Theoretical Study of the Benzyl + O2 Reaction: Kinetics, Mechanism, and Product Branching Ratios

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• 作者：Yoshinori Murakami ; Tatsuo Oguchi ; Kohtaro Hashimoto ; Yoshio Nosaka
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：December 20, 2007
• 年：2007
• 卷：111
• 期：50
• 页码：13200 - 13208
• 全文大小：375K
• 年卷期：v.111,no.50(December 20, 2007)
• ISSN：1520-5215

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.