Overhauser effect spectroscopic technique

It is not only chemical shifts or couplings that can be used to define correlations in a 2D NMR spectrum information from through-space interactions as in the Nuclear Overhauser Effect (NOE, Section 4.11.4) can also be used. The NOES Y (Nuclear Overhauser Effect Spectroscop Y) experiment is one of the most useful in this context. It is a homonuclear technique that allows correlation of nuclei through space separated by less than 5 A. The occurrence of a cross peak therefore indicates that the corresponding two nuclei are close in... 

LC-NMR plays a central role in the on-line identification of the constituents of crude plant extracts (Wolfender and others 2003). This technique alone, however, will not provide sufficient spectroscopic information for a complete identification of natural products, and other hyphenated methods, such as LC-UV-DAD and LC-MS/MS, are needed for providing complementary information. Added to this, LC-NMR experiments are time-consuming and have to be performed on the LC peak of interest, identified by prescreening with LC-UV-MS. NMR applied to phenolic compounds includes H NMR,13 C NMR, correlation spectroscopy (COSY), heteronuclear chemical shift correlation NMR (C-H HECTOR), nuclear Overhauser effect in the... 

A wide variety of chemical and spectroscopic techniques has been used to determine functionality in humic substances. Although nuclear magnetic resonance (NMR) spectroscopy has been used for a much shorter period of time than most other techniques for determining functional group concentrations, this technique has provided far more definitive information than all other methods combined. However, substantially more work must be done to obtain the quantitative data that are necessary for both structural elucidation and geochemical studies. In order to increase the accuracy of functional group concentration measurements, the effect of variations in nuclear Overhauser enhancement (NOE) and relaxation times must be evaluated. Preliminary results suggest that spectra of fractions isolated from humic substances should be better resolved and more readily interpreted than spectra of unfractionated samples. 

Phytochemical studies on Narcissus obesus have resulted in the isolation of a new alkaloid named obesine (420) (see Fig. 24), accompanied by several known Amaryllidaceae alkaloids (74). The stereochemistry and structural determination of the alkaloid 420 have been carried out by spectroscopic analyses and by application of 2D NMR techniques. Although the H-6)8 proton (5 4.38) is masked by the H-3 proton (5 4.30-4.40), the H-6a proton (54.02) was assigned at higher field on the basis of the nuclear Overhauser effect (NOE) with H-12 endo (5 3.10) observed in the 2D NMR experiment. On the other hand, the a disposition of H-3 was confirmed by the NOE between H-3 (5 4.30-4.40) and H-12 exo (5 3.01). In the C-NMR spectrum of 420 a characteristic signal due to the C-11 carbon was observed at 82.7 (singlet) ppm. Also, a comparison of the H and NMR spectra of obesine (420) with those of the related alkaloid 3-epi-marconine (303) (50) was performed. 

One interesting facet of the germacrane-type sesquiterpenoids is the conformation of the ten-membered ring. This aspect has previously been discussed in terms of transannular electronic effects (anomalous u.v. spectra) and transannular chemical reactions (Cope rearrangement and cyclisations to eudesmane and/or guaiane types). Recently, the power of two spectroscopic techniques has been brought to bear on this problem. The first of these is the use of the Nuclear Overhauser Effect (NOE) and the second is the A"-ray analysis of a suitable derivative. 

Older methods based on solubility changes upon complexation, or on partition coefficients between aqueous solutions and hydrophobic solvents, have been shown to lead to gross errors as compared to spectroscopic techniques (40) that are also less sensitive to the formation of emulsions, micelles, and so on. The traditional X-ray analysis of inclusion compounds is of limited significance for establishing complexation between lipophilic substrates and macrocyclic host, particularly in aqueous solution. The essential hydrophobic driving force for complexation, of course, is nonexistent in the crystal. The future development of NMR methods including shielding calculations and measurements of intermolecular nuclear Overhauser effects is expected to provide the most reliable information on intercavity inclusion complexes in solution as the basis for catalytic applications. 

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