Magnetic Resonance Studies on Copper(II) Complex Ions in Solution. I. Temperature Dependences of the 17O NMR and Copper(II) EPR Linewidths of Cu(H2O)62+

The temperature dependences of copper (II) EPR and 17O NMR spectra are analyzed in terms of a tetragonally distorted Cu(H2O)62+ ionic species in which only the equatorial water molecules form strong σ bonds to copper (II). By reconstructing the EPR spectra at temperatures in the range −10° to 100°C, the contributions to the linewidth from spin—lattice relaxation, tumbling of an ionic complex having an anisotropic g factor and an anisotropic hyperfine coupling constant, and from isotropic hyperfine splitting, are separated. It is found that the spin—lattice relaxation time T1e has components from both spin—rotational and Van Vleck processes. The 17O NMR linewidth is due to scalar hyperfine interaction with the copper (II) electron spin, and the spin-exchange correlation time τe for this mechanism is determined over the same temperature range. While T1e and τe have similar temperature dependences, τe is 6–8 times smaller than T1e, suggesting that it may be related to inversion of tetragonal distortion in the complex, rather than to electron relaxation.