Theoretical study of potential energy surface and thermal rate constants for the C₆H₅ + H₂ and C₆H₆ + H reactions

The potential energy surface for the C₆H₅-H₂ system has been calculated with a modified Gaussian-2 method (G2M). The system includes the reactions C₆H₅ + H₂ ⇌ C₆H₆ + H (1) and H + C₆H₆ ⇌ C₆H₇ (2). The computed molecular parameters and energetics are employed to calculate the thermal rate constants for these reactions. For the direct abstraction reaction (1), the energy barrier was found to be 8.8 kcal/mol at our best G2M(rcc,MP2) level of theory, with the tunneling corrected transition-state-theory rate constant k₁ = 9.48 × 10^-20T^2.43 exp(-3159/T) cm³/(molecule s) covering 300-5000 K. This result is consistent with scattered kinetic data available in the literature. For the addition reaction (2), the barrier was found to be 8.9 kcal/mol. The rate constant calculated by solving the master equation, with tunneling corrections based on the RRKM theory, gave k₂ = 5.27 × 10^-11 exp(-1605/T) cm³/(molecule s) at the high-pressure limit and 300 ≤ T ≤ 1000 K. In this temperature regime, where most addition kinetics have been measured, the calculated results between 1 and 100 Torr encompass all experimental data. k₂ was found to be strongly pressure dependent above room temperature. Additionally, the effects of isotope substitution and possible secondary reactions on reported experimental data have been discussed.