EXCHANGE INTERACTION BETWEEN COPPER(II) IONS THROUGH GLUTAMIC-ACID MOLECULES

EPR measurements at omega0 = 9.7 and 33.3 GHz microwave frequencies in single crystals of the copper complex of glutamic acid [Cu(glu)] and at 9.8 GHz in the zinc complex of glutamic acid [Zn(glu)] doped with Cu-63 have been performed at room temperature. A single EPR line was observed in Cu(glu) for any orientation of the magnetic field B at both frequencies. The gyromagnetic tensor exhibits axial symmetry, and the line width DELTAB(pp)(theta,phi) is strongly dependent on omega0. We calculate the dependence of the line width with omega0 in terms of the exchange interactions between the copper ions. This is achieved with a model based on Kubo and Tomita's theory, which allows us to evaluate selectively the exchange coupling constants Absolute value of J between nonequivalent copper ions in the unit cell. Pathways for superexchange between copper ions in Cu(glu) are provided by carboxylate bridges and hydrogen bonds, as well as by glutamic acid units. From the data we estimate Absolute value of J(sigma) 1 = 0.19 K between copper ions in a distorted octahedral coordination, connected equatorially through the sigma skeleton of a glutamic acid molecule. This bridge involves six diamagnetic atoms. A lower limit for the exchange parameter assigned to the carboxylate paths is estimated as Absolute value of J(c) > 1 K. The results obtained for Zn(glu) doped with Cu-63 are used to study the electronic properties of the isolated copper ions, which in turn allow one to understand the overall axial symmetry of the g2 tensor, and the line width data obtained in Cu(glu).