Tunneling effects in the unimolecular decay of (CH 3 ) 2 COO Criegee intermediates to OH radical products.

Unimolecular decay of the dimethyl substituted Criegee intermediate (CH ₃ ) ₂ COO is observed at energies significantly below the transition state barrier associated with hydrogen atom transfer [Y. Fang et al., J. Chem. Phys. 144, 061102 (2016)] with time-resolved detection of the resultant OH radical products. (CH ₃ ) ₂ COO is prepared at specific energies in the 3900-4600 cm ^-1 region through IR excitation of combination bands involving CH stretch and another lower frequency mode, and the OH products are detected by UV laser-induced fluorescence. OH appearance times on the order of microseconds are observed in this deep tunneling regime, which are about 100 times slower than that in the vicinity of the barrier. The experimental rates are in good accord with Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the microcanonical dissociation rates for (CH ₃ ) ₂ COO that include tunneling. Master equation modeling based on these microcanonical rates is used to predict the thermal decay rate of (CH ₃ ) ₂ COO to OH products under atmospheric conditions of 276 s ^-1 at 298 K (high pressure limit). Thermal unimolecular decay of (CH ₃ ) ₂ COO to OH products is shown to have significant contributions from tunneling at energies much below the barrier to H-atom transfer.