Multigenerational Theoretical Study of Isoprene Peroxy Radical 1–5-Hydrogen Shift Reactions that Regenerate HOx Radicals and Produce Highly Oxidized Molecules
Piletic, I., R. Howell, L. Bartolotti, Tad Kleindienst, S. Kaushik, AND E. Edney. Multigenerational Theoretical Study of Isoprene Peroxy Radical 1–5-Hydrogen Shift Reactions that Regenerate HOx Radicals and Produce Highly Oxidized Molecules. JOURNAL OF PHYSICAL CHEMISTRY A. American Chemical Society, Washington, DC, 123(4):906-919, (2019). https://doi.org/10.1021/acs.jpca.8b09738
This product will examine the isomerization reactions of important free-radical species such as isoprene peroxy radicals in the environment. These reactive species play an important role in the atmosphere by affecting the total suspended PM mass and ozone concentrations which have climate change and air quality implications. This product will consist of one manuscript. The manuscript will detail the isomerization kinetics of a variety of isoprene peroxy radicals in low NOx environments.
A computational protocol is employed to glean new insight into the kinetics of several 1,5-hydrogen atom (H) shift reactions subsequent to first- and second-generation OH/O2 additions to isoprene. The M06-2X density functional was initially used with the Nudged Elastic Band (NEB) method to determine the potential energy surface of OH/O2 addition reactions, the 1,5-H shift reactions, and the fragmentation exit channels. The Master Equation Solver for Multi-Energy Well Reactions (MESMER) was applied to determine the rate constants for OH addition and the 1,5-H shifts. M06-2X was capable of quantifying the rate constants of OH addition to the first and second double bonds of isoprene with deviations less than 17% from the experimentally determined values. However, M06-2X underestimated the 1,5-H shift rate constants of second-generation isoprene peroxy radicals. Consequently, MN15, ωB97X-D, and CBS-QB3 methods were employed to compute average barrier heights to first- and second-generation 1,5-H shifts. In the first generation, the rate constants of H abstraction by β-(1,2) and (4,3) isoprene hydroxy-peroxy radicals from the neighboring hydroxyl group are 1.1 × 10–3 and 2.4 × 10–3 s–1, respectively. These values are determined primarily by the barrier of the H shift reaction and, to a smaller albeit nonnegligible extent, by the stability of the resulting alkoxy radical and the exit barrier leading to C–C bond dissociation. In contrast, the average second-generation rate constant of 1,5-H shifts from H–R–OH sites to the peroxy radical is 1.8 × 10–1 s–1, with tunneling playing the significant role of increasing this value relative to first-generation 1,5-H shifts. Under low NOx conditions, first-generation isoprene oxidation reactions may recycle HOx at levels ranging from 10 to 30% due in large part to 1,5-H shifts, with the recycling efficiency being sensitive to HO2 concentrations and temperature. HOx recycling is expected to increase to levels beyond 80% in second-generation reactions of oxidized isoprene species because of isoprene epoxydiol (IEPOX) formation and further 1,5-H shifts that are kinetically favorable.