Ligand endor study of Cu(II)-doped l-histidine deuterochloride monodeuterohydrate single crystals at 4.2 K

Electron-nuclear double resonance (ENDOR) spectroscopy has been applied to Cu(II)-doped l -histidine deuterochloride monodeuterohydrate single crystals at 4.2 K. Detailed analysis of the ENDOR spectra allowed the determination of the principal values and direction cosines of the hyperfine coupling, and nuclear quadropole coupling tensors for two 14 N and the 35 Cl nuclei. Also, we evaluated the asymmetry parameters η for those nuclei. The assignment of the tensors to the molecular frame enabled us to identify one of the nitrogen nuclei, N1, as belonging to the amino group, and N3 as one of the imidazole nitrogen atoms. The analysis of the data also shows that the Cu(II) site has octahedral symmetry and it possesses C 2 symmetry along the C(I)Cu(II)Cl(II) axis. Theoretical calculations of the two 14 N and the 35 Cl quadrupole coupling constants, and the charge distributions in the bonds, were performed using the Townes-Dailey approximation. In addition we calculated theoretical values for the components of the g tensor using the method introduced by Lin (J. Mag. Reson., 68 (1986) 146) in which the complete energy matrix including, the ligand field, electron—electron repulsion, and spin—orbit coupling are simultaneously diagonalized. To evaluate the ligand-field matrix elements, the angular overlap model (AOM) was used. The theoretical calculations are in excellent agreement with the experimental values.