基态乙酸甲硫醇酯单分子反应机制的理论研究

A THEORETICAL STUDY ON THE UNIMOLECULAR DECOMPOSITION MECHANISM OF S-METHYL THIOACETATE ON THE GROUND STATE

运用B3LYP和MP2方法在6-31G**水平上对基态乙酸甲硫醇酯单分子反应机制进行了理论研究.结果表明,基态乙酸甲硫醇酯有2种异构体,其中反式比顺势能量高21.7 kJ.mol-1,而二者相互转化的能垒为47.7 kJ.mol-1,表明室温下基态乙酸甲硫醇酯主要以顺式存在,这与实验结果是一致的.本文所研究的分子解离反应全部为协同过程,而自由基解离过程都是无势垒的吸热过程.其中分子解离反应中生成CH3SH+H2CCO的反应,在反应动力学上最有利,这与实验上该反应为主要反应的结论是一致的.自由基解离反应中生成CH3CO+SCH3和CH3C(O)S+CH3的反应热比该反应的能垒稍高,可与之竞争.而其他反应则因能垒或反应热更高而难以在基态上发生.

The energy profiles of eight reactions on the ground-state S-methyl thioacetate are investigated by using B3LYP and MP2 methods in combination with 6-31G^＊＊ basis set. It is found that there are two isomers for the ground-state S-methyl thioacetate with 21.7 kJ·mol^-1 higher of the anti isomer with respect to the syn one. The barrier of the transformation of them is 47.7 kJ·mol^-1 predicted by the MP2/6-31G^＊＊ calculation, suggesting that the ground-state S-methyl thioacetate mainly exists as syn isomer which is in agreement with the experimental result. The three molecular decomposition reactions studied here are all concerted, and the radical reactions all endothermal and barrierless. The reaction to CH3 SH and H2 CCO is the most favorable dynamically, which is experimentally considered to be the dominant process. The reaction heat of CH3C（O） SCH3 →CH3CO＋ SCH3 and CHiC （O） SCH3→CH3C （O） S＋ CH3 is somewhat higher than the barrier of CH3 C（O） SCH3→CH3 SH＋ H2 CCO, suggesting that they are competitive. However, other channels take place with less possibility because of the high barrier or heat of reaction.