On unimolecular decomposition of phenyl radical

The unimolecular decomposition of the phenyl radical and ortho-benzyne was examined by ab initio quantum chemical calculations, Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, and numerical simulation of shock tube pyrolysis of phenyl and benzene. The rate constants of CH fission in the phenyl radical was first determined by re-examining the rates of H-atom production from nitrosobenzene pyrolysis at temperatures from 1450 to 1730 K and pressures from 1.5 to 7 atm. The experimental rate constant was successfully reproduced and extrapolated with RRKM calculations using molecular parameters obtained with the complete active space self-consistent field approach. The theoretical rate constants were then fitted in Tree's fall-off expressions for C6H5 (+Ar) --> o-C6H4 + H(+Ar) in the temperature range of 500 to 2500 K, which gives k(infinity) (s(-1)) = 4.3 x 10(12) T-0.62 exp(-38,900/T) k(0)/[Ar] (cm(3) mol(-1) s(-1)) = 1.0 x 10(84) T-18.87 exp(-45,340/T) F-c = (1 - 0.902) exp(-T/696) + 0.902 exp(-T/358) + exp(-3856/T) The concerted unimolecular decomposition of ortho-benzyne, O-C6H4 (+Ar) --> C4H2 + C2H2 (+Ar) was studied similarly. The high-pressure limit rate parameters were estimated to be k(infinity) (S-1) = 1.2 x 10(18) T-0.34 exp(-44,200/T) It was shown that a revised Bauer-Aten mechanism, featuring H ejection from phenyl followed by the concerted decomposition of o-benzyne, describes very well benzene decomposition in a shock tube.