Nuclear Overhauser effect chemical elucidation

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

Later in 1996 the first example of a dimeric indoloquinoline alkaloid, crypt-omisrine (22), was isolated and its structure elucidated.73 Inverse-detected 2D-NMR spectra were recorded using a 3 mm micro-inverse probe 13C chemical shift data were recorded for the sample using a 3 mm micro-dual probe. In addition to the mass spectral data, a 0.9% nuclear overhauser effect (NOE) observed between the HI and H9 protons was used to confirm the structure. 

Finally, mention should be made of the increasing application of the intramolecular Nuclear Overhauser Effect (NOE) which has played an important role not only as an aid to structural elucidation but also in the conformational determination of some of the germacrane sesquiterpenes. These results have a significant bearing on the understanding of transannular effects and chemical reactivity. 

Due to the great complexity of this class of molecules, nuclear magnetic resonance (NMR) and mass spectroscopy (MS) are the tools most widely used to identify cucurbitacins. Both one- and two-dimensional NMR techniques have been employed for the structural elucidation of new compounds 2D NMR, 1H-NMR, 13C-NMR, correlated spectroscopy (COSY), heteronuclear chemical shift correlation (HETCOR), attached proton test (APT), distortionless enhancement by polarization transfer (DEPT), and nuclear Overhauser effect spectroscopy (NOESY) are common techniques for determining the proton and carbon chemical shifts, constants, connectivity, stereochemistry, and chirality of these compounds [1,38,45-47]. 

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. 

C Spin lattice relaxation times, Tj, spin-spin relaxation times, T2, and nuclear Overhauser enhancements, NOE, for the a-carbons of PBLG of various molecular weights have been used to study transitions from rigid to flexible forms of this polymer (Allerhand and Oldfield, 1973). Effective rotational correlation times, reff, calculated from 7 - and NOE-values, for the a-carbons were 24-32 nanoseconds for the helical form and approximately 0-8 nanoseconds for the random coil (Allerhand and Oldfield, 1973). The transition from the a-helix to the random-coil of PLM causes the resonances of the a- and carbonyl carbons to move upfield 2-3 and 3-4 ppm respectively (Tadokoro et al., 1973), which is consistent with results obtained for PBLG and PCBO. Further work is required before the reasons for the chemical shift differences between the corresponding carbons in the helical and random-coil forms in deuterochloroform-TFA systems can be elucidated. Plots of chemical shifts and relaxation times vs. pH have been used to study the helix-coil transition of poly-L-lysine hydrochloride in aqueous solution (Saito and Smith,... 

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