Rate constants for the title reactions have been measured over a range of temperatures by using laser flash photolytic techniques. At 300 K the rate constants are 10.8, 2.01, 2.34, 1.35, 2.47, and 1.84 × 106 M-1 s-1 for methyl, ethyl, n-butyl, isopropyl, cyclohexyl, and tert-butyl, respectively. The Arrhenius preexponential factors range from 109.39 M-1 s-1 for methyl to 108.43 M-1 s-1 for tert-butyl and activation energies from 3.97 kcal/mol for cyclohexyl to 2.95 kcal/mol for tert-butyl. The rate constant (k10) for the 1,5-cyclization of 5-hexenyl radical to cyclopentylmethyl radical can be represented by log k10/s-1 = (10.37 ± 0.32) - (6.85 ± 0.42)/θ, where θ = 2.3RT kcal/mol. © 1981 American Chemical Society.