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

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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 anisotropicg‐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 anisotropicg 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 τ i (25° C )=1.6× 10 −11 sec −1 , ΔEi =1.2 kcal/mole for the aqueous complex and τ i (25° C )=1.2× 10 −11 sec −1 , ΔEi =1.1 kcal/mole for the methanol complex. NMRrelaxation 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/ τ M (25° C )=7.4× 10 7 sec −1 with an activation energy ΔE=6.6 kcal/mole; the molecular tumbling time is τ R (25° C )=4.6× 10 −11 sec with ΔER =3.0 kcal/mole; the isotropic ligand hyperfineinteraction constants are A OH /h=8.8× 10 5 Hz , A CH 3 /h=10.6× 10 5 Hz for the protons and A O /h=2.9× 10 7 Hz for 17O. Previous NMR results in aqueous solutions are discussed and reinterpreted.

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