Hydrogen Shift Reactions in Nonhydrocarbon Peroxy Radicals

Hydrogen shift reactions (H-shifts) are important unimolecular steps in the autoxidation of volatile organic compounds (VOCs). These potentially lead to the formation of highly oxygenated organic molecules (HOMs) that contribute to the formation of secondary organic aerosol (SOA). Although H-shift chemistry in hydrocarbon peroxy radicals (RO₂) is well-studied, the impact of incorporating a heteroatom, oxygen, nitrogen, sulfur, or phosphorus (O, N, S, or P), into the carbon backbone remains largely unexplored. We have used a multiconformer transition state theory approach to calculate H-shift rate coefficients in RO₂s that contain a heteroatom. We find that heteroatoms accelerate H-shift reactions, particularly when the heteroatom is positioned α with respect to the carbon from which the H atom is abstracted. For the 1,6 H-shift with the heteroatom β to the abstraction site, the rate coefficients are more similar to those in the hydrocarbon. For large H-shift spans, the variation in the H-shift rate coefficient with heteroatoms is large, and the reactions are faster than in hydrocarbons. For an ether, we find that the 1,8 H-shift is as fast as the 1,5 H-shift and both are competitive with bimolecular reactions under pristine atmospheric conditions. We explain the trends in selectivity and reactivity based on steric, inductive, and stereoelectronic effects. These findings highlight the importance of isomerization reactions in atmospheric chemistry, including those involving nonhydrocarbon peroxy radicals.