No abstract

Isotropic ESR spectra have been determined for the following α-nitronyl aminoxyl radicals showing different substituents at the 2-position of the imidazolyl ring:  1H (1); 2H (2); 3‘,5‘-(t-C4H9)2-4‘-(HO)C6H2 (3); 4‘-HOC6H4 (4); 3‘,5‘-(HO)2C6H3 (5); 3‘-HOC6H4 (6); 3‘,4‘-(HO)2C6H3 (7); C6H5 (8); 4‘-NO2C6H4 (9); 2‘-HOC6H4 (10); 2‘,4‘-(HO)2C6H3 (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.

The variation of the number and position of OH substituents a t the phenyl ring of a-phenyl nitronyl nitroxide radicals yields different hydrogen-bonded molecular self-assem blies with distinct dimensionalities; the mono-ortho-su bstituted isomer is most remarkable since it shows a three-dimensional network of weak hydrogen bonds exhibiting a bulk ferromagnetic transition at 0.45 K.

An extensive investigation of the conformations adopted by the family of phenyl a-nitronyl nitroxides has been carried out. A database containing 110 crystal structures was used in a statistical study of the solid-state geometries and conformations of these radicals. This study revealed that the favoured conformations involve a twisted distortion in the imidazolyl rings and a twist between the aromatic and heterocyclic rings in the molecules. As a consequence, these radicals show two types of preferred conformations in the solid state: the pseudo-anti enantiomeric pair and the pseudo-eclipsed pair, the latter type being the most statistically probable. A new chiral member of this group of radicals that bears a lactate moiety, (R)-1, and its corresponding racemic compound, (R,S)-1, have been prepared in order to study the influence of chiral induction from the stereogenic centre on the torsion angle between the aromatic and heterocyclic rings of the anitronyl nitroxides. The X-ray crystal structures of the enantiopure and race-mic compounds, which both reveal chains of molecules sustained by strong OÀH´´´O hydrogen bonds between the carboxylic acid group and the ON group of the radical in the solid, as well as their magnetic properties have been determined. Remarkably, the molecules with a given stereogenic centre have a single helical sense between their component rings, even in the racemic crystal. Chiral induction from the stereogenic centre to the radical unit has also been proved by CD spectroscopy in the solid state. The results of these experiments have been rationalised by ab initio calculations of the spectra.