Laser-induced fluorescence spectra of 4-methylcyclohexoxy radical and perdeuterated cyclohexoxy radical and direct of Automated Methods and Management in Chemistry 7 kinetic studies of their reactions with O2

The laser-induced fluorescence (LIF) excitation spectra of the 4-methylcyclohexoxy and d11-cyclohexoxy radicals have been measured for the first time. LIF intensity was used as a probe in direct kinetic studies of the reaction of O2 with trans-4-methylcyclohexoxy and d11-cyclohexoxy radicals from 228 to 301 K. Measured rate constants near room temperature are uniformly higher than the Arrhenius fit to the lower-temperature data, which can be explained by the regeneration of cyclic alkoxy radicals from the product of their â-scission and the effect of O2 concentration on the extent of regeneration. The Arrhenius expressions obtained over more limited ranges were kO2) (1.4-1+8) 10-13 exp[(-810 ( 400)/T] cm3 molecule-1 s-1 for trans-4-methylcyclohexoxy (228-292 K) and kO2) (3.7-1+4) 10-14 exp)[(-760 ( 400) /T] cm3 molecule-1 s-1 for d11-cyclohexoxy (228-267 K) independent of pressure in the range 50-90 Torr. The room-temperature rate constant for the reaction of trans-4-methylcyclohexoxy radical with O2 (obtained from the Arrhenius fit) is consistent with the commonly recommended value, but the observed activation energy is 3 times larger than the recommended value of 0.4 kcal/mol and half the value previously found for the reaction of normal cyclohexoxy radical with O2