Ab initio potential energy surfaces of the ion-molecule reaction:C_2H_2+O~+

摘要

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double delta) and cc-pVTZ (triple delta) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction,C_2H_2(~1 SIGMA_g~+)+O~+(~4S),for which collision experiment has been performed by Chiu et al [J.Chem.Phys.109,5300 (1998)].The minor low-energy process leading to the weak spin-forbidden product C_2H_2+ (~2II_u)+O(~1D) has been studied previously and will not be discussed here.The major pathways to form charge-transfer (CT) products,C_2H_2+(~2II_u)+O(~3P) (CT1) and C_2H_2~+(~4A_2)+O(~3p) (CT2),and the covalently bound intermediates are investigated.The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Intl between CT2 products.This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states.As the C-O distance becomes shorter beyond the above intermediate,the C_1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol.Although this TS connects diabatically to the covalent intermediate Int2,there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions.These findings are consistent with the experiment,in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies,respectively.