Kinetics and Mechanism for the Reaction of Phenyl Radical with Formaldehyde

Abstract

The kinetics and mechanism for the C6H5 + CH2O reaction were investigated by the cavity ringdown spectrometric (CRDS) and pulsed laser photolysis/mass spectrometric (PLP/MS) methods at temperatures between 298 and 1083 K. With the CRDS method, the rate constant was measured by monitoring the decay times of injected probing photons in the absence (tc0) and presence (tc) of the C6H5 radical. In the PLP/MS experiment at higher temperatures, the rate constant was determined by kinetic modeling of the absolute yields of C6H6. The values of the rate constants obtained by the two different methods agree closely, suggesting that the C6H5 + CH2O → C6H6 + CHO reaction 1 is the dominant channel. A weighted least-squares analysis of the two sets of data gave k1 = (8.55 ± 0.25) × 104 T2.19±0.25 exp[−(19 ± 13)/T] cm3 mol-1 s-1 for the temperature range studied. The mechanism for the C6H5 + CH2O reaction was also elucidated with a quantum-chemical calculation employing a hybrid density functional theory (B3LYP) using the aug-cc-PVTZ basis set. The theory predicts the barriers for the abstraction producing C6H6 and the addition giving C6H5CH2O and C6H5OCH2 to be 0.8, 1.4, and 9.1 kcal/mol, respectively. The rate constant calculated for the H-abstraction process using the canonical variational transition-state theory with a 1.1 kcal/mol barrier agrees closely with the experimental result over the entire range of temperatures studied.