Differential nuclear Overhauser effects

Quantitative information may also be gained from the data. If the amino acid composition is accurately known, it may prove possible to integrate portions of the 13C-n.m.r. spectra in order to gain quantitative information about the oligosaccharide structure. This also assumes that no differential nuclear Overhauser effects (n.O.e. s) exist between the various resonances and that none of the resonances in question are attenuated by the use of short recycle-times. 

Tetracoordinate l,2A4-oxaselenetane 47 is synthesized by a ring-closure reaction of /3-hydroxylalkyl selenide in 54% yield <1997CC1671>. Similarly, oxaselenetanes 48, 35, and 36 are obtained in 44%, 56%, and 65% yields, respectively (Scheme 14) <1997CC1671, 1998PS501>. These compounds were found to be air stable, colorless plates at room temperature, and the relative stereochemistry between the 3- and 4-positions of 35 and 36 has been determined by differential nuclear Overhauser effect (NOE) experiments. 

The protons in nonaroma tie pyrrolo[l,2-a]imidazoles show normal chemical shifts for this type of compound <89CB95>. In some cases <92JOC2347>, differential nuclear Overhauser effect (DIFNOE) experiments were used to assign signals. 

Differential nuclear Overhauser effects are either small or absent, so that integrals of peak areas can be used to calculate the relative % incorporation. 

From the NMR data it can be concluded that the propylene units occur in predominantly isotactic head-to-tail seqnences and that the ethylene units are incorporated as isolated units only. The NMR method can be used to provide reference standards for the less time consuming infrared method. Provided the infrared method is calibrated in this way, excellent agreement is obtained between the infrared and NMR methods for copolymers containing >95% propylene. No corrections need to be made for differential nuclear Overhauser effects since these have been shown to be constant for the major resonances in low ethylene - propylene copolymers. 

The structures of the di- and trimeric profisetinidins from Pithecellobium dulce (Guamii-chil) were rigorously corroborated via synthesis.The synthetic approach was additionally motivated by the precariousness of unequivocally differentiating between 2,3-cis-3,4-trans-and 2,3-c7.s-3,4-c7.s-confugurations of the chain-extension units on the basis of H NMR coupling constants.Furthermore, the powerful nuclear Overhauser effect (NOE) method for differentiating between 2,4-cis- and 2,4-tra i -substitution is less useful at the di- and trimeric levels due to the adverse effects of dynamic rotational isomerism about the interflavanyl bond(s) on NMR spectra at ambient temperatures. 

Nuclear Overhauser effect (NOE) difference measurements were used to assign structure 79 for the product of reaction of diphenylnitrile imine with 5-ethylsulfonyl-2-methyl(27/)pyridazinone. Thus in the H NMR spectrum the ot/, o-protons of the arylhydrazino moiety (which were identified by two-dimensional heteronuclear multiple quantum correlation (2-D HMQC) spectroscopy) were shown in differential NOE (DNOE) experiment to be significantly enhanced on irradiation of pyridazine hydrogen H-7, proving their steric proximity <2000JST13>. 

However, the differences in the chemical shifts of the olefinic protons are generally not very large. Moreover, an overlap of the ranges is often observed. Dahumel and coworkers24 examined a number of enamines and were able to differentiate the E- and Z-isomers by the nuclear Overhauser effect (NOE) see Figures 1 and 2. 

Nuclear magnetic resonance spectroscopy plays a major part in the elucidation of the structures of these bases, and continues to be the most valuable technique in structural work in the series. Pulsed nmr difference spectroscopy of small nuclear Overhauser effects between aromatic protons and o-methyl and /-methyl groups has permitted differentiation between two possible isomeric structures for dihydrodaphnine (D. Neuhaus et al.. Tetrahedron Letters, 1981, 2933. 

The differentiation between the 15N chemical shifts in dyes existing practically completely in their hydrazone forms is easy because —NaH— signals are shifted upfield and they are detected as strong negative signals because of the negative nuclear Overhauser effect (Table 18), 88... 

Abbreviations CCK, cholecystokinin RNase. ribonuclease G-protein, guanine nucleotide binding protein GPCR, G-protein-coupled receptor. SDS sodium dodecylsulfate, CTAH. hexadecyltrimethyl ammonium hydroxide DMPC. di-myristoylphosphatidylcholine DPPC, di-palmitoylphosphatldylcholine CMC, critical micellar concentration SUV, small unilamellar vesicles CD, circular dichroism NMR, nuclear magnetic resonance hs-DC, high sensitivity differential scanning calorimetry IR-ATR, infrared attenuated total reflection spectroscopy NOE, nuclear Overhauser effect MD, molecular dynamics DMSO, dimethylsulfoxide TFE, trifluoroethanol for abbreviations of peptides see tables land 2, and fig. 11. 

The analytical situation becomes much more complicated with open-chain tetrapyrrole pigments, which occur as bile pigments in nature (Falk, 1989). Their conformation is by no means limited to planar or ruffled-planar, but all kinds of cw-tran -configured diastereomers are known and produce extremely differentiated and often complicated UV/vis spectroscopic shifts and extreme changes in solubility, caused by enormous changes in neighbor group interactions. The latter are best analyzed by NOE (nuclear Overhauser effects) (Sanders, 1996). 

For the chemist today, the importance of solution-state NMR is well established. Individual nuclei within a molecule are differentiated on account of their chemical shift, while connectivities, which permit spectral assignment, are identified by through-bond J couplings. Through-space proximities, which yield information about three-dimensional structure, are accessible by experiments which exploit the nuclear Overhauser effect (NOE). Moreover, a host of multi-dimensional experiments have been developed which further enhance the information content [1, 2]. In many cases, however, the most appropriate sample to study molecular structure and dynamics is the solid. The purpose of this article is to give an overview of the different solid-state NMR methods which are available in such cases. Our focus is on the structural and dynamic information which a particular method can dehver, and, at most, only a simple qualitative explanation of how the experiment works will be given, although the relevant literature will always be cited, such that the interested reader can find details about, e. g., the experimental implementation. 

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