Atmospheric Photochemical Oxidation of Benzene:  Benzene + OH and the Benzene−OH Adduct (Hydroxyl-2,4-cyclohexadienyl) + O_2

Abstract

The addition of hydroxyl radical to benzene leading to the formation of the hydroxyl-2,4-cyclohexadienyl radical (benzene−OH adduct, BOH) initiates the atmospheric oxidation of benzene. This reaction and subsequent reactions of the BOH adduct with O_2 are chemically activated reactions. Rate constants of these chemically activated bimolecular reactions and unimolecular decompositions are analyzed using a quantum version of Rice−Ramsperger−Kassel theory (QRRK) for k(E) and a modified strong collision approach for falloff. Results of QRRK analyses show that stabilization channels of energized BOH and benzene−OH−O_2 (BOHO_2) adducts are dominant in chemically activated reaction systems under atmospheric condition. Unimolecular reactions of stabilized adducts to products are also important. Thermodynamic parameters (ΔH_f^°_(298), S^°_(298), and C_p(T)s) are calculated using group additivity techniques with evaluated bond energies (for ΔH_f^°_(298)) and semiempirical PM3 molecular orbital calculations (for S6°_(298) and C_p(T)s). A limited elementary reaction mechanism that includes 29 reactions and 26 species is developed with reverse reaction rates determined from species thermodynamic parameters and microscopic reversibility for each step. Simulation results of three reaction systems indicate that pseudo-equilibrium is attained and that equilibrium levels of the important BOH and BOHO_2 adducts are controlled by thermodynamic properties. The most important bicyclic intermediate leading to ring cleavage products is adduct III. Rate constants of important bimolecular reactions are (k = A(T/K)^n exp(−E_a/RT), A in cm^3/(mol s), Ea in kcal/mol): k^4, (4.65 × 10^(15))(T/K)^(-1.18) e^(-1.23/RT) for C)6H)6 + OH ⇒ BOH; k_7, (3.55 × 10^(36))(T/K)^(-8.86) e^(-3.79/RT) for BOH + O_2 ⇒ BOHO_2; k_8, (1.73 × 10^(10))(T/K)^(-0.26) e^(-8.28/RT) for BOH + O_2 ⇒ hexadienedial + OH; k_9, (7.06 × 10^(14))(T/K)^(-1.83) e^(-5.36/RT) for BOH + O_2 ⇒ phenol + HO_2; k_(10), (2.14 × 10^(15))(T/K)^(-2.05) e^(-4.69/RT) for BOH + O_2 ⇒ adduct III. Rate constants of important unimolecular reactions are (A in s^(-1)): k_6, (2.04 × 10^(25))(T/K)^(-4.2) e^(-24.5/RT) for BOH ⇒ phenol + H; k_(15), (6.30 × 10^(40))(T/K)^(-10.86) e^(-19.4/RT) for BOHO)2 ⇒ phenol + HO)2; k_(16), (1.43 × 10^(42))(T/K)^(-11.34) e^(-18.8/RT) for BOHO_2 ⇒ adduct III.