Deep tunneling in the unimolecular decay of CHCHOO 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 methyl-substituted Criegee intermediate, syn-CHCHOO, 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-CHCHOO. IR excitation of syn-CHCHOO at energies nearly 2000 cm 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-CHCHOO combination bands in the 4100-4350 cm region are identified by comparison with the computed IR absorption spectrum. The experimental decay rates are found to be ca. 10 s 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-CHCHOO at lower energies. Thermal unimolecular decay of syn-CHCHOO is predicted to have significant contribution from microcanonical rates at energies that are much below the barrier.