Nuclear Overhauser effect measurement, factor

Relaxation is the process by which the spins in the sample come to equilibrium with the surroundings. At a practical level, the rate of relaxation determines how fast an experiment can be repeated, so it is important to understand how relaxation rates can be measured and the factors that influence their values. The rate of relaxation is influenced by the physical properties of the molecule and the sample, so a study of relaxation phenomena can lead to information on these properties. Perhaps the most often used and important of these phenomena in the nuclear Overhauser effect (NOE) which can be used to probe internuclear distances in a molecule. Another example is the use of data on relaxation rates to probe the internal motions of macromolecules. 

A check on the effectiveness of degassing for some nuclei may be possible by examining the nuclear Overhauser enhancement (NOE) factor. If you have independent knowledge of exactly what the NOE factor should be, then its measurement will reveal whether oxygen is playing a significant role in the relaxation by decreasing the NOE (Levy and Peat, 1975). 

The nuclear Overhauser effect (NOE) is a powerful nuclear magnetic resonance (NMR) tool for structure elucidation because it can be used to identify pairs of nuclei that are close together, typically within 0.5 nm (5 A). The size of the effect is strongly dependent upon the internuclear separation r in optimal cases, it is proportional to r. However, as outlined below, complicating factors often reduce this dependence. We therefore stress throughout that structural analyses that rely on exact quantitative measurements should be avoided, and the NOE is best used in a semiquantitative manner. 

As a matter of fact, the vast majority of experimental studies focuses on a relatively well-defined set of parameters. Taking as an example the important case of NMR spectroscopy of organic molecules, the characterization is usually based on measurements of proton and carbon chemical shifts in solution, homonuclear (and possibly heteronuclear) coupling constants, and proton-proton nuclear Overhauser enhancements [or the corresponding rotating-frame effects (ROEs)]. This set of data is certainly reductive if compared with the information content potentially accessible by NMR measurements however, it does represent a reasonable balance of such factors as operator and instrument time, apparatus availabihty, costs, amounts of material required, completeness of information, and ease of interpretation. 

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