TY - JOUR

T1 - Electrochemistry of electron transfer probes. Observation of the transition from activation to counterdiffusion control in the fragmentation of α-aryloxyacetophenone radical anions

AU - Andersen, Mogens L.

AU - Long, William

AU - Wayner, Danial D.M.

PY - 1997/7/16

Y1 - 1997/7/16

N2 - The cleavage of radical anions of substituted α-phenoxyacetophenones, X-C6H4COCH2OPh, IIa-k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E°) for formation of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with digital simulation and previously reported laser flash photolysis data. The rate constants cover a range of almost eight orders of magnitude (0.4 s-1 for X = p-MeCO- to 1.3·107 s-1 for X = p-MeO-). The relative driving forces, ΔΔG°(het)(RX.-), for the heterolytic cleavage of the radical anions (to give R. + X-) were estimated from thermochemical cycles. A combined plot of log(k) versus ΔΔG°(het)(RX.-) for the radical anions of IIa-k and of α-aryloxyacetophenones gave a curve with α = 0.5 at high driving forces and α = 1 at low driving forces, where α = δΔG0(paragraph)/δΔG°. The plot was analyzed using a model in which reversible cleavage of the radical anions takes place inside the solvent cage followed by (counter)diffusion of the fragments out of the solvent cage. The change in the value of α is interpreted as a change in the rate limiting process from chemical activation (i.e., fragmentation) to counterdiffusion. The model allowed the determination of the absolute values of ΔG°(het)(RX.-) and the intrinsic barrier, ΔG0(paragraph), for the fragmentation of the radical anions (8 ± 1 kcal mol-1, 0.35 eV). This leads to an estimate of the homolytic bond dissociation free energy of the C-OPh bond in the unsubstituted α-phenoxyacetophenone.

AB - The cleavage of radical anions of substituted α-phenoxyacetophenones, X-C6H4COCH2OPh, IIa-k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E°) for formation of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with digital simulation and previously reported laser flash photolysis data. The rate constants cover a range of almost eight orders of magnitude (0.4 s-1 for X = p-MeCO- to 1.3·107 s-1 for X = p-MeO-). The relative driving forces, ΔΔG°(het)(RX.-), for the heterolytic cleavage of the radical anions (to give R. + X-) were estimated from thermochemical cycles. A combined plot of log(k) versus ΔΔG°(het)(RX.-) for the radical anions of IIa-k and of α-aryloxyacetophenones gave a curve with α = 0.5 at high driving forces and α = 1 at low driving forces, where α = δΔG0(paragraph)/δΔG°. The plot was analyzed using a model in which reversible cleavage of the radical anions takes place inside the solvent cage followed by (counter)diffusion of the fragments out of the solvent cage. The change in the value of α is interpreted as a change in the rate limiting process from chemical activation (i.e., fragmentation) to counterdiffusion. The model allowed the determination of the absolute values of ΔG°(het)(RX.-) and the intrinsic barrier, ΔG0(paragraph), for the fragmentation of the radical anions (8 ± 1 kcal mol-1, 0.35 eV). This leads to an estimate of the homolytic bond dissociation free energy of the C-OPh bond in the unsubstituted α-phenoxyacetophenone.

UR - http://www.scopus.com/inward/record.url?scp=0030861477&partnerID=8YFLogxK

U2 - 10.1021/ja963374u

DO - 10.1021/ja963374u

M3 - Journal article

AN - SCOPUS:0030861477

VL - 119

SP - 6590

EP - 6595

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 28

ER -