Direct Dynamics Simulation of the Thermal ³CH₂ + ³O₂ Reaction. Rate Constant and Product Branching Ratios

Abstract

The reaction of CH with O is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CHOO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, HCO + O(P), while the singlet surface leads to eight product channels with their relative importance as CO + HO > CO + OH + H ∼ HCO + O(D) > HCO + OH ∼ CO + H ∼ CO + H + O(D) > CO + H + H > HCO + O(D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10 cm molecule s in comparison to the recommended experimental rate constant of 3.3 × 10 cm molecule s. The simulation product yields for the singlet PES are compared with experiment, and the most significant differences are for H, CO, and HO. The reaction on the triplet surface is also barrierless, inconsistent with experiment. A discussion is given of the need for future calculations to address (1) the barrier on the triplet PES for CH + O → CHOO, (2) the temperature dependence of the CH + O reaction rate constant and product branching ratios, and (3) the possible non-RRKM dynamics of the CHOO Criegee intermediate.

Direct Dynamics Simulations of the 3CH2 + 3O2 Reaction at High Temperature

“…Reaction Pathways and Reaction Probabilities. A schematic of the PES for the simulation of the reaction between 3 CH 2 and 3 O 2 on the ground state singlet surface was calculated in our previous study 16 at the UM06/6-311+ +G(d,p) level of theory and is shown in Figure 1. As discussed in the methodology section, the simulations were performed by choosing collision impact parameter b randomly between 0 and b max = 5 Å at a temperature of 1000 K. At the end of the trajectories, seven product channels, CO + H 2 O (R2), CO 2 + H 2 (R1), CH 2 O + O( 3 P) (R7), CO 2 + H + H (R4), CO + H 2 + O( 1 D) (R9), CO + OH + H (R8), and HCO + OH (R5), were identified.…”

“…In our recent studies, , we showed that the density functional UM06 with the 6-311++G­(d,p) basis set , provided a good description of the PES for the 3 CH 2 + 3 O 2 reaction. Hence, in the present work, direct dynamics simulations for the 3 CH 2 + 3 O 2 reaction were performed with UM06/6-311++G­(d,p) level of theory.…”

“…Quantum chemical calculations − show that there is a potential energy minimum for 3 CH 2 + 3 O 2 on the singlet surfaces for the Criegee intermediate 1 CH 2 O 2 , which further rearranges into dioxirane and then dissociates into products. However, chemical dynamics simulations show that trapping in this intermediate is unimportant for the reaction dynamics CASSCF studies by Chen et al and Fang et al reported energy barriers of 1.9 and 1.6 kcal/mol, respectively, for formation of 1 CH 2 O 2 on the ground state singlet surface.…”

Direct Dynamics Simulations of the Unimolecular Decomposition of the Randomly Excited 1CH2O2 Criegee Intermediate. Comparison with 3CH2 + 3O2 Reaction Dynamics

“…For reaction on the triplet surface only O( 3 P) + CH 2 O are formed. 4 In contrast, the reaction dynamics on the singlet surface are quite complex with seven different reaction pathways and nine different products. 5 Though 3 CH 2 + 3 O 2 interact to form a potential energy minimum for the CH 2 O 2 Criegee intermediate on both the triplet and singlet surfaces, trapping in this intermediate is unimportant for the reaction dynamics.…”

“…Potential energy profile for the 3 CH 2 + 3 O 2 reaction on the singlet PES, calculated at the UM06/6-311++G­(d,p) level of theory (adapted from ref ).…”