Ab initio study of the methyl internal rotation and aldehyde hydrogen wagging of thioacetaldehyde in the X1A′ and a3A″ states

Ab initio SCF calculations have been performed for the methyl internal rotation and for aldehyde hydrogen wagging of thioacetaldehyde in the first triplet excited state, a³A, as well as in the singlet ground state, X¹A. The preferred conformations for these states are the anti-eclipsed and the eclipsed ones, respectively. The calculated barrier heights to methyl rotation (118.3 and 455.6 cm^–1 for a³A and X¹A, respectively) are in good agreement with the available experimental data. The singlet ground state and the triplet excited state exhibit a planar and pyramidal configuration, respectively. The inversion barrier of the pyramidal configuration is found to be very low 67.4 cm^–1. Finally, the change of conformation and structure with the transition is explained by a change of hydridization of the aldehyde carbon atom due to an n ^* excitation.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday