Relaxation time, nitroxide complexes

Collision of nitroxides with fast-relaxing radicals, such as oxygen and metal ion complexes, causes spin exchange that effectively shortens the spin-lattice relaxation time T of the nitroxide (Hyde and Subczynski, 1989). This effect can be measured either by continuous wave (CW) power saturation techniques or by saturation recovery methods. Collision frequency is directly proportional to the accessibility of the paramagnetic reagent to the nitroxide radical and is defined as... 

Li Relaxation times have been used to study complex formation with nitroxides in the aqueous phase, and the interaction of Li with Mn. Dynamic NMR spectroscopy has been used to study equihbria of chelated lithium phenolates. The formation of a stable betaine Hthium salt adduct has... 

The use of Gd(iii) chelate complexes as the second spin label instead of N-labelled nitroxides ° or Cu(ii) complexes offers important advantages. Spectroscopic separation of the signals from nitroxide radicals and Gd(iii) centres relies to a lesser extent on the difference in resonance fields for the two types of paramagnetic species. The main factors that allow spectroscopic separation are the 2-3 orders of magnitude difference in longitudinal relaxation time (at optimum repetition rate for Gd(iii) species nitroxide radicals are nearly completely saturated) and the difference in the transition moments between high-spin [S = 7/2) Gd(iii) centres and low-spin (S = l/2) nitroxide radicals. 

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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