In the present work, the CASSCF method was employed to optimize structures of thiocamphor in the lowest five electronic states (S₀, T₁, S₁, T₂, and S₂), which was followed by single-point energy calculations at the MR-CI level. The energy gaps among the S₁, T₁ and T₂ states are very small, but the S₂ state is much higher than the T₂ state in energy. Similar results were found for other thiocarbonyl compounds. The T₁ potential energy profiles of the β-insertion and Norrish II-like reactions were characterized by the B3LYP calculations and the barriers were predicted to be respectively 314.1 and 332.6 kJ/mol with respect to the S₀ zero-level. Upon irradiation of thiocamphor at 400 nm, the molecules excited to S₁ state do not have enough internal energy to overcome the barrier on the T₁ surface. Thus, the internal conversion to the ground state could be the most probable pathway for the S₁ deactivation. However, photoexcitation at 254 nm results in the molecules populated in the S₂ state. In this case, the system has sufficient internal energy to undergo the β-insertion and Norrish II-like reactions. Indeed, the two reactions have been observed experimentally.

Key words: thiocamphor, photochemistry, ab initio