Theoretical study of alkoxyl radical decomposition reactions: structure–activity relationships

Simple expressions for predicting independently the kinetic and thermodynamic parameters for the decomposition reactions of alkoxyl radicals are proposed. These relationships have been parametrized, using quantum chemistry ab initio BAC-MP4 and density functional theory calculated activation energies and reaction enthalpies on a set of linear and branched C1 to C5 alkoxyl radical decomposition processes. Once parametrized, and validated against experimental data, the proposed relationships can be extrapolated to larger alkoxyl radical decomposition reactions relevant to atmospheric modelling. They only involve the ionisation potential of the departing alkyl group of the alkoxyl radicals and the parameter nH, defining whether the radical is primary, secondary or tertiary. One of the main interests of these findings is to predict with a reasonable accuracy the activation energies without knowing a priori the enthalpies of the reactions. In addition, RRKM calculations have shown that rate constant corrections for fall-off behavior are less than a factor of 2, at room temperature and atmospheric pressure, which is within experimental uncertainties. Nevertheless, Arrhenius pre-exponential factors can be very low at that pressure, compared with their high pressure limit value. The relationships proposed in this work provide kinetic parameters that are in reasonable agreement with available experimental data.