A strong dependence of the CH ₃ internal rotation barrier on conformation in thioacetic acid: Microwave measurements and an energy decomposition analysis

Rotational spectra of thioacetic acid (CH ₃ COSH) 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 ³⁴ S and ¹³ C 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, V ₃ , 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 CH ₃ 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 V ₃ 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.