Nuclear magnetic resonance spectroscopy interpretation

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

High-resolution nuclear magnetic resonance spectroscopy also has the potential for rapid sample throughput and could expand the range of metabolites readily detectable. The instrumentation is expensive and interpretation requires considerable... 

Nuclear magnetic resonance spectroscopy is a powerful and widely-used tool for investigating structure. Its utility is enhanced by the use of computational chemistry to aid in the interpretation. In this work we present an example of the use of calculated nuclear magnetic resonance parameters to help elucidate the role of alkoxyalkylsilanes in Ziegler-Natta catalysis. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

Detailed descriptions of the theory and instrumentation of nuclear magnetic resonance Spectroscopy can be found elsewhere (see Bibliography). In this chapter the important features of the NMR spectrum and their use in the interpretation of spectra are described, together with details of certain special procedures which assist interpretation. 

Nuclear magnetic resonance spectroscopy can be used to distinguish between the phosphate esters of steroids. The free steroids can be distinguished by infra-red spectrophotometry, but the phosphate esters are sufficiently polar to give rise to absorption bands that dominate the IR spectra and make distinction difficult. The NMR spectra of these steroid esters are not subject to this interference, and although they may be very difficult to interpret, they do provide the necessary distinction. 

Solid state nuclear magnetic resonance spectroscopy provides information on the environment of individual atoms. In essence, the change in environment of any atom can arise from two factors, which usually are not separable in the interpretation of the SSNMR spectra, but are conceptually independent. Since different polymorphs are different crystal structures, it is expected that the crystal environment of at least some atoms will differ from polymorph to polymorph (Section 2.4.2). In addition, since the molecular conformation may also vary among polymorphs (Section 5.6), the change in the environment of an atom due to conformational differences will also be reflected in the SSNMR (Levy et al. 1980 Bugay 2001 Strohmeier et al. 2001). 

To determine the nature of the silicon moieties in a polymer, clearly the easiest method would be a technique that provides a direct observation of the silicon atom and meaningful, interpretable information on the atom. Nuclear magnetic resonance spectroscopy tuned to the Si isotope ( Si NMR) is a tool of this nature it can directly probe the state of the silicon atom, and with it one can often readily determine the extent to which Si-O-Si crosslinks (fi-om silanol condensation), have formed. One can observe spectra of silicon-containing compounds either dissolved in a solvent or in the solid state. Liquid-state Si NMR, while the most sensitive, cannot be used quantitatively on heterogeneous systems such a latex formulations. Therefore, one must separate the liquid and solid portions of the latex (without heat, which would promote hydrolysis and condensation) and use the solid residue for the Si NMR experiments. 

Besides the direct relations between orbitals and spectroscopy outlined above, there are many indirect relations which have to do with the interpretation of various spectral parameters in other branches of spectroscopy. We shall illustrate this with the main spectral parameters in nuclear magnetic resonance spectroscopy NMR chemical shifts and nuclear spin-spin coupling constants. 

G. J. Boender, S. Vega and H. J. M. de Groot, A physical interpretation of the Floquet description of magic angle spinning nuclear magnetic resonance spectroscopy. Mol. Phys., 1998, 95, 921-934. 

Nuclear magnetic resonance spectroscopy provides the most conclusive evidence of both identity and purity [90] but few laboratories are equipped with such a resource and even fewer researchers with the experience to interpret the resulting data. Gas chromatography can be used to assess the chiral purity of derivatives, and mass spectrometry (MS) is a particularly sensitive and accurate measure of product purity. Use of electrospray ioni-... 

The most common and important complex ions are hydrated metal ions. The coordination numbers and structures of some of these simple complexes have been determined. Isotope dilution techniques were used to show that Cr and Al are bonded rather firmly to six water molecules in aqueous solutions. The interpretation of the visible spectra of solutions of transition metal ions using CFT indicates that ions such as Mn, Fe, Co, Ni, Cr, and Fe are octahedral [M(H20)6] species. For non-transition metal ions it has been more difficult to obtain structural information. Flowever, nuclear magnetic resonance spectroscopy demonstrates that Be in aqueous solution is surrounded by four water molecules. These data support the importance of six coordination. The only exception cited here is Be, an element which obeys the octet rule. 

Nuclear magnetic resonance spectroscopy has been used to study self-ossociation in promethazine hydrochloride, in 2-butyl-3-benzofuranyl 4-[2-(diethylamino) ethoxy]-3,5-diiodo-phenyl ketone hydrochloride (SKF 33134A),40ond in d-propoxyphene hydrochloride. Florence has measured the properties of p-diethylaminoethyl diphenylpropylacetate hydrochloride (SKF 525-A) by light scattering, surface tension, and microelectrophoretic techniques. He suggests that caution should be exercised in the interpretation of enzyme inhibition results obtained with a compound of this type since it exhibits surface activity, and surfactants are known to exert an appreciable effect on certain enzyme systems. 

As mentioned before (Section II,E,3), the determination of tac-ticity by X-ray analysis is limited by the requirement that the polymer be crystalline. For the study of poly (methyl methacrylate), which may or may not be crystalline, nuclear magnetic resonance spectroscopy has been more useful. In order to interpret the spectra, it has been found necessary to describe the stereochemistry of a unit by the configurations on both sides. Therefore, an isotactic configuration, or isotactic triad, is one where the central unit is fianked by units of the same asymmetry, that is ddd or III. Similarly, for a syndiotactic triad, the stereochemistry is did or Idl. To overcome the disadvantages of the term atactic, a new term heterotactic was introduced. The stereochemistry for heterotactic configurations is, therefore, Idd, dll, lid, and ddl. 

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