Torsional Spectra of Ethyl Methyl Sulfide
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979 511
Analysis of Torsional Spectra of Molecules with Two Internal CSvRotors. 1l.+ Low Frequency Vibrational Spectra, Methyl Torsional Potentia! Functions, and Internal Rotation of Ethyl Methyl Sulfide J. R. Durig," D. A. C. Compton, and M.-R. Jalilian' Department of Chemistiy, University of South Carolina, Columbia, South Carolina 29208 (Received August 28, 1978) Publication costs assisted by the University of South Carolina
The infrared and Raman spectra of gaseous ethyl methyl sulfide have been investigated below 500 cm-'. A number of torsiond transitions were observed and have been assigned on the basis of an equilibrium between the gauche and high energy s-trans conformers. The asymmetric potential function for the CH2-S torsion has been calculated from observed transitions which leads to a value for the enthalpy difference between s-trans and gauche conformers of 46 f 14 cal/mol. The values of the potential coefficients were Vl = -44.7 f 5.7 cm-' (-128 cal/mol); V2 = 23.7 f 5.9 cm-l (68 cal/mol); V, = 1230 f 8 cm-l (3517 eal/mol); V, = -124.7 f 2.8 cm" (-357 cal/mol). One series of torsional transitions in the Raman spectrum and another in the infrared spectrum have been assigned to the coupled methyl torsions of gauche-ethyl methyl sulfide which lead to barrier heights for the C-CH3 and S-CH3 symmetric torsions of 3.33 and 2.05 kcal/mol, respectively.
Introduction Torsional vibrations of moleculers such as dimethyl ether1 and dimethyl sulfide,2 which have two symmetric CSutops on a common atom, have been studied in this laboratory in order to characterize the torsional coupling present between the tops. Results showed that this coupling is strong and results in radical changes in the torsional potential functions from those calculated for single CSu rotors. A study of the low frequency vibrational spectra of ethyl methyl ether? indicated that the two methyl tops in this compound also had coupled torsions, even though the tops were not on a common atom, as in dimethyl ether, but were separated by one bond length. Transitions present below 120 cm-l in the infrared spectrum of ethyl methyl ether? were assigned to the asymmetric CH2-0 torsions of both s-trans and high energy gauche conformers; the asymmetric potential function was calculated from these data and indicated a value of 1.11kcal/mol for AH, the enthalpy difference between the conformers. Evidence has been put f ~ r w a r c l that ~ - ~ an equilibrium between s-trans and gauche conformers also exists for ethyl methyl sulfide. Early results indicated that the more stable conformer was s-trans: but more recent vibrational studies have proposed5p6that the gauche conformer is slightly more stable than the s-trans conformer, with values for AH calculated to be 30 f 50 cal/mol. Microwave studies7s8on ethyl methyl sulfide have observed the spectrum of the s-trans conformer, which led to a partial determination of the structure8by utilizing the spectra from four isotopic molecules. Ilnfortunately, the spectrum of the gauche conformer was not assigned due to the richness of the spectrum. The barrier to internal rotation of the S-CH3 torsion of the s-trans conformer was calculateds to be 2.07 kcal/mol from observed splitting of the ground state rotational lines. The other torsions were not observed directly, but previous calculations7 gave an estimate for the asymmetric CH2-S torsion of 65 cm-l, which is in reasonable agreement with normal coordinate calculations4 For part 10,see J. Chem. Phys., in press. :Taken in part from the thesis of M.-R. Jalilian which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. Degree. 0022-3654/79/2083-0511$01 .OO/O
which estimate 80 cm-l for this vibration. A recent electron diffraction studyg on ethyl methyl sulfide concluded that the gauche conformer was predominant (75 f 15%) near room temperature and had a skeletal structure significantly different from the s-trans form and a dihedral angle of 114 f go. Very little information about the torsions of ethyl methyl sulfide has been obtained by vibrational spectroscopy. A band in the Raman spectrum of the liquid phase a t approximately 210 cm-l has been assigned to the gauche C-CH3 torsion,6 whereas a weak band at 232 cm-l in both liquid and solid phases was assigned to this vibration in a later Raman s t ~ d y . ~ Raman and infrared spectra of gaseous ethyl methyl sulfide have not been studied previously below 500 can-l. It was considered likely by the present authors that the torsional vibrations would be observed in the gaseous phase because the corresponding vibrations were observed as relatively strong modes in ethyl methyl ether3 and therefore the spectra have been recorded. A number of torsional transitions were observed and those above 100 cm-l have been assigned as the methyl torsions of the gauche conformer, whereas bands near 90 cm-l were assigned as the asymmetric CH2-S torsions of both conformers. From these transitions the asymmetric potential function and methyl torsional barriers have been calculated. Experimental Section Ethyl methyl sulfide was purchased from Pfaltz and Bauer, Inc., Stamford, Conn., and purified by using a low temperature sublimation column. Traces of water in the final sample were removed using BaO. Far-infrared spectra between 500 and 40 cm-l were recorded by using a Digilab FTS-15B Fourier transform interferometer. Gaseous samples were examined in 12-cm and 1-m cells at the vapor pressure of approximately 160 torr. Interferograms for both the sample and empty reference cell were recorded 3000 times, averaged, and computed using a boxcar apodization function to give 0.5-cm-l resolution. The spectrum above 110 cm-l was examined using a 6.25-pm beam splitter and below 110 cm-' using a 25-pm beam splitter. The region between 96 @ 1979 American Chemical Society
512
The Journal of Physical Chemistry, Vol. 83,No. 4, 1979
J. R. Durig, D. A. C. Compton, and M. R. Jalilian
TABLE I: Observed Bands (cm-*) and Proposed Assignment of the Vibrational Spectrum of Gaseous Ethyl Methyl Sulfide below 500 cm-' Raman em'
'
infrared re1 inta
351 272 229 212.5 206.0 201.5
vs vvw m m m
190.8
wsh
S
cm- '
re1 inta
conformerb
assignment
353.4 352 351.4 27 0
m w,sh
1st hot band
S
g t g
s
w,br
g
6
10 +- 00 11+- 01 20 +- 1 0
W
g g g t
201
92
m
vw m W S
W
m vs
a s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad. conformers.
94
90
0.0 -0.8
cscc
t t
g
1 5 +- O T
0.4 -0.1 0.1 -0.1 0.0 0.0 0.0 -0.2 0.2 -0.1
f
+-
g and t represent the gauche and s-trans
350
-
0.4
g g g t g
g
W W
6
cscc cscc cscc
30 +- 20 01 00 02 +- 01 03 02 3i+-2~ 3+-2 2*+-1T 2+-l 1
