Spectroscopy carbon chemical shifts

The presence of two different probes, namely, Gal C-l and Thr C7, giving rise to signals situated in two different, uncrowded regions of the spectrum, with chemical-shift differences (AdC-l up to 8.35 p.p.m., and Ad Thr C7, up to 6.7 p.p.m.), allows the facile detection of any racemiza-tion occurring at either, or both, of the asymmetric carbon atoms of the threonine. It would be difficult to distinguish a-D-Gal — L-Thr (27) from its P analog (28), or / -D-Gal — D-alloThr (58) from / -D-Gal — L-alloThr (56), on the basis of the anomeric-carbon chemical-shifts only. However, they can be differentiated on the basis of Thr Cr chemical-shift data. It should be noted that neither optical rotation nor -n.m.r. spectroscopy could have elucidated this point.85... 

Laurie was one of the first to apply two-dimensional (2D) NMR to carbohydrates. With students Subramaniam Sukumar and Michael Bernstein, and visiting scientist Gareth Morris, he demonstrated and extended the application of many of the directly observed 2D NMR techniques of the time. These included the homo- and hetero-nuclear 2D /-resolved techniques, delayed proton /-resolved NMR that allowed broad resonances to be suppressed, for example, those of dextran in the presence of methyl /Lxvlopyranoside. proton-proton chemical shift correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), proton-carbon chemical shift correlation (known later as HETCOR), and spin-echo correlated spectroscopy (SECSY). Trideuteriomethyl 2,3,4,6-tetrakis-<9-trideuterioacetyl-a-D-glucopyranoside served as a commonly used model compound for these studies. 

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

In that early work we relied much more on proton than carbon chemical shift trends. This was, in part, driven by the limited quantities of some of the twelve pure, synthetic, model compounds. However, the proton shift trends were also more meaningful than the carbon for this particular set of model compounds. It is our contention that proton chemical shift data should be used more frequently for this purpose and that this underutilization is largely a bias of technological origin. From the advent of C NMR spectroscopy chemical shift trends were recognized to be of primary importance. Relatively large field dispersion and the routine lack of coupling data predisposed... 

The early chapters introduce classical NMR spectroscopy. A thorough understanding of proton and carbon chemical shifts (Chapter 3) is required in order to initiate any analysis of spectra. The role of other nuclei is key to the examination of molecules containing various heteroatoms. An analysis of coupling constants (Chapter 4) provides information about stereochemistry and connectivity relationships between nuclei. The older concepts of chemical shifts and coupling constants are emphasized, because they provide the basis for the application of modern pulse sequences. 

Figure 7.16 Chart of carbon chemical shift in XPS spectra. (Reproduced with permission from D. Briggs and J.T. Grant, Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications and Surface Spectra Ltd, Chichester. 2003 IM Publications.)...

The dynamics of the molecular rotation of 2-pyridone in toluene, carbon tetrachloride, methanol, and water have been investigated at 305 K by 13C and 2H NMR spectroscopy. Both chemical shifts and relaxation times show that it forms stable hydrogen-bonded complexes in methanol and in water, reorienting as a complete unit and taking with it two solvent molecules. These solvated species are stable within the liquid-state temperature range, and reorient according to the hydrodynamic law as indicated by the 14N line width measurements (85MRC460). 

For proton and carbon spectroscopy, the chemical shifts of the solvent resonances need to be anticipated as these may occur in a particularly unfortunate place and interfere with resonances of interest. In proton spectroscopy, the observed solvent resonance arises from the residual protonated species (NMR solvents are typically supplied with deuteration levels in excess of 99.5%). For routine studies, where a few milligrams of material may be available, the lower specification solvents should suffice for which the residual protonated resonance is often comparable in magnitude to that of the solute. Solvents with... 

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]. 

Although two structures with double bonds can be identified by means of the combination of homonuclear 2D CX)SY and 2D HOHAHA spectroscopy, it can not be excluded that other possible structures with double bonds are present. For this reason we applied heteronuclear 2D experiments, with correlations between proton and carbon chemical shifts, resulting in unambigious identification of the unsaturated constituents (de Waard et al., 1993)). 

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