Determination of Channel-Specific Unimolecular Reaction Rate Coefficients of Peroxy Radicals Formed from the OH-Initiated Oxidation of α-Pinene

Abstract

Unimolecular reactions of organic peroxy radicals (RO2) play a central role in the atmospheric oxidation of volatile organic compounds (VOCs), yet experimental constraints on their reaction kinetics remain limited. Ring-opened RO2 formed in the OH-initiated oxidation of α-pinene undergoes rapid unimolecular reactions, including 1,5 H-shift, 1,6 H-shift, and endocyclization. However, previously reported experimental rate coefficients represent the overall unimolecular reaction rate from all channels and do not resolve branching among chemically distinct channels. As these channels lead to different oxidation products, channel-specific rate coefficients are needed to constrain product distributions under different atmospheric conditions. Here, we employ site-specific deuterium-labeled α-pinene isotopologues to selectively suppress individual H-shift channels and isolate the contributions of competing unimolecular channels. For each deuterated isotopologue, the overall unimolecular rate coefficient of the ring-opened RO2 was determined at 296 K using hydroxy nitrate (HN)-based kinetic analysis and compared with that of unlabeled α-pinene to derive channel-specific rate coefficients. We obtain 0.7−0.4+0.5 s–1 for the 1,6 H-shift,0.5−0.3+0.5 s–1 for the 1,5 H-shift, and 3.0−1.6+1.7 s–1 for endocyclization. These values are consistent with theoretical calculations and provide direct experimental constraints on individual channel rate coefficients in α-pinene-derived RO2 unimolecular chemistry. These channel-specific rate coefficients provide a quantitative basis for predicting oxidation product distributions under different atmospheric conditions and serve as experimental benchmarks for theoretical calculations of RO2 unimolecular chemistry.