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

Simple expressions for predicting independently the kinetic and thermodynam ic parameters for the decomposition reactions of alkoxyl radicals are propo sed. These relationships have been parametrized, using quantum chemistry ab initio BAC-MP4 and density functional theory calculated activation energie s and reaction enthalpies on a set of linear and branched C-1 to C-5 alkoxy l radical decomposition processes. Once parametrized, and validated against experimental data, the proposed relationships can be extrapolated to large r alkoxyl radical decomposition reactions relevant to atmospheric modelling . They only involve the ionisation potential of the departing alkyl group o f the alkoxyl radicals and the parameter n(H), defining whether the radical is primary, secondary or tertiary. One of the main interests of these find ings is to predict with a reasonable accuracy the activation energies witho ut knowing a priori the enthalpies of the reactions. In addition, RRKM calc ulations have shown that rate constant corrections for fall-off behavior ar e less than a factor of 2, at room temperature and atmospheric pressure, wh ich is within experimental uncertainties. Nevertheless, Arrhenius pre-expon ential factors can be very low at that pressure, compared with their high p ressure limit value. The relationships proposed in this work provide kineti c parameters that are in reasonable agreement with available experimental d ata.