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 6 H 5 ) with ketene (H 2 C β C α O) were studied by the cavity ring‐down spectrometric (CRDS) technique and hybrid DFT and ab initio molecular orbital calculations. The C 6 H 5 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 CD 2 CO, can be expressed by the Arrhenius equation k =(5.9±1.8)×10 11  exp[−(1160±100)/ T ] cm 3  mol −1  s −1 over a temperature range of 301–474 K. 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 E α = E β =1.2 kcal mol −1 ; they can be represented by the following expression in units of cm 3  mol −1  s −1 : k α =6.2×10 19   T −2.3  exp[−7590/ T ] and k β =3.2×10 4   T 2.4  exp[−246/T].