An EPR/ENDOR study of aminoxyls (nitroxides) capable of intramolecular bonding: hydroxyalkyl radical spin adducts of nitrones

Abstract

A comparison of the effect of solvent and 13 C labeling (of the radical addend) on the EPR (electron paramagnetic resonance) spectra of hydroxyalkyl vs. alkyl radical adducts of α-phenyl-N-tert-butyl nitrone (PBN) and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) has been investigated. The solution ENDOR spectra in toluene and in ethanol are the first examples studied of aminoxyls with hydroxyl substituents in close proximity to the free radical centre. Diastereomeric mixtures of hydroxyalkyl radical adducts are clearly resolved by ENDOR spectroscopy. Conformations of these radicals have been assigned based on the 1 H and 13 C hyperfine splittings (HFS's) and by differential optimum ENDOR temperatures. Definitive assignments of carbon-centered radical adducts of DMPO and PBN are shown to be feasible by monitoring the β- 13 C HFS's of the radical addend. Long-range γ-H HFS's from the added radical group can be observed when the deuterated spin trap PBN-d 14 is used. The production of 13 C labeled hydroxyalkyl adducts of nitrones (e.g., DMPO– 13 CH 2 OH, from the reaction of hydroxyl radicals with added 13 CH 3 OH) is shown to be useful as an improved EPR spectroscopic method for the verification of the presence of hydroxyl radicals.