Rapid unimolecular reactions of acyl peroxy radicals: extending the structure–activity relationships

Acyl peroxy radicals are especially efficient at forming organic accretion products in the troposphere, but they also have short lifetimes due to rapid unimolecular reactions. For this reason, we find it important to accurately represent the reactions of these species in structure-activity relationships estimating the unimolecular reactivity of atmospheric peroxy radicals. To address this, we performed multi-conformer transition state theory calculations to determine H-shift and ring closure reaction rates for aldehyde-substituted and unsaturated acyl peroxy radicals over a wide temperature range. Similar calculations were performed for enol-substituted peroxy radicals, which are also underrepresented in SAR models. As a results, we found that H-shifts from aldehyde groups are highly competitive, that ring closures are overwhelmingly the major atmospheric fate of unsaturated acyl peroxy radicals, and that H-shifts from <i>Z</i>-enols outcompete all other unimolecular and bimolecular reactions whenever they are possible. In conclusion, in extending the SAR models we have gained valuable insight on some of the most rapid reactions for any peroxy radicals in the atmosphere.