A semi-empirical correlation for the rate coefficients for cross- and self-reactions of peroxy radicals in the gas-phase

Peroxy radicals play an important role as reaction intermediates in the atmospheric oxidation of volatile organic compounds (VOCs). The rate coefficients for the self-reactions vary by up to six orders of magnitude (e.g. 3.0×10−17 cm3 molecule−1 s−1 for t-C4H9O2 and 1.5×10−11 cm3 molecule−1 s−1 for CH3C(O)O2) with, up to now, no clear pattern. This work represents the first rationalisation of peroxy radical self-reaction trends in reactivity. A correlation between the logarithm of the rate coefficient of peroxy radical self-reactions with the stabilisation energy, the difference in the enthalpy of formation of peroxy radicals with that of the tetraoxide adduct {ΔHf(RO4R)−2ΔHf(RO2)}, is presented. The equation, log10 k=−{ΔHf(RO4R)−2ΔHf(RO2)+235.95}/13.32 (where energy is in kJ mol−1) is given to predict the room temperature rate coefficients of peroxy radical self-reactions. The correlation was extended to predict rate coefficients for the cross-reactions of peroxy radicals with CH3O2. Furthermore, favourable comparisons are made between the predicted rate coefficients and very recent studies of complex peroxy radical systems.