Gas phase reactivity of the cyclohexadienyl radical with O2 and NO and thermochemistry of the association reaction with NO

Abstract

The kinetics of the reaction of the cyclohexadienyl radical (C6H7) with O2 and NO have been investigated for the first time using flash photolysis coupled to UV absorption spectrometry. The kinetic study was complemented by a thermochemical study of the association reaction of the C6H7 radical with NO. The reaction with O2 was found to be fairly slow, k(C6H7 + O2) = (4.0 ± 2.0) × 10−14 cm3 molecule−1 s−1 at 298 K, thus confirming the low reactivity of cyclohexadienyl-type radicals towards O2. The association reaction of C6H7 with NO is faster: k(C6H7 + NO) = (1.8 ± 0.5) × 10−12 cm3 molecule−1 s−1 (280 K, 1 atm pressure H2) and yet the rate constant is one order of magnitude smaller than the usual values observed for radical reactions with NO. RRKM calculations have indicated that pressure effects are small under these conditions, the rate constant being only 30 to 40% smaller than the high pressure limiting value. The reaction of C6H7 with NO was found to be equilibrated at T>300 K, under our experimental conditions. The equilibrium constant was measured at 4 different temperatures between 300 and 373 K, yielding the following expression: ln(Kc/cm3 molecule−1) = (−62.6 ± 1.0) + [(8680 ± 700) K/T]. A thermodynamic treatment of the data, using the Third Law method of analysis, yielded ΔH298° = − (74.9 ± 7.0) kJ mol−1, corresponding to the calculated value of ΔS298° = − (159 ± 8) J K−1 mol−1 (using AM1 calculations). This weak value of the R–NO bond dissociation energy is related to the resonance stabilisation energy of the C6H7 radical. The present results are discussed by comparison with those obtained for other cyclohexadienyl-type radicals. In particular, an explanation is suggested for the discrepancies observed in the literature concerning the kinetics of the hydroxycyclohexadienyl (HOC6H6) radical reaction with NO.