EPR, ENDOR, TRIPLE resonance spectroscopy and MO studies on radical anion, neutral radical and radical cation of vitamin K3 in a selection of solvents

Abstract

All three radical species (radical anion, neutral radical and radical cation) from 2-methyl-1,4-naphthoquinone (vitamin K₃) were prepared in liquid solutions and measured by EPR and ENDOR/TRIPLE resonance spectroscopy. The methyl group protons showed the largest variation of isotropic hyperfine coupling, from 8.3 MHz (anion) to 19.1 MHz (neutral) to 9.3 MHz (cation). Isotropic hyperfine couplings of a proton in the quinoid ring and the methyl group protons in different solutions were shown to be dependent on the relative permittivity of the solvent. A prominent alternating linewidth effect was detected in the EPR spectra of the radical anion of 2-methyl-1,4-naphthoquinone at different concentrations and at different temperatures in 1,2-dimethoxyethane. This was explained as being due to the counter-cation fluctuation between the two quinodic oxygens. A solid dipotassium radical salt of vitamin K₃ was prepared and its EPR spectra measured. Isotropic hyperfine coupling constants were also calculated for all three radical species using the UB3LYP density functional method.