Correlation-type structure activity relationships for the kinetics of the decomposition of simple and β-substituted alkoxyl radicals

Structure activity relationship (SARs) have been developed for the estimation of rate data for the decomposition of RO to alkyl radical and carbonyl fragments. The SARs are based upon strong, non-linear, correlations between the logarithm of measured room temperature rate coefficients and the average measured ionisation potential (IP) of the reaction products, for the decomposition of a range of simple unsubstituted, β-chlorinated and β-hydroxylated alkoxyl radicals. The role of chemical activation in the decomposition chemistry of substituted-alkoxyl radicals is briefly discussed. Distinct correlations are observed for simple and for lone pair-bearing β-substituted radicals, and each is rationalised in terms of simple Marcus theory. By assuming an Arrhenius A-factor of 1×1013 s−1, for a single decomposition pathway, activation barriers (Ea) have been estimated from selected room temperature rate coefficient data taken from the literature. These estimated Ea values, and barriers obtained directly from recent temperature-dependent experimental studies, are also shown to correlate with the average product IP. The correlation expressions presented allow the prediction of decomposition rate data with a precision comparable to, or within, that of the experimental data. Of the 10 simple and 11 heteroatom-substituted RO. species used to construct the correlations, 18 (85%) of the predicted room temperature rate coefficients are within a factor of two of their measured (or theoretically calculated) values, and 100% are within a factor of three.