Abstract
| - The lowest-lying triplet and singlet potential energy surfaces for the O(3P) + C6H6 reaction were theoreticallycharacterized using the “complete basis set” CBS-QB3 model chemistry. The primary product distributionsfor the multistate multiwell reactions on the individual surfaces were then determined by RRKM statisticalrate theory and weak-collision master equation analysis using the exact stochastic simulation method. It isnewly found that electrophilic O-addition onto a carbon atom in benzene can occur in parallel on two tripletsurfaces, 3A‘ and 3A‘ ‘; the results predict O-addition to be dominant up to combustion temperatures. Majorexpected 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 formedvia a spin-conservation mechanism on the lowest-lying triplet surface, phenol and/or benzene oxide/oxepinare mainly generated from the lowest-lying singlet surface after inter-system crossing from the initial tripletsurface. 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, incontrast with previous theoretical conclusions (J. Phys. Chem. A 2001, 105, 4316). Efficient H-abstractionpathways are newly identified, occurring on two different electronic state surfaces, 3B1 and 3B2, resulting inhydroxyl plus phenyl radicals; they are predicted to play an important role at higher temperatures in hydrocarboncombustion, 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 × T1.66 ×exp(−1830 K/T) cm3 molecule-1 s-1, in good agreement with the experimental data available.
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