Ligand ENDOR studies of Cu(II)-doped l-alanine single crystals. The nuclear quadrupole coupling tensor and the hyperfine coupling tensor in ligand molecules

Abstract

The electron-nuclear double-resonance technique was applied to Cu(II)-doped l-alanine single crystals at 4.2 K. Analysis of ENDOR spectra allows us to determine not only the hyperfine coupling tensor, but also the nuclear quadrupole coupling tensor and the relative sign between them for nitrogen in such a paramagnetic system. The hyperfine coupling tensor; the nuclear quadrupole coupling constant, e 2 Qq; and the asymmetry parameter, η, for nitrogen in l-alanine ligand closer to Cu(II) were determined, and these values are found to be different from those determined by NQR in pure polycrystalline l-alanine. Simple theoretical calculation for the quadrupole coupling constant was performed by using the Townes and Dailey approximation for this system. The results indicate that the electronic state of nitrogen in l-alanine ligand has sp 2 hybrid orbitals rather than spa hybrid orbitals. Four proton ENDOR signals were assigned to be three amino protons and one α proton and the hyperfine coupling tensors were determined. It was found that the two amino protons and one a proton belong to the l-alanine ligand closer to Cu(II) and a fourth proton belongs to the distant l-alanine ligand.