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

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 methylsubstituted 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 o f syn-CH3CHOO at energies nearly 2000 cm(-1) below the barrier is achievedthrough 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. 10(6) 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-KasselMarcus (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. Published by AIP Publishing.