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 O-2. The Arrhenius expression obtained was k(O2) = (5.8 +/- 2.3) x 10(-12) exp[(-14.3 +/- 0.8) kJ/mol/RT] cm(3) 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 beta C-C scission of cyclohexoxy radical. However, the resulting Arrhenius preexponential factor of 4.5 x 10(15) s(-1) is unreasonably high compared with the value of similar to2 x 10(13) 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 O-2 rate constant measured here is only for a single conformer of cyclohexoxy radical, whereas the relative rate experiments represent some averaging over both conformers.