Hyperfine-axis orientation in rhombic metal complexes from ENDOR-induced EPR and quadrupolar coupling determination

Abstract

The ENDOR resonances from ligand nuclei in paramagnetic transition-metal complexes exhibit a shift from the nuclear Larmor frequency because of the dipolar and Fermi contact interactions with the unpaired electron. The dipolar interaction has orientation dependence associated with its magnitude, where the position of the nuclei with respect to the metal atom determines the resonant frequencies and spectral lineshape. In this paper effects of rhombic g and metal hyperfine tensors on the EPR powder pattern and ENDOR spectra are illustrated. The EPR angular selection is modified from that encountered in axial systems and the technique of ENDOR-induced EPR is utilized to elucidate the orientation of magnetic axes with respect to one another. Quadrupole couplings in ENDOR spectra have often been analyzed using an equation from which cross terms have been omitted. The cross terms make important contributions to the spectra in some cases and the effect of these terms has been examined in order to determine when the complete equation for the quadrupole interaction must be utilized to analyze correctly ENDOR spectra.