Electron Spin Resonance Linewidths of Manganese(II) Ions in Concentrated Aqueous Solutions

ESR spectra of concentrated solutions of manganese(II) perchlorate (1–3.2M) do not indicate strong spin exchange and the dipole—dipole interaction is considered to be the predominant intermolecular-relaxation process. A model of the ESR spectrum is constructed using 30 Lorentzian distribution functions positioned according to a relation developed by Hurd, Sachs, and Hershberger. Ratios of the second moment for the various electron-magnetic transitions are used to approximate the linewidth ratios of the transitions and thereby reduce the number of linewidth parameters to one, ΔHD½, the dipolar-linewidth contribution to the −½→½ absorption. ΔHD½ is calculated from the experimental spectra by trial and error. The variations of ΔHD½ with increasing temperature indicates that for concentrations less than 1.9M a simple rapid-motion diffusion model is sufficient to explain the narrowing of the spectrum. Above 1.9M, ΔHD½ follows a modified exponential function, ΔHD½=AD—BD exp(—u/kT), in which AD varies linearly with concentration and BD and U are very nearly concentration independent. A weak exchange interaction is suggested for the more concentrated solutions.