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

Yi Fang; Fang Liu; Victoria P Barber; Stephen J Klippenstein; Anne B McCoy; Marsha I Lester

doi:10.1063/1.4972015

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

J Chem Phys. 2016 Dec 21;145(23):234308. doi: 10.1063/1.4972015.

Authors

Yi Fang¹, Fang Liu¹, Victoria P Barber¹, Stephen J Klippenstein², Anne B McCoy³, Marsha I Lester¹

Affiliations

¹ Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
² Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
³ Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.

PMID: 28010089
DOI: 10.1063/1.4972015

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.