Theoretical study of the extremely small torsional barriers of thiophenol in the ground and the first excited electronic states

The extremely small (around 0.80 kcal/mol) barrier for the –SH rotation of thiophenol is studied by using MP2, CASSCF(12,11), and three DFT (B3LYP, HCTH, and ωB97X-D) methods with various basis sets. The 6-311++G(3df,3pd) turns out to be the proper minimal basis-set for reliable calculations. CCSD(T) and CASPT2//CASSCF calculations confirm that the planar conformation of the –SH is the minimum-energy structure of not only the ground S0 but also the first excited S1 electronic states. The calculated torsional barrier of the S1, 1.89 kcal/mol, is also quite small.