Nuclear spin dipolar relaxation in paramagnetic systems undergoing multiple internal motions

Abstract

A theoretical treatment is proposed for the relaxation induced by the dipolar interaction between two spins separated by several bonds about which jump motions occur among three sites, two of them at least being equivalent. The influence of the internal and overall molecular motions upon the carbon-13 longitudinal relaxation is discussed for the model of n-butylamine coordinated to a paramagnetic ion having a comparatively long T _1e like Mn²⁺. The ¹³C paramagnetic dipolar relaxation appears to be mainly dependent upon the ratios of the transition rates among the different conformers. A computer program has been written to analyse the ¹³C longitudinal dipolar relaxation in terms of internal motions and transient conformations for flexible and partially rigid molecules such as butylamine, allylamine and benzylamine coordinated to Mn(AA)₂ and Gd(DPM)₃. The extension of these calculations to complexes where the nuclear relaxation is governed by the electron spin relaxation times is examined for the example of n-butylamine-Ni(AA)₂ and Dy(DPM)₃ adducts. The calculations of ¹³C relaxation rates have been applied to the conformational study of leucine and norleucine co-ordinated to Gd³⁺ in aqueous solution.