Variable reaction coordinate study of the benzyl + HO2 ? benzylhydroperoxide and benzoxyl + OH ? benzylhydroperoxide radical-radical barrierless association reactions

We present a theoretical kinetic study, based on variable reaction coordinate calculations, of the radical-radical barrierless association reactions of benzyl with HO2 and benzoxyl with OH, both yielding benzylhydroperoxide, over the broad temperature range 200-3000 K. These reactions are important in the combustion of aromatic fuels at low and intermediate temperatures, and are involved in soot formation. Our results are significantly different to those previously reported by G. da Silva and J. W. Bozzelli (Proc. Comb. Inst. 31 (1) (2009) 287-294), who used the variational transition state theory and obtained much larger rate constants. We attribute these discrepancies to the different methodologies used in both studies. Variable reaction coordinate calculations are recommended for barrierless association reactions, as the variational transition state theory may fail to locate the best dividing surface and therefore to variationally minimize the rate constant. Our results are useful to update the kinetic models of fuels involving aromatic species and therefore improve predictive capabilities in combustion simulations.