ELECTROCHEMISTRY OF ELECTRON-TRANSFER PROBES - OBSERVATION OF THE TRANSITION FROM ACTIVATION TO COUNTERDIFFUSION CONTROL IN THE FRAGMENTATION OF ALPHA-ARYLOXYACETOPHENONE RADICAL-ANIONS

The cleavage of radical anions of substituted alpha-phenoxyacetophenon es, X-C6H4COCH2OPh, IIa-k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E degrees) for forma tion of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with dig ital simulation and previously reported laser flash photolysis data. T he rate constants cover a range of almost eight orders of magnitude (0 .4 s(-1) for X = p-MeCO- to 1.3.10(7) s(-1) for X = p-MeO-). The relat ive driving forces, Delta Delta G degrees(het)(RX.-), for the heteroly tic cleavage of the radical anions (to give R-. + X-) were estimated f rom thermochemical cycles. A combined plot of log(k) versus Delta Delt a G(het)degrees(RX.-) for the radical anions of IIa-k and of alpha-ary loxyacetophenones gave a curve with alpha = 0.5 at high driving forces and alpha = 1 at low driving forces, where alpha = partial derivative Delta G(0)(double dagger)/partial derivative Delta G degrees. The plo t was analyzed using a model in which reversible cleavage of the radic al anions takes place inside the solvent cage followed by (counter)dif fusion of the fragments out of the solvent cage. The change in the val ue of a is interpreted as a change in the rate limiting process from c hemical activation (i.e.,, fragmentation) to counterdiffusion. The mod el allowed the determination of the absolute values of Delta G(het)deg rees(RX.-) and the intrinsic barrier, Delta G(0)(double dagger), for t he fragmentation of the radical anions (8 +/- 1 kcal mol(-1), 0.35 eV) . This leads to an estimate of the homolytic bond dissociation free en ergy of the C-OPh bond in the unsubstituted alpha-phenoxyacetophenone.