Solvent effects of 1,4-benzoquinone and its anion radicals probed by resonance Raman and absorption spectra and their correlation with redox potentials

Abstract

Resonance Raman (RR) and absorption spectra and cyclic voltammograms were measured for 1,4-benzoquinone (BQ) and its anion radical (BQ-•) in various solvents. The absorption maxima of BQ-• around 440 and 320 nm exhibit a good linear correlation both with acceptor numbers (AN) of solvents and the standard reduction potentials of BQ. The wavenumbers of ν8a (Wilson’s notation, C=C stretch) and ν9a (C–H in-plane deformation) of BQ show linear correlations with the polarizabilities and dielectric constants of solvents, respectively. The ν8a and ν7a (C=O stretch) wavenumbers of BQ-• can be linearly correlated with solvent AN. In addition, the standard reduction potentials of BQ also show good linear correlations with both solvent AN and wavenumbers of ν8a of BQ-•. These results cannot be interpreted by simple hydrogen bonding interactions of carbonyl oxygen with solvent but rather suggest that the π-electrons of BQ-• and BQ serve as an H-bond acceptor, forming novel H-bonds with the solvent. The magnitude of the novel H-bond interaction is estimated to be ca. 7 and ca. 3.5 kcal mol-1 or less for BQ-• and BQ in water, respectively. © 1998 John Wiley & Sons, Ltd.