SINGLE-CRYSTAL ELECTRON-SPIN ECHO ENVELOPE MODULATION STUDY OF CU(II)-DOPED ZINC BIS(1,2-DIMETHYLIMIDAZOLE) DICHLORIDE

Single crystals of copper-doped zinc bis(1,2-dimethylimidazole) dichloride were studied with electron spin-echo envelope modulation (ESEEM) spectroscopy. Modulation frequencies apparent through Fourier transformation of the ESEEM decay curves were attributed to interactions between the Cu(II) unpaired electron and remote N-14 nuclei of the two coordinated dimethylimidazole ligands. Hyperfine and quadrupole coupling tensors were fit to the observed magnetic field angular dependencies of these frequencies according to a least-squares criterion. The two remote nitrogen atoms, although formally magnetically distinct, give essentially equivalent coupling tensor principal values. This agrees with the noncrystallographic, 2-fold rotation relationship between the two dimethylimidazole rings around the metal ion in the host structure. The isotropic part of the N-14 hyperfine tensor was found to be smaller than reported in other model imidazole compounds previously studied by ESEEM spectroscopy in frozen solutions. This trend is consistent with the change in geometry of ligands around the copper from presumed approximate square planar in frozen solution to near tetrahedral in the present system. The maximum hyperfine principal direction is not along the Cu(II)-N(remote) vector but subtends an angle of approximately 40-degrees from this direction. A simple multipoint electron spin-nucleus dipolar hyperfine calculation failed to reconcile this difference. This result and others suggest that a nonnegligible contribution to the hyperfine coupling originates from spin density located on the imidazole ring. The quadrupole tensor principal directions, on the other hand, correlate with the local geometry about the remote nitrogen. In addition, the quadrupole principal values were found to be similar to those reported previously for solid N-benzylimidazole.