An EPR and NMR study of some tetramethylisoindolin-2-yloxyl free radicals

Abstract

Isoindolin-2-yloxyls are stable free radicals whose potential as spin probes/labels has not yet been exploited. We have prepared 1,1,3,3-tetramethylisoindoline, its 5-nitro derivative (paving the way to adding prosthetic groups) and a version with fully deuteriated methyl groups. These isoindolines have been oxidized to give the corresponding free radicals. It was found that the isotropic EPR spectra of the radicals have linewidths much smaller than those of the TEMPO family of radicals. The parent radical has a spectrum characterized by hyperfine interactions from nitrogen and from protons of the methyl groups. By measuring the NMR chemical shift differences between the isoindolines and the isoindolinyloxyls, values and signs of hyperfine splittings have been determined for the aromatic protons (too small to measure even in well resolved EPR spectra) and for some of the carbon-13 nuclei (at natural abundance). An important advantage of the NMR method is that assignments can be made with near certainty. This advantage has been taken further by obtaining ¹H–¹³C chemical shift correlation spectra of the free radicals.

For spin probe measurements, accurate values of the anisotropic g-factors and nitrogen hyperfine interactions are needed: these have been derived from the EPR powder spectrum of the deuteriated radical.