Ligand ENDOR spectra of Cu(II) impurity complexes in α-glycine crystals

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Abstract
Symmetrical Cu(II) impurity complexes in α‐glycine crystals have been studied with ENDOR of ligand proton and 14N nuclei at 4.2° K. It has been found that the inversion symmetry in Cu(gly)2 complexes is significant in interpreting the ENDOR spectra showing additional splittings. We propose that a pair of equivalent nuclear spins in the complex are weakly coupled via their hyperfine interactions, resulting in the observed splittings. The 14N nuclear quadrupole tensors have also exhibited the symmetry of the complex, reflecting a coupling between the identical spins. The ENDOR analysis of close protons have revealed that two amino protons and one methylene proton make a predominant contribution to the anisotropic EPR linewidths. The results indicate that the ligand amino groups are deprotonated, and the proton vacancies are localized consistent with the complex symmetry, and responsible for the electric field gradient symmetry at the 14N nuclei.