Kinetics and mechanisms for the reactions of phenyl radical with ketene and its deuterated isotopomer: An experimental and theoretical study

Kinetics and mechanism for the reaction of phenyl radical (C ₆H₅) with ketene (H₂Cβ=Cα=O) were studied by the cavity ring-down spectrometric (CRDS) technique and hybrid DFT and ab initio molecular orbital calculations. The C₆H₅ transition at 504.8 nm was used to detect the consumption of the phenyl radical in the reaction. The absolute overall rate constants measured, including those for the reaction with CD₂CO, can be expressed by the Arrhenius equation k = (5.9 ± 1.8) × 10¹¹ exp[-(1160 ± 100)/ T] cm³mol^-1s^-1 over a temperature range of 301-474K. The absence of a kinetic isotope effect suggests that direct hydrogen abstraction forming benzene and ketenyl radical is kinetically less favorable, in good agreement with the results of quantum chemical calculations at the G2MS//B3LYP6-31G(d) level of theory for all accessible product channels, including the above abstraction and additions to the C_α, C _β, and O sites. For application to combustion, the rate constants were extrapolated over the temperature range of 298-2500 K under atmospheric pressure by using the predicted transition-state parameters and the adjusted entrance reaction barriers Eα = Eβ = 1.2 kcalmol ^-1; they can be represented by the following expression in units of cm³mol^-1s^-1: k_α = 6.2 × 10¹⁹T^-2.3 exp[-7590/1] and k_β = 3.2 × 10⁴T^2.4 exp[-246/T].