Theoretical study on the reaction mechanism of NH2-Â +Â SCX (XÂ =Â O, S)

Ab initio calculations of the two title reactions have been made to compare reactivities of OCS and CS2 toward a nucleophile, NH2-. MP2/6-311++G(d, p) geometry optimizations on the singlet potential energy surface have demonstrated that respective channels start from key intermediates and have revealed that multistep paths give the most favorable products: (1) NH2- + OCS → H2NS− + CO; (2) NH2- + OCS → HS− + HNCO; (3) NH2- + OCS → NCS− + H2O; (4) NH2- + OCS → NCO− + H2S; (5) NH2- + CS2 → NCS− + H2S; (6) NH2- + CS2 → HS− + HNCS. Furthermore, to get more reliable energetic data, single-point calculations are carried out at CCSD/6-311++G(d, p) level. The calculated result is consistent with the measured large rate constant in experiment. A comparison of reaction mechanisms is offered between NH2- + OCS and NH2- + CS2.