Spin density distribution of a-nitronyl aminoxyl radicals from experimental and ab initio calculated ESR isotropic hyperfine coupling constants

Isotropic ESR spectra have been determined for the following alpha -nitronyl aminoxyl radicals showing different substituents at the 2-position of the imidazolyl ring: H-1 (1); H-2 (2); 3',5'-(t-C4H9)2-4'-(HO)C6H2 (3); 4'-HOC6H4 (4); 3',5'-(HO)(2)C6H3 (5); 3'-HOC6H4 (6); 3',4'-(HO)(2)C6H3 (7); C6H5 (8); 4'-NO2C6H4 (9); 2'-HOC6H4 (10); 2',4'-(HO)(2)C6H3 (11); and 2'-ClC6H4 (12). Solvent dependence in a large variety of solvents of the isotropic ESR hyperfine coupling constants (hfcc's) for 1, 4, 6, 8, 9, 10, and 12 has been studied for the first time by the linear solvation energy relationship (LSER) methodology. From this study, the most important solvent-solute interactions governing the spin density distribution in these radicals as well as the estimates of their hyperfine coupling constants in the absence of any significant solvent-solute interaction have been determined. Such solvent-independent hyperfine coupling constants are the expected values in gas phase and, therefore, they are used to evaluate the theoretically calculated hfcc's, at the DFT/B3LYP/EPR-II level, to validate the level of precision of this theoretical method. It is found that ab initio calculations reproduce the order of magnitude and the trends of the solvent-independent hfcc's. Ab initio calculations also reproduce the main features of the atomic spin populations and the spin density maps experimentally found in the solid state by polarized neutron diffraction for radicals 8 and 9.