SplCtrochimlcn Acta, Vol. 49A, No. 13/14, pp. 2007-2026, 1993 Printed in Great Britain 0584-8539/93 56.00 +0.00 CO 1993Perglmon Press Ltd Vibrational spectra of monothiocarbamates-II.'" IR and Raman spectra, vibrational assignment, conformational analysis and ab initio calculations of S-rnethyl-N,N-dimethylthiocarbamate G. KERESZTURY, S. HOLLY, G. BESENYEI and J. VARGA Central Research Institute for Chemistry, Hungarian Academy of Sciences, P.O. Box 17, H-1525, Budapest, Hungary and AIYING WANG and J. R. DURIG Department of Chemistry, University of South Carolina, Columbia, SC 29208, U.S.A. (Received 17 December 1991; accepted 21 February 1992) Abstract-The IR (3200-30 cm ") spectra have been recorded for S-methyl-N,N-dimethylthiocarbamate, (CH3)2NC(O)SCH3, and its isotopomers, S-d 3 , N-d 6 and N-d 9 , for the gas and liquid. Additionally, the Raman spectra (3200-10cm· l ) for the solid and liquid, with qualitative depolarization ratios, have been obtained for all the isotopes. These data are interpreted on the basis that the s-cis conformer (the S-methyl group oriented cis to the carbonyl group) with C, symmetry is the only form existing in all three phases for this molecule. A complete vibrational assignment proposed for the -do molecule is facilitated by the availability of spectral data for five different isotopomers. A normal coordinate analysis has been carried out utilizing ab initio calculations with the 3-21G· basis set. The potential energy distributions and ab initio calculated frequencies have allowed a clarification of some of the corresponding results obtained from experiment. Structural optimizations and potential surface scan have also been carried out by ab initio calculations with the 3-210· basis set. These results are compared with some previous studies on this molecule as well as on similar molecules. INTRODUCTION IN the first part of this series [1], we have reported some preliminary results of our vibrational studies of S-methyl-N,N-dimethylthiocarbamate (SMDMTC), (CH3)2NC(O)SCH3, the smallest fully alkylated monothiocarbamate, We have given a tentative assignment of the fundamental vibrational bands observed for five isotopomers in the IR and Raman spectra at room temperature and the proposed assignment was supported by a tentative normal coordinate analysis using a 45-parameter simplified valence force field. For the analysis of the sods (Z) conformation (with the S-methyl group cis relative to the C=O bond) C. molecular symmetry was assumed. While the assignment of the bands above 400cm" seemed to be acceptable and in full agreement with the observed isotope shifts, there remained some uncertainties regarding the true molecular geometry and the assignment of the low frequency bands. There has been some controversy in the literature about the stability of different conformers in similar molecules like esters, amides, carbamates and thiocarbamates [2-7]. Doubling of the C=O stretching mode has been observed in the IR spectra of most esters in solution and has been interpreted in different ways in different esters by OKI and NAKANISHI [2-5]. In the case of esters of formic acid with bulky alkyl groups it has been attributed to conformational heterogeneity brought about by internal rotation around the C-O bond [2]. In alkyl acetates and pivalates it has been interpreted as due to Fermi resonance [3,4], while in propionates and isobutirates it has been explained by rotational isomerism around the CC=o-C a bond [4]. In primary, secondary and tertiary carbamates alike, the doublet has been interpreted as arising from two conformers, with the s-trans (E) form (R 1 trans relative to the C=X bond as shown in Fig. 1) believed to be more stable than the s-cis (Z) conformer [5]. Contrary to this result, a theoretical study by KYDD and RAUK [6] led to the s-cis(Z) conformation as being energetically favored in • For Part I, see J. Malec. Struct. 218,237 (1990). 2007