Exchange interactions and magnetic dimension in Cu(L-alanine)2.

Abstract

A study of the magnetic properties of the copper (II) complex of the amino acid l-alanine [Cu(l-alanine${)}_{2}$] is reported. The susceptibility of a powder sample has been measured between 0.013 and 240 K. A linear-spin-chain model with antiferromagnetic exchange coupling J=-0.52 K fits well the susceptibility data above 0.3 K. Room-temperature electron paramagnetic resonance (EPR) spectra of single crystals of Cu(l-alanine${)}_{2}$ at 9 and 35 GHz show a single, exchange-narrowed resonance. The g tensor, with principal values ${\mathit{g}}_{1}$=2.0554\ifmmode\pm\else\textpm\fi{}0.0005, ${\mathit{g}}_{2}$=2.1064\ifmmode\pm\else\textpm\fi{}0.0005, and ${\mathit{g}}_{3}$=2.2056\ifmmode\pm\else\textpm\fi{}0.0005, reflects the crystal structure of Cu(l-alanine${)}_{2}$ and the electronic properties of the copper ions. The observed angular variation of the linewidth is attributed to the magnetic interactions, narrowed by the exchange coupling between copper ions, and shows a contribution characteristic of the dipole-dipole interaction in a spin system with a predominant two-dimensional spin dynamics. Considering the exchange-collapsed resonance corresponding to the two lattice sites for copper in Cu(l-alanine${)}_{2}$, we evaluate an exchange constant \ensuremath{\Vert}J(${\mathit{AB}}_{1}$)\ensuremath{\Vert}=0.47 K between nonequivalent copper neighbors in a spin chain, similar to the value obtained from the susceptibility data. The one-dimensional magnetic behavior suggested by the susceptibility data in Cu(l-alanine${)}_{2}$, where the metal ions are distributed in layers, is explained by proposing that carboxylate bridges provide electronic paths for superexchange interactions between coppers. Considering the characteristics of the molecular structure of Cu(l-alanine${)}_{2}$, the layers seem to be magnetically split off into one-dimensional zigzag ribbons. The apparent disagreement between the one-dimensional behavior suggested by the susceptibility data and the two-dimensional behavior of the spin dynamics suggested by the EPR linewidth is analyzed.