Paramagnetic relaxation

In the low-field condition, the quantization axis is defined by the EFG main component In this situation, and rj can both be determined from powder spectra when recorded in an externally applied field. Figure 4.14 shows simulated spectra as is often encountered in practice such as in applied-field measurements of diamagnetic compounds or fast-relaxing paramagnetic compounds at high temperatures. The simulated traces differ in detail from a single-crystal spectrum as shown in Fig. 4.13, but their features still correlate in a unique manner with rj and the sign of... 

In addition to line broadening due to accelerated relaxation, paramagnetic salts and chelates give rise to isotropic shifts when added to samples containing molecules with groups susceptible to coordination with metal ions, e.g. —OH, —NH2, — SH, — COOH, -C = 0. The isotropic shift A of a nucleus i in a sample S is the difference between chemical shifts measured before and after addition of the paramagnetic shift reagent SR [103] ... 

Heisenberg exchange between spin-labels and fast relaxing paramagnetic species including dissolved molecular oxygen or transition metals provides a thermal contact with the lattice and is an effective T, mechanism [43]. The effect has been used to measure oxygen concentrations [44]. 

Hyde and Sarna [43] showed that when spin labels collide with fast relaxing paramagnetic species, Heisenberg exchange provides a thermal contact between the slow relaxing species and the lattice. Thus, there is a change in effective This effect has been observed directly by Kusumi et al. [42]. 

NOE intensities can be reduced if alternative relaxation mechanisms compete with dipolar cross-relaxation. Paramagnetic relaxation may broaden the signals. Select diamagnetic analogs if possible. Nuclei with low gyromagnetic ratios (such as C) are less sensitive than protons... 

If concentrations are between 50 and 500 xM and relaxation times are not diminished by proximity of other, very fast relaxing paramagnetic centers, maximum dipolar evolution times of at least 1 p,s can usually be achieved. This allows for measurement of distances up to 4.5 nm. In most systems, = 2.5 ps is feasible, corresponding to a distance range of up to 6 nm. In fairly rigid protonated matrices without methyl groups, such as o-terphenyl, = 6 ps can be achieved, so that... 

There are a number of processes that can produce the fluctuating fields that cause relaxation dipole-dipole (DD) interactions between NMR-active nuclei, chemical shift anisotropy (CSA), scalar (SC) interactions, spin-rotation (SR) relaxation, paramagnetic (P) relaxation, and quadmpolar (Q) interactions. 

Another case of interest is the influence of paramagnetic quenchers, which are fast relaxing paramagnetic species, on the nitroxide spectrum. Such quenchers are the ground-state triplet molecule oxygen, transition metal complexes with group spin S > 1/2, and lanthanide ions. If the longitudinal relaxation time of the quencher is much shorter than the lifetime of the collisional encounter complex, the effect on the nitroxide is a pure relaxation enhancement. 

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