EPR and ligand-ENDOR measurements of Cu ( i i ) in l -alanine single crystals

EPR experiments have been performed in l-alanine single crystals doped with copper in order to measure the gyromagnetic tensor of Cu(ii) and the hyperfine coupling tensor with 63Cu. The hyperfine coupling and the nuclear quadrupole interaction tensors of a ligand nitrogen have been obtained by ENDOR at 4 K. The principal values of the hyperfine coupling of 14N are: AN(1)=27 190 KHz, AN(2)=27 560 KHz, and AN(3)=41 740 KHz and those for the nuclear quadrupole interaction are PN(1)=97 KHz, PN(2)=987 KHz and PN(3)=−1084 KHz. The principal directions of the four tensors evaluated in this work are noncoincident. ENDOR resonances of another nitrogen ligand and at least three protons have been detected. Our ENDOR measurements show a pseudonuclear spin–spin interaction between 63Cu and 14N nuclei. This effect is very large and arises from a high order mechanism involving the hyperfine couplings of the unpaired electrons of Cu(ii) with those nuclei. The data are explained with a second order perturbation calculation and it is shown that these effects should be considered in systems with low symmetry where the electrons are coupled to more than one magnetic nuclei.