ROTATIONAL-ISOMERISM IN CH3CH2-C(=S)SR (R=CH3, CH2CH3) - A COMBINED VIBRATIONAL SPECTROSCOPIC AND AB-INITIO STUDY

The vibrational and conformational properties exhibited by CH3CH2C(=S)SCH3 and CH3CH2C(=S)SCH2.CH3 were studied by Raman and infrared spectroscopies for the liquid and solid phases, and by ah initio calculations for the isolated molecule. It is shown that these molecules tend to adopt nonsymmetric conformations near the C(=S)S group, in contrast to their oxygen analogues whose most stable conformers correspond to structures having a planar skeleton. For the conformers differing by internal rotation about the C-alpha-C bond, the most stable conformer-the skew form having the CC-C=S dihedral angle equal to +/- 100 degrees-is more stable than the symmetric syn conformer (CC-C=S equal to 0 degrees) by ca. 1.0 kJ mol(-1). In the annealed solid, CH3CH2-C(=S)SCH3 exists in the skew conformation, which is also the most stable form in the liquid and isolated molecule. As the liquid is rapid-froze, a glassy state where both conformers exist is obtained, which enables a spectroscopic characterization of the less stable syn form, taking advantage of the usual solid-state band-narrowing effect. The conformational dependence of some relevant structural parameters was used to characterize the most important intramolecular interactions present in the various conformers. In addition, the ab initio vibrational spectra were calculated and used for the assignment of the experimentally observed bands. In particular, the Raman and IR spectra of the molecules in the liquid and solid phases were assigned and the results were used to explain details of the resonance Raman spectra of methyl dithiopropionate [Ozaki, Y.; Storer, A, C.; Carey, P. R. Can J. Chem. 1982, 60, 190].