Nuclear Overhauser effect and

Methods of disturbing the Boltzmann distribution of nuclear spin states were known long before the phenomenon of CIDNP was recognized. All of these involve multiple resonance techniques (e.g. INDOR, the Nuclear Overhauser Effect) and all depend on spin-lattice relaxation processes for the development of polarization. The effect is referred to as dynamic nuclear polarization (DNP) (for a review, see Hausser and Stehlik, 1968). The observed changes in the intensity of lines in the n.m.r. spectrum are small, however, reflecting the small changes induced in the Boltzmann distribution. 

It is not usually possible to integrate routine C spectra directly unless specific precautions have been taken. However with proper controls, NMR spectroscopy can be used quantitatively and it is a valuable technique for the analysis of mixtures. To record C NMR spectra where the relative signal intensity can be reliably determined, the spectra must be recorded with techniques to suppress the Nuclear Overhauser Effect and with a long delay between the acquisition of successive spectra to ensure that all of the carbons in the molecule are completely relaxed between spectral acquisitions. 

The NOESY spectrum relies on the Nuclear Overhauser Effect and shows which pairs of nuclei in a molecule are close together in space. The NOESY spectrum is very similar in appearance to a COSY spectrum. It is a symmetrical spectmm that has the Iff NMR spectmm of the substance as both of the chemical shift axes (Fi and F2). A schematic representation of NOESY spectmm is given below. Again, it is usual to plot a normal (one-dimensional) NMR spectmm along each of the axes to give reference spectra for the peaks that appear in the two-dimensional spectmm. 

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. 

Lipid bilayers have been studied in vesicles of about 500 A diameter. The bilayers can be made of many lipids. The most common lipid is lecithin, phosphatidyl choline (PC). Packing the lipids in the vesicle results in two-thirds of the lipid head groups on the external face about one-third on the internal face. The head group of the lipid PC is phosphocholine -O-POJ-O-CH2-CH2N (013)3 and the head group is studied by P, H, D or C NMR and the long fatty chains by C, H, or D NMR. Conformational studies of the molecules by conventional de-coupling, nuclear Overhauser effects and by Ln(III) probes... 

Krems, B., Bachert, P., Zabel, H. J. and Lorenz, W. J. (1995) F- H nuclear Overhauser effect and proton decoupling of 5-fluoruracil and alph-fluoro-beta-alanine. Journal of Magnetic Resonance B 108(2), 155-164. 

NMR spectroscopy has been the most useful tool in cephalosporin C chemistry. In cephalosporins the carbons are unsaturated or highly substituted with heteroatoms, and the protons are usually widely separated in chemical shift and have simple coupling patterns. Recently, solvent induced chemical shifts, nuclear Overhauser effects, and the anisotropy of the sulfoxide bond have been utilized in chemical studies of cephalosporin C derivatives. Analytical information may be derived from NMR spectra of cephalothin by observing the contribution of the 0-lactam protons, thiophene protons, methylene groups, and methyl protons (from acetate). 

Several different analytical methods are needed to determine the structure and dynamics and to map the intermolecular interactions that prevail in supramolecular systems in solution [1]. NMR is one of the most powerful of these methods [Id-e]. The conventional NMR parameters that are used to characterize the structure and dynamics of supramolecular systems in solution are chemical shifts, spin-spin coupling, relaxation times, NOEs (Nuclear Overhauser Effect) and the correlation thereof. We shall demonstrate that the diffusion coefficient, which is currently underused, should be added to this arsenal of NMR parameters when characterizing supramolecular systems in solution. 

The most important relaxation mechanism for many spin- /2 nuclei arises from the dipolar interaction between spins. This is also the source of the tremendously important nuclear Overhauser effect and further discussions on... 

The identification of both short chain and long chain branches in polyethylene at concentrations of 1 per 10,000 carbon atoms has become feasible with the availability of improved probes and improved computer hardware/ software capabilities. Reviewed in this chapter are the methods and computations as well as the basic requirements for sound quantitative analyses namely, correct choice of solvent, a consideration of concentration effect on line widths and satisfying nuclear Overhauser effects and spin lattice relaxation time requirements. Finally, the NMR generated structural information is put to use in correlations with polyethylene physical properties and measurements of number average molecular weight. 

There are a number of considerations that must be addressed when formulating quantitative 13c NMR procedures - these include solvent effects, spectral overlap, line widths, dynamic and nuclear Overhauser effects and detailed assignments. The steps required to develop sound quantitative methods will be the subject of this chapter. It is imperative that excellent quantitative methods be established so that NMR can be utilized in studies of polymer structure-property relationships. Polymer molecular structure needs to be related to the incipient solid state structure and ultimately to observed solid state physical properties such as density, flexural moduli, environmental stress cracking behavior, to name a few. 

The formation of cyclic structures and polymerization of aryl cyanates was followed from the changes in signal intensities of the reaction products. All intermediate spectra consist of two well separated signals arising from cyanate and triazine groups, respectively. Since a chromium acetylacetonate is added in order to eliminate the Nuclear Overhauser Effect and reduce the spin-lattice... 

Nuclear Magnetic Resonance Spectroscopy.—As noted above, conformational analysis of bicyclo[3.3.1]nonanes is still a topic of considerable interest. A variable-temperature n.m.r. analysis now provides the first case in which the boat-chair-chair-boat equilibrium is directly observed in the amines (17) and (18). In a related case, re-examination of the acetal (19) suggests that the preferred conformation involves a chair carbocyclic ring and a boat heterocyclic ring. This conclusion was made by n.m.r. analysis, using lanthanide shift reagents, by a study of nuclear Overhauser effects, and by measurement of relaxation times of protons. Details have been reported for other 3-azabicyclo[3.3.1]nonanes, and the non-additivity of substituent effects on chemical shifts in 9-thiabicyclo[3.3.1]non-2-enes has been analysed. Both and n.m.r. data have been reported for a series of 9-borabicyclo[3.3.1]non-anes and their pyridine complexes. 

Consider now two magnetic nuclei, A and B, which are in sufficiently close spatial proximity to influence each other s relaxation times. If the nucleus A is observed while the nucleus B is simultaneously irradiated (see Sec. 12.4 for multiple irradiation), the relaxation process in nucleus A becomes more efficient because nucleus B, which is undergoing rapid up-and-down transitions, becomes effectively a rotating magnetic field. This results in a perturbation of the usual Boltzmann distribution of nuclei A towards the lower state and increases up to 50% the intensity of the signal due to the nucleus A. This enhancement of intensity is known as the Nuclear Overhauser Effect and is diagnostic for the presence of magnetic nuclei in close spatial proximity. Structural information can thus be obtained, for example, in coimection with cis-trans isomerism. 

K. Umemoto, S. Oikawa, M. Aida, and Y. Sugawara, Intermolecular nuclear Overhauser effect and atomic pair potential approaches to wheat-germ agglutinin-sugar binding, J. Biomol. Struct. Dyn., 6 (1988) 593-608. 

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