文摘
Ab initio calculations at the level of CBS-QB3 theory have been performed to investigate the potential energysurface for the reaction of benzyl radical with molecular oxygen. The reaction is shown to proceed with anexothermic barrierless addition of O2 to the benzyl radical to form benzylperoxy radical (2). The benzylperoxyradical was found to have three dissociation channels, giving benzaldehyde (4) and OH radical through thefour-centered transition states (channel B), giving benzyl hydroperoxide (5) through the six-centered transitionstates (channel C), and giving O2-adduct (8) through the four-centered transition states (channel D), in additionto the backward reaction forming benzyl radical and O2 (channel E). The master equation analysis suggestedthat the rate constant for the backward reaction (E) of C6H5CH2OO C6H5CH2 + O2 was several orders ofmagnitude higher that those for the product dissociation channels (B-D) for temperatures 300-1500 K andpressures 0.1-10 atm; therefore, it was also suggested that the dissociation of benzylperoxy radicals proceededwith the partial equilibrium between the benzyl + O2 and benzylperoxy radicals. The rate constants for productchannels B-D were also calculated, and it was found that the rate constant for each dissociation reactionpathway was higher in the order of channel D > channel C > channel B for all temperature and pressureranges. The rate constants for the reaction of benzyl + O2 were computed from the equilibrium constant andfrom the predicted rate constant for the backward reaction (E). Finally, the product branching ratios formingCH2O molecules and OH radicals formed by the reaction of benzyl + O2 were also calculated using thestationary state approximation for each reaction intermediate.