Unimolecular Decay of Criegee Intermediates to OH Radical Products: Prompt and Thermal Decay Processes
Marsha I. Lester, Stephen J. Klippenstein
Index: 10.1021/acs.accounts.8b00077
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Abstract
The dimethyl-substituted Criegee intermediate, (CH3)2COO, is utilized in this Account to showcase recent efforts to experimentally measure and theoretically predict the rates for prompt and thermal unimolecular decay processes of prototypical Criegee intermediates under laboratory and atmospheric conditions. The experimental laboratory studies utilize an alternative synthesis method to efficiently generate Criegee intermediates via the reaction of iodoalkyl radicals with O2. Infrared excitation is then used to prepare the (CH3)2COO Criegee intermediates at specific energies in the vicinity of the transition state barrier or significantly below the barrier for 1,4-hydrogen transfer that leads to OH products. The rate of unimolecular decay is revealed through direct time-domain measurements of the appearance of OH products utilizing ultraviolet laser-induced fluorescence detection under collision-free conditions. Complementary high-level theoretical calculations are carried out to evaluate the transition state barrier and the energy-dependent unimolecular decay rates for (CH3)2COO using Rice–Ramsperger–Kassel–Marcus (RRKM) theory, which are in excellent accord with the experimental measurements. Quantum mechanical tunneling through the barrier, incorporated through Eckart and semiclassical transition state theory models, is shown to make a significant contribution to the unimolecular decay rates at energies in the vicinity of and much below the barrier. Master equation modeling is used to extend the energy-dependent unimolecular rates to thermal decay rates of (CH3)2COO under tropospheric conditions (high pressure limit), which agree well with recent laboratory measurements [Smith et al. J. Phys. Chem. A 2016, 120, 4789 and Chhantyal-Pun et al. J. Phys. Chem.A 2017, 121, 4–15]. Again, tunneling is shown to enhance the thermal decay rate by orders of magnitude. The experimentally validated unimolecular rates are also utilized in modeling the prompt and thermal unimolecular decay of chemically activated (CH3)2COO formed upon ozonolysis of 2,3-dimethyl-2-butene under atmospheric conditions [Drozdet al. J. Phys. Chem. A 2017, 121, 6036−6045]. Future challenges lie in extension of these spectroscopic and dynamical methods to Criegee intermediates derived from more complex ozonolysis reactions involving biogenic alkenes.