ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation

Abstract

ESR line-shape measurements of Cu(ClO4)2 solutions in methanol and water at X- and Q-band frequencies are reported. In both frequencies, above room temperature, the over-all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g-tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/mole for the aqueous complex and τi(25°C)=1.2× 10−11 sec−1, ΔEi=1.1 kcal/mole for the methanol complex. NMR relaxation measurements of Cu(ClO4)2 solutions in methanol between −90°C and 100°C are also reported and are interpreted in terms of kinetic and magnetic parameters of the Cu2+ solvation complex. Since there is fast intramolecular inversion of the complex, only weighted averaged parameters over the axial and equatorial ligand molecules are obtained. The pseudo-first-order rate constant for the exchange of solvation shell molecules is 1/ τM(25°C)=7.4× 107 sec−1 with an activation energy ΔE=6.6 kcal/mole; the molecular tumbling time is τR(25°C)=4.6× 10−11 sec with ΔER=3.0 kcal/mole; the isotropic ligand hyperfine interaction constants are AOH/h=8.8× 105 Hz, ACH3/h=10.6× 105 Hz for the protons and AO/h=2.9× 107 Hz for 17O. Previous NMR results in aqueous solutions are discussed and reinterpreted.