ANGLE-SELECTED N-14-ENDOR STUDY OF COPPER(II) COMPLEXES WITH DISTORTED N2S2 AND N2O2 COORDINATION STRUCTURES

N-14 hyperfine coupling (HFC) tensors of some copper(II) complexes having tetrahedrally distorted N2S2 and N2O2 and trigonal bipyramidal N2O-O2 coordination structures were examined by the angle-selected ENDOR method. By the use of this spectroscopic method, not only the principal values but also the relative orientations with respect to the g tensors were determined from disordered systems. On the basis of the observed N-14 HFC tensors, (1) the effects of tetrahedral distortion on N-14 HFC constants and the unpaired electron distribution, (2) hybridization of the nitrogen coordinating orbitals, and (3) orientations of the g tensor and the copper unpaired electron orbital in the molecular framework were discussed. In the tetrahedrally distorted cis-N2S2 type complexes, the g(perpendicular-to) plane was found to be on the S-Cu-S plane. This is attributed to that the orientation of the copper unpaired electron orbital (3d orbital) is tilting toward the S-Cu-S plane and also to that the spin-orbit interactions of the coordinating sulfurs contribute appreciably to the g tensor. The obtained information will be useful for studies of electronic states of the copper binding sites in copper proteins, most of which have distorted coordination structures.