Theoretical Study of the Reaction Mechanism and Kinetics of the Phenyl + Allyl and Related Benzyl + Vinyl Associations.

Abstract

Potential energy surfaces for the allyl + phenyl and benzyl + vinyl barrierless radical association reactions have been studied at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G** level of theory. Variable reaction coordinate transition state theory (VRC-TST) has been employed to evaluate high-pressure limit rate constants for the barrierless channels. Then, Rice-Ramsperger-Kassel-Marcus master equation (RRKM-ME) calculations have been performed to assess phenomenological rate constants and product branching ratios of various reaction channels at different temperatures and pressures. The initial step of both radical association reactions produces 3-phenylpropene which can further dissociate into a variety of bimolecular products including the indene precursor 1-phenylallyl + H. The results showed that at typical combustion conditions the collisional stabilization of 3-phenylpropene dominates both the phenyl + allyl and benzyl + vinyl reactions at temperatures below 1000 K and remains important at high pressures up to 2500 K. The main bimolecular products of the two reactions at high temperatures are predicted to be benzyl + vinyl and phenyl + allyl, respectively. The well-skipping mechanism to form 1-phenylallyl directly in the allyl + phenyl and benzyl + vinyl reactions appeared to be not significant, however, the reactions can provide some contributions into the formation of the indene precursor via the 3-phenylpropene stabilization/dissociation sequence and most of all, via the formation of 3-phenylpropene itself, which then can undergo H-abstraction by available radicals to produce 1-phenylallyl. The allyl + phenyl reaction can also contribute to the formation of two-ring PAH by producing benzyl radical at high temperatures, either by the well-skipping or stabilization/dissociation mechanisms; in turn, benzyl can readily react with acetylene or propargyl radical to form indene or naphthalene precursors, respectively. Rate expressions for all important reaction channels in a broad range of temperatures and pressures have been generated for kinetic modeling.