A strong dependence of the CH3 internal rotation barrier on conformation in thioacetic acid: Microwave measurements and an energy decomposition analysis.

Rotational spectra of thioacetic acid (CH3COSH) have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for both the syn and anti conformers of the parent species, as well as the 34S and 13C carbonyl isotopologues. Transitions arising from the lowest A and E internal rotor states of the methyl group have been observed and analyzed. Experimental values of the three-fold internal rotation barrier, V3, for the syn and anti conformers of the parent isotopologue are 76.300(12) and 358.056(51) cm-1, respectively, indicating a large effect of the S-H orientation on the CH3 internal rotation potential. M06-2X/6-311+G(d,p) calculations are in good agreement with these results. The block localized energy decomposition method has been applied to understand the origins of this strong dependence of V3 on conformation. The results indicate that π conjugation from the SH to the carbonyl group and steric repulsion between the SH and the methyl group in the anti form are main contributors to the difference.