Thermochemical Properties, Pathway, and Kinetic Analysis on the Reactions of Benzene with OH: An Elementary Reaction Mechanism

Abstract

Kinetics for the chemical activation reaction of the OH radical with benzene and unimolecular dissociation of the adduct are analyzed using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) and master equation analysis for pressure falloff. Thermochemical properties and reaction path parameters are determined by ab initio and density functional calculations. Molecular structures and vibration frequencies are determined at the B3LYP/6-31G(d,p) and MP2(full)/6-31G(d) levels, with single point calculations for the energy at the B3LYP/6-311++G(2df,p)//B3LYP/6-31G(d,p), composite methods of CBS-Q, CBS-QB3 and G3(MP2) and the G3 methods. The OH addition to benzene forms a chemically activated prereactive complex with a shallow well (ca. 3 kcal mol-1), which predominantly dissociates back to reactants. Additional reactions of the energized precomplex include stabilization, or forward reaction to form hydroxycyclohexadienyl radical, C•HDOH, which has a well depth of 16 kcal mol-1. This C•HDOH adduct can either eliminate H atom to form phenol, undergo intramolecular addition of the radical to an unsaturated carbon site to form bicyclo[3.1.0]hexan-6-ol radical (I in Figure 2), or react back through the prereactive complex. The radical (I) can cleave a strained exocyclic, cyclopropane bond forming cyclopenta-2,4-dienylmethan-1-ol radical (II in Figure 2). Rate coefficients for reactions of the energized complex are obtained from canonical transition state theory. The high-pressure addition rate constant for OH + benzene → prereactive complex is calculated from variational transition state theory with a center-of-mass reaction coordinate approximation. A detailed mechanism with mass conservation and microscopic reversibility is assembled and used to identify the intermediates and products of the benzene + OH reaction for comparison with experiment. The prereactive complex has a small effect on the overall kinetics and can be considered negligible over the temperature and pressure range investigated. The most important product formation channel in the OH + benzene addition reaction system is formation of phenol plus H atom. Comparisons of our calculated rate constants with experimental data that exhibit complex temperature and pressure dependence of [OH] vs time shows very good agreement and illustrate that microscopic reversibility needs to be included in analysis of experimental data on this reaction system. The important products for benzene + OH addition are predicted to be C•HDOH and phenol + H. At 800 K, product formation from cyclopentadiene intermediates is at least 3 orders of magnitude lower than phenol + H.