Nuclear Overhauser effect factor

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

Hence, provided that I g is known and that R has been determined by means of an independent experiment, provides the cross-relaxation rate ct. This enhancement is called nuclear Overhauser effect (nOe) (17,19) from Overhauser (20) who was the first to recognize that, by a related method, electron spin polarization could be transferred to nuclear spins (such a method can be worked out whenever EPR lines are relatively sharp it is presently known as DNP for Dynamic Nuclear Polarization). This effect is usually quantified by the so-called nOe factor p... 

G. M. Lipkind, A. S. Shashkov, S. S. Mamyan, and N. K. Kochetkov, The nuclear Overhauser effect and structural factors determining the conformations of disaccharide glycosides, Carbohydr. Res., 181 (1988) 1-12. 

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-... 

Abbreviations NMR, nuclear magnetic resonance 2D-NMR, two-dimensional NMR HOHAHA, 2D-NMR homonuclear Hartman-Hahn spectroscopy NOE, nuclear Overhauser effect NOESY, 2D-NMR nuclear Overhauser effect spectroscopy rf, radio frequency FID, free induction decay CD, circular dichroism PF4, platelet factor-4 IL-8, interleukin-8 Gro-a, growth-related protein a. 

Heteronuclear incoherent magnetization transfer is the transfer of longitudinal magnetization. It can proceed in the laboratory frame and in the rotating frame. The nuclear Overhauser effect (NOE) [Nogl] is a manifestation of polarization tranter in the laboratory frame. In the extreme narrowing limit saturation of dipolar relaxation of the I doublet of a heteronuclear IS two-spin- system leads to an enhancement of the S-spin polarization by a 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. 

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. 

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