The impact of unimolecular reactions on acyl peroxy radical initiated isoprene oxidation

Abstract. Unimolecular H-shift and endoperoxide ring formation reactions were studied for several different acyl peroxy radicals (APRs) using quantum-mechanical methods. Also, for structures with slow unimolecular reactions, accretion reactions with isoprene were investigated. The goal of the study was to determine which APRs could work as atmospheric oxidants of unsaturated hydrocarbons. The reaction rate coefficients were calculated at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31+G* level using multi-conformer transition state theory. Unimolecular reactions of acyl peroxy radicals were shown to have rate coefficients of up to 0.1 s−1 and bimolecular accretion reactions with isoprene of up to 10−15 cm3 s−1. Both smaller and larger acyl peroxy radicals with rigid structures were observed to be more likely to initiate oxidation of isoprene because of their inability in fast unimolecular reactions. The pseudo-first-order reaction rates were calculated for accretion reactions of isoprene with OH and six APRs at different temperatures. The significance of APR-initiated isoprene oxidation was shown to increase with increasing temperature. APR-initiated oxidation could lead to dimeric products with atmospheric impact through formation of low-volatility compounds.