title: 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-1below 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-1region are identified by comparison with the computed IR absorption spectrum. The experimental decay rates are found to be ca. 106s-1in 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.
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
authors.forename: Yi
authors.initials: Y
authors.lastname: Fang
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
authors.forename: Fang
authors.initials: F
authors.lastname: Liu
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
authors.forename: Victoria P
authors.initials: VP
authors.lastname: Barber
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Klippenstein
authors.affiliation: Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Anne B
authors.initials: AB
authors.lastname: McCoy
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
authors.forename: Marsha I
authors.initials: MI
authors.lastname: Lester