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

Abstract

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 → π ^* excitation.