Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2

Abstract

The kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C₆H₇ with O₂ have been investigated using both experimental and theoretical approaches. The rate constant has been measured using conventional flash photolysis in the temperature range 302–456 K, 1 atm pressure. c-C₆H₇ radicals were produced by reacting Cl atoms with 1,4-cyclohexadiene. The rate expression is k₁ = (1.4 ± 0.26) × 10⁻¹³ exp[−(300 ± 74) K/T] cm³ molecule⁻¹ s⁻¹ (2σ error bars). The reaction can proceed either by association, yielding a peroxy radical RO₂ or by H-abstraction, yielding benzene + HO₂, the two reaction channels involving two distinct transition states. In contrast to what is observed for the c-C₆H₆OH radical, no equilibrium with the peroxy radical could be characterised. The theoretical approach, involving both DFT and ab initio methods, was used to determine if the measured rate constant should be assigned to the association or to the H-abstraction channel. Comparison of experimental and theoretical results shows that H-abstraction must be the only significant reaction channel.