Nitrogen hyperfine splitting of nitroxide solutions: Differently structured and charged nitroxides as probes of environmental properties

Abstract

AbstractThe dependence of the nitrogen isotropic coupling constants, aN, on the structure and charge of the radicals, temperature and solvent polarity was analysed for solutions containing the following nitroxides: 4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (Tempol); 4‐trimethylammonium‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl, iodide salt (TempTMA+); 2,2,5,5‐tetramethylpyrrolin‐1‐oxyl‐3‐carboxamide (Tempyo); and 2,2,5,5‐tetramethylpyrrolin‐1‐oxyl‐3‐carboxylate, sodium salt (Tempyca−). Polar hydrogen bond acceptor solvents and hydroxylic solvents were used. In each solvent used, Tempol and Tempyca− showed aN values which were higher, respectively, than those of TempTMA+ and Tempyo due to inductive, structural and charge effects. Such effects were analysed on the basis of the proton and 13C coupling constants. Non‐hydroxylic solvents gave temperature coefficients for aN that were almost independent of solvent polarity. Lower daN/dT values were found for charged radicals. A gradual decrease of temperature coefficients with decreasing hydrogen bond donor (HBD) properties were related to free volume changes of radical molecules. The analysis of the aN dependence on polarity parameters gave the best fitting using a simplified form of the Wertheim reaction field. Tempyca− was greatly affected by changes in polarity. A parameter to account for HBD abilities was also analysed.