Decomposition of substituted alkoxy radicals—part I: a generalized structure–activity relationship for reaction barrier heights

Abstract

An update and expansion of our readily applicable structure–activity relationship (SAR) for predicting the barrier height E_b to decomposition by β C–C scission of (substituted) alkoxy radicals is presented. Such alkoxy radicals are key intermediates in the atmospheric oxidation of volatile organic compounds, and a correct description of their chemistry is vital to the understanding of atmospheric chemistry; nevertheless, experimental data on these reactions remain scarce. The SAR is based on quantum chemical characterizations of a large set of alkoxy radicals, and accommodates alkoxy radicals with alkyl- (–R), oxo- (O), hydroxy- (–OH), hydroperoxy (–OOH), alkoxy (–OR), alkylperoxy- (–OOR), nitroso- (–NO), nitro- (–NO₂), nitrosooxy- (–ONO), and nitroxy- (–ONO₂) functionalities, as well as 3- to 6-membered rings and some unsaturated side chains. The SAR expresses the barrier height to decomposition, E_b = 17.9 kcal mol⁻¹ + ΣN_s×F_s, as a linear function of the number N_s of these substituents on the relevant carbons, and the substituent-specific activities F_s derived from the quantum chemical calculations, allowing facile predictions based solely on the molecular structure. For low barriers, ≤7 kcal mol⁻¹, a simple curvature correction is required. The SAR-predicted barrier height E_b can be used to predict the high-pressure rate coefficient for alkoxy decompositionk_diss at or around 298 K.