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 sigma*) 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 sigma*-bond. In the case of 3-nitrosubstituted DPI (NDPI) the one-electron-reduced intermediate was observed both as a sigma*-radical (in polar glasses) and as a,pi*-radical (when intercalated into DNA). Calculations suggest that the change from sigma* to pi* on intercalation into DNA is driven both by electric field of the DNA backbone and by pi-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.