Electron spin resonance and molecular oribtal study of one-electron-reduced O,O '-diphenylenehalonium cations: First evidence for a diaryliodine radical, Ar2I center dot or simply a new sigma*-radical?

One-electron reduction of o,o'-diphenylenebromonium (DPB) and o,o'-diphenyl eneiodonium (DPI) cations in low-temperature glasses produces free radical intermediates whose halogen hyperfine couplings suggest significant spin de nsities on bromine (0.13) and iodine (0.30). An adequate theoretical descri ption of these species has been obtained at both semiempirical (PM3) and de nsity functional levels of theory. These calculations show these species ar e a planar conformation of the 2-halobiphenyl-2'-yl radicals, stabilized th rough intramolecular three-electron (or sigma*) carbon-halogen bonding. The ory 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 i odine 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 sigm a* 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-reduc ed diphenylenehalonium derivatives were not found to undergo intramolecular free radical addition leading to a cyclohexadienyl-type adduct. This resul t is supported by theoretical calculations indicating that such a process w ould be endothermic by 13.9 kcal mol(-1) at the ROMP2/6-31G* level.