Environmental effects on phenoxyl free radical spin densities and hyperfine couplings

The effect of the environment, as modelled by hydrogen bonding, ion-pairing and/or general continuum model effects, is investigated for the phenoxyl free radical. All are shown to lead to a redistribution of spin density from the phenoxyl O atom to the C_ipso position. Isotropic and anisotropic hyperfine couplings are calculated at the B3LYP level of theory confirming this trend. Introduction of the continuum model has a significant effect the Na-O bond length of the ion-pair model significantly altering its calculated EPR properties in comparison with gas-phase values. The trends identified are of immediate significance for biological environmental effects on tyrosyl free radicals.