Deep tunneling in the unimolecular decay of CH3CHOO Criegee intermediates to OH radical products

Abstract

Unimolecular decay of Criegee intermediates produced in alkene ozonolysis is known to be a significant source of OH radicals in the troposphere. In this work, unimolecular decay of the methyl-substituted Criegee intermediate, syn-CH₃CHOO, to OH products is shown to occur at energies significantly below the transition state barrier for a 1,4 hydrogen transfer that leads to these products [Y. Fang et al., J. Chem. Phys. 144, 061102 (2016)]. The rate of appearance of OH products arising from tunneling through the barrier is obtained through direct time-domain measurements following the vibrational activation of syn-CH₃CHOO. IR excitation of syn-CH₃CHOO at energies nearly 2000 cm^-1 below the barrier is achieved through combination bands involving CH stretch and another lower frequency mode, and the resultant OH products are detected by UV laser-induced fluorescence. The observed syn-CH₃CHOO combination bands in the 4100-4350 cm^-1 region are identified by comparison with the computed IR absorption spectrum. The experimental decay rates are found to be ca. 10⁶ s^-1 in this deep tunneling regime, which is approximately 100-times slower than that in the vicinity of the barrier.The experimental results are consistent with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the microcanonical decay rates with tunneling through the barrier, and notable deviations may originate from the sparsity in the density of states for syn-CH₃CHOO at lower energies. Thermal unimolecular decay of syn-CH₃CHOO is predicted to have significant contribution from microcanonical rates at energies that are much below the barrier.