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The vibrational spectra and conformational behavior of methyl chloroacetate
The vibrational spectra and conformational
behavior of methyl chloroacetate* J. E.
Department
KATON and DEEPALI SINHA of Chemistry, Miami University, Oxford, OH 45056, U.S.A.
(Received 28 October 1975; revised 18 February 1976) Abstract-The i.r. spectra of methyl chloroacetate and methyl chloroacetate-ds have been recorded in both the liquid and solid states and Raman spectra of the two compounds have been recorded in the liquid phase. A tentative vibrational assignment has been made for both compounds.
The conforrnational properties of methyl chloroacetate are discussed and it is shown that the molecule does exist as a conformational equilibrium in the liquid phase. The doublet observed in the solution spectrum of methyl chloroacetate in the carbonyl stretching region has been shown to be a poor indication of this conformational
equilibrium, however.
Since methyl chloroacetate has not undergone a thorough vibrational analysis and since it is widely quoted as an example of an ester existing as a conformational equilibrium in the liquid state, it seemed worthwhile to carry out a detailed analysis of its vibrational spectra in an effon to put its structural features on a firmer footing. This is particularly true since the existence of doublets in the carbonyl stretching region is not a very good criterion for conformational equilibria, in general [2].
INTRODUCXION
detailed molecular structures and conformations of carboxylic acid esters have been investigated by many workers. This work has been reviewed in detail recently by JONES and OWEN [l]. Conformational equilibria in esters can arise by internal rotation of three basic types (1) rotation about the C-O bonds; (2) rotation about the C-C bonds of the alkyl group; and (3) rotation about the C-C bonds of the acyl group. Substituted methyl acetates are particularly simple examples of the class since in these molecules conformers may arise only by rotation about (1) one of the C-O bonds and (2) the single C-C bond of the acyl group. There have been, however, very few detailed studies of the vibrational spectra of simple acyl substituted methyl acetates. Apparently the only molecules possessing this structure which have undergone complete spectral assignment and investigation of conformational behavior are methyl cyanoacetate [2, 31 and methyl propionate [4]. The latter has also been investigated with regard to its conformational equilibrium by GEORGE’ et al. (5). Although methyl monochloroacetate has been stated to exemplify the existence of conformational equilibria in esters [ 1, 61 the only experimental data relating to this conclusion seems to be the fact that two bands have been reported in the carbonyl stretching region in carbon tetrachloride solution [7, 81. The conformers have been assumed by these workers to be those due to rotation about the C-C
The
bond
EXPERIMENTAL.
Reagent grade methyl chloroacetate and chloroacetyl chloride (Eastman Organic Chemical) were purified by distillation. Methyl chloroacetate-d3 was prepared by the slow addition of CD30D to purified chloroacetyl chloride followed by distillation. Infrared spectra in the 400050 cm-’ region were recorded on a Perkin-Elmer Model 180 i.r. spectrophotometer. The spectra of the crystalline films were obtained utilizing a Research and Industrial Instrument Corporation VLT-2 cell and liquid nitrogen coolant. Raman spectra of the liquid compounds were recorded on a Cary Model 81 Raman spectrometer using an Argon ion laser source. Infrared spectra of a 0.15% solution of methyl chloroacetate in Ccl., in a 0.1 mm sealed cell were recorded as a function of temperature from 28” to 61°C in a heated compartment of conventional design. The liquid phase i.r. and Raman spectra of methyl chloroacetate and methyl chloroacetate-d, are reproduced in Figs. l-4. Crystal mid-i.r. spectra of methyl chloroacetate and methyl chloroacetate-d, are reproduced in Figs. 5-6. The observed ix. and Raman bands are given in Table 1. RESULTS
of the acyl group.
a.
Conformational
AND
DISCUSSION
behavior
Comparison of Figs. 1 and 5 indicates that the existence of a conformational equilibrium in methyl
*Supported m part by the United States Air Force under Contract #F33615-73 C-5013. 45
J. E. &-ION and DEEPAL~SINHA Wavelength, 4.
6
5
microns e
7
9
la
I2
x,
30
,50
200
m f
60
z E
40
+ 1
20
i? ::
0 4
Wavenumber,
cm-’
Fig. 1. The i.r. spectrum of liquid methyl chloroacetate.
2400
2000
I600
1600
1200
1400
1000
800 '--
600
400
200
Roman shift Acm-’
Fig. 2. The Raman spectrum of liquid methyl chloroacetate.
Wavelength,
4000
3500
3000
2500
2000
1800
1600
Wavenumber,
microns
1400
1200
1000
800
cm-1
Fig. 3. The i.r. spectrum of liquid methyl chloroacetate-d,.
600
400
200
0
The vibrational spectra and conformational
k=iL;
4000
1J--I
3600
I
I 32Ob
:
2800
I:/
/ 2400
!
!
1
I600
2000
47
behavior of methyl chloroacetate
I
I
1600
MOO
I
i
/
! 1000
1200
-6Oi-
--6co--400
/
200
-
0
Roman shift A cm-’
Fig. 4. The Raman spectrum of liquid methyl chloroacetate-d3.
Wavelength, 3
4
5
2xX)
2000
6
microns
7
6
9
IO
12
15
20
30
50
200
0 4000
3500
3000
I600
1600
1400
Wavenumber,
1200
1000
800
600
400
200
0
cm-’
Fig. 5. The i.r. spectrum of crystalline methyl chloroacetate. Wovelength,
4000
3500
3000
4
5
2500
2003
6
I600
microns 7
1600
Wovenumber,
MOO
a
9
1200
IO
IO00
I2
600
cm-’
Fig. 6. The i.r. spectrum of crystalline methyl chloroacetate-d,.
I5
20
600
30
400
50
200
200
0
J. E.
48
KATON
and DEEPALISINHA
Table 1. Observed vibrational frequencies of methyl chloroacetate and methyl chloroacetate-ds (in cm-‘) tentative assignments. Bands enclosed in braces are crystal splitting components i.r. (lis) -3040 sh 3007 w 2954 w-m 2844 vw
CH2ClC02CHs i.r. (sol) 3475 w 3036 w-m 3009 m-s 2958 m
Assignment
(& 3475 w -3040 sh 3010 m
3008 w
3007 s
3005 m
2960 s
2958 w
2961 m
2958 vs
2510 vw 2531 w
2850 w 2510~~
2420 vw 2398 vw 2275 sh
2335 w
2280 m
2256 w 2195 w-m
228 1 w-m 2257 m-s 2190vw 2141 vw
2260 m 2195 m
2330 vw
2120 w
2118m
2085 vw 2077 w
1751 s 1719 w 1709 w
2080 m 2033 1982 1912 1868 1832 1790
1774 m-s 17.55 vs
1753 m, b
1758 vs
1
vw vw vw vw vw vw
1768 m 1747 s 1714 w
1748m,b
1650 w
1593 v-w 1562 vw 1505 vw 1461 1443 1437 1432
w-m m w-m w-m
1505 vw 1485 vw 1455 m 1440 sh
1416 m
1404 w-m I 1320 w-m
1275 vw
1199 m-s
1204 s
1172 m-s
1175 m-s
(sym) (~4
1188+1006 1087 + 1049 2 x 1083 1204+888 2 x 1058 1087 + 936 1049+936 1087+823 2x936 936+898 968+823 2x888 823 + 936 vc=o
sCH3
(~4
aCH3
b.w)
=H3
(~4
6CH2 (scissors) 1006 + 390
1325 w
1320 s, b 1328 m-s 1
1310sh 1268 vw 1264 w 1207 m-s, b
1185 w-m, b 1178m
1188m
118Ow >
~CHZ (wag) isomer 888 +390 968+297 vCOC (asym)
1259 w 1229 m-s 1215 sh I 1192~
1193 m
vCD3 vCD3
1333 m
1320 w-m 1305 sh 1263 w, sh
1415 w-m
1411 m
1403 w-m 1 1391 vw 1325 m 1314m-s
1440+ 1416 1747+782 1322+ 1193 1229+1188 1204 + 1193 vCD3 (asym) 1322 + 1006
1415 m
1412 w-m 1410 m
2 x 1751 (1747) vCH, (ash) I&H? (asym) uCH3 (asym) vCHl (sym)
936 + 786 (782) 1330+374 1006+674 2x823 1204+390 2x768 936+573 (571) 1188+297
1702 w-m
1670 vw
1452 sh 1435 m-s
and their
6CH2 (twist) 6CH3 (rock)
The vibrational spectra and conformational behavior of methyl chloroacetate
49
Table 1 (Cont’d) i.r. (liq) 1145 sh
CH2ClC02CHS i.r. (sol)
(1%
CH2CLC0rCDS i.r. (sol)
i.r. (ho) 1145i 1083 s 1058 sh
1154vw
1087 s 1049 s 1017 w-m
1006 w-m 936 m
1005 m 938vw
929 w
1085 w-m 1060 w-m 1020 w-m
1016 w 1002 m
(G)
965 m, b
1006 w-m 968 m
927 w
936 m-s
898 w
898 m
968 m
Assigmnent 2x573 (571) 6CDs (asym) 6CD3
b.vd
6CD3
(~4
vcoc
(sym)
6CH2 (rock)
931m 1 884~
888 w-m
886 s
816w,b
900 vw
ED3
827 m-s
vcc
785 m-s
isomer VCCI
755 w-m
isomer
(rock)
823 w, 830 w b1 828 vw 788 m
786 s
790 s
782 s 761 vw 750vw 700 vw
783 m 752~
709 w
701w 683 w -580 sh 567 w 416 w 392w 306 w-m
667 w
-200 w, sh 177 w
204 w 175 w
573 m 390 w-m 310m
690 591 -570 422 395 315 245
w, sh m sh w-m s m w
685 w 658 w 585 sh 573 w 409 w 375 w-m 293 w-m
-170w,sh
649 w-m 571 m-s 374 w 297 m
690 w 665 w 580 w-m -570 sh 412~ 375 s 299 w-m 240 w -175w,sh
isomer isomer 6C02 (scissors) isomer 6 skeleton isomer 6 skeleton skeleton skeleton skeleton CH30 torsion?
s = strong, v = very, m = medium, w = weak, sh = shoulder, b = broad, v = stretch, 6 = bend, asym = antisymmetric, sym = symmetric.
chloroacetate is not nearly as obvious as earlier work would imply. The carbonyl stretching mode is a single rather broad band in the pure liquid, but the crystal spectrum shows two additional absorption features, one of which is apparently crystal split. We have recorded the spectrum of a dilute solution of methyl chloroacetate in carbon tetrachloride in this small region and confirm the two bands previously reported, [8] although our frequencies are somewhat different (1748 and 1770 cm-’ vs the previously reported 1750 and 1775 cm-‘). The two frequencies are quite close to the two major features of the crystal spectrum in this region, however. Furthermore, the absorbances of these two bands are independent of temperature, within experimental error (=tO.O5 absorbance units), over the temperature range 28-61”C. These two facts indicate that two bands do not originate from a conformational equilibrium. A careful examination of the remainder of the spectra indicates that it is diflicult
to find obvious cases of liquid phase absorptions which disappear on crystallization, although there are a few examples. Part of the difficulty is due to the observation of a good deal of crystal splitting which leads to inconclusive results since the observation of two bands in the crystal may be due to either crystal splitting or enhanced resolution of liquid phase bands due to band narrowing on woling. It appears that features at about 1300, 820, 701 and 580 cm-’ disappear on crystallization. The clearest example of band disappearance, however, occurs with the weak band at 416 cm-‘. This is confirmed by the analogous band in methyl chloroacetate-d,. Observation of the liquid phase Raman spectrum shows the last three of these bands to be quite clear. It therefore seems clear that methyl chloroacetate does exist as a conformational equilibrium in the liquid phase, but the usefulness of the doublet in the carbonyl stretching region as diagnostic for this molecular feature seems quite doubtful. By implication, at least, this
50
J. E. KATONand DEEPALJSINHA
confirms the conclusion of CHARLES et al. [2] that the existence of a doublet in the carbonyl stretching region is not good evidence for the existence of a conformational equilibrium. It should be noted that there is a second possibility for the formation of conformers which has not been discussed by earlier authors with respect to methyl chloroacetate. There exists the possibility of conformers formed by rotation about the carbonyl carbon-oxygen bond (the cis and tram ester structures). This type of conformational equilibrium has been noted with dimethyl carbonate and discussed in some detail [9]. One conformer is expected to be present in considerably smaller quantities than the other in this case and the weak bands which disappear upon crystallization of methyl chloroacetate are consistent with such an equilibrium. It must therefore be concluded that although methyl chloroacetate exists as a conformational equilibrium in the liquid phase, the structures of the conformers are not known. b. Frequency assignment There are five CH stretching modes expected in methyl chloroacetate, but observation of Figs. 1 and 3 indicates that only two clear bands, one of which has a shoulder, are observed. In the crystal spectrum (Fig. 4) the shoulder is resolved into a distinct band. Comparison with the spectra of methyl chloroacetate-d, indicates that the highest frequency band is due to the methyl group and must be the antisymmetric methyl CH stretch. The two lower frequency bands must be due to the two CH, stretching modes and the two symmetric methyl CH stretches must then be accidentally degenerate with one or both of the methylene CH stretches. The CD stretching region of the liquid phase spectrum of methyl chloroacetate-d, is, on the other hand, quite complex. There are clearly five absorption features in both the i.r. and the Raman spectra. This spectral region is greatly simplified in the crystal, however, where only three reasonably intense bands are noted. It is relatively common to see several additional features in the CD stretching region of CD, compounds. KATON and GRIFFIN [9] have discussed these to some extent with regard to methyl chloroformate-d, and assigned the extra bands as overtones of CD, bending modes in Fermi Resonance with the fundamentals. These features remain in the spectrum of the crystal, however. The general intensities and band shapes of these extra liquid phase absorption features are not typical of those expected for conformer bands and CH or CD stretching frequencies
are notably insensitive to conformation. It would appear that the best explanation lies in band shifts upon crystallization which lead to the overtones being sufficiently different in energy that Fermi Resonance is no longer a significant factor in their intensities. This lack of resonance would lead to the appearance of even greater band shifts for the fundamentals since they would no longer be perturbed by the overtones. It must be noted, however, that CD stretching modes have never been studied carefully in compounds which exist in a conformational equilibrium due to the internal rotation about a carbonyl carbon-oxygen bond. In such compounds the CD stretches might be conformation sensitive. There are three bands observed in the liquid in the 1400-1500cm-1 region, two of which are crystal split in the solid spectrum. One expects three CH3 deformations and one CH, deformation (scissors) in this region but it is quite common for two of the methyl deformations to be accidentally degenerate. Since the -L& compound has only one band in this region, the differentiation of the CH, and CH2 deformations is quite straightforward. There are two strong bands in the 14001200cm-’ region in the spectra of both the light and heavy compounds. On the basis of group frequencies these are assigned as the CH, wag and the antisymmetric COC stretching modes, respectively. In the 1200-lOOOcm_’ region we observe three bands in the crystal spectrum of methyl chloroacetate, one of which (1175 cm-‘), is not present in the spectrum of the deuterated compound. This band is assigned as a CH3 rocking mode while the 1193 cm-’ solid band is assigned as the CH, twisting mode. The 1002 cm-’ band is assigned as the symmetric COC stretching mode on the basis of its intensity in the Raman spectrum. The lOOO-SOOcm-’ region is probably the most ambiguous region in the spectrum for assignment. One expects the CH, rocking mode and the CC stretching mode in this region and, in addition, there is one CH, rocking mode which remains to be assigned. There are, however, only two bands in this region. On the basis of Raman intensities, it would appear that the 886 cm-’ band in the light compound should be assigned as the CC stretch, but the intense Raman band in the deuterated compound occurs at 827 cm-‘. This seems a very large shift for this mode. The band near 935 cm-’ remains essentially constant in both compounds and has been assigned as the CH, rock. The final CH3 rock must then be accidentally degenerate with one of the other fundamentals.
The vibrational spectra and conformational behavior of methyl chloroacetate The strong Raman band at about 790 cm-’ which has a strong i.r. counterpart is certainly the CC1 stretching mode and the lower skeletal bending modes are assigned on the basis of assignments for other simple esters. Several of these very low frequency modes are highly coupled skeletal deformations and no attempt has been made to characterize them more definitively.
51
[2] S. w. CHARLES,G. I. L. JONESand N. L. &fEN, J. Chem Sot.. Furadav 11. 69. 1454 (1973).
[3] D. SINHAahd J. E. iDLT;N, ban J. ‘hem. 52,3057 (1975). [4] R. M. MORAVIEand J. CORSET, J.Mol. Structure 24, 91 (1975). [5] W. 0. GEORGE, D. V. HASSIDand W. F. MADDAMS, J. Chem Soc..Perkin 12, 1029 (1972). [6] L. J. BELLAMY, The Infrared Spectra of Complex Molecules, 2nd Ed., Wiley, New York, 1958. [7] J. LAATO,Ann. Uniu. Turku. A, 1,82, 86 (1965). [8] M. L. JOSIEN and R. CALLOS,Compt. rend. 240, 1641 (1955). r91 - _ J. E. KATON and hl. D. COHEN. Can. J. Chem. 53. REFERENCES 1378 (1975). [l] G. I. L. JONESand N. L. OWEN,J. MoL Structure 18, [lo] J. E. K&TON and M. G. GRIFFIN,J. Chem Phys. 59, 1 (1973). 5868 (1973).
behavior of methyl chloroacetate* J. E.
Department
KATON and DEEPALI SINHA of Chemistry, Miami University, Oxford, OH 45056, U.S.A.
(Received 28 October 1975; revised 18 February 1976) Abstract-The i.r. spectra of methyl chloroacetate and methyl chloroacetate-ds have been recorded in both the liquid and solid states and Raman spectra of the two compounds have been recorded in the liquid phase. A tentative vibrational assignment has been made for both compounds.
The conforrnational properties of methyl chloroacetate are discussed and it is shown that the molecule does exist as a conformational equilibrium in the liquid phase. The doublet observed in the solution spectrum of methyl chloroacetate in the carbonyl stretching region has been shown to be a poor indication of this conformational
equilibrium, however.
Since methyl chloroacetate has not undergone a thorough vibrational analysis and since it is widely quoted as an example of an ester existing as a conformational equilibrium in the liquid state, it seemed worthwhile to carry out a detailed analysis of its vibrational spectra in an effon to put its structural features on a firmer footing. This is particularly true since the existence of doublets in the carbonyl stretching region is not a very good criterion for conformational equilibria, in general [2].
INTRODUCXION
detailed molecular structures and conformations of carboxylic acid esters have been investigated by many workers. This work has been reviewed in detail recently by JONES and OWEN [l]. Conformational equilibria in esters can arise by internal rotation of three basic types (1) rotation about the C-O bonds; (2) rotation about the C-C bonds of the alkyl group; and (3) rotation about the C-C bonds of the acyl group. Substituted methyl acetates are particularly simple examples of the class since in these molecules conformers may arise only by rotation about (1) one of the C-O bonds and (2) the single C-C bond of the acyl group. There have been, however, very few detailed studies of the vibrational spectra of simple acyl substituted methyl acetates. Apparently the only molecules possessing this structure which have undergone complete spectral assignment and investigation of conformational behavior are methyl cyanoacetate [2, 31 and methyl propionate [4]. The latter has also been investigated with regard to its conformational equilibrium by GEORGE’ et al. (5). Although methyl monochloroacetate has been stated to exemplify the existence of conformational equilibria in esters [ 1, 61 the only experimental data relating to this conclusion seems to be the fact that two bands have been reported in the carbonyl stretching region in carbon tetrachloride solution [7, 81. The conformers have been assumed by these workers to be those due to rotation about the C-C
The
bond
EXPERIMENTAL.
Reagent grade methyl chloroacetate and chloroacetyl chloride (Eastman Organic Chemical) were purified by distillation. Methyl chloroacetate-d3 was prepared by the slow addition of CD30D to purified chloroacetyl chloride followed by distillation. Infrared spectra in the 400050 cm-’ region were recorded on a Perkin-Elmer Model 180 i.r. spectrophotometer. The spectra of the crystalline films were obtained utilizing a Research and Industrial Instrument Corporation VLT-2 cell and liquid nitrogen coolant. Raman spectra of the liquid compounds were recorded on a Cary Model 81 Raman spectrometer using an Argon ion laser source. Infrared spectra of a 0.15% solution of methyl chloroacetate in Ccl., in a 0.1 mm sealed cell were recorded as a function of temperature from 28” to 61°C in a heated compartment of conventional design. The liquid phase i.r. and Raman spectra of methyl chloroacetate and methyl chloroacetate-d, are reproduced in Figs. l-4. Crystal mid-i.r. spectra of methyl chloroacetate and methyl chloroacetate-d, are reproduced in Figs. 5-6. The observed ix. and Raman bands are given in Table 1. RESULTS
of the acyl group.
a.
Conformational
AND
DISCUSSION
behavior
Comparison of Figs. 1 and 5 indicates that the existence of a conformational equilibrium in methyl
*Supported m part by the United States Air Force under Contract #F33615-73 C-5013. 45
J. E. &-ION and DEEPAL~SINHA Wavelength, 4.
6
5
microns e
7
9
la
I2
x,
30
,50
200
m f
60
z E
40
+ 1
20
i? ::
0 4
Wavenumber,
cm-’
Fig. 1. The i.r. spectrum of liquid methyl chloroacetate.
2400
2000
I600
1600
1200
1400
1000
800 '--
600
400
200
Roman shift Acm-’
Fig. 2. The Raman spectrum of liquid methyl chloroacetate.
Wavelength,
4000
3500
3000
2500
2000
1800
1600
Wavenumber,
microns
1400
1200
1000
800
cm-1
Fig. 3. The i.r. spectrum of liquid methyl chloroacetate-d,.
600
400
200
0
The vibrational spectra and conformational
k=iL;
4000
1J--I
3600
I
I 32Ob
:
2800
I:/
/ 2400
!
!
1
I600
2000
47
behavior of methyl chloroacetate
I
I
1600
MOO
I
i
/
! 1000
1200
-6Oi-
--6co--400
/
200
-
0
Roman shift A cm-’
Fig. 4. The Raman spectrum of liquid methyl chloroacetate-d3.
Wavelength, 3
4
5
2xX)
2000
6
microns
7
6
9
IO
12
15
20
30
50
200
0 4000
3500
3000
I600
1600
1400
Wavenumber,
1200
1000
800
600
400
200
0
cm-’
Fig. 5. The i.r. spectrum of crystalline methyl chloroacetate. Wovelength,
4000
3500
3000
4
5
2500
2003
6
I600
microns 7
1600
Wovenumber,
MOO
a
9
1200
IO
IO00
I2
600
cm-’
Fig. 6. The i.r. spectrum of crystalline methyl chloroacetate-d,.
I5
20
600
30
400
50
200
200
0
J. E.
48
KATON
and DEEPALISINHA
Table 1. Observed vibrational frequencies of methyl chloroacetate and methyl chloroacetate-ds (in cm-‘) tentative assignments. Bands enclosed in braces are crystal splitting components i.r. (lis) -3040 sh 3007 w 2954 w-m 2844 vw
CH2ClC02CHs i.r. (sol) 3475 w 3036 w-m 3009 m-s 2958 m
Assignment
(& 3475 w -3040 sh 3010 m
3008 w
3007 s
3005 m
2960 s
2958 w
2961 m
2958 vs
2510 vw 2531 w
2850 w 2510~~
2420 vw 2398 vw 2275 sh
2335 w
2280 m
2256 w 2195 w-m
228 1 w-m 2257 m-s 2190vw 2141 vw
2260 m 2195 m
2330 vw
2120 w
2118m
2085 vw 2077 w
1751 s 1719 w 1709 w
2080 m 2033 1982 1912 1868 1832 1790
1774 m-s 17.55 vs
1753 m, b
1758 vs
1
vw vw vw vw vw vw
1768 m 1747 s 1714 w
1748m,b
1650 w
1593 v-w 1562 vw 1505 vw 1461 1443 1437 1432
w-m m w-m w-m
1505 vw 1485 vw 1455 m 1440 sh
1416 m
1404 w-m I 1320 w-m
1275 vw
1199 m-s
1204 s
1172 m-s
1175 m-s
(sym) (~4
1188+1006 1087 + 1049 2 x 1083 1204+888 2 x 1058 1087 + 936 1049+936 1087+823 2x936 936+898 968+823 2x888 823 + 936 vc=o
sCH3
(~4
aCH3
b.w)
=H3
(~4
6CH2 (scissors) 1006 + 390
1325 w
1320 s, b 1328 m-s 1
1310sh 1268 vw 1264 w 1207 m-s, b
1185 w-m, b 1178m
1188m
118Ow >
~CHZ (wag) isomer 888 +390 968+297 vCOC (asym)
1259 w 1229 m-s 1215 sh I 1192~
1193 m
vCD3 vCD3
1333 m
1320 w-m 1305 sh 1263 w, sh
1415 w-m
1411 m
1403 w-m 1 1391 vw 1325 m 1314m-s
1440+ 1416 1747+782 1322+ 1193 1229+1188 1204 + 1193 vCD3 (asym) 1322 + 1006
1415 m
1412 w-m 1410 m
2 x 1751 (1747) vCH, (ash) I&H? (asym) uCH3 (asym) vCHl (sym)
936 + 786 (782) 1330+374 1006+674 2x823 1204+390 2x768 936+573 (571) 1188+297
1702 w-m
1670 vw
1452 sh 1435 m-s
and their
6CH2 (twist) 6CH3 (rock)
The vibrational spectra and conformational behavior of methyl chloroacetate
49
Table 1 (Cont’d) i.r. (liq) 1145 sh
CH2ClC02CHS i.r. (sol)
(1%
CH2CLC0rCDS i.r. (sol)
i.r. (ho) 1145i 1083 s 1058 sh
1154vw
1087 s 1049 s 1017 w-m
1006 w-m 936 m
1005 m 938vw
929 w
1085 w-m 1060 w-m 1020 w-m
1016 w 1002 m
(G)
965 m, b
1006 w-m 968 m
927 w
936 m-s
898 w
898 m
968 m
Assigmnent 2x573 (571) 6CDs (asym) 6CD3
b.vd
6CD3
(~4
vcoc
(sym)
6CH2 (rock)
931m 1 884~
888 w-m
886 s
816w,b
900 vw
ED3
827 m-s
vcc
785 m-s
isomer VCCI
755 w-m
isomer
(rock)
823 w, 830 w b1 828 vw 788 m
786 s
790 s
782 s 761 vw 750vw 700 vw
783 m 752~
709 w
701w 683 w -580 sh 567 w 416 w 392w 306 w-m
667 w
-200 w, sh 177 w
204 w 175 w
573 m 390 w-m 310m
690 591 -570 422 395 315 245
w, sh m sh w-m s m w
685 w 658 w 585 sh 573 w 409 w 375 w-m 293 w-m
-170w,sh
649 w-m 571 m-s 374 w 297 m
690 w 665 w 580 w-m -570 sh 412~ 375 s 299 w-m 240 w -175w,sh
isomer isomer 6C02 (scissors) isomer 6 skeleton isomer 6 skeleton skeleton skeleton skeleton CH30 torsion?
s = strong, v = very, m = medium, w = weak, sh = shoulder, b = broad, v = stretch, 6 = bend, asym = antisymmetric, sym = symmetric.
chloroacetate is not nearly as obvious as earlier work would imply. The carbonyl stretching mode is a single rather broad band in the pure liquid, but the crystal spectrum shows two additional absorption features, one of which is apparently crystal split. We have recorded the spectrum of a dilute solution of methyl chloroacetate in carbon tetrachloride in this small region and confirm the two bands previously reported, [8] although our frequencies are somewhat different (1748 and 1770 cm-’ vs the previously reported 1750 and 1775 cm-‘). The two frequencies are quite close to the two major features of the crystal spectrum in this region, however. Furthermore, the absorbances of these two bands are independent of temperature, within experimental error (=tO.O5 absorbance units), over the temperature range 28-61”C. These two facts indicate that two bands do not originate from a conformational equilibrium. A careful examination of the remainder of the spectra indicates that it is diflicult
to find obvious cases of liquid phase absorptions which disappear on crystallization, although there are a few examples. Part of the difficulty is due to the observation of a good deal of crystal splitting which leads to inconclusive results since the observation of two bands in the crystal may be due to either crystal splitting or enhanced resolution of liquid phase bands due to band narrowing on woling. It appears that features at about 1300, 820, 701 and 580 cm-’ disappear on crystallization. The clearest example of band disappearance, however, occurs with the weak band at 416 cm-‘. This is confirmed by the analogous band in methyl chloroacetate-d,. Observation of the liquid phase Raman spectrum shows the last three of these bands to be quite clear. It therefore seems clear that methyl chloroacetate does exist as a conformational equilibrium in the liquid phase, but the usefulness of the doublet in the carbonyl stretching region as diagnostic for this molecular feature seems quite doubtful. By implication, at least, this
50
J. E. KATONand DEEPALJSINHA
confirms the conclusion of CHARLES et al. [2] that the existence of a doublet in the carbonyl stretching region is not good evidence for the existence of a conformational equilibrium. It should be noted that there is a second possibility for the formation of conformers which has not been discussed by earlier authors with respect to methyl chloroacetate. There exists the possibility of conformers formed by rotation about the carbonyl carbon-oxygen bond (the cis and tram ester structures). This type of conformational equilibrium has been noted with dimethyl carbonate and discussed in some detail [9]. One conformer is expected to be present in considerably smaller quantities than the other in this case and the weak bands which disappear upon crystallization of methyl chloroacetate are consistent with such an equilibrium. It must therefore be concluded that although methyl chloroacetate exists as a conformational equilibrium in the liquid phase, the structures of the conformers are not known. b. Frequency assignment There are five CH stretching modes expected in methyl chloroacetate, but observation of Figs. 1 and 3 indicates that only two clear bands, one of which has a shoulder, are observed. In the crystal spectrum (Fig. 4) the shoulder is resolved into a distinct band. Comparison with the spectra of methyl chloroacetate-d, indicates that the highest frequency band is due to the methyl group and must be the antisymmetric methyl CH stretch. The two lower frequency bands must be due to the two CH, stretching modes and the two symmetric methyl CH stretches must then be accidentally degenerate with one or both of the methylene CH stretches. The CD stretching region of the liquid phase spectrum of methyl chloroacetate-d, is, on the other hand, quite complex. There are clearly five absorption features in both the i.r. and the Raman spectra. This spectral region is greatly simplified in the crystal, however, where only three reasonably intense bands are noted. It is relatively common to see several additional features in the CD stretching region of CD, compounds. KATON and GRIFFIN [9] have discussed these to some extent with regard to methyl chloroformate-d, and assigned the extra bands as overtones of CD, bending modes in Fermi Resonance with the fundamentals. These features remain in the spectrum of the crystal, however. The general intensities and band shapes of these extra liquid phase absorption features are not typical of those expected for conformer bands and CH or CD stretching frequencies
are notably insensitive to conformation. It would appear that the best explanation lies in band shifts upon crystallization which lead to the overtones being sufficiently different in energy that Fermi Resonance is no longer a significant factor in their intensities. This lack of resonance would lead to the appearance of even greater band shifts for the fundamentals since they would no longer be perturbed by the overtones. It must be noted, however, that CD stretching modes have never been studied carefully in compounds which exist in a conformational equilibrium due to the internal rotation about a carbonyl carbon-oxygen bond. In such compounds the CD stretches might be conformation sensitive. There are three bands observed in the liquid in the 1400-1500cm-1 region, two of which are crystal split in the solid spectrum. One expects three CH3 deformations and one CH, deformation (scissors) in this region but it is quite common for two of the methyl deformations to be accidentally degenerate. Since the -L& compound has only one band in this region, the differentiation of the CH, and CH2 deformations is quite straightforward. There are two strong bands in the 14001200cm-’ region in the spectra of both the light and heavy compounds. On the basis of group frequencies these are assigned as the CH, wag and the antisymmetric COC stretching modes, respectively. In the 1200-lOOOcm_’ region we observe three bands in the crystal spectrum of methyl chloroacetate, one of which (1175 cm-‘), is not present in the spectrum of the deuterated compound. This band is assigned as a CH3 rocking mode while the 1193 cm-’ solid band is assigned as the CH, twisting mode. The 1002 cm-’ band is assigned as the symmetric COC stretching mode on the basis of its intensity in the Raman spectrum. The lOOO-SOOcm-’ region is probably the most ambiguous region in the spectrum for assignment. One expects the CH, rocking mode and the CC stretching mode in this region and, in addition, there is one CH, rocking mode which remains to be assigned. There are, however, only two bands in this region. On the basis of Raman intensities, it would appear that the 886 cm-’ band in the light compound should be assigned as the CC stretch, but the intense Raman band in the deuterated compound occurs at 827 cm-‘. This seems a very large shift for this mode. The band near 935 cm-’ remains essentially constant in both compounds and has been assigned as the CH, rock. The final CH3 rock must then be accidentally degenerate with one of the other fundamentals.
The vibrational spectra and conformational behavior of methyl chloroacetate The strong Raman band at about 790 cm-’ which has a strong i.r. counterpart is certainly the CC1 stretching mode and the lower skeletal bending modes are assigned on the basis of assignments for other simple esters. Several of these very low frequency modes are highly coupled skeletal deformations and no attempt has been made to characterize them more definitively.
51
[2] S. w. CHARLES,G. I. L. JONESand N. L. &fEN, J. Chem Sot.. Furadav 11. 69. 1454 (1973).
[3] D. SINHAahd J. E. iDLT;N, ban J. ‘hem. 52,3057 (1975). [4] R. M. MORAVIEand J. CORSET, J.Mol. Structure 24, 91 (1975). [5] W. 0. GEORGE, D. V. HASSIDand W. F. MADDAMS, J. Chem Soc..Perkin 12, 1029 (1972). [6] L. J. BELLAMY, The Infrared Spectra of Complex Molecules, 2nd Ed., Wiley, New York, 1958. [7] J. LAATO,Ann. Uniu. Turku. A, 1,82, 86 (1965). [8] M. L. JOSIEN and R. CALLOS,Compt. rend. 240, 1641 (1955). r91 - _ J. E. KATON and hl. D. COHEN. Can. J. Chem. 53. REFERENCES 1378 (1975). [l] G. I. L. JONESand N. L. OWEN,J. MoL Structure 18, [lo] J. E. K&TON and M. G. GRIFFIN,J. Chem Phys. 59, 1 (1973). 5868 (1973).