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

An update and expansion of our readily applicable structure-activity relationship (SAR) for predicting the barrier height E(b) to decomposition by beta 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- (-NO(2)), nitrosooxy- (-ONO), and nitroxy- (-ONO(2)) 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) + Sigma N(s) x 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 decomposition k(diss) at or around 298 K.