Unimolecular Decay of the Dimethyl-Substituted Criegee Intermediate in Alkene Ozonolysis: Decay Time Scales and the Importance of Tunneling.

We used the steady-state master equation to model unimolecular decay of the Criegee intermediate formed from ozonolysis of 2,3-dimethyl-2-butene (tetramethylethylene, TME). Our results show the relative importance and time scales for both the prompt and thermal unimolecular decay of the dimethyl-substituted Criegee intermediate, (CH)COO. Calculated reactive fluxes show the importance of quantum mechanical tunneling for both prompt and thermal decay to OH radical products. We constrained the initial energy distribution of chemically activated (CH)COO formed in TME ozonolysis by combining microcanonical rates k(E) measured experimentally under collision-free conditions and modeled using semiclassical transition-state theory (SCTST) with pressure-dependent yields of stabilized Criegee intermediates measured with scavengers in flow-tube experiments. Thermal decay rates under atmospheric conditions k(298 K, 1 atm) increase by more than 1 order of magnitude when tunneling is included. Accounting for tunneling has important consequences for interpreting pressure dependent yields of stabilized Criegee intermediates, particularly with regard to the fraction of Criegee intermediates formed in the zero-pressure limit.