Nuclear Magnetic Resonance Study of the Solvation of Co++ in Methanol and Methanol—Water Mixtures

Abstract

Proton magnetic resonance spectra of solutions of Co(ClO₄)₂ in acidified methanol, and in methanol—water mixtures are reported. At temperatures below about —50°C separate peaks for the bulk methanol and for the methanol bonded to Co⁺⁺ are observed, and methanol exchange is too slow to contribute appreciably to the observed linewidths. Peak intensity measurements give a coordination number of six for Co⁺⁺ with respect to methanol. From the temperature dependence of the peak frequencies, values of the hyperfine interaction between the bonded methanol protons and the Co⁺⁺ ion were obtained. The results are +8.8×10⁵ cps for the OH protons and +3.9×10⁵ cps for the CH₃ protons. The linewidths are shown to be predominately determined by electron—proton dipole—dipole interaction. The relevant correlation time is the electron relaxation time, which is approximately 5×10^—13 sec at —60°C. This time decreases with increasing temperature, with an apparent activation energy of 0.9 kcal mole^—1. In methanol—water solutions, additional lines, due to mixed methanol—water complexes of cobalt, are observed. Equilibrium constants for the formation of the mono‐ and dihydrated mixed complexes were obtained from measurements of peak intensity as function of water concentration. At temperatures above about —50°C the rate constant of methanol exchange between complexes and bulk solvent was obtained from the excess widths of the peaks. Comparison of the OH and CH₃ linewidths shows that whole methanol molecules exchange, i.e., proton exchange between the complexes and the solution is relatively unimportant.