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

Yuxuan Yao, Sandhiya Lakshmanan, Subha Pratihar, William L. Hase

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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.

Original languageEnglish
JournalJournal of Physical Chemistry A
StateAccepted/In press - 2020

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