硫代羰基化合物激发态结构及光化学反应的理论预示

运用精确的量子化学计算方法，优化了几种硫代羰基化合物（H2CS，CH3CHS和 CH3CSCH3）的基态和激发态的结构，计算了它们的相对能量。不同的方法得到的结 构参数比较一致，并与可用的实验结果相符。计算结果表明，三个分子的S1，T1和 T2态的能量非常接近，而S2态的能量明显高于T2态，这个理论结果与先前关于CH2 ＝CH2，CH2CHO和CH2＝CH－CH＝CH2的研究的结论是一致的。用不同的方法对基态 和最低三态的角途径进行了理论优化，得到的结果较好地一致。并且利用优化的基 态和最低三态解离的势能剖面，和光激发到各个电子态的可用能量，对硫代羰基化 合物的光解离反应的机理进行了理论预示。

Theoretical Predication of the Structures and Reactivity of Thiocarbonyl Compounds in Excited Sates

Carbonyl compounds are a kind of the most important organic compounds. Because of the unique electronic structures of carbonyl compounds, their photochemistry has been extensively investigated both theoretically and experimentally in the past decades. In comparison, thiocarbonyl compounds have received less attention, particularly their excited-state properties and photochemical reactions. In the present work, high-level quantum chemical ab initio methods were used to investigate the structures and energies of H_2CS, CH_3CHS and CH_3CSCH_3 in the ground and excited states. It is found that the structural parameters by different computational methods and different basis sets are similar, which are in agreement with the experimental results. The present calculations predict that the S_1, T_1 and T_2 states are close to each other in energy. However, there is a large difference in energy between the S_2 and T_2 states. The similar conclutions were found for molecules of CH_2CH_2 , CH_2CHO and CH_2CH-CHCH_2. Upon photoexcitation at 500 nm, the systems do not have enough internal energy to overcome the barrier on the T_1 pathway. Thus internal conversion to the ground state is the most probable pathway to deactivate. Photoexcitation at 210 nm results in the molecules in the S_2 states. From this state, the molecules that relax to the T_1 state are left with sufficient internal energy to undergo the C-H or C-C bond dissocition along the T_1 pathway.