Nuclear relaxation processes of paramagnetic complexes The slow-motion case

Abstract

The nuclear relaxation mechanism in tumbling paramagnetic molecules is a superposition of two processes, the usual ‘S-mechanism’ and an additional ‘χ-mechanism’. The latter arises from the thermal average of the electron spin polarization and is conveniently described in terms of the molecular susceptibility χ. Expressions for T ₁ and T ₂ due to χ-relaxation are derived. The χ-contribution is appreciable in cases where the rotational correlation time is four orders of magnitude longer than the electron spin relaxation time. The χ-relaxation rates increase quadratically with the external field. The effects of anisotropic susceptibility and of chemical exchange are considered. It is shown that in certain important cases the N.M.R. line width is not determined by the chemical exchange rate, even when it is much faster than the tumbling rate.