Journal of Physical Chemistry A vol:111 issue:19 pages:3836-3849
The lowest-lying triplet and singlet potential energy surfaces for the O(P-3) + 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, (3)A' and (3)A' '; the results predict O-addition to be dominant up to combustion temperatures. Major expected end-products of the addition routes include phenoxy radical + H-center dot, phenol and/or benzene oxide/oxepin, in agreement with the experimental evidence. While c-C6H5O center dot + H-center dot 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 center dot + (CHO)-C-center dot 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, B-3(1) and B-3(2), 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 x 10(-16) x T (1.66) x exp(-1830 K/T) cm(3) molecule(-1) s(-1), in good agreement with the experimental data available.