ELECTRON-TRANSFER QUENCHING OF EXCITED DIPHENYLMETHYL RADICALS

The intermolecular reactivity of the excited diphenylmethyl radical (DPM*) has been studied with particular emphasis on electron-transfer reactions. These studies allow the determination of the rate constants for the reaction of DPM* with electron acceptors. For example, carbon tetrachloride, methyl benzoate, and benzyl bromide quench DPM* with rate constants of 3.3 x 10(8), 1 x 10(6), and 3.6 x 10(6) M-1 s-1 in acetonitrile. The corresponding carbocation is an observable product of the reaction, leaving no doubt that the reaction involves electron transfer. A kinetic salt effect is observed for the reaction of DPM* with carbon tetrachloride, where the carbocation yield increases from ca. 61% to ca. 100% with the addition of small concentrations of tetrabutylammonium perchlorate. The lower limits to the rates of back electron transfer (BET) and ion pair escape (ESC) for the product cation and radical anions in acetonitrile solution have been estimated using reductive dehalogenation of aryl halides as clock reactions where the rates of fragmentation have been estimated.