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(K_c/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.