Ligand ENDOR of Cu-8-Hydroxyquinolinate Substituted into Organic Single Crystals

The technique of electron–nuclear double resonance (ENDOR) has been applied to copper‐8‐hydroxyquinolinate substituted into single crystals of phthalimide and of 8‐hydroxyquinoline. This method has permitted determination of the nitrogen hyperfine tensor, with sufficient accuracy to detect small orthorhombic deviations from axial symmetry, and the nitrogen quadrupole tensor. These tensors have the same principal axes and the coupling constants of the two interactions have opposite sign. The axis corresponding to the largest quadrupole splitting is oriented along the nitrogen lone pair in the plane of the metal complex. One $g$ axis of the slightly monoclinic copper‐8‐hydroxyquinolinate complex is oriented approximately along the oxygen–copper axis, and the other in‐plane $g$ axis is at a small angle with respect to the nitrogen–copper direction. The delocalization of the unpaired electron density to the ligand protons depends on the host lattice. The quadrupole tensor and its dependence on the host lattice are discussed. It is shown that the unpaired electron in the aromatic rings of 8‐hydroxyquinoline resides mainly in the $\sigma$ system, in accordance with the symmetry of the ground state. The isotropic and anisotropic coupling constants of the proton adjacent to the nitrogen and some coupling components of other protons in the aromatic ligands and in the neighbouring molecules of the host lattice have been determined. The spin densities in the aromatic rings are positive.