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 C-H 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 Troe's fall-off expressions for
in the temperature range of 500 to 2500 K, which gives β«1Χβ¬ 1 20 . 6 2 k (s ) β«Χ‘β¬ 4.3 β«Χβ¬ 10 T exp(β«/009,83Χβ¬T) Ο± 3 β«1Χβ¬ β«1Χβ¬ 8 β«78.81Χ4β¬ k /[Ar] (cm mol s ) β«Χ‘β¬ 1.0 β«Χβ¬ 10 T exp(β«/043,54Χβ¬T) 0 F β«Χ‘β¬ (1 β«Χβ¬ 0.902) exp(β«Χβ¬T/696) β«Χβ¬ 0.902 exp(β«Χβ¬T/358) β«Χβ¬ exp(β«/6583Χβ¬T) c The concerted unimolecular decomposition of ortho-benzyne, o-C H (β«Χβ¬Ar) β C H β«Χβ¬ C H (β«Χβ¬Ar) 6 4 4 2 2 2
was studied similarly. The high-pressure limit rate parameters were estimated to be
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.