Chemistry of the Cyclopentoxy and Cyclohexoxy Radicals at Subambient Temperatures

The Cl-atom initiated oxidation mechanisms of both cyclopentane and cyclohexane have been studied as a function of temperature using an environmental chamber/FTIR technique. The oxidation of cyclohexane leads to the formation of the cyclohexoxy radical, the chemistry of which is characterized by a competition between ring-opening (R5) and reaction with O 2 (R6) to form cyclohexanone. The yield of cyclohexanone is shown to increase with decreasing temperature, and a rate coefficient ratio k 6 / k 5 = (1.3 ± 0.3) × 10 -27 exp(5550 ± 1100/ T ) cm 3 molecule -1 is obtained. The energy barrier to ring-opening is estimated to be 11.5 ± 2.2 kcal/mol. The dominant fate of the cyclopentoxy radical, formed in the Cl-atom initiated oxidation of cyclopentane, is ring-opening under all conditions studied here (230−300 K, 50−500 Torr O 2 ), with only a minor contribution from the O 2 reaction at the lowest temperatures studied. The barrier to ring-opening for the cyclopentoxy radical is probably less than 10 kcal/mol.