Theoretical Reinvestigation of the O(³P) + C₆H₆ Reaction:  Quantum Chemical and Statistical Rate Calculations^†

The lowest-lying triplet and singlet potential energy surfaces for the O(³P) + C₆H₆ reaction were theoretically characterized using the “complete basis set” CBS-QB3 model chemistry. The primary product distributions for the multistate multiwell reactions on the individual surfaces were then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. It is newly found that electrophilic O-addition onto a carbon atom in benzene can occur in parallel on two triplet surfaces, ³A‘ and ³A‘ ‘; the results predict O-addition to be dominant up to combustion temperatures. Major expected end-products of the addition routes include phenoxy radical + H^•, phenol and/or benzene oxide/oxepin, in agreement with the experimental evidence. While c-C₆H₅O^• + H^• are nearly exclusively formed via a spin-conservation mechanism on the lowest-lying triplet surface, phenol and/or benzene oxide/oxepin are mainly generated from the lowest-lying singlet surface after inter-system crossing from the initial triplet surface. CO + c-C₅H₆ are predicted to be minor products in flame conditions, with a yield ≤ 5%. The O + C₆H₆ → c-C₅H₅^• + ^•CHO channel is found to be unimportant under all relevant combustion conditions, in contrast with previous theoretical conclusions (J. Phys. Chem. A 2001, 105, 4316). Efficient H-abstraction pathways are newly identified, occurring on two different electronic state surfaces, ³B₁ and ³B₂, resulting in hydroxyl plus phenyl radicals; they are predicted to play an important role at higher temperatures in hydrocarbon combustion, with estimated contributions of ca. 50% at 2000 K. The overall thermal rate coefficient k(O + C₆H₆) at 300−800 K was computed using multistate transition state theory:  k(T) = 3.7 × 10^-16 × T ^1.66 × exp(−1830 K/T) cm³ molecule^-1 s^-1, in good agreement with the experimental data available.