Adjusting the top end of the alkyl radical kinetic scale. Laser flash photolysis calibrations of fast radical clocks and rate constants for reactionsof benzeneselenol

Rate constants for 5-exo cyclizations of the 6,6-diphenyl-5-hexenyl radical (la), the 1-methyl-6,6-diphenyl-5-hexenyl radical (Ib), and the 1,1-dimeth yl-6,6-diphenyl-5-hexenyl radical (Ic) were measured by laser flash photoly sis methods, and Arrhenius parameters for these cyclizations were determine d. Relative rate constants for cyclizations of radicals 1 and reactions wit h benzeneselenol were determined by indirect kinetic methods, and the relat ive Arrhenius parameters for the competing reactions were combined with the parameters for the cyclization reactions to give absolute Arrhenius parame ters for the PhSeH reactions. At 20 degrees C, PhSeH reacts with the 1 degr ees, 2 degrees, and 3 degrees radicals 1 with nearly the same rate constant s, (1.2 +/- 0.1) x 10(9) M-1 s(-1). Absolute Arrhenius parameters for react ions of PhSH and t-BuSH with the primary alkyl radical la were calculated u sing literature values for the competition between cyclization of la and re actions with the appropriate thiol and the absolute values for cyclization of la determined in this work. The results suggest that rate constants for reactions of primary alkyl radicals with t-BuSH are about 20% smaller than those previously reported. In the case of PhSH, the results are in good agr eement with one previously reported set of rate constants but about 35% sma ller than another set of rate constants that was subsequently incorporated into fast alkyl radical kinetics. The rate constants for alkyl radical reac tions calibrated by competition against reaction with PhSeH and PhSH appare ntly are 30-40% smaller than those previously reported, and the derived rat e constants for the fast radical reactions should be adjusted. An especiall y noteworthy example is ring opening of the cyclopropylcarbinyl radical, th e Arrhenius function for which was determined in part from PI ISH trapping results. Using the adjusted rate constants for PhSH and recalculating the A rrhenius parameters for the cyclopropylcarbinyl radical ring opening gives log(k/s(-1)) = (13.04(5) +/- 0.10) - (6.99 +/- 0.09)/theta (kcal/mol, error s at 2 sigma); the rate constant at 20 degrees C of 6.7 x 10(7) s(-1) is ab out 13% smaller than that previously calculated.