Molecular motion and solvent effects in several Cr⁵⁺ compounds using nonstationary nuclear-electron resonance

Abstract

Dynamic proton polarization in several diols Cr⁵⁺ complex ion solutions has been studied at various magnetic field strengths and temperatures. Normally, in most organic radical solutions the dynamic enhancements of solvent proton resonances decay with increasing field and decreasing temperature due to the dipole-dipole intermolecular interactions, modulated by translational diffusion of individual molecules. In contrast to free radical, a strong solvation effect has been discovered in paramagnetic Cr⁵⁺ compounds. Drastic departures from the usual DNP behaviour have been observed. An explanation of the observed effects is proposed by considering the occurrence of hydrogen-bridged solvates. The important motional mechanism that modulates the proton-electron interaction switches from “translation” to “rotation” of the complex molecules. A new type of pulse double resonance equipment is used.