Direct Dynamics Simulations of the CH₂ + O₂ Reaction on the Ground- and Excited-State Singlet Surfaces

In a previous work [ Lakshmanan , S. ; J. Phys. Chem. A 2018 , 122 , 4808 - 4818 ], direct dynamics simulations at the M06/6-311++G(d,p) level of theory were reported for ³CH₂ (X³B₁) + ³O₂ (X³∑_g^-) reaction on its ground-state singlet potential energy surface (PES) at 300 K. However, further analyses revealed the simulations are unstable for the ³CH₂ (X³B₁) + ³O₂ (X³∑_g^-) reactants on the ground-state singlet surface and the trajectories reverted to an excited-state singlet surface for the ¹CH₂ (ã¹A₁) + ¹O₂ (b¹∑_g⁺) reactants. Thus, the dynamics reported previously are for this excited-state singlet PES. The PESs for the ³CH₂ (X³B₁) + ³O₂ (X³∑_g^-) and ¹CH₂ (ã¹A₁) + ¹O₂ (b¹∑_g⁺) reactants are quite similar, and this provided a means to perform simulations for the ³CH₂ (X³B₁) + ³O₂ (X³∑_g^-) reactants on the ground-state singlet PES at 300 K, which are reported here. The reaction dynamics are quite complex with seven different reaction pathways and nine different products. A consistent set of product yields have not been determined experimentally, but the simulation yields for the H atom, CO, and CO₂ are somewhat lower, higher, and lower respectively, than the recommended values. The yields for the remaining six products agree with experimental values. Product decomposition was included in determining the product yields. The simulation ³CH₂ + ³O₂ rate constant at 300 K is only 3.4 times smaller than the recommended value, which may be accommodated if the ³CH₂ + ³O₂ → ¹CH₂O₂ potential energy curve is only 0.75 kcal/mol more attractive at the variational transition state for ³CH₂ + ³O₂ → ¹CH₂O₂ association. The simulation kinetics and dynamics for the ³CH₂ + ³O₂ and ¹CH₂ + ¹O₂ reactions are quite similar. Their rate constants are statistically the same, an expected result, since their transition states leading to products have energies lower than that of the reactants and the attractive potential energy curves for ³CH₂ + ³O₂ → ¹CH₂O₂ and ¹CH₂ + ¹O₂ → ¹CH₂O₂ are nearly identical. The product yields for the ³CH₂ + ³O₂ and ¹CH₂ + ¹O₂ reactions are also nearly identical, only differing for the CO₂ yield. The reaction dynamics on both surfaces are predominantly direct, with negligible trapping in potential energy minima, which may be an important contributor to their nearly identical product yields.