Rotational isomerism in CH₃C(S)SCH₃ and CH₃C(S)SCH₂CH₃: a combined vibrational spectroscopic and ab initio study

Rotational isomerism in CH₃C(S)SCH₃ and CH₃C(S)SCH₂CH₃ is examined by combining a vibrational spectroscopic with an ab initio MO study. Particular emphasis is given to the gauche ⇌ anti conformational equilibrium originated by internal rotation about the dithioester SCH₂CH₃ bond in CH₃C(S)SCH₂CH₃. The ab initio equilibrium geometries of the molecules studied are calculated by gradient geometry refinement, and the conformational dependence of some relevant structural parameters is used to characterize the most important intramolecular interactions present in the various conformers. For ethyl dithioacetate, correlation between the calculated and the observed frequencies for the condensed phases and the isolated molecule clearly shows that the most stable form corresponds to the non-symmetric s-cis/gauche form (the ab initio calculated values of the relevant dihedral angles are 0° (SCSC) and ≈ ±84° (CSCC)). Variable temperature Raman experiments show that the enthalpy difference between the s-cis/gauche and s-cis-anti forms is 1.9 ± 0.2 kJ mol^-1. Observed frequency shifts for vSC and vCS are successfully correlated with substitution of the alkyl and acyl fragments. A consistent interpretation of the resonance Raman spectra of both methyl and ethyl dithioacetate and their isotopomers is also achieved.