Electron Spin Resonance of Copper (II) Citrate Chelates

Abstract

Electron spin resonance (ESR), paramagnetic susceptibility, and polarographic measurements have been made of copper (II) citrate chelates over a wide range of pH. The signal observed at pH 4 is typical of that found for copper (II) in a distorted octahedral environment. With increasing pH the signal becomes less intense and is replaced by a much broader spin resonance observable at relatively high concentrations of copper (II) and in frozen solutions. At pH's of about 11, a third signal is observed. The broad signal has been interpreted in terms of dipole—dipole coupling of copper (II) ions arising from dimer formation. Measurements show that the paramagnetic susceptibility is independent of pH. Solution of the spin Hamiltonian including the dipole—dipole interaction has made possible calculation of the line shape for the dimer signal. The calculations have led to a value of 3.1 Å as the distance between the copper (II) dipoles and to the conclusion that each copper (II) ion is in a | 3z2—r2〉 ground state rather than | x2—y2〉. The ESR data provides very strong evidence for the formation of a dimer in solution and allows reinterpretation of the data found in the literature. The signal observed beyond pH 11, with its partly resolved hyperfine structure at room temperature, is attributed to a monomer formed as a result of replacement of the hydrogen from the hydroxy group of the citrate anion by copper (II).