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

Abstract

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.

Jet-cooled laser spectroscopy of the cyclohexoxy radical

“…It is also worth comparing our jet-cooled spectrum with the near ambient spectra of Zhang et al 16 They assign a progression (in their paper denoted by A) involving 4 transitions to the CO stretch in the B state with a 1-0 interval of 689 cm −1 . This is consistent with our band F at 688 cm −1 in Table 7, and while Table 7 does not extend into the higher frequency region, additional members of the progression can easily be identified in the broad scan of Fig.…”

“…Fig. 2 expands the scale of the low frequency end of the LIF spectrum, upon which this paper concentrates, and shows it under three temperature conditions, hard expansion (bottom), moderate expansion (middle), and a spectrum (top) taken in a flow tube at about 226 K by Zhang, et al 15,16 In addition a number of bands have been recorded with higher resolution, yielding partially resolved rotational structure.…”

“…Direct spectral observation of cyclohexoxy would provide both information about its structure and a convenient diagnostic for its monitoring. Recently, Zhang et al 15,16 demonstrated these advantages by observing the laser-induced fluorescence (LIF) excitation spectrum of cyclohexoxy in a near-to-ambienttemperature (225-300 K) cell and used the LIF intensity as a probe in the kinetic study of the radical's reaction with O 2 . While a very significant experiment, the relatively low resolution and congestion of the observed spectrum leaves many questions unanswered about cyclohexoxy's spectroscopy and it's use as a diagnostic.…”

Mechanism of the Aerobic Oxidation of α‐Pinene

“…A likely mechanism for Q (a) =O formation from R (a) ÀO · is the abstraction of the weakly bonded aH atom by O 2 . The analogous reaction of O 2 with cyclohexoxyl radicals has been kinetically characterized in the range 225-302 K; [34] extrapolation of the rate constant to 363 K predicts a value of 3 10 7 m À1 s À1 . Given an O 2 concentration of approximately 35 mm, the pseudo-first-order rate constant can be estimated at ca.…”

Laster spectroscopic investigations of organic alkoxy and peroxy radicals.

“…The cyclohexoxy radical and its methyl substitutions are important reaction intermediates in combustion and atmospheric chemistry. 148,149 Spectroscopic investigations can serve as diagnostics of these free radicals in kinetic studies, 36,37,150,151 and provide information about their structure and dynamics. Previously, laser-induced fluorescence (LIF) 37 and dispersed fluorescence (DF) 93 spectra of 2-, 3-, and 4-methylcyclohexoxy (MCHO) radicals have been obtained under jet-cooled conditions.…”