RATE CONSTANTS FOR TERMINATION AND TEMPO TRAPPING OF SOME RESONANCE STABILIZED HYDROAROMATIC RADICALS IN THE LIQUID-PHASE

The rate constants for the termination reaction (2k(1)) of some resonance stabilized carbon centered radicals (SR(.)) derived from hydroaromatics (SR(.) + SR(.) --> P) have been determined at 294 +/- 2 K by laser flash photolysis with UV-vis detection. The radicals were generated by hydrogen atom abstraction by t-BuO(.) radicals from the corresponding hydrocarbon (SRH + t-BuO(.) --> SR(.) + t-BuOH, k(4)). The extinction coefficients (epsilon) Of the SR(.), essential to calculate 2k(1), were obtained using a relative kinetic technique. The change in 2k(1) for the radicals derived from 1,4-cyclohexadiene, fluorene, 9,10-dihydroanthracene, diphenylmethane, tetralin, indan, indene, and phenol appeared to be modest; a range of 2k(1) = 2-10 x 10(9) M(-1) s(-1) in mixtures of benzene and di-tert-butyl peroxide was observed. Most of the rate constants are near the diffusion controlled limit. In contrast, quenching the radicals with a persistent radical, 2,2,5,5-tetramethylpiperidin-1-oxyl (TEMPO), resulted in a larger variation of <0.01-23 x 10(7) M(-1) s(-1). The strength of the N-O bond formed in the latter process may have an important contribution to the observed rate constant.