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 ab 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_α-C bond, the most stable conformer - the skew form having the CC-C=S dihedral angle equal to ±100° - is more stable than the symmetric syn conformer (CC-C=S equal to 0°) 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].