Crossed molecular beam experiments and theoretical simulations on the multichannel reaction of toluene with atomic oxygen†

Abstract

Despite extensive experimental and theoretical studies on the kinetics of the O(³P) + C₇H₈ (toluene) reaction and a pioneering crossed molecular beam (CMB) investigation, the branching fractions (BFs) of the CH₃C₆H₄O(methylphenoxy) + H, C₆H₅O(phenoxy) + CH₃, and spin-forbidden C₅H₅CH₃ (methylcyclopentadiene) + CO product channels remain an open question, which has hampered the proper inclusion of this important reaction in the chemical modelling of various chemical environments. We report a CMB study with universal soft electron-ionization mass-spectrometric detection of the reactions O(³P,¹D) + toluene at the collision energy of 34.7 kJ mol⁻¹. From CMB data we have inferred the reaction dynamics and quantified the BFs of the primary products and the role of intersystem crossing (ISC). The CH₃-elimination channel dominates (BF = 0.69 ± 0.22) in the O(³P) reaction, while the H-displacement and CO-formation channels are minor (BF = 0.22 ± 0.07 and 0.09 ± 0.05, respectively), with ISC accounting for more than 50% of the reactive flux. Synergistic transition-state theory (TST)-based master equation simulations including nonadiabatic TST on ab initio coupled triplet/singlet potential energy surfaces were employed to compute the product BFs and assist in the interpretation of the CMB results. In the light of the good agreement between the theoretical predictions for the O(³P) + toluene reaction and the CMB results as well as the absolute rate constant as a function of temperature (T) (from literature), the so-validated computational methodology was used to predict channel-specific rate constants as a function of T at 1 atm.