Rate constants for the reactions of isobutyl, neopentyl, cyclopentyl, and cyclohexyl radicals with molecular oxygen

Rate constants have been measured for the reactions of four hydrocarbon radicals with O 2 in the gas phase at room temperature. Laserflash photolysis was used to generate low concentrations of radicals. A photoinization mass spectrometer followed the radical loss as a function of time. The measured pseudo first‐order decay rate of the radical and the absolute oxygen concentration were combined to give the absolute rate constants (in units of 10 −12 cm 3 molec −1 s −1 ): isobutyl (2.9 ± 0.7); neopentyl (1.6 ± 0.3); cyclopentyl (17 ± 3); and cyclohexyl (14 ± 2). The cycloalkyl radicals have rate constants similar to those of other secondary radicals. However, the isobutyl and neopentyl radicals react more slowly than similar primary radicals. These new rate constants are compared in Figure 2 with the recently published correlation of reactive cross section with radical ionization potential.