EPR of layered magnetic metal-amino acid salts. II. Cu(L-Met)2

Single crystals of bis(L-methioninato)copper(II), Cu(L-Met)₂, were studied by EPR at 9.7 and 33.6 GHz, at 300 K. The position and the peak-to-peak linewidth of the single observed EPR line were measured in three perpendicular planes of the samples. This single resonance is due to the collapse of the resonances of the two magnetically inequivalent copper ions in the lattice caused by the exchange interaction. The components of the molecular g tensor for isolated copper ions were obtained using a model which assumes axial symmetry. The results indicate that the unpaired electron occupies the d(x² − y²) orbital, and the orientation of the molecule obtained from the EPR data agrees with the crystallographic result. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic lattice, an incomplete collapse of the hyperfine structure, and a frequency-dependent contribution in the planes where the g factors for the two sites of copper are different. An analysis of this latter contribution, allows to evaluate an exchange coupling constant |J′| = 0.10 K between inequivalent copper neighbors. Besides, the analysis of the hyperfine contribution to the linewidth gives |J| = 0.18 K for the average value of the exchange interaction of one copper ion with its six nearest neighbors within the layer. Concerning the possible superexchange paths between inequivalent copper ions, we suggest they can be of two types: the Cu---N---H…O---Cu one, involving hydrogen bonds between equatorial nitrogens and equatorial oxygens, and the other consisting of Cu---O---C---O---Cu carboxylate bridges involving apical and equatorial oxygens. They are discussed in view of the experimental results.