Longitudinal relaxation rates paramagnetic systems

While keeping in mind the general picture of nuclear relaxation in paramagnetic systems as described in Section 3.1, it is appropriate to consider first the simple case of dipolar coupling between two point-dipoles as if the unpaired electrons were localized on the metal ion. The enhancement of the nuclear longitudinal relaxation rate Rim due to dipolar coupling with unpaired electrons can be calculated starting from the Hamiltonian for the system ... 

The information content of nuclear longitudinal relaxation measurements in both paramagnetic and diamagnetic systems can be greatly increased by performing such measurements as a function of the magnetic field. For paramagnetic species, the reason is apparent from the functional form of the equations discussed in Chapter 3 and from the relevant experimental data, reported in Chapter 5. The field dependence of a relaxation rate is called relaxation dispersion, and is abbreviated as NMRD. In principle, NMRD would be helpful for any chemical system, but practical limitations, as will be shown, restrict its use, with a few exceptions, to water protons. 

Y(Mn,Fe)2- Substitution within the Mn lattice can also lead to lattice contraction and hence to a destabilization of Mn moments. An example is Y(Mhi combined neutron scattering and pSR study was carried out by Cywinski et al. (1990). Neutrons show that already at x = 0.025 the Mn moment is reduced to 0.2pb and the system reverts to Pauli paramagnetic behavior. They further revealed the presence of AFM correlations over a wide temperature range, but could not distinguish between static (frozen spins) or dynamic (longitudinally fluctuating spins) correlations. With the help of TF-p.SR measurements it was possible to pin down the dynamical nature of the correlations. The temperature dependence of relaxation rate followed a simple power law (A(J) oc T " )... 

The basic experimental condition required for distance measurements from a paramagnetic center to a ligand nucleus in enzyme systems is that the paramagnetic (p) contribution to the longitudinal (l/T,) or transverse (l/7 2) relaxation rates be measurably larger than the diamagnetic (d) contribution of the enzyme itself ... 

---