Abstract: The B3LYP, MP2 and CASSCF calculations with cc-pVDZ and 6-31++G** basis sets were carried out to investigate the dehydrosulfide and C—C, C—N photodissociation reactions on their ground and the lowest triplet states. The effects that one water molecule from solvent directly takes part the dehydrosulfide reaction were studied carefully. It was concluded that none of these reactions was predicted to happen on the excited states. On the ground state, the proton transfer reaction is the main routine, while in the water or ethanol solution, the dehydrosulfide reaction can be predicted as the main reaction. We also use CASSCF method to optimize the stationary points and the conical intersection and intersystem cross points on the first excited states. Our calculations show that the chemical reactions except internal conversion on the S1 and T1 surfaces would not easy to happen. The possible mechanism could be that CH3CSNH2 was excited to S2 state T1, following a S2/S1 crossing. There are two paths to arrive the ground state from the first excited state: by a S1/S0 cross, or S1/T1 intersystem crossing to T1. It can be known based on our calculations that it is low possibility that CH3CSNH2 can process a photodissociation. This conclusion can help to consider thioamides in biology studies.

Key words: CH3CSNH2, Density functional theory, Photoreaction mechanism, Solvent effect