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

The mechanisms and kinetics of O(³P,¹D) + OCS(X¹Σ⁺) 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(³P) + CO(X¹Σ⁺) and S(³P) + CO₂(X¹ Σ_g⁺) by passing 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)O¹), are formed barrierlessly from O(¹D) + OCS(X¹Σ⁺) in the singlet surface, which lie at -40.5 and -50.1 kcal·mol^-1 relative to O(³P) + OCS(X¹Σ⁺) reactants and decompose to CO(X¹Σ⁺) + SO(a¹Δ) and S(¹D) + CO₂(X¹Σ_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(³P) + OCS(X¹Σ⁺) reaction including the crossings, 9.2 × 10^-11 exp(-5.18 kcal·mol^-1/RT) cm³ 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).