Copper(II) dimers in solution: evidence for motional averaging of coupling tensors without chemical dissociation

Abstract

The stability of the copper(II)–carnosine dimer in aqueous solutions at pH 7.2 has been investigated with Fourier transform–i.r. and e.s.r. spectroscopies. At subsaturated concentrations, the dimer dissociates into two identical monomers. However, the dimer does not dissociate in saturated solutions even at temperatures as high as 80 °C according to Fourier-transform i.r. spectra. Motional averaging of the dipolar coupling tensors occurs at temperatures at least as low as 15 °C. Thus at temperatures above the freezing point of the solvent the copper(II) dimer in saturated solution exhibits a spectrum with only four hyperfine lines. These are broader than the hyperfine lines of monomeric copper(II) complexes because of dipole–dipole interaction. This is the first spectroscopic confirmation to our knowledge that a copper(II) dimer need not dissociate in aqueous solution at temperatures above the freezing point of water.