The electronic structure and spin density distribution of peroxyl radicals are investigated by density functional theory (DFT) at the B3LYP level. Results found for superoxide anion and tert-butyl peroxyl radicals at a variety of basis sets suggest that 6-31G is the most appropriate basis set for calculation of hyperfine coupling constants (hfcc's) of carbon-based peroxyl radicals. Calculation of parallel 17O hfcc's [A ∥(17O)] for a series of substituted methyl peroxyl radicals with the 6-31G basis set yielded calculated values with a maximum deviation of 2.2% from experiment. Spin density distributions estimated from experiment A ∥(17O) are compared to theoretical estimates from Mulliken orbital population analysis. Electronegative substitution at the carbon alpha to the peroxyl group results in an increase of terminal oxygen hyperfine coupling and spin density, shortening of C−O, and lengthening of O−O. In cases involving significant steric hindrance, however, C−O bond shortening was prevented. A ∥(17O) values for the terminal peroxyl oxygen atom correlate well with Taft σ* substitutent parameters for the R group in the peroxyl radicals (ROO•). Thiyl peroxyl radicals are reinvestigated using B3LYP for comparison to previous theoretical work at UHF level. This resulted in confirmation that the effect of the addition of an electron pair donor (hydroxide ion) to CH3SOO• is to alter the spin density distribution in the peroxyl group. Structural models of lipid peroxyl radicals show that vinyl peroxyl radicals may be distinguished from saturated, allylic, and ester-based peroxyl radicals on the basis of hyperfine coupling constants.

One-electron reduction of o,o‘-diphenylenebromonium (DPB) and o,o‘-diphenyleneiodonium (DPI) cations in low-temperature glasses produces free radical intermediates whose halogen hyperfine couplings suggest significant spin densities on bromine (0.13) and iodine (0.30). An adequate theoretical description of these species has been obtained at both semiempirical (PM3) and density functional levels of theory. These calculations show these species are a planar conformation of the 2-halobiphenyl-2‘-yl radicals, stabilized through intramolecular three-electron (or σ*) carbon−halogen bonding. Theory also predicts a nonequivalence of the C−X bonds and unsymmetrical spin density distribution over the two C−X bonding carbons. As compared to DPB, the DPI radical gives evidence for more equivalent bonding of the iodine to both carbons, accompanied by lower potential barriers for intramolecular iodine atom migration (1−2 kcal mol-1) along the σ*-bond. In the case of 3-nitrosubstituted DPI (NDPI) the one-electron-reduced intermediate was observed both as a σ*-radical (in polar glasses) and as a π*-radical (when intercalated into DNA). Calculations suggest that the change from σ* to π* on intercalation into DNA is driven both by electric field of the DNA backbone and by π-stacking of NDPI with DNA bases. One-electron-reduced diphenylenehalonium derivatives were not found to undergo intramolecular free radical addition leading to a cyclohexadienyl-type adduct. This result is supported by theoretical calculations indicating that such a process would be endothermic by 13.9 kcal mol-1 at the ROMP2/6-31G* level.