LIF Spectra of Cyclohexoxy Radical and Direct Kinetic Studies of Its Reaction with O2

The laser-induced fluorescence (LIF) excitation spectrum of cyclohexoxy radical has been measured for the first time. The dominant vibrational progression is consistent with computations, at CIS/6-31+G(d), of the C−O stretch frequency of the axial conformer of cyclohexoxy radical. LIF intensity was used as a probe in direct kinetic studies of the reaction of cyclohexoxy radicals with O2. The Arrhenius expression obtained was k O2 = (5.8 ± 2.3) × 10-12 exp[(−14.3 ± 0.8) kJ/mol/RT] cm3 molecule-1 s-1 (225−302 K), independent of pressure in the range 50−125 Torr. The room temperature rate constant for this reaction is a factor of 2 higher than the commonly recommended value, but the observed activation energy is 9 times larger than the recommended value of 1.6 kJ/mol. Combining our results with the ratio of rate constants, k O2/k scission, measured in chamber experiments, we obtained an Arrhenius expression for k scission, the rate constant for β C−C scission of cyclohexoxy radical. However, the resulting Arrhenius preexponential factor of 4.5 × 1015 s-1 is unreasonably high compared with the value of ∼2 × 1013 s-1 obtained in our RRKM/Master Equation calculations as well as in calculations and experiments reported for other alkoxy radicals. The apparent discrepancy is resolved by examining the uncertainties in the values of k scission and the limited temperature range spanned by the relative rate experiments. A part of the discrepancy might also be explained by the observation that the O2 rate constant measured here is only for a single conformer of cyclohexoxy radical, whereas the relative rate experiments represent some averaging over both conformers.