Free electron transfer from several phenols to radical cations of non-polar solvents

Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating diffusion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the effect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.

[1]  Kenneth B. Wiberg,et al.  Solvent effects. 3. Tautomeric equilibria of formamide and 2-pyridone in the gas phase and solution: an ab initio SCRF study , 1992 .

[2]  D. Kahaner,et al.  Acuchem: A computer program for modeling complex chemical reaction systems , 1988 .

[3]  E. Land,et al.  Primary photochemical processes in aromatic molecules. Part 7.—Spectra and kinetics of some phenoxyl derivatives , 1963 .

[4]  W. Naumann,et al.  Kinetics of excitation and charge transfer reactions in non-polar media , 1987 .

[5]  H. Orthner,et al.  Radical Cations of Sterically Hindered Phenols as Intermediates in Radiation-Induced Electron Transfer Processes , 1996 .

[6]  W. Naumann,et al.  Spectral properties and kinetics of cationic transients generated in electron-pulse irradiated liquid alkanes , 1986 .

[7]  O. Brede,et al.  Encounter geometry determines product characteristics of electron transfer from 4-hydroxythiophenol to n-butyl chloride radical cations , 1999 .

[8]  H. Orthner,et al.  Phenol-assisted photolytic and radiolytic radical cation formation of a sterically hindered tertiary amine , 1994 .

[9]  J. Mittal,et al.  TWO CHANNELS OF ELECTRON TRANSFER OBSERVED FOR THE REACTION OF N-BUTYL CHLORIDE PARENT RADICAL CATIONS WITH NAPHTHOLS AND HYDROXYBIPHENYLS , 1998 .

[10]  Kenneth B. Wiberg,et al.  Solvent effects. 1. The mediation of electrostatic effects by solvents , 1991 .

[11]  Jin‐Pei Cheng,et al.  Substituent effects on the stabilities of phenoxyl radicals and the acidities of phenoxyl radical cations , 1991 .

[12]  A. Becke.  Density-functional thermochemistry. III. The role of exact exchange , 1993 .

[13]  Ralf Hermann,et al.  Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches , 2000 .

[14]  E. Land,et al.  Primary photochemical processes in aromatic molecules. Part 6.—The absorption spectra and acidity constants of phenoxyl radicals , 1961 .

[15]  Steen Steenken,et al.  One-electron redox potentials of phenols. Hydroxy- and aminophenols and related compounds of biological interest , 1982 .

[16]  W. T. Dixon,et al.  Determination of the acidity constants of some phenol radical cations by means of electron spin resonance , 1976 .

[17]  E. Land,et al.  Pulse radiolysis studies of aqueous phenol. Water elimination from dihydroxycyclohexadienyl radicals to form phenoxyl , 1967 .

[18]  R. H. Schuler,et al.  Reaction of azide radicals with aromatic compounds. Azide as a selective oxidant , 1985 .