Reaction of phenyl radicals with acetylene: Quantum chemical investigation of the mechanism and master equation analysis of the kinetics

The mechanism of the C₆H₅ + C₂H ₂ reaction has been investigated by various quantum chemical methods. Electrophilic addition to the CC triple bond is found to be the only important mode of phenyl radical attack on acetylene. The initially formed chemically activated C₆H₅C₂H₂ adducts may follow several isomerization pathways in competition with collisional stabilization and H-elimination. Thermochemistry of various decomposition and isomerization channels is evaluated by the G2M method. For key intermediates, the following standard enthalpies of formation have been deduced from isodesmic reactions: 94.2 ± 2.0 kcal/mol (C₆H₅CHCH), 86,4 ± 2.0 kcal/mol (C₆H₅CCH₂), and 95.5 ± 1.8 kcal/ mol (o-C₆H₄C₂H₃). The accuracy of theoretical predictions was examined through extensive comparisons with available experimental and theoretical data. The kinetics and product branching of the C₆H₅ + C₂H ₂ reaction have been evaluated by weak collision master equation/Rice-Ramsperger-Kassel-Marcus (RRKM) analysis of the truncated kinetic model including only kinetically important transformations of the isomeric C₈H₇ radicals. Available experimental kinetic data can be quantitatively reproduced by calculation with a minor adjustment of the C ₆H₅ addition barrier from 3.7 to 4.1 kcal/mol. Our predicted total rate constant, k_R1 = (1.29 x 10¹⁰)T ^0.834 exp(-2320/T) cm³ mol^-1 s^-1, is weakly dependent on P and corresponds to the phenylation process under combustion conditions (T > 1000 K).