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 -nitrony l 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'-HOC 6H4 (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 ES R hyperfine coupling constants (hfcc's) for 1, 4, 6, 8, 9, 10, and 12 has b een studied for the first time by the linear solvation energy relationship (LSER) methodology. From this study, the most important solvent-solute inte ractions governing the spin density distribution in these radicals as well as the estimates of their hyperfine coupling constants in the absence of an y significant solvent-solute interaction have been determined. Such solvent -independent hyperfine coupling constants are the expected values in gas ph ase 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 o f this theoretical method. It is found that ab initio calculations reproduc e 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 sta te by polarized neutron diffraction for radicals 8 and 9.