Direct Dynamics Simulations of the Unimolecular Decomposition of the Randomly Excited ¹CH₂O₂ Criegee Intermediate....

The ³CH₂ + ³O₂ reaction has a quite complex ground state singlet potential energy surface (PES). There are multiple minima and transition states before forming the 10 possible reaction products. A previous direct chemical dynamics simulation at the UM06/6-311++G(d,p) level of theory ( J. Phys. Chem. A 2019, 123, 4360-4369) found that reaction on this PES is predominantly direct without trapping in the potential minima. The first minima ³CH₂ + ³O₂ encounters is that for the ¹CH₂O₂ Criegee intermediate and statistical theory assumes the reactive system is trapped in this intermediate with a lifetime given by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. In the work presented here, a direct dynamics simulation is performed with the above UM06 theory, with the trajectories initialized in the ¹CH₂O₂ intermediate with a random distribution of vibrational energy as assumed by RRKM theory. There are substantial differences between the dynamics for ¹CH₂O₂ dissociation and ³CH₂ + ³O₂ reaction. For the former there are four product channels, while for the latter there are seven in agreement with experiment. Product energy partitioning for the two simulations are in overall good agreement for the CO₂ + H₂ and CO + H₂O product channels, but in significant disagreement for the HCO + OH product channel. Though ¹CH₂O₂ is excited randomly in accord with RRKM theory, its dissociation probability is biexponential and not exponential as assumed by RRKM. In addition, the ¹CH₂O₂ dissociation dynamics follow non-intrinsic reaction coordinate (non-IRC) pathways. An important finding is that the nonstatistical dynamics for the ³CH₂ + ³O₂ reaction give results in agreement with experiment.