Carbon-13, Overhauser effect spectroscopy

Coordination of [6Li]-a-(phenylthio)benzyllithium with 9 was studied by H Li-HOESY NMR technique (HOESY = heteronuclear Overhauser effect spectroscopy) <1998JOM(550)359>. This interaction results in the formation of contact ion pair and ligand and tetrahydrofuran (THF) solvent molecules compete for three coordination sites. The fourth site is occupied by the anionic benzylic carbon atom in an qMike manner. 

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

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]. 

The heteronuclear variant of NOESY is HOESY (Heteronuclear Overhauser Effect SpectroscopY). Figure 4.60 shows a HOESY spectmm for the tetramethylethylenediamine (tmeda) adduct of 2-lithio-l-phenylpyrrole, whose dimeric structure is also shown in the figure. The normal H and Li NMR spectra are shown along the axes of the 2D contour plot, which contains just three peaks. The lithium atom is therefore close (i.e. less than about 3.5 A) to three different sets of three protons, which can be readily identified as H(7) and H(ll), equivalent by virtue of fast rotation about the N(l)-C(6) bond in solution, H(3), and the methyl protons of the tmeda ligand. Note that the hydrogen atoms are numbered according to the numbers of the carbon atoms to which they are attached. The close contact between Li and H(11) seen in the crystal structure is thus maintained in solution, and it is of chemical significance, as it leads to... 

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. 

Unfortunately, in l3C NMR spectroscopy one cannot simply relate the relative intensities to the number of equivalent carbon atoms in a molecule relaxation phenomena and something called the nuclear Overhauser effect have to be taken into account But it is not our purpose to drag you through the subtleties of this technique, you must get this from a more specialized treatment. Here we will simply give you a feel for the type of information on polymer microstructure... 

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). 

The structures of some of these compounds have been established by proton and carbon-13 spectroscopy <83JA5390>. In some cases nuclear Overhauser effect (NOE) <85T603> or NMR (NOESY) experiments <94CC1383> have been used. 

See also Liquid Chromatography Liquid Chromatography-Nuclear Magnetic Resonance Spectrometry. Nu-ciear Magnetic Resonance Spectroscopy Principles Instrumentation. Nuclear Magnetic Resonance Spectroscopy-Applicable Elements Carbon-13 Nitrogen-15. Nuclear Magnetic Resonance Spectroscopy Techniques Nuclear Overhauser Effect. 

UzAWA, J., and S. Takeuchi Application of Selective C- H Nuclear Overhauser Effects with Low-power H-Irradiation in Carbon-13 NMR Spectroscopy. Org. Mag. Reson. 11, 502 (1978). 

Uzawa J, Takeuchi S 1978 Application of selective C-[ H] nuclear Overhauser effects with low-power H irradiation in carbon-13 NMR spectroscopy. Org Magn Res 11 502-506... 

N.m.r. spectroscopy has been used extensively in the structural elucidation of halogenated monoterpenoids e.g. refs. 424,426,427) two further papers which report useful data concern C n.m.r. y-effects and the differentiation of some brominated carbons from chlorinated carbons due to shortened C n.m.r. spin-relaxation times and reduced values of nuclear Overhauser enhancement. 

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