硫代乙酰胺光化学反应机理的理论研究

用B3LYP, MP2和CASSCF方法, 采用cc-pVDZ和6-31++G**基组, 研究了硫代乙酰胺在基态和最低三态上消除硫化氢以及其它光解离反应, 并考虑了单个溶剂分子参与反应对质子迁移反应的影响, 得到了消除硫化氢反应的反应机理, 计算结果可以很好地解释实验结果. 进而用CASSCF方法计算了第一激发单态上的各驻点, 以及各交叉点. 计算结果表明, 在S1和T1态上发生除分子内转动以外的化学反应的可能性比较小, 当分子被激发到S2态上时, 将通过S2/S1交叉点到S1态, 在S1态上的分子有两条途径去活化, 通过S1/S0交叉点到热基态, 通过S1/T1交叉点系间窜越到T1态. 因而得出CH3CSNH2发生光解离反应的可能性不大. 基于此, 可将硫代酰胺结构引入蛋白或多肽中, 有望在不破坏分子整体结构的情况下对其进行光化学研究.

Theoretical Studies on the Photochemical Reaction of Thioacetamide

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.