硫代樟脑光化学反应的理论研究

运用量子化学方法优化了硫代樟脑的最低5个电子态(S₀, T₁, S₁, T₂和S₂)的结构, 并计算了它们的相对能量. 计算结果表明: S₁, T₁和T₂态的能量非常接近, 而S₂的能量远远高于T₂态, 这与之前对几种小的硫代羰基化合物的研究结论一致. 确定了硫代樟脑分子在T₁态发生β-插入反应和类Norrish II型反应的机理, 计算的势垒相对于S₀的振动零点分别为314.1和332.6 kJ/mol. 在400 nm波长的光的照射下, 分子被激发到S₁态, 此时分子没有足够的能量发生反应, 只能通过内转换回到基态. 当激发光波长在254 nm时, 硫代樟脑分子被激发到S₂态, 这时候体系有了足够的内部能量使反应发生. 实验上已经观察到此激发光波长下, 气态硫代樟脑可以发生β-插入反应和类Norrish II型反应.

Theoretical Study on Photochemical Reaction of Thiocamphor

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.