Anomalous paramagnetism of copper acetate

TLDR

Copper acetate’s paramagnetic resonance spectrum is anomalous, resembling a spin‑1 ion and weakening with cooling, a behavior linked to the decreasing susceptibility reported by Guha (1951) and attributed to crystal‑field effects and strong exchange interactions between isolated copper ion pairs that form singlet and triplet states. The study seeks to explain the fine and hyperfine structure of this spectrum and to predict the experimentally observed small splitting of the triplet state. The authors model the crystal as containing isolated copper ion pairs subject to crystal‑field and exchange interactions, producing singlet and triplet states whose energy separation accounts for the spectral features. With an empirical exchange parameter, the model predicts two differently oriented ion pairs per unit cell and reproduces Guha’s susceptibility measurements with fair agreement.

Abstract

The paramagnetic resonance spectrum of copper acetate is anomalous in that it resembles that of an ion of spin 1, and its intensity decreases as the temperature is lowered. The latter is correlated with the decreasing susceptibility found by Guha (1951). The following hypo­theses are suggested: (1) the crystalline field acting on each copper ion is similar to that in other salts such as the Tutton salts; (2) isolated pairs of copper ions interact strongly through exchange forces, each pair forming a lower singlet state and an upper triplet state, the latter only being paramagnetic. On this basis both the fine structure and the hyperfine structure of the spectrum have a simple explanation, and the theory also predicts a small initial split­ting of the triplet state of the same order as that found experimentally. The unit cell of the crystal contains two differently oriented pairs of ions, and, using an empirical value for the exchange parameter, fair agreement with the susceptibility measurements of Guha is obtained.