A theoretical and experimental study of the electron spin resonance of a number of low symmetry copper(II) dimers

Abstract

Theoretical considerations of the interactions in pair systems of identical copper(II) ions have shown that the symmetry of the pair system plays an important role in determining the form of the zero-field splitting appropriate to the triplet state e.s.r. spectra observed in such circumstances. To test the theoretical predictions, a number of compounds, for which X-ray structural evidence was available, have been studied either as powders, doped into isomorphous diamagnetic hosts, or in suitable solvents. The particular systems studied are copper(II) complexes of diethyldithiocarbamate, pyridine N-oxide, salicylaldehyde, and tartaric acid. These examples involve dimeric units such that the overall dimer symmetry is monoclinic. Theoretical results are discussed in the limit of weak exchange coupling, small compared with the X-band microwave energy, as well as for exchange coupling large compared with X-band energy but not greater thanca. 30 cm^–1. Within these limits, the zero-field splitting which manifests itself in the e.s.r. spectrum, is believed to be due entirely to dipole–dipole coupling between the copper(II) pairs. A perturbation theory approach leads to analytical expressions for transition field positions and intensities which are then used in the computer simulation of spectra. Excellent agreement is found between X-ray and e.s.r. structural parameters, except for the tartrate which is thought to possess a different dimer structure in frozen solution than in the pure copper compound.