Li Xiao-Hong, Ren Si-Jia, Wei Xi-Guang, Zeng Yi, Gao Guo-Wei, Ren Yi, Zhu Jun, Lau Kai-Chung, Li Wai-Kee
College of Chemistry and Key State Laboratory of Biotherapy, Sichuan University , Chengdu 610064, China.
J Phys Chem A. 2014 May 15;118(19):3503-13. doi: 10.1021/jp5021559. Epub 2014 May 6.
The water-mediated neutral hydrolysis mechanism of carbonyl sulfide (OCS) has been re-examined using the hybrid supramolecule/continuum models with n = 2-8 explicit water cluster at the level of MP2(fc)(CPCM)/6-311++G(d,p)//MP2(fc)(CPCM)/6-31+G(d). Present calculations indicate that the potential energy surface in water solution is different from the one in the gas-phase, and only stepwise mechanism is observed in aqueous solution, i.e., monothiocarbonic acid (H2CO2S) is formed via monothiocarbonate (OCSOH(-), MTC) and its counterion, protonated water cluster, (H2O)nH3O(+). The predicted rate-determining step (RDS) barrier for the stepwise mechanism in water solution, about 90 kJ/mol, shows good agreement with the experimental values, 83.7-96.2 kJ/mol using six- or eight-water model including two cooperative water molecules. Moreover, two reaction pathways, the nucleophilic addition of water molecule across the C═O or the C═S bond of OCS are competitive.
利用混合超分子/连续介质模型，在MP2(fc)(CPCM)/6 - 311++G(d,p)//MP2(fc)(CPCM)/6 - 31+G(d)水平下，采用含n = 2 - 8个显式水簇的模型，重新研究了羰基硫(OCS)的水介导中性水解机理。目前的计算表明，水溶液中的势能面与气相中的不同，且在水溶液中仅观察到分步机理，即通过一硫代碳酸根(OCSOH(-)，MTC)及其抗衡离子——质子化水簇(H2O)nH3O(+)形成一硫代碳酸(H2CO2S)。水溶液中分步机理的预测速率决定步骤(RDS)势垒约为90 kJ/mol，与实验值83.7 - 96.2 kJ/mol（使用包含两个协同水分子的六水或八水模型）吻合良好。此外，两条反应途径，即水分子对OCS的C═O或C═S键的亲核加成是相互竞争的。
