Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase

Article abstract of DOI:10.1021/j100020a047

The rate constants for the termination reaction (2k₁) 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. k₄).The extinction coefficients (e) of the SR^., essential to calculate 2k₁, were obtained using a relative kinetic technique.The change in 2k₁ for the radicals derived from 1,4-cyclohexadiene, fluorine, 9,10-dihydroanthracene, diphenylmethane, tetralin, indan, indene, and phenol appeared to be modest; a range of 2k₁ = 2-10 x 1E9 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 1E7 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.