J. R. DURIG,C. K. TONG,C. W. HAWLEY, AND J. BRAGIN
44 cals (produced by X-rays) in single crystals of ice at 4.2"Rand suggested that the three absorptions they observed are consistent with what is expected for OH in
substitutional positions. It seem reasonable to suggest that interstitial OH radicals at 4.2"K might also be trapped in certain preferred orientations.
Vibrational Spectra of truns- and cis-Crotononitrile' by J. R. Durig,* C. K. Tong, C. W. Hawley, and J. Bragin Department of Chemistry, University of South Carolina, Columbia, South Carolina
$9308 (Received August $1, 1970)
P,ublication costs borne completely by The Journal of Physical chemistry
The infrared spectra of truns- and cis-crotononitrile, CHaCHCHCN, in the liquid and gaseous states have been recorded from 4000 to 100 cm-1. The infrared spectra of the solids have also been investigated from 33 to 500 cm+. The Raman spectra of the liquids have been recorded and quantitative depolarization values have been measured. A complete vibrational assignment based on band contours, positions, and depolarization is given for both the cis and trans isomers and the spectra are intekpreted in detail. The interesting methyl torsional modes were observed in the spectra of the solids at 178 and 219 cm-l for the trans and cis isomers, respectively. The calculated barriers are 1.76 and 3.08 kcal/mol for the truns- and cis-crotononitrile molecules. The lattice frequencies of the trans isomer show marked temperature dependence, particularly in the Raman spectrum. It is concluded that this isomer may exist in two distinct crystalline forms.
Introduction Although many investigators have reported carbonnitrogen stretching frequencies for a number of nitriles, few complete vibrational assignments have been given for this important class of compounds. Crotononitrile is no exception. Only four reports have dealt with the vibrational spectra of the two isomers (cis and trans) of this m o l e c ~ l e . ~ -Two ~ Raman studies were done on ~ Hemptinne and crotononitrile during the 1 9 3 0 ' ~when Wouters2" observed 14 of the 48 Raman active fundamentals of the two molecules and Reitz and SabathyZb obtained 24 additional frequencies but were unable to complete the vibrational assignment. Tramer and Wier~chowski~ attempted to assign the planar fundamentals of both isomers by calculating the frequencies of these modes and comparing the calculated values with those reported by Reitz and Sabath y ; however, the assignment of the former authors is inconsistent with any reasonable assignment of the out-of-plane vibrations. Wyss and Gunthard4 obtained the infrared spectrum of the liquid trans isomer to a low frequency limit of 650 cm-l and proposed a complete assignment for the vibrations of this isomer. Unfortunately the Raman spectrum of the trans isomer reported by Reitz and Sabathy contains a strong band due to cis impurity and this led Wyss and Gunthard t o a misassignmcnt of the fundamentals in the former molecule. Thus, the assignments for both isomers are incomplete and subject to doubt. I n addition, no The Journal of Phys6eal Chemistry, Vol. 76, No. 1 , 1971
frequencies have been recorded for the gaseous molecules and these data are important for the calculation of statistical thermodynamic q ~ a n t i t i e sneither ;~ is there a valid assignment for the interesting methyl torsional frequency in either molecule.
Experimental Section The sample of crotononitrile containing both isomers was obtained from Columbia Organic Co. and was separated by fractional distillation with a Teflon spinning band. The trans crotononitrile was further purified on an Aerograph Model A90-P3 gas chromatograph using a 20-ft TECP column under a flow rate of He of 90 ml/min, a column temperature of 130", detector temperature 165", and injector temperature of 150". The cis isomer was better than 99% pure under the same conditions. The Raman spectra of the two isomers were taken with a Cary Model 81 Raman spectrometer which used a filter solution composed of ethyl violet and O-nitro(1) Taken in part from the thesis of 6 . W. Hawley. (2) (a) M. Hemptinne and J. Wouters, A n n . BruxeEles, 53, 215 (1933); (b) A. W. Reitz and R. Sabathy, Sitzber. ALad. Wiss. Wien Math-Naturwiss. Kl, Abt. BB, 146, 577 (1938). (3) A. Tramer and K. L. Wierzchowski, Bull. Acad. Pol. Sci. Cl. Troisieme, 5, 335 (1957). (4) H. R. Wyss and Hs. 1%.GCmthard, Helv. Chim. Acta, 44, 625 (1961). (5) W. G. Fateley, I. Matsubara, and R. E. Witkowski, Spectrochim. Acta, 20, 1461 (1964).
VIBRATIONAL 8PECT13A OB'
45
trans- AND C'k-CLEOTONONITRILE
l*r---
..~ - .
--
-1
--
I
1nl
2Mo
IW
1 n l
nl
WAVENUMBER C Y 1 --
,w
~
WAVENUMBER OM.!
WAVZNUMBER OM
Figure 1. A, Infrared spectrum of gaseous cis-crotononitrile. B, Infrared spectrum of liquid cis-crotononitrile. C, Raman spectrum of liquid cm-orotononitrile.
toluene to isolate the 4358-A mercury excitation line. Depolarization values were measured by the standard Polaroid technique. Some spectrt were also taken with He-Ne laser axeitation of 6328-A wavelength. The instrument was calibrated with emission lines from a neon lamp over the spectral range from 0 to 4000 em-'. Typical Raman spectra are shown in Figures 1 and 2 and the frequencies are listed in Tables I and 11. The frequencies for all sharp lines should be good to 1 2 CM-1.
The mid-infrared. spectra in the region 4000 to 200 em-* were recorded with a Perkin-Elmer Model 621. The instrument, which is equipped with an extended source, was continuously purged with dry air to remove atmospheric water vapor and was calibrated with standard gases.6 The infrared spectra of the liquids were recorded as films between CsI plates and the spectra of the vapors were recorded by using 20-em cells equipped with C8Br w h d h s . The frequencies are listed in Tables E and 11 and are expected to be accurate to *2 ern'-'. 'Typical spectra are shown in Figures 1 and 2. The far-infrared spectra in the region from 500 to 33 cm-' were taken with a Beckman IR-11 double-beam grating spectrometer which was purged with dry air and calibrated with water vapor.'~~For the spectra of the solids the samples were condensed onto a silicon support plate w hioh was cooled with liquid nitrogen. The samples were repeatedly annealed until no further
W A n m E R MJ
Figure 2. A, Infrared spectrum of gaseous Irans-crotononitrile. B, Infrared spectrum of liquid trans-crotononitrile. 6, Raman spectrum of liquid trans-crotononitrile.
L3----L-.-/ L..!L--LAA i
300
zoo
120
100
80
WAVENUMBER CM.1
Figure 3. Far-infrared spectrum of cis-crotononitrile.
changes were noted. The far-infrared spectra are shown in Figures 3 and 4 and the observed frequencies are listed in Table 111.
Results Both isomers of crotononitrile have a plane of symmetryg~10 and belong to the C6 point group. Twentyfour normal modes are expected for each isomer of (6) IUPAC, "Tables of Wave Numbers for Calibration of Infrared Spectrometers," Butterworths, Washington, 13. C., 1961. (7) E. M.Randall, D. M. Dennison, H. Ginsberg, and L. R. Weber, Phys. Rev., 5 2 , 160 (1939). (8) R.T.Hall and J. M. Dowling, J . C h m . Phys., 47, 2454 (1967). (9) N. Suzuki and K. Kozima, Mol. Spectrosc. J., 33, 407 (1970). (10) R. A . Beaudet, J . Chem. Phys., 38, 2548 (1963). The Journal of Physical Chemistry, Vol. '76,No. 1 , 1971
J. R. DURIG, C . E(. TONG, C . W. HAWLEY, AND J. BRAGIN
46
Table I : Infrared and Raman Frequencies of cis-Crotononitrile Infrared-------Liquid-D,
om-1
3077 R 3070 ctr 3062 P 2987 R 2972 ctr 2970 P 295% Q 2931 R 2922 ctr 2914 P 2870 R 2861 ctr 2852 P
2236 R 2230 ctr 2223 P 1997 R 1990 ctr 1980 P
1780 R 1773 ctr 1763 P 1690 R 1683 ctr 1673 P 1637 R 1631 ctr 1621 P 1560 et? 1457 R 1450 Q 1447 Q 1440 R 1406 R 1399 ctr 1390 I? 1380 R 1376 ctr 1365 P
Bend type
Re1 int
ms
A
- ------------Raman--------,
___._I_
Re1 int
D,
om-1
AD*
Liquid--Re1
om -1
int
Depol ratio
Assignment
3845 3235 3170 3110
W
v5
W
v5
+ = 3854 + 3240 4- vzo = 3171 + 3111
3070
m
3078
6
0.86
v1
(a’) HCCH unsymmetrical stretch
3052 3010
w, sh
3054
20
0.30
v2
(a‘) HCCH symmetrical stretch
vw vw
V5
V6
VI
W
vi6
VIS
VE
+ va = 3033
(a’) CH3 antisymmetric stretch
m
A
2967
S
2969
7
0.86
v3
rns
C
2947
S
2956
7
0.86
Vi7
ms
A
2917
S
2926
36
0.15
v4
w
B
2870
mw
2868
1
2v7 = 2866
mw
2852
1
VB
m
W
2852 2800 2780 2735 2719 2610 2513 2458 2388 2344 2281 2252
vw vw vw mw vw vw mw mw mw
2219
(a’) CH3 symmetric stretch
+ = 2860 + vs = 2793 2vs = 2796 + vzz = 2740 vi1
v7
W
W
(a”) CHa antisymmetric stretch
2781 2748 2722
