Computational Study on the Mechanisms and Rate Constants for the O(P-3,D-1) + OCS Reactions

Abstract

The mechanisms and kinetics of O(P-3,D-1) + OCS(X-1 Sigma(+)) reactions have been studied by the high-level G2M(CC2) and CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) methods in conjunction with the transition-state theory and variational Rice-Ramsperger-Kassel-Marcus theory calculations. The result shows that the triplet surface proceeds directly by abstraction and substitution channels to produce SO(P-3) + CO(X-1 Sigma(+)) and S(P-3) + CO2(X-1 Sigma(+)(g)) bypassing the barriers of 7.6 and 9.1 kcal.mol(-1) at the G2M(CC2)//B3LYP/6-311+G(3df) level, respectively, while two stable intermediates, LM1 (OSCO1) and LM2 (SC(O)O-1), are formed barrierlessly from O(D-1 ) + OCS(X-1 Sigma(+)) in the singlet surface, which lie at -40.5 and -50.1 kcal.mol(-1) relative to O(P-3) + OCS(X-1 Sigma(+)) reactants and decompose to CO (X-1 Sigma(+)) + SO (a(1)Delta) and S(D-1) + CO2(X-1 Sigma g(+)). LM1 and LM2 may also be produced by singlet-triplet surface crossings via MSX1 and MSX2; the predicted total rate constant for the O(P-3) + OCS(X-1 Sigma(+)) reaction including the crossings, 9.2 X 10(-11) exp(-5.18 kcal-mol(-1)/RT) cm(3) molecule(-1) s(-1), is in good agreement with available experimental data. The branching ratio of the CO2 product channel, 0.22-0.32, between 1200 and 1600 K, is also in excellent agreement with the value of 0.2-0.3 measured by Isshiki et al. (J. Phys. Chem. A. 2003, 107, 2464).