Enhanced diagnostic EPR and ENDOR spectroscopy of radical spin, adducts of deuterated α-phenyl N-tert-butyl nitrone†

Deuterated analogues of C-phenyl N-tert-butyl nitrone (PBN) were synthesized to provide significant gains in spectral sensitivity and resolution in electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) applications. Three deuterated α-phenyl N-tert-butyl nitrones (PBNs) were prepared. EPR spectra of the corresponding radical spin adducts with the phenyl ring (PBN-d5-R˙) or tert-butyl moiety (PBN-d9-R˙) deuterated were found to enhance disclosure of the structure of the added radical. The most dramatic increases in EPR resolution, however, were not realized until both the phenyl and tert-butyl groups were deuterated (PBN-d14-R˙). Here, baseline resolution of unique long-range (e.g. γ- and δ-) hyperfine splittings from the radical addend could be displayed. Representative radical spin adducts of PBN-d14 (methyl, hydroxyl, aminyl, cyanyl, carbamoyl, and vinyl) were prepared and compared with those of PBN to illustrate this point. It is also shown that when even higher spin adduct resolution is desired the combination of spin trap deuteration and ENDOR may be applied to advantage.