Nuclear Overhauser effect consequences

In most C spectra, nuclei which have directly attached protons receive a significant (but not easily predictable) signal enhancement when the protons are decoupled as a result of the Nuclear Overhauser Effect (see Section 7.3) and as a consequence, peak intensity does not necessarily reflect the number of nuclei giving rise to the signal. 

The nuclear Overhauser effect (NOE) is a consequence of the modulation of the dipole-dipole interactions (through space) between different nuclei and is correlated with the inverse sixth power of the internuclear distance. Experimentally, the NOE is the fractional change in intensity of one resonance when another resonance is irradiated in a double-irradiation experiment. The NOE phenomenon is intimately related to spin relaxation. The NOE varies as a function of the product of the Larmor frequency, co0, and the rotational correlation time, tc. In small molecules, tc is short relative to uo"1. In this extreme motional narrowing situation, the frequency... 

The nuclear Overhauser effect (NOE), which is manifested in certain changes in the intensities of NMR lines, is a consequence of magnetic dipolar relaxation. The name comes from a phenomenon predicted by Albert Overhauser in 1953, when he showed theoretically that saturating the electron magnetic resonance in a metal would cause the nuclear resonance intensity to be enhanced by a factor of the order of 103 (the ratio of "/electron/"/nucleus)- lonel Solomon later found that a similar effect occurs between two nuclei, but with a much smaller intensity enhancement—the nuclear Overhauser effect.90 Because the NOE is of great practical im-... 

Those familiar with the routine acquisition of 13c NMR spectra are aware of the consequences of the nuclear Overhauser effect (NOE). Saturation of protons has the effect of increasing the net 13c magnetization of those carbons relaxed by the protons of up to a factor of three times the equilibrium magnetization. Most analytical or survey 13c spectra are obtained with continuous broadband proton decoupling and any resultant NOE. Characteristics of this mode of operation are, (1) the possibility of variable NOE, (2) repetition rate governed by 13c T and (3) both protonated and non-protonated carbons are detected. The first aspect makes quantitation difficult. The second affects net sensitivity, and the third has the prospect of having undesirable signals in certain situations. 

Dipolar Couplings and Distance Information. - The nuclear Overhauser effect (NOE) arises from dipolar interactions between magnetic moments associated with nuclear spins and it has become a powerful tool to extract relevant pieces of structural information about small molecules, as well as in molecules of biological interest. As a consequence, accurate NOE measurement is a very crucial issue. Walker et presented a comparison between direct and a new inverse HOESY experiment aimed at the detection of heteronuclear NOE between H and which is particularly well suited for symmetric compounds. It transpires that directly detected data are more suitable for quantitative assessment even if they suffer from lower sensitivity, whereas inverse detection is quite appropriate for a quick and quahtative assessment. In the latter experiment, unwanted cross-correlation effects may hide valuable NOE data (cross-relaxation), this drawback can be circumvented by a slight modification of the pulse sequence. 

One of the most powerful structural NMR tools is the ability to detect pairs of nuclei that are close in space, even if they are noncovalently bound, using the nuclear overhauser effect (NOE). The NOE is a relaxation effect that originates from the participation of a neighbor nucleus in the process as a consequence of which the other nucleus returns to its equilibrium situation after it has been perturbed. In the standard NOE experiment, the initial perturbation is a change in the relative populations of the two spin states (a and P) of the nucleus of a proton from its equilibrium... 

Those familiar with the routine acquisition of NMR spectra are aware of the consequences of the nuclear Overhauser effect (NOE). Saturation of protons has the effect of increasing the net mag-... 

Nuclear magnetic dipole relaxation interactions may occur with other nuclei, or with unpaired electrons. These processes usually dominate the relaxation of spin - nuclei. Both intra- and intermolecular interactions may contribute to dipole-dipole nuclear relaxation times. The value of due to the intramolecular dipole-dipole process is proportional to the sixth power of the internuclear separation. Consequently, this process becomes rather inefficient in the absence of directly bonded magnetic nuclei. However, it follows that a measurement of can be provide an estimate of internuclear separation that can be of chemical interest. The nuclear Overhauser effect (NOE) depends upon the occurrence of dipole-dipole relaxation processes and can similarly provide an estimate of internuclear separation. 

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