcy G T

Cl

GT

TT

TG

J.

FIG. 2. Conformations of some primary alkyl chlorides and skeletal atom model con(a) galcche-wpropyl rhloridp; (b) pounds used in the normal coordinate calculations. /Tans-n-propyl chloride; and (c) ethyl chloride.

which alter the value of the off-diagonal term between A and B ma.y also cause a change in the v(C-CI) frequency. In the chloropropanes this higher order coupling is much less important than the direct coupling. To give :L simple demonstration of this effect, normal coordinate calculations have been made fol some models using the skeletal atoms of ethyl, wpropyl, and I/-butyl chloridr in different conformations. NORMAL

COORDINATE

CALCULATIONS

The calculations were carried out using the standard Wilson GF mt%hod (16). The conformations of the alkyl chlorides and model compounds are tihown in Vig. 2. It was felt that only an approximate force field would be required to obtain a semiquantitative description of the kinetic coupling effects in the normal vibration involving principally C-C1 bond stretching. Consecluently, ;1 simple general vnlence force field in terms of internal symmrtr\. coordinates ~11s used.

DEMPSTER

22

TABLE A FORCE FIELD 1 CHI sym. str. 2 CH3 asym. str. 3 CH2 sym. str. 4 CH+o” sym. str.

FOR

I

PRIMARY

ALKVL

CHLORIDES~

8 CH, sym. def. 9 CH, asym. def. 10 CH, rock

0.575 0.537 0.680

19 CH, rock 20 CHz twist 21 CH&Cl) rock 22 CHS(~‘) twist 23 CH&Cl~-CH2 tors. 24 CHZ-CH2 tom. def. 25 C-C str./CCC 26 C-C str./CHP wag 27 C-C str./CH2’C11 wag 28 C-Cl str./CH&C1l wag

11 12 13 14

0.981 0.520 0.663 1.02Q 0.524

29 30 31 32 33

0.679 4.533 4.828

t.w. 34 CH2’C1’ tw./CH, ro. 35 CHZ(“‘) ro./CH, 36 C-C str./CHs wag

5 C’C”-c str. 6 C-C str. 7 C-Cl st.r.

C-C-C def. CH? bend CH? wag

Cl-C-C def. 15 CHZ(~~’ bend 16 CHJC” wag 17 CH: a.sym. str. 18 CHZ(~~) asym. str.

4.809 4.730 4.557 4.852 4.300 4.573 2.850

0.703 0.661 0.620

C-C str./CCCl def. C-Cl str./CCCl def. C-C str./C-C str. C-Cl str./C-C str. C-C str./CHa sym. def.

0.705 0.014 0.010 0.351 0.290 -0.290 0.370 0.176 0.249 0.124 0.520 -0.370 -0.070b O.llOb -0.290

” Stretching force constants are in mdyn/A; bending force constants in mdyn _&/(rad)2; and stretch-bend interactions in mdyn/rad. b This term was introduced for trans- but not gauche-n-propyl chloride as much better fitting could be obtained.

The required force constants were derived from consideration of several literature sources (17-21) and adjusted by hand sufficiently to give a reasonable fit with the experimental frequencies. The force constants used are given in Table I. Since Snyder and Schachtschneider (22) have recently presented the results of normal coordinate calculations for several primary alkyl chlorides, the detailed results of the calculations made in this work will not be given. (They may be obtained from the author on request.) The discussion is confined to the normal vibration which is principally a C-Cl bond stretching motion. DISCUSSION

From the potential energy distribution (PED) of the normal vibration of trws-ti-propyl chloride calculated at 724.6 cm-’ it was found that the major contribution arises from the C-Cl str. while there is also a considerable contribution from both the C-C-C def. and the Cl-C-C def. On the other hand, for gauche-r,-propyl chloride the major contribution is again from the C-Cl str. with a smaller contribution from the Cl-C-C def. However, the contribution from

the

CC-C

def. is replaced

by a contribution

from

the

adjacent

CH2

rock. In other words the C-Cl stretching motion in the tram isomer couples with the low frequency bending motion of the adjacent C-C-C angle. Since the

\-IBIZATIONAL

COUPLING

IN ALKYL

“‘_I _I

CHLOIlIIlll;S

C-C1 stretching motion is of higher frequency it is raised to even higher frequency by the coupling. No coupling occurs with the CH? rocking motion around the adjacent carbon atom since it belongs to the other symmetry cluss. For the gauche isomer the geometrical conditions determine that the effrctive coupling of the C-Cl motion with the adjacent C-C-C bending motion is small and may be discounted; but the CH, motion now couples significantl>T with the C-Cl motion. The CH? rocking motion is of higher frequency than the natural C-Cl stretching frequency. Consequently, coupling of the two motions t.ettds t,o push the v(C-Cl) frequency even lower. The PED suggests that the contribution to the v(C-Cl) vibration from tBhta C-C-C bending motion in the tratls isomer is not much greater than the contribution from the CH, rocking motion in the gauche isomer, but in t,he. opposite) direction. This conclusion is confirmed empirically by further considering the two isomers of I-chloro , Q-methyl propane along with the two isomers of )/-propyl chloride as shown in Fig. 3. The two bans isomers are observed to have the same frequency (730 cm-l), showing that introduction of :L carbon atom gauche to the chlorine atom does not affect the v(C-Cl) frequency in this case. I’or the gauche isomers, however, introduction of the carbon atom {/au&e to the chlorine atom causes a rise in frequency of the v(C-Cl) vibration. The explanat.ion is not that there is a further coupling with the new low frequency vibration which raisrs the v(C-Cl) the lowering

frequency,

but that the CH? group is effectively

of tht: v(C-Cl)

frequency

G : 686

cm-’

through

this coupling

removrd,

no lorlgcr

and twkr::

1 : 730 cm-’

HH*H ;* Cl

H

H

G: 645 cm-’

T:726

cm-’

FIG. 3. Conformations of gauche (G)- and lrans (T)-l-chloro-2-methylpro1~arle with analogous isomers of n-propyl chloride.

r~~mpared

( ohs,) ( talc.)

72 6 crri+ ”

725

ohs.)

cm”(calc.)

*.

-2%

HYDROGEN CARBON

0

cx

t

0

0

CHLORiNE

FIG. 4. Cartesian chloride

during

’

displacements of the atoms of gauche (U)the normal vibrations at the given frequencies.

L.

Cl-C-

and trans

(T)-n-prop)

c

G

Cl -c-c-c

GT

Cl-C-C-C-C

GG

Cl-C-C-C-C

T

Cl-C-C-C

TT

Cl-C-C-C-C

TG

Cl-C-C-C-C FIG. 5. The v(C-Cl)

I

I I I. 700 frequencies

750

cm-’

of some model alkyl chlorides

‘ 800

1XBKATIOPU‘AL

COUPLING

IN ALKYL

2:?

CHLORIDES

place. In gauche-1-chloro-Z-methyl propane no large coupling with either tlrtl C-C-C bending or CH, rocking motions occurs and the vibration experiment,ally observed at 686 cm-l may be considered as the uncoupled v(C-Cl) frtlcluency of a CH,Cl group. The fact that the frequency at 686 cm-l is approximately midway between the 645 and 730 cm-’ values of gauche- and haps-Upropyl chloride thus appears to confirm that the two forms of coupling are tqually effective in perturbing the natural C-Cl frequency, but in different dirrctions. The frequency for ethyl chloride is very near that found for yauche-rl-propyl chloride. This may be expected; there are no low frequency angle bending vibrations around the neighboring carbon atoms and coupling with the CHa rocking coordinate replaces the coupling with the CH, rocking coordinate. To show the effects of the coupling even more clearly, the Cartesian displaccments of the atoms for trcws- and yauche-n-propyl chloride during the normal vibrations which involve principally the (:-Cl coordinate are given in Fig. 4. lpor the truns isomer the stretching of the GCI bond involves a large amplitude motion of the carbon atom which automatically implies a large change in the adjacent C-C-C angle. In the corresponding vibration of the ~uuche isomer, thP carbon atom moves in a direction which leaves the adjacent (:-C-C angle rrl:itivel\y unchanged. But the hydrogen atoms on the adjacent, carbon atom art’ now much involved in the vibration, primarily in the form of a rocking motion. Although not shown here, the hydrogen atoms of thP CH, group in ethyl chloride also move considerably during the principally v(C-Cl) vibration. The effects of chain length are demonstrated for the models in Fig. 5, in the form of a stick diagram. As shown, the frequencies-which are high compared with the corresponding real alkgl chlorides owing to the fact that the hydrogen atoms were ignored in the calculations-fall into two distinct sets, depending on the dihedral angle of the Cl-C-C-C group, i.e., for the set with the chlorint~ atom Irwts to the carbon atom the frequency is some 50 cm-* higher than in tllcx scat \vherr it is pmche. However, small differences in frequency do occur withill we11 wt. These differences result from mixing of internal coordinates couplt~tl to the C-Cl motion with other internal coordinates not dir&l), coupled to tlw C’-~C’Imotion, as explained earlier. The point to note is that the conformational dependent direct coupling effect is of much greater importance than these WCondary effects. For real alkyl halides coupling between the C-Cl bond and C’H, groupq should be considered;

but this would not affect the basic argument

given

abovc. CONCLUPIO?; It,

is reasonable

quency of primar), plied to the similar (29). P’urthermore,

to conclude

that

the caxplanation

wlsting

the v(C-Cl)

fre-

chlorine atoms to local molecular structure can also be apcorrelations observed for primary alkyl bromides and iodides it is tempting to suggest that this mechanism also plays a

DEMPSTER

26

major role in determining the frequency correlations observed for different conformations of secondary and tertiary halides with these halogen atoms. Finally, it is interesting to note that in the series @ZH&l .CH(CH,) - CH3, CH&l. CH3, (CH&CHCl, (CHs)J2C1, the respective Y(C-Cl) frequencies for these compounds in which the C-Cl stretching mode may mix with 0, 1, 2, and 3 methyl groups, respectively, are 686, 657, 611, and 569 cm-‘. Thus it may be that the general fall in v(C-Cl) frequency from a primary to a tertiary alkyl halide may also result in part from the same mechanism, although there may also be effects from a change in the C-Cl force constant and differences in the nonconformational dependent coupling terms between the C-Cl stretching and other internal modes in passing down the series. ACKNOU’LEDGMENTS The versity Tatsuo Sugeta debted

main part of this work was carried out at the Institute for Protein Research, Uniof Osaka, Japan. The author would like to express his sincere thanks to Professor Miyazawa for the hospitality extended to him during his stay and to Dr. Hiromu for the use of his computer programs and patient instruction. The author is into Rotary International for the award of a Graduate Fellowship for study in Japan.

RECEIVED:

October

6, 1969 REFERENCES

1. I(. W. F. KOHLRAUSCH, 2. Phys. Chem. Abt. B 18, 61 (1932). 2. S. MIzUSHIM.~, Y. MORINO, AND S. NOJIRI, Sci. Pap. Inst. Phys. Chem. Res., Tokyo 22, 63, 188 (1936). S. J. K. BROWN AND N. SHEPPARD, Trans. Faraday Sot. 48, 128 (1952). 4. J. K. BROWN AND N. SHEPPARD, Trans. Faraday Sot. 60, 1164 (1954). 6. J. K. BROWN >~NDN. SHEPPARD, Proc. Roy. Sot., Ser. A a31, 555 (1955). 6. I. NAKAGA~~ AND S. MIZUSHIMA, J. Chem. Phys. 21, 2195 (1953). 7. C. KOMAKI, I. ICHISHIMA, K. KURATANI, T. MIY~z.\w.\, T. SHIMANOUCHI,AND S. MIZUSHIMA, Bull. Chem. Sot. Jap. 28, 330 (1965). 8. S. MIZUSHIM~, T. SHIM.~NOUCHI,K. NAKAMUR‘Z, M. H~YASHI, AND S. TSUCHIYA, .Z. Chem. Phys. 26,970 (1957). 9. T. SHIMANOUCHI,S. TSUCHIY~, AND S. MIZUSHIM~, J. Chem. Phys. 30, 1365 (1959). 10. M. T.~SUMI AND T. SHIMANOIJCHI,Spectrochim. Ada 17, 731 (1961). 11. M. T.\SUMI AND T. SHIMANOUCHI, Spectrochim. Ada 1’7, 755 (1961). 12. S. KRIMM, A. R. BIXRENS, V. L. FOLT, -*ND J. J. SHIPMAN, Chem. Znd. (London), 1969, 433. 1s. J. J. SHIPM~N, V. L. FOLT, AND S. KRIMM, Spectrochim. Acta 18,1603 (1962). 14. N. B. COLTHUP, Spectrochim. Acta 20, 1843 (1964). f5. C. G. OP.ISKAR AND 8. KRIMM, Spectrochim. Acta, Part A 23, 2261 (1967). Vibrations”, McGraw-Hill, 16. E. B. WILSON, J. C. DECIUS, .~NDP. C. CROSS, “Molecular New York, 1955. 17. J. H. SCHICTSCHNXIDER AND R. G. SNYDER, Spectrochim. Acta 19, 117 (1963). 18. R. G. SNYDER AND J. H. SCHACHTSCHNEID~:R,Spectrochim. Acta 21, 169 (1965). 19. J. L. DUNC.IN, Spectrochim. Acta 20, 1197 (1964). 20. T. SHIMANOUCHI et al., Syn/nLp.Mol. Struct., Osaka, 1966. 21. Y. ABE !.ND T. SHIMANOUCHI, Prepr. Symp. Mol. Struct., Osaka, lW6. 2:. R. G. SNYDER AND J. H. SCHBCHTSCHNEIDER,J. Mol. Spectrosc. 30, 290 (1969). 23. F. F. BF:NTLY, T. N. MCDEVITT, .~ND A. L. ROZEEC,SpectrochinL. Acta 20, 105 (1967).