Proton spin relaxations in solutions of paramagnetic complex with S=3/2 in slow motion region

Abstract

Proton NMR relaxation in paramagnetic systems of S=3/2 with electron spin relaxation in slow-motional region is examined. The proton relaxations arising from the modulations of scalar, dipolar, and scalar–dipolar cross interactions from paramagnetic spin by slow molecular motion together with their field dependence, the variation of zero field splitting (zfs) interaction, and internal rotation interference are investigated. The overall reorientational motion of the complex presumably gives rise to the fluctuation of zfs interaction. The model for independent modulations of dipolar and zfs interactions is considered also. The proton relaxation rates are directly calculated from the approach of solving stochastic Liouville equation for slow-motional ESR relaxations. Slow-motional effects on individual proton relaxation rates are discussed. The final results are appropriately modified to incorporate the influence of internal rotation for proton. The field-dependent methyl proton relaxation rates in tris-acetylacetonatochromium (III) complex are calculated. Effects due to various potential barriers in the internal rotation are also presented.