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Acetyl halides—I
Bpectrochimicadcta, 1961,Vol. 17, pp. 1205to 1218. Perganmn Press Ltd. Printed in Northern Ireland
Acetyl halides-I Infrared and Raman spectra and vibrational assignment CD,COCI and CH,DCOCl
of CH,COCl,
JOHN OVEREND*, R. A. NYQUIST, J. C. EVANS and W. J. POTTS Chemical Physics Research
Lahoratory,t
The Dow Chemical Company,
Midland,
Michigan
(Received 7 July 1961)
Abstract-The infrared and Raman spectra of acetyl chloride, acetyl chloride-& and acetyl chloride-d, have been studied and a vibrational assignment has been made on the basis of an approximate normal-co-ordinate calculation with a Urey-Bradley field. It is proposed that the two intense bands at 1109 and 958 cm-l in the infrared spectrum of acetyl chloride correspond to vibrational modes which are mixtures of the C-C stretching co-ordinate and the CH, rocking co-ordinate, and that the similar pair of intense bands in the CDsCOCl spectrum arise from modes which are mixtures of CC stretching and CD, deformation co-ordinates. Thermodynamic functions were calculated for acetyl chloride in the ideal gaseous state.
Introduction acetyl halides are the simplest typical members of an extensive family of chemically important molecules and a full understanding of their vibrational modes is of real importance to chemical spectroscopists. In particular, the intense band, which occurs at ca. 958 cm-r in CH,COCl, appears at roughly the same frequency 0 THE
in the spectra of all molecules
containing
II C
the skeleton c
where X may be Cl, x
Br, S or I, [l, 2, 31 and this band has been used for several years in our laboratory as an empirical group frequency. However, fully recognizing the dangers of such empirical correlations, we have turned to a normal-co-ordinate analysis of acetyl chloride with a view to establishing a detailed description of its normal modes. It was our hope that these results would lead to a logical interpretation of the spectra of many structurally similar molecules. Reports of several Raman studies of acetyl chloride [4, 5, 61 and acetyl chlorided, [7] are to be found in the scientific literature and tentative vibrational assignments have been made [5]. Our Raman results confirm the earlier work but do not * Present address: Department of Chemistry, t Formerly Spectroscopy Laboratory. :l] .2] :3] 41 :5] 61 .7]
N. R. R. K. J. H. W.
1
University
of Minnesota,
Minneapolis
SEEPPARD, Trans. Farada?/ Sot., 45, 693 (1949). MECKE and H. SPIESECKE, Be?., dtich. &em. @es. 89, 1110 (1956). A. NYQUIST and W. J. POTTS, Spectmchim. Acta 7, 514 (1959). W. F. KOFILRAUSCH, Rawmnspektren. Edwards, Ann Arbor (1945). C. EVANS and H. J. BERNSTEIN, Canad. J. Chem. a4, 1083 (1956). SEEWAN-ALBERT and L. KAHOVEC, Acta. Phys. Austriaca 1,352 (1948). ENQLER, 2. Phys. Chem. 85B, 433 (1937).
1205
14, Minnesota.
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS md W. J. POTTS
add anything essentially new except in the case of CH,DCOCl which had not been studied previously. However, the infrared spectra reveal several extremely interesting features which, taken with the normal-co-ordinate analysis, allow an unambiguous vibrational assignment. We have found these results a very real help in interpreting the general class of molecules discussed in the preceeding.
Experimental Acetyl chloride (Baker reagent grade) was fractionally distilled to remove acetic acid impurity immediately before use for infrared and Raman studies. The acetyl chloride-d, was prepared, from a sample of acetic acid-d, supplied by Merck, by reaction with SOCI, in CCI, solution. The product was fractionally distilled before use. The acetyl chloride-d, was prepared by reacting ketene with DCl; both gases were condensed in stoichiometric proportions in a liquid-nitrogen trap and allowed to warm up to room temperature: reaction was apparently immediate and complete. Infrared spectra were measured from 3800 to 450 cm-l on the Hersher grating spectrometer [8] and are shown in Figs. l-6. Gasphase samples were contained in glass absorption cells of length 5 cm and 12-5 cm fitted with KBr windows. For the solution spectra, the solvents were Ccl, (1340-3800 cm-l) and CS, (450-1340 cm-l) and the spectra shown were obtained from 10 per cent solutions (by weight) contained in O-1 mm cells. Raman spectra were measured with a Hilger Raman spectrometer using the 4358-A exciting line and the usual filter solutions. Spectra were recorded photoelectrically, dispersion 7 A/mm; and photographically, dispersion 16 A/mm. Depolarization measurements were made by the Edsall-Wilson method although in the case of the heavy compounds the sample volume was insufficient to allow quantitative conclusions. The observed vibrational frequencies of CH,COCl and CD,COCl are listed in Tables 1 and 2 and those of CH,DCOCl in Table 3.
Vibrational assignment The infrared spectra of the acetyl halides present a peculiar problem in vibrational assignment, particularly when data on the deuterium-substituted molecules are available. Starting with the methyl-group frequencies, which are expected to fall at about 3050 (deg.), 2930, 1450 (deg.), and 1375 cm-l in the hydrogen compound, and at about 2275 (deg.), 2110, 1040 (deg.) and 1060 cm-l in the deuterium compound, we can immediately assign those bands which occur at approximately these frequencies to the methyl group. When these are assigned we are left with nine fundamentals, two of which are associated with the in-plane bending of the carbonyl and C-Cl bonds, two with the in-plane and out-of-plane rocking of the methyl group, one with the torsional vibration of the methyl group, one with CO out-of-plane, and three with the stretching of the CC, CO and CC1 bonds. It is possible to proceed with the vibrational assignment along quite plausible lines until there remain only the three skeletal stretching modes to be assigned; at this [S] L. W. HERSHER,Spectrochina.
Actn
901 (1959).
1206
WUQqrQGq’f
$
B e
$
-
-
0.6
i.0
‘5.4
0*2-
0.1
”
b-izmm
Frequency, Cm-'
cm-i
Fig. 4. Bdution infkxmd spectrum of CD,COCi. From 3600 to 1340 cm--’ solvent is CCI,; from 1340 to 450 cm-” C8,. Path length 0.1 mm. Con~~nt~ti~n~; 10 per cem by weight.
Frequency,
Fig. 3. C&s-phase inframd spectrrrm of CD,COCl,
Path lenqt” :12.7 cm Pressure * G 2m mm
cm :a 224,
length: 4
Pressures
Acetyl halicies-I
T E ”
1209
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS and W. J. POTTS
point we find that in each molecule we are left with four unassigned, intense bands at 1822, 1109, 958 and 606 cm-l in CH,COCl and at 1820, 1132, 962 and 563 cm-’ in CD,COCl. The problem of the acetyl halides is to explain this surfeit of bands. Table 1. CHsCOCl. Infrared (gas)
Raman* (liquid)
3629 3029 (2950) (2850)
1212 1159 1140 1109 1047 1029 958 791 663 659 I 608 514 (445)
-
Infrared (liquid)
Infrared int .
-
~
Raman int.
Assignment
Calc. ---
3024 2993 2934 2826
(2760) 2390 2312 2268 2060 1902 1822 1780 1712 1555 1470 1432 1370
Observed vibrational frequencies and assignments
3582 3010 2930
1810
1416 1358
m -
2 1
VW
8,P l/2
VW
2745
2248 2050 1900 1807 1765 (1700)
it,,,
1032 953
1 j 2851;2864
VW
(vg(VP + + Yra), v12) (114+ Z’,)’ ~ 2390, 2399
VW
(% + v,) (%I + va) (ra + v,), 2i.13 219,
w
2b, 0.6
V.V.S.
;v5 ;
w(s) S S
I
VI!, (1’6 +
)I6 V8 (%I + 1’13) VP, v12
2b, 0.89 l/S, 0.69
w
v5 (v, + VI&
w
(VI3 + %a)
(1180)
w(s)
1098
w(s) V.S. m(s) S V.S.
I 2780 1 2328 1 2258 ~ 2067, 2058 / 1916
v3 @a + VI01
w(s) w
(650)
2yp, (~5 + +,),
(v4 + v12) h + 9)
m
1021 953
-
VW
VW
1098
3644
Vl’ %I
VW
1421 1361 1290 1242
__-
215
m
V6 2”14, b’s +
l/2 1, 0.87
Vl3
~
1545
1718, 1717
1306 1267 1216 1196 1122 -
b, + V16) (Vg + v14)
1, 0.6
1780
1465 -
2Vs
i i
~ 1’s)
)‘2)
1028, 1044 -
VW
V? (v, + %I))
784
VW
(VI5 + %I
674
VW
589
594 514
436 348 (238)
V.S.
6, sb. 0.21
t:, -
7, sb. 0.35 1, 0.69
-
-
* Ref. [S].
An approximate normal-co-ordinate calculation, details of which are given in Tables 4 and 5 and in a following paper, supplies a clue to the vibrational assignment. A reasonable set of Urey-Bradley force constants, shown in Table 4,t t Following the precedent set by SHIMANOUCHI [9] we have assumed the linear terms in the potential energy F’ equal to -O.lF. [9] T. SRIMANOUCHI, J. Chem. Phya. 17, 848 (1949).
1210
Acetyl halides--I
chosen appropriate of CH,COCl side by side
by comparison with similar molecules and these were taken with 2 and G matrices [lo] in the calculation of the vibrational frequencies and CD,COCl. In Table 5 the calculated frequencies are displayed with the frequencies of the prominent bands in the spectrum which
was
Table 2. CD,COCI. Infrared (gas)
-3628 3300 3220 (2950) (2780) 2638 2385 2289 2280 2257 2085 1959 1940 1915
1820 1743 1687 1632 1378 1328 1280 1132 1090 1040 1003 962 877
j ‘Raman 1 (liquid) -i-
Observed vibrational frequencies and assignments
-
Infrared int.
Infrared (liquid) _‘_
_‘-
/ I&man int. i -.-_-
Assignment
m w w W W TY
3640 3320 3236 2952 2782 2638 2383 -
W
/ )
2248 2104
/ j i / ! ’
m
D(?)27b
W
P 55 1950 1924 1917 1858
111
m m -
1877
T.S. m
181,”
P(P) 4
: Pl6b 1754 1695 1636 1375 1316 1279
W W
m U’ m
:
i
/ 1 /
V.S.
(797) 777 604
1037
m
j /
948
V.S.
/ ! /
823
563 522 438
1126
V.W.
I
818
Calc.
i D(P) 8b I
S
m m m m UT
I ,571
/
437 317
/ / I
I
V.S.
v5 1000
I’(?) 5b
! P12 1 I / I335
754 634
W W(S)
have been assigned as fundamentals. The correspondence between observed and calculated frequencies provides considerable support for our proposed assignment although we freely admit that this argument depends on our assumption that the Urey-Bradley force constants are transferable from molecule to molecule and that [IO] J.
OVERBND
and J. R.
‘SCHERJX%, J.
Chem. Phye.
32, 1289 (1960).
1211
JOI~N
OVEREND,
R. A.
J. C.
NYQUIBT,
EVANS
and
W.
J.
POTTS
the force constants of acetyl chloride may be represented adequately by a linear hybrid of those of acetone and carbonyl chloride. With these approximate normal co-ordinates we return to the assignment Table
3. CH,DCOCl.
Observed vibrational frequencies and assignment
-7
Infrared (gas)
i Infrared (liquid)
Raman (liquid)
Infrared int.
Raman
pol.
Approximate description
Assignment _.._ .
~_
@runs)
I-
_._ 3610 3020 2972 2918 2849 2797 2665 (2400)
I 3008 2956
/
I I
3583 3005 2955 2812 2772 2650 (2400)
m w w
Dp? P
2245 2190
1935 1820 1730 1690 1454 1422 1408
1923 1802 1712 (1693) 1412 1397 1361 1281 1254
1796
1395
1290 1265 (1200)
91
VCH
92 b4
+
9g)
W
(93
+
911)
(94
+
9d
/
(911 + +a
W
m vvs VW
‘I’Cll
1’
P?
(98 + y7)
(9&I +
9L‘o
v3
94
2
!
(92
W
(6
m
SC&J
Dp?
(SCH2
+
9121
-G, -
(9,
1073 1011 1000(s) 973 908 852 (786) 592
1020 987 909 853
S
9x2
s
95
-
VS
-
(608)
688
565 507 489
553
8
96
W
fv3
8
P P
8
97
wCD -+- v,, (93
(93
+
912)
+
912)
+
914)
97 (910
9a -
+
-
%)
99’ 2912 +
v10 %of
(913 +
914)
S
I
8 W W
0.5 DP -
911)
(910
913
@CD
VW
0.4
554 503 487
9131
+
9%
VW
W
4
(93
95
-t- 99)
9lfJ)
(93
94
W
910) 91,)
93
W
m
+
P
1095(s)
1088
851
2% 9UH
W
2220 2255
(gauche)
-
-
%O
vco
98
Yco
914
F
9s
0001
sCCC1
/
I
910
Vll
-
-
“12 “13
-
“14
problem. The totally symmetric CH and CD stretching vibrations may be identified immediately from the polarization results in the Raman effect and are assigned The two remaining CH stretching to the bands at 2934 and 2 104 cm-’ respectively. modes, one of which belongs to the A’ species and one to the A”, and which are 1212
Acetyl halides-I
degenerate if the molecule has the full symmetry of the methyl group, may be expected to be almost coincident in acetyl chloride and accordingly the band at 3029 cm-l in the gas-phase infrared spectrum of CH,COCl has been attributed to an unresolved doublet consisting of v1 and vI1. The band at 2280 cm-l in the CD,COCl spectrum has been similarly assigned. The assignment of the methyl-group deformation modes presents certain problems and this assignment will be deferred until the skeletal modes have been discussed. There is no difficulty in assigning the band at 1822 cm-l in CH,COCl Table 4. Force constants of CH,COCl transferred from COCl, and (CH,),CO. K and F are in mdyn/A. H and .?Y in 1011 erg/rad2. (CH,),CO Coordinate
/
[ill
-/__
UBFC
I
Coordinate
4.632
i
Arccl
2.929 9.851 0.474 ! 0.535 , 1.526 0.723 0.244 (0.316)[13]~ 0.497 0.076 0.298 -0.067
CH,COCl
COCl, [ 121
1
UBFC
1.99 12.61 0.442 * 0.335 * 0.860 0.523
Arc0 Au,,,,
bxc, QOCl 4ClCl
I
Coordinate
Arm
AToo Arc0 Arc,1 Auacc AWCH &co &Xl Aaocc, AY A, WC 9aa 4co PCCl
POCl 2
-__
UBFC
4.632 2.929 11.23 2.00 0.535 0.474 0.723 0.930 0.442 0.400 [ 121 0.014t 0.298 0.076 0.497 0.419 0.860 -0.060
* Corrected for the revised definition of the internal coordinate. t Calculated from barrier height of 1.35 kcal/mole quoted in Ref. [16]. p. 1215.
and at 1820 cm-l in CD,COCI to the carbonyl stretching mode vS, nor is there any question that the bending modes (calculated frequencies 496 and 391 cm-l in CH,COCl and 462 and 355 cm-l in CD,COCl) should be assigned to the Raman bands at 436 and 348 cm-l in the hydrogen compound and 437 and 317 cm-l in the deuterium compound. Also the correspondence between observed and calculated frequencies and the high infrared intensity leaves little doubt that the bands at 608 and 563 cm-l should be assigned to the carbon-chlorine stretching vibration. The remaining A’ type modes involve the C-C stretching co-ordinate and the methyl-group deformation and rocking co-ordinates but it is not possible to match each defined co-ordinate with a spectral band. Further, although the spectra of [Ill J. OVEREND,J. R. SCRBRERand R. R. HOLMES. Unpublished work. [12] J. OVERENDand J. R. SCHERER,J. Chenz. Phys. 22, 1296 (1960). [ 131 J. OVERENDand J. R. SCHERER,J. Opt. Sot. Amer. 50, 1203 (1960). Value transferred from (CR,)&.
1213
JORN
OVEREND,
R. A.
NYQUIST,
J. C. EVANS and W. J. POTTS
CH,COCl and CD,COC!l are superficially similar in that they both have very intense bands at ea. 1100 and ca. 950 cm-l, the assignments cannot be made along parallel lines because account must be taken of frequency differences resulting from the differences in mass between H and D. Table 5. Frequencies of CHaCOCl, CDaCOCl and CHaDCOCl Cttlculated from assumed force constants, see Table 4. CDaCOCl
CHaCOCl Approximate* description
Gale, (cm-l) _---
A’
v3 VP A'.V5 V6 v7 V8 vfi ,VlO Vll
it”
i%a
VI3
1% iv15
-
Calc.
Mode
--
3002 2939 1780 1441 1378 1104 962 661 469 391
3029 2934 1822 1432 1370 1109 958 608 436 348
3000 1444 1040 568 136
3029 1432 1029 514 238
WE8
D-eclipsed
iv1 aye
1.Assignment
2962 2192 1778 1418 1266 1033 899 607 467 381
v3 v4
%x3
voo 6CH8
wcwz WC
WCD WC1 dOCCi &xl VCVCA 6CHZ OCD Ye0 Par@
/
"5 v6 v7 VS VS %l %1 %2 818 "14 v15,
CHaDCOCl (trans)
VCW
3000 1306 974 568 120
,
V2
A”
/ hip1 ment, ^._..__ -_
2232 2114 1777 1147 1031 986 802 601 462 355
v1
A’
Approximate* description
(cm+) --.
I'CD
2227 1031 854 516 100
WCD3 Yco 7CD3
CH,DCOCl
3002 2962 2189 1780 1417 1300 1260 I.085 968 847
A
3020 1290 987 507 -
-
(gauche)
I
@CD
657 539 467 378 121
-
2280 1040 877 498
%D 6 CD3
H-eclipsed 2972 2255 1620 1408 1265 1020 909 565 437 340
2280 2104 1820 1132 1040 962 818 563 437 317
vCD
3020 2972 2255 1820 1422 1290 1265 1088 987 851
+ WC
GOCCl
/
SCCCI
1
7CH2D
588 489 437 340
!
-
WC1
we
-
* This approximate description ix determined by inspection of the potential-onorgy-dist,ribution matrices.
We submit that the vibrational assignment should be made along indicated in Table 5; that the two intense bands at 1109 and 958 cm-r are as mixtures of methyl-rooking and carbon-carbon-stretching vibrations the two similar intense bands in the spectrum of CD,COC1 at 1132 and I.214
the lines described and that 962 cm-r
Acetyl halides-I
are described as mixtures of the symmetrical CD, deformation and the carboncarbon-stretching modes. Although one would not normally expect the methyl deformation and rocking vibrations to be so intense in the infrared spectrum, the mixing indicated by the normal-co-ordinate analysis and the assumption that the carbon-carbon bond has a high dipole-moment slope* would qualitatively account for the observed spectra. The assignment of the two A’ CH, deformations in CH,COCl to the bands at 1432 and 1370 cm-l and the A’ CD, deformation and wagging modes to the bands at 1040 and 818 cm-l completes the assignment of the A’ modes for these two molecules. The A” CH and CD stretching vibrations have already been assigned. There is a band at 1432 cm-l in the gas-phase infrared spectrum of CH,COCI and one at 1040 cm-l in the fully deuterated species which appear to have sharp central Q branches and which are assigned to the A” methyl-deformation vibrations. The A” methyl-rocking modes are assigned to the bands at 1029 cm-l and 877 cm-l respectively, and, as OVEREND and EVANS have already determined [15], the carbonyl out-of-plane vibration should be assigned to the band at 514 cm-l in CH,COCl and that at 498 cm-l in CD,COCl. The assignment of the torsional frequency by KOHLRAUSCH [4] to the weak band? at ca. 238 cm-l in the Raman spectrum is somewhat tenuous, and since it is markedly different from the torsional frequency value calculated from the barrier to internal rotation determined by the microwave method, we shall accept the calculated value; no low frequency line was observed in the spectrum of CD,COCl. The remaining bands in the spectra of both molecules can be satisfactorily assigned to combination and overtone bands arising from these fundamentals although in several cases it is not possible to choose between alternative possibilities.
Vibrational assignment of CH,DCOCl The equilibrium configuration of acetyl chloride has been shown to have the carbonyl bond eclipsed by one of the CH bonds: [16] there is a three-fold potential barrier to the rotation of the methyl group and there are three possible equilibrium conformations. In the symmetrical molecule these are necessarily equivalent and for a normal-co-ordinate analysis it is sufficient to consider a single species in the equilibrium conformation. In the case of CH,DCOCl the three positions of the methyl group are no longer equivalent and we must consider the geometry more carefully. The rate at which one molecule passes from one conformer to the other should be given roughly by the Arrhenius equation k = A exp (-E/RT) * J. PETRO[14] has pointed out that such a high value for the dipole-moment slope might be expected in cases where the bond is between two differentlv hvbridized carbon atoms. t KORLRAUSCR reported the relative intensity of &is band as 1/2b; however, it has not been reported by later workers. [14] J. PETRO,J. Amer. Chem. Sot. 4230 (1958). [15] J. OVERENDand J. C. EVANS, Tram Fmm?q Sot. 55, 1817 (1959). [16] K. M. SINNOTT,BUZZ. Amer. Phys. Sot. 1,198 (1956).
1215
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS and W. J. POTTS
where A is the frequency factor and E the barrier height; taking E = 1.350 cal/ mole, at room temperature the rate is about O-1 A. The two equilibrium confirmations are: (1) The CH bond eclipses the carbonyl and the CD bond is gauche to the CC1 bond (gauche CH,DCOCl), and (2) The CD bond eclipses the carbonyl and is trans to the CC1 bond (trans-CH,DCOCl). If we use twice the torsional frequency (ca. 200 cm-l) as an approximate value for A, we might then expect the remainder of the vibrational spectrum, which is at much higher frequencies, to be understandable in terms of two conformers which will be present in the ratio 2 : 1. In Table 5 we show the frequencies of both trans and gauche forms calculated from the assumed potential constants and a tentative vibrational assignment made on the basis of these calculations. Comparing the frequencies of the two conformers calculated from the potential function assumed in the preceding (Table 5), we see that in a few instances there is a significant difference which we may reasonably expect to show up as a splitting in the observed spectra. In Table 6 we have collected the calculated splittings and these are compared with the prominent doublets in the infrared spectrum. A simple model suggests that the intensities of trans and gauche forms will be in the statistical ratio of 1:2. However, as the splitting arises from significant differences in the mixing of internal co-ordinates in the normal modes (arising from the different symmetry properties of the two conformers) it is extremely unlikely that the aP/aQ’s are equal; any differences in this quantity will give rise to a deviation from the statistical ratio. However, in support of our proposed assignment it is interesting to note that the fairly intense doublet at 565-607 cm-l has an intensity ratio of approximately 1: 2, and that the component assigned to the gazcche form is the more intense. The final vibrational assignment was made on the assumption that the spectrum is that of a mixture of the two conformers, and is shown in Tables 3 and 5, although there is some uncertainty in the precise interpretation of the complex of bands between 950 and 1100 cm-l, and further experimental data at much higher resolution are desirable.
Discussion Deuterium substitution is a powerful tool for the elucidation of a vibrational assignment and has been successfully employed on countless occasions. From the most na’ive viewpoint, one would expect a shift of about l/1/2 in the frequency of any mode which involves a hydrogen atom. If we take into account the fact that the normal vibrations do not correspond to simple distortions of a single bond, then the shift in a particular band on deuteration will depend on the extent to which the internal co-ordinate involving the hydrogen atom participates in the particular normal co-ordinate. However, even in this case we should expect to see a marked frequency shift in any vibration which involves a hydrogen atom. It is therefore apparently contradictory when we assign the CH, rocking mode to a band in the spectrum of CH,COC’l which does not appear to shift when we replace the hydroWe have explained this contradiction by saying that, although gens by deuterium. the intense bands occur at approximately the same frequencies in both light and heavy compound, there is a fundamental difference in the normal co-ordinates. If we further postulate that the infrared intensity stems entirely from the dipolemoment slope of the carbon-carbon bond, then it is easy to see that any vibration 1216
Acetyl halides-I
might occur at about 1000 cm-l and which is appropriately coupled to the carbon-carbon stretching co-ordinate will give rise to an intense infrared band in this region of the spectrum. We believe that this phenomenon occurs frequently in 0 which
the spectra of molecules containing
I/ C
the
skeleton and that in all these cases
(IL ’ 3 “xone must exercise caution in interpreting the results of deuterium Table 6. Obsecrved and calculated conformational Mode
Approx. description
Calc. (trans)
(k/YWbS)
I
1
I
2 3 4 5 6 7 8 9 10 11 12 13 14 15
:
L
I
2962 2192 1778 1418 1266 1033 899 607 467 381 3000 1306 974 668 120
splittin
I / / ;
Y~_
-2 1 6 -52 52 .._.60
voc OCD
I
VCCl
0 3
&Cl
I
6fC1
!
WE
-2 6 6 29
&HZ SC0 -1
We feel that the nature of the vibration
producing 0
in cm-l
Observed splitting
Cak. splitting ___-.0 3
WE rcu wo GCHB 6CHa
!
substitution.
(~~a~s~u~c~e) in CHsDCOCI
;
0 0 0 14 0 -68 58 -43 0 0 0 0 0 18 -
the intense absorption
at
II ca. 950 cm-l and characteristic of the structure CH,-C-X is fairly well explained. Not so clear, however, is the fact that a completely analogous absorption in the 0 I/ range 850-1000 cm-l is characteristic of any molecule of structure C-C-X, no matter what the nature of the a-carbon atom. In view of the above discussion, it 0 // would appear that molecules of form CH,-C-X constitute a somewhat special case, and probably will not serve as model compounds which can be used to estimate the nature of this normal mode for more complex molecules. We hope to investigate this point more fully in future studies of related molecules.
Appendix Thermodynamic functions for acetyl chloride in the ideal gaseous state at one atmosphere pressure were calculated using the standard statistical methods based 1217
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS and W. J. POTTS
on the rigid-rotator, harmonic oscillator approximation. Molecular dimensions determined by Sinnott using the microwave method and quoted by KRISHER and WILSON [ 171 were used; these authors quote all dimensions except the C-Cl bond properties of act“tJ11chloride (ideal
Table 7. Thermodynamic C”,,
T”, -___ 298.15 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
!_
16.21 16.26 18.86 21.19 23.18 24.86 26.30 27.54 28.60 29.51 30.30 30.98 31.57 32.08
-
/
so 70.47 70.57 75.61 80.17 84.11 87.82 91.24 94.40 97.36 100.13 102.73 105.18 107.50 109.70
__
(H” - H,“)/T
-@ -(F”
11.80 11.82 13.26 14.62 15.88 17.05 18.12 19.10 20.00 20.82 21.58 22.28 22.92 23.52
-
H,“)/T
58.67 58.74 62.34 65.55 68.23 70.76 73.11 75.30 77.36 79.30 81.15 82.90 84.58 86.18
Units: cal/deg. mole.
length, and for this, the value 1.80 .L% indicated by electron diffraction studies was assumed [18]. The principal moments of inertia thus calculated were 48.84, 104.35 and 150.07 a.m.u. x A2. The symmetry number for external rotation is one, while for the hindered internal rotation the symmetry number is three. The contributions made by this latter degree of freedom were determined using PITZER and GWINN’S methods [19], taking the barrier height to be 1300 Cal/mole [17] and the reduced moment of inertia to be 4.92 x lO& g cm2. Calculated values are collected in Table 7. [17] L. C. KRISHER and E. B. WILSON, J. Chem. Ph?/a.31,888 (1959). [IS] Innteratonaic Distances, Special Publication No. 11. Chemical Society, London (1958). [IQ] K. S. PITZER and W. D. GWINN, J. Chem. Phys. 10,428 (1942).
1218
Acetyl halides-I Infrared and Raman spectra and vibrational assignment CD,COCI and CH,DCOCl
of CH,COCl,
JOHN OVEREND*, R. A. NYQUIST, J. C. EVANS and W. J. POTTS Chemical Physics Research
Lahoratory,t
The Dow Chemical Company,
Midland,
Michigan
(Received 7 July 1961)
Abstract-The infrared and Raman spectra of acetyl chloride, acetyl chloride-& and acetyl chloride-d, have been studied and a vibrational assignment has been made on the basis of an approximate normal-co-ordinate calculation with a Urey-Bradley field. It is proposed that the two intense bands at 1109 and 958 cm-l in the infrared spectrum of acetyl chloride correspond to vibrational modes which are mixtures of the C-C stretching co-ordinate and the CH, rocking co-ordinate, and that the similar pair of intense bands in the CDsCOCl spectrum arise from modes which are mixtures of CC stretching and CD, deformation co-ordinates. Thermodynamic functions were calculated for acetyl chloride in the ideal gaseous state.
Introduction acetyl halides are the simplest typical members of an extensive family of chemically important molecules and a full understanding of their vibrational modes is of real importance to chemical spectroscopists. In particular, the intense band, which occurs at ca. 958 cm-r in CH,COCl, appears at roughly the same frequency 0 THE
in the spectra of all molecules
containing
II C
the skeleton c
where X may be Cl, x
Br, S or I, [l, 2, 31 and this band has been used for several years in our laboratory as an empirical group frequency. However, fully recognizing the dangers of such empirical correlations, we have turned to a normal-co-ordinate analysis of acetyl chloride with a view to establishing a detailed description of its normal modes. It was our hope that these results would lead to a logical interpretation of the spectra of many structurally similar molecules. Reports of several Raman studies of acetyl chloride [4, 5, 61 and acetyl chlorided, [7] are to be found in the scientific literature and tentative vibrational assignments have been made [5]. Our Raman results confirm the earlier work but do not * Present address: Department of Chemistry, t Formerly Spectroscopy Laboratory. :l] .2] :3] 41 :5] 61 .7]
N. R. R. K. J. H. W.
1
University
of Minnesota,
Minneapolis
SEEPPARD, Trans. Farada?/ Sot., 45, 693 (1949). MECKE and H. SPIESECKE, Be?., dtich. &em. @es. 89, 1110 (1956). A. NYQUIST and W. J. POTTS, Spectmchim. Acta 7, 514 (1959). W. F. KOFILRAUSCH, Rawmnspektren. Edwards, Ann Arbor (1945). C. EVANS and H. J. BERNSTEIN, Canad. J. Chem. a4, 1083 (1956). SEEWAN-ALBERT and L. KAHOVEC, Acta. Phys. Austriaca 1,352 (1948). ENQLER, 2. Phys. Chem. 85B, 433 (1937).
1205
14, Minnesota.
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS md W. J. POTTS
add anything essentially new except in the case of CH,DCOCl which had not been studied previously. However, the infrared spectra reveal several extremely interesting features which, taken with the normal-co-ordinate analysis, allow an unambiguous vibrational assignment. We have found these results a very real help in interpreting the general class of molecules discussed in the preceeding.
Experimental Acetyl chloride (Baker reagent grade) was fractionally distilled to remove acetic acid impurity immediately before use for infrared and Raman studies. The acetyl chloride-d, was prepared, from a sample of acetic acid-d, supplied by Merck, by reaction with SOCI, in CCI, solution. The product was fractionally distilled before use. The acetyl chloride-d, was prepared by reacting ketene with DCl; both gases were condensed in stoichiometric proportions in a liquid-nitrogen trap and allowed to warm up to room temperature: reaction was apparently immediate and complete. Infrared spectra were measured from 3800 to 450 cm-l on the Hersher grating spectrometer [8] and are shown in Figs. l-6. Gasphase samples were contained in glass absorption cells of length 5 cm and 12-5 cm fitted with KBr windows. For the solution spectra, the solvents were Ccl, (1340-3800 cm-l) and CS, (450-1340 cm-l) and the spectra shown were obtained from 10 per cent solutions (by weight) contained in O-1 mm cells. Raman spectra were measured with a Hilger Raman spectrometer using the 4358-A exciting line and the usual filter solutions. Spectra were recorded photoelectrically, dispersion 7 A/mm; and photographically, dispersion 16 A/mm. Depolarization measurements were made by the Edsall-Wilson method although in the case of the heavy compounds the sample volume was insufficient to allow quantitative conclusions. The observed vibrational frequencies of CH,COCl and CD,COCl are listed in Tables 1 and 2 and those of CH,DCOCl in Table 3.
Vibrational assignment The infrared spectra of the acetyl halides present a peculiar problem in vibrational assignment, particularly when data on the deuterium-substituted molecules are available. Starting with the methyl-group frequencies, which are expected to fall at about 3050 (deg.), 2930, 1450 (deg.), and 1375 cm-l in the hydrogen compound, and at about 2275 (deg.), 2110, 1040 (deg.) and 1060 cm-l in the deuterium compound, we can immediately assign those bands which occur at approximately these frequencies to the methyl group. When these are assigned we are left with nine fundamentals, two of which are associated with the in-plane bending of the carbonyl and C-Cl bonds, two with the in-plane and out-of-plane rocking of the methyl group, one with the torsional vibration of the methyl group, one with CO out-of-plane, and three with the stretching of the CC, CO and CC1 bonds. It is possible to proceed with the vibrational assignment along quite plausible lines until there remain only the three skeletal stretching modes to be assigned; at this [S] L. W. HERSHER,Spectrochina.
Actn
901 (1959).
1206
WUQqrQGq’f
$
B e
$
-
-
0.6
i.0
‘5.4
0*2-
0.1
”
b-izmm
Frequency, Cm-'
cm-i
Fig. 4. Bdution infkxmd spectrum of CD,COCi. From 3600 to 1340 cm--’ solvent is CCI,; from 1340 to 450 cm-” C8,. Path length 0.1 mm. Con~~nt~ti~n~; 10 per cem by weight.
Frequency,
Fig. 3. C&s-phase inframd spectrrrm of CD,COCl,
Path lenqt” :12.7 cm Pressure * G 2m mm
cm :a 224,
length: 4
Pressures
Acetyl halicies-I
T E ”
1209
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS and W. J. POTTS
point we find that in each molecule we are left with four unassigned, intense bands at 1822, 1109, 958 and 606 cm-l in CH,COCl and at 1820, 1132, 962 and 563 cm-’ in CD,COCl. The problem of the acetyl halides is to explain this surfeit of bands. Table 1. CHsCOCl. Infrared (gas)
Raman* (liquid)
3629 3029 (2950) (2850)
1212 1159 1140 1109 1047 1029 958 791 663 659 I 608 514 (445)
-
Infrared (liquid)
Infrared int .
-
~
Raman int.
Assignment
Calc. ---
3024 2993 2934 2826
(2760) 2390 2312 2268 2060 1902 1822 1780 1712 1555 1470 1432 1370
Observed vibrational frequencies and assignments
3582 3010 2930
1810
1416 1358
m -
2 1
VW
8,P l/2
VW
2745
2248 2050 1900 1807 1765 (1700)
it,,,
1032 953
1 j 2851;2864
VW
(vg(VP + + Yra), v12) (114+ Z’,)’ ~ 2390, 2399
VW
(% + v,) (%I + va) (ra + v,), 2i.13 219,
w
2b, 0.6
V.V.S.
;v5 ;
w(s) S S
I
VI!, (1’6 +
)I6 V8 (%I + 1’13) VP, v12
2b, 0.89 l/S, 0.69
w
v5 (v, + VI&
w
(VI3 + %a)
(1180)
w(s)
1098
w(s) V.S. m(s) S V.S.
I 2780 1 2328 1 2258 ~ 2067, 2058 / 1916
v3 @a + VI01
w(s) w
(650)
2yp, (~5 + +,),
(v4 + v12) h + 9)
m
1021 953
-
VW
VW
1098
3644
Vl’ %I
VW
1421 1361 1290 1242
__-
215
m
V6 2”14, b’s +
l/2 1, 0.87
Vl3
~
1545
1718, 1717
1306 1267 1216 1196 1122 -
b, + V16) (Vg + v14)
1, 0.6
1780
1465 -
2Vs
i i
~ 1’s)
)‘2)
1028, 1044 -
VW
V? (v, + %I))
784
VW
(VI5 + %I
674
VW
589
594 514
436 348 (238)
V.S.
6, sb. 0.21
t:, -
7, sb. 0.35 1, 0.69
-
-
* Ref. [S].
An approximate normal-co-ordinate calculation, details of which are given in Tables 4 and 5 and in a following paper, supplies a clue to the vibrational assignment. A reasonable set of Urey-Bradley force constants, shown in Table 4,t t Following the precedent set by SHIMANOUCHI [9] we have assumed the linear terms in the potential energy F’ equal to -O.lF. [9] T. SRIMANOUCHI, J. Chem. Phya. 17, 848 (1949).
1210
Acetyl halides--I
chosen appropriate of CH,COCl side by side
by comparison with similar molecules and these were taken with 2 and G matrices [lo] in the calculation of the vibrational frequencies and CD,COCl. In Table 5 the calculated frequencies are displayed with the frequencies of the prominent bands in the spectrum which
was
Table 2. CD,COCI. Infrared (gas)
-3628 3300 3220 (2950) (2780) 2638 2385 2289 2280 2257 2085 1959 1940 1915
1820 1743 1687 1632 1378 1328 1280 1132 1090 1040 1003 962 877
j ‘Raman 1 (liquid) -i-
Observed vibrational frequencies and assignments
-
Infrared int.
Infrared (liquid) _‘_
_‘-
/ I&man int. i -.-_-
Assignment
m w w W W TY
3640 3320 3236 2952 2782 2638 2383 -
W
/ )
2248 2104
/ j i / ! ’
m
D(?)27b
W
P 55 1950 1924 1917 1858
111
m m -
1877
T.S. m
181,”
P(P) 4
: Pl6b 1754 1695 1636 1375 1316 1279
W W
m U’ m
:
i
/ 1 /
V.S.
(797) 777 604
1037
m
j /
948
V.S.
/ ! /
823
563 522 438
1126
V.W.
I
818
Calc.
i D(P) 8b I
S
m m m m UT
I ,571
/
437 317
/ / I
I
V.S.
v5 1000
I’(?) 5b
! P12 1 I / I335
754 634
W W(S)
have been assigned as fundamentals. The correspondence between observed and calculated frequencies provides considerable support for our proposed assignment although we freely admit that this argument depends on our assumption that the Urey-Bradley force constants are transferable from molecule to molecule and that [IO] J.
OVERBND
and J. R.
‘SCHERJX%, J.
Chem. Phye.
32, 1289 (1960).
1211
JOI~N
OVEREND,
R. A.
J. C.
NYQUIBT,
EVANS
and
W.
J.
POTTS
the force constants of acetyl chloride may be represented adequately by a linear hybrid of those of acetone and carbonyl chloride. With these approximate normal co-ordinates we return to the assignment Table
3. CH,DCOCl.
Observed vibrational frequencies and assignment
-7
Infrared (gas)
i Infrared (liquid)
Raman (liquid)
Infrared int.
Raman
pol.
Approximate description
Assignment _.._ .
~_
@runs)
I-
_._ 3610 3020 2972 2918 2849 2797 2665 (2400)
I 3008 2956
/
I I
3583 3005 2955 2812 2772 2650 (2400)
m w w
Dp? P
2245 2190
1935 1820 1730 1690 1454 1422 1408
1923 1802 1712 (1693) 1412 1397 1361 1281 1254
1796
1395
1290 1265 (1200)
91
VCH
92 b4
+
9g)
W
(93
+
911)
(94
+
9d
/
(911 + +a
W
m vvs VW
‘I’Cll
1’
P?
(98 + y7)
(9&I +
9L‘o
v3
94
2
!
(92
W
(6
m
SC&J
Dp?
(SCH2
+
9121
-G, -
(9,
1073 1011 1000(s) 973 908 852 (786) 592
1020 987 909 853
S
9x2
s
95
-
VS
-
(608)
688
565 507 489
553
8
96
W
fv3
8
P P
8
97
wCD -+- v,, (93
(93
+
912)
+
912)
+
914)
97 (910
9a -
+
-
%)
99’ 2912 +
v10 %of
(913 +
914)
S
I
8 W W
0.5 DP -
911)
(910
913
@CD
VW
0.4
554 503 487
9131
+
9%
VW
W
4
(93
95
-t- 99)
9lfJ)
(93
94
W
910) 91,)
93
W
m
+
P
1095(s)
1088
851
2% 9UH
W
2220 2255
(gauche)
-
-
%O
vco
98
Yco
914
F
9s
0001
sCCC1
/
I
910
Vll
-
-
“12 “13
-
“14
problem. The totally symmetric CH and CD stretching vibrations may be identified immediately from the polarization results in the Raman effect and are assigned The two remaining CH stretching to the bands at 2934 and 2 104 cm-’ respectively. modes, one of which belongs to the A’ species and one to the A”, and which are 1212
Acetyl halides-I
degenerate if the molecule has the full symmetry of the methyl group, may be expected to be almost coincident in acetyl chloride and accordingly the band at 3029 cm-l in the gas-phase infrared spectrum of CH,COCl has been attributed to an unresolved doublet consisting of v1 and vI1. The band at 2280 cm-l in the CD,COCl spectrum has been similarly assigned. The assignment of the methyl-group deformation modes presents certain problems and this assignment will be deferred until the skeletal modes have been discussed. There is no difficulty in assigning the band at 1822 cm-l in CH,COCl Table 4. Force constants of CH,COCl transferred from COCl, and (CH,),CO. K and F are in mdyn/A. H and .?Y in 1011 erg/rad2. (CH,),CO Coordinate
/
[ill
-/__
UBFC
I
Coordinate
4.632
i
Arccl
2.929 9.851 0.474 ! 0.535 , 1.526 0.723 0.244 (0.316)[13]~ 0.497 0.076 0.298 -0.067
CH,COCl
COCl, [ 121
1
UBFC
1.99 12.61 0.442 * 0.335 * 0.860 0.523
Arc0 Au,,,,
bxc, QOCl 4ClCl
I
Coordinate
Arm
AToo Arc0 Arc,1 Auacc AWCH &co &Xl Aaocc, AY A, WC 9aa 4co PCCl
POCl 2
-__
UBFC
4.632 2.929 11.23 2.00 0.535 0.474 0.723 0.930 0.442 0.400 [ 121 0.014t 0.298 0.076 0.497 0.419 0.860 -0.060
* Corrected for the revised definition of the internal coordinate. t Calculated from barrier height of 1.35 kcal/mole quoted in Ref. [16]. p. 1215.
and at 1820 cm-l in CD,COCI to the carbonyl stretching mode vS, nor is there any question that the bending modes (calculated frequencies 496 and 391 cm-l in CH,COCl and 462 and 355 cm-l in CD,COCl) should be assigned to the Raman bands at 436 and 348 cm-l in the hydrogen compound and 437 and 317 cm-l in the deuterium compound. Also the correspondence between observed and calculated frequencies and the high infrared intensity leaves little doubt that the bands at 608 and 563 cm-l should be assigned to the carbon-chlorine stretching vibration. The remaining A’ type modes involve the C-C stretching co-ordinate and the methyl-group deformation and rocking co-ordinates but it is not possible to match each defined co-ordinate with a spectral band. Further, although the spectra of [Ill J. OVEREND,J. R. SCRBRERand R. R. HOLMES. Unpublished work. [12] J. OVERENDand J. R. SCHERER,J. Chenz. Phys. 22, 1296 (1960). [ 131 J. OVERENDand J. R. SCHERER,J. Opt. Sot. Amer. 50, 1203 (1960). Value transferred from (CR,)&.
1213
JORN
OVEREND,
R. A.
NYQUIST,
J. C. EVANS and W. J. POTTS
CH,COCl and CD,COC!l are superficially similar in that they both have very intense bands at ea. 1100 and ca. 950 cm-l, the assignments cannot be made along parallel lines because account must be taken of frequency differences resulting from the differences in mass between H and D. Table 5. Frequencies of CHaCOCl, CDaCOCl and CHaDCOCl Cttlculated from assumed force constants, see Table 4. CDaCOCl
CHaCOCl Approximate* description
Gale, (cm-l) _---
A’
v3 VP A'.V5 V6 v7 V8 vfi ,VlO Vll
it”
i%a
VI3
1% iv15
-
Calc.
Mode
--
3002 2939 1780 1441 1378 1104 962 661 469 391
3029 2934 1822 1432 1370 1109 958 608 436 348
3000 1444 1040 568 136
3029 1432 1029 514 238
WE8
D-eclipsed
iv1 aye
1.Assignment
2962 2192 1778 1418 1266 1033 899 607 467 381
v3 v4
%x3
voo 6CH8
wcwz WC
WCD WC1 dOCCi &xl VCVCA 6CHZ OCD Ye0 Par@
/
"5 v6 v7 VS VS %l %1 %2 818 "14 v15,
CHaDCOCl (trans)
VCW
3000 1306 974 568 120
,
V2
A”
/ hip1 ment, ^._..__ -_
2232 2114 1777 1147 1031 986 802 601 462 355
v1
A’
Approximate* description
(cm+) --.
I'CD
2227 1031 854 516 100
WCD3 Yco 7CD3
CH,DCOCl
3002 2962 2189 1780 1417 1300 1260 I.085 968 847
A
3020 1290 987 507 -
-
(gauche)
I
@CD
657 539 467 378 121
-
2280 1040 877 498
%D 6 CD3
H-eclipsed 2972 2255 1620 1408 1265 1020 909 565 437 340
2280 2104 1820 1132 1040 962 818 563 437 317
vCD
3020 2972 2255 1820 1422 1290 1265 1088 987 851
+ WC
GOCCl
/
SCCCI
1
7CH2D
588 489 437 340
!
-
WC1
we
-
* This approximate description ix determined by inspection of the potential-onorgy-dist,ribution matrices.
We submit that the vibrational assignment should be made along indicated in Table 5; that the two intense bands at 1109 and 958 cm-r are as mixtures of methyl-rooking and carbon-carbon-stretching vibrations the two similar intense bands in the spectrum of CD,COC1 at 1132 and I.214
the lines described and that 962 cm-r
Acetyl halides-I
are described as mixtures of the symmetrical CD, deformation and the carboncarbon-stretching modes. Although one would not normally expect the methyl deformation and rocking vibrations to be so intense in the infrared spectrum, the mixing indicated by the normal-co-ordinate analysis and the assumption that the carbon-carbon bond has a high dipole-moment slope* would qualitatively account for the observed spectra. The assignment of the two A’ CH, deformations in CH,COCl to the bands at 1432 and 1370 cm-l and the A’ CD, deformation and wagging modes to the bands at 1040 and 818 cm-l completes the assignment of the A’ modes for these two molecules. The A” CH and CD stretching vibrations have already been assigned. There is a band at 1432 cm-l in the gas-phase infrared spectrum of CH,COCI and one at 1040 cm-l in the fully deuterated species which appear to have sharp central Q branches and which are assigned to the A” methyl-deformation vibrations. The A” methyl-rocking modes are assigned to the bands at 1029 cm-l and 877 cm-l respectively, and, as OVEREND and EVANS have already determined [15], the carbonyl out-of-plane vibration should be assigned to the band at 514 cm-l in CH,COCl and that at 498 cm-l in CD,COCl. The assignment of the torsional frequency by KOHLRAUSCH [4] to the weak band? at ca. 238 cm-l in the Raman spectrum is somewhat tenuous, and since it is markedly different from the torsional frequency value calculated from the barrier to internal rotation determined by the microwave method, we shall accept the calculated value; no low frequency line was observed in the spectrum of CD,COCl. The remaining bands in the spectra of both molecules can be satisfactorily assigned to combination and overtone bands arising from these fundamentals although in several cases it is not possible to choose between alternative possibilities.
Vibrational assignment of CH,DCOCl The equilibrium configuration of acetyl chloride has been shown to have the carbonyl bond eclipsed by one of the CH bonds: [16] there is a three-fold potential barrier to the rotation of the methyl group and there are three possible equilibrium conformations. In the symmetrical molecule these are necessarily equivalent and for a normal-co-ordinate analysis it is sufficient to consider a single species in the equilibrium conformation. In the case of CH,DCOCl the three positions of the methyl group are no longer equivalent and we must consider the geometry more carefully. The rate at which one molecule passes from one conformer to the other should be given roughly by the Arrhenius equation k = A exp (-E/RT) * J. PETRO[14] has pointed out that such a high value for the dipole-moment slope might be expected in cases where the bond is between two differentlv hvbridized carbon atoms. t KORLRAUSCR reported the relative intensity of &is band as 1/2b; however, it has not been reported by later workers. [14] J. PETRO,J. Amer. Chem. Sot. 4230 (1958). [15] J. OVERENDand J. C. EVANS, Tram Fmm?q Sot. 55, 1817 (1959). [16] K. M. SINNOTT,BUZZ. Amer. Phys. Sot. 1,198 (1956).
1215
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS and W. J. POTTS
where A is the frequency factor and E the barrier height; taking E = 1.350 cal/ mole, at room temperature the rate is about O-1 A. The two equilibrium confirmations are: (1) The CH bond eclipses the carbonyl and the CD bond is gauche to the CC1 bond (gauche CH,DCOCl), and (2) The CD bond eclipses the carbonyl and is trans to the CC1 bond (trans-CH,DCOCl). If we use twice the torsional frequency (ca. 200 cm-l) as an approximate value for A, we might then expect the remainder of the vibrational spectrum, which is at much higher frequencies, to be understandable in terms of two conformers which will be present in the ratio 2 : 1. In Table 5 we show the frequencies of both trans and gauche forms calculated from the assumed potential constants and a tentative vibrational assignment made on the basis of these calculations. Comparing the frequencies of the two conformers calculated from the potential function assumed in the preceding (Table 5), we see that in a few instances there is a significant difference which we may reasonably expect to show up as a splitting in the observed spectra. In Table 6 we have collected the calculated splittings and these are compared with the prominent doublets in the infrared spectrum. A simple model suggests that the intensities of trans and gauche forms will be in the statistical ratio of 1:2. However, as the splitting arises from significant differences in the mixing of internal co-ordinates in the normal modes (arising from the different symmetry properties of the two conformers) it is extremely unlikely that the aP/aQ’s are equal; any differences in this quantity will give rise to a deviation from the statistical ratio. However, in support of our proposed assignment it is interesting to note that the fairly intense doublet at 565-607 cm-l has an intensity ratio of approximately 1: 2, and that the component assigned to the gazcche form is the more intense. The final vibrational assignment was made on the assumption that the spectrum is that of a mixture of the two conformers, and is shown in Tables 3 and 5, although there is some uncertainty in the precise interpretation of the complex of bands between 950 and 1100 cm-l, and further experimental data at much higher resolution are desirable.
Discussion Deuterium substitution is a powerful tool for the elucidation of a vibrational assignment and has been successfully employed on countless occasions. From the most na’ive viewpoint, one would expect a shift of about l/1/2 in the frequency of any mode which involves a hydrogen atom. If we take into account the fact that the normal vibrations do not correspond to simple distortions of a single bond, then the shift in a particular band on deuteration will depend on the extent to which the internal co-ordinate involving the hydrogen atom participates in the particular normal co-ordinate. However, even in this case we should expect to see a marked frequency shift in any vibration which involves a hydrogen atom. It is therefore apparently contradictory when we assign the CH, rocking mode to a band in the spectrum of CH,COC’l which does not appear to shift when we replace the hydroWe have explained this contradiction by saying that, although gens by deuterium. the intense bands occur at approximately the same frequencies in both light and heavy compound, there is a fundamental difference in the normal co-ordinates. If we further postulate that the infrared intensity stems entirely from the dipolemoment slope of the carbon-carbon bond, then it is easy to see that any vibration 1216
Acetyl halides-I
might occur at about 1000 cm-l and which is appropriately coupled to the carbon-carbon stretching co-ordinate will give rise to an intense infrared band in this region of the spectrum. We believe that this phenomenon occurs frequently in 0 which
the spectra of molecules containing
I/ C
the
skeleton and that in all these cases
(IL ’ 3 “xone must exercise caution in interpreting the results of deuterium Table 6. Obsecrved and calculated conformational Mode
Approx. description
Calc. (trans)
(k/YWbS)
I
1
I
2 3 4 5 6 7 8 9 10 11 12 13 14 15
:
L
I
2962 2192 1778 1418 1266 1033 899 607 467 381 3000 1306 974 668 120
splittin
I / / ;
Y~_
-2 1 6 -52 52 .._.60
voc OCD
I
VCCl
0 3
&Cl
I
6fC1
!
WE
-2 6 6 29
&HZ SC0 -1
We feel that the nature of the vibration
producing 0
in cm-l
Observed splitting
Cak. splitting ___-.0 3
WE rcu wo GCHB 6CHa
!
substitution.
(~~a~s~u~c~e) in CHsDCOCI
;
0 0 0 14 0 -68 58 -43 0 0 0 0 0 18 -
the intense absorption
at
II ca. 950 cm-l and characteristic of the structure CH,-C-X is fairly well explained. Not so clear, however, is the fact that a completely analogous absorption in the 0 I/ range 850-1000 cm-l is characteristic of any molecule of structure C-C-X, no matter what the nature of the a-carbon atom. In view of the above discussion, it 0 // would appear that molecules of form CH,-C-X constitute a somewhat special case, and probably will not serve as model compounds which can be used to estimate the nature of this normal mode for more complex molecules. We hope to investigate this point more fully in future studies of related molecules.
Appendix Thermodynamic functions for acetyl chloride in the ideal gaseous state at one atmosphere pressure were calculated using the standard statistical methods based 1217
JOHN OVEREND, R. A. NYQUIST, J. C. EVANS and W. J. POTTS
on the rigid-rotator, harmonic oscillator approximation. Molecular dimensions determined by Sinnott using the microwave method and quoted by KRISHER and WILSON [ 171 were used; these authors quote all dimensions except the C-Cl bond properties of act“tJ11chloride (ideal
Table 7. Thermodynamic C”,,
T”, -___ 298.15 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
!_
16.21 16.26 18.86 21.19 23.18 24.86 26.30 27.54 28.60 29.51 30.30 30.98 31.57 32.08
-
/
so 70.47 70.57 75.61 80.17 84.11 87.82 91.24 94.40 97.36 100.13 102.73 105.18 107.50 109.70
__
(H” - H,“)/T
-@ -(F”
11.80 11.82 13.26 14.62 15.88 17.05 18.12 19.10 20.00 20.82 21.58 22.28 22.92 23.52
-
H,“)/T
58.67 58.74 62.34 65.55 68.23 70.76 73.11 75.30 77.36 79.30 81.15 82.90 84.58 86.18
Units: cal/deg. mole.
length, and for this, the value 1.80 .L% indicated by electron diffraction studies was assumed [18]. The principal moments of inertia thus calculated were 48.84, 104.35 and 150.07 a.m.u. x A2. The symmetry number for external rotation is one, while for the hindered internal rotation the symmetry number is three. The contributions made by this latter degree of freedom were determined using PITZER and GWINN’S methods [19], taking the barrier height to be 1300 Cal/mole [17] and the reduced moment of inertia to be 4.92 x lO& g cm2. Calculated values are collected in Table 7. [17] L. C. KRISHER and E. B. WILSON, J. Chem. Ph?/a.31,888 (1959). [IS] Innteratonaic Distances, Special Publication No. 11. Chemical Society, London (1958). [IQ] K. S. PITZER and W. D. GWINN, J. Chem. Phys. 10,428 (1942).
1218