Hydrogen-nuclear magnetic resonance structural information

We saw in Chapter 12 that mass spectrometry provides information about a molecule s formula and that infrared spectroscopy provides information about a molecule s functional groups. Nuclear magnetic reson r spectroscopy does not replace either of these techniques rather, it comp.c ments them by providing a map of the carbon-hydrogen framework of an organic molecule. Taken together, NMR, IR, and mass spectrometry often make it possible to determine the complete structures of even very complex molecules. 

Under these circumstances, it is inevitable to estimate the active conformation of a chemical by another approach. The quantitative structure-activity relationship (QSAR) O) is one of the important approaches, particularly when the target site of a biologically active compound is unknown. Although X-ray crystallography is also helpful to estimate the active conformation, it provides the conformational information in a solid phase. More important is the conformation of a chemical in solution, which can be assigned in part by spectroscopic studies. Nuclear magnetic resonance (NMR) spectroscopy has been utilized to estimate the relative orientation of each atom in a molecule (2-5). Infra-red ( IR) spectroscopy is sometimes a useful tool, especially when hydrogen bonds are present ( ). Recently,... 

Today, a number of different instrumental techniques are used to identify organic compounds. These techniques can be performed quickly on small amounts of a compound and can provide much more information about the compound s structure than simple chemical tests can provide. We have already discussed one such technique ultraviolet/visible (UVA/is) spectroscopy, which provides information about organic compounds with conjugated double bonds. In this chapter, we will look at two more instrumental techniques mass spectrometry and infrared (IR) spectroscopy. Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features of the compound. Infrared spectroscopy allows us to determine the kinds of functional groups a compound has. In the next chapter, we will look at nuclear magnetic resonance (NMR) spectroscopy, which provides information about the carbon-hydrogen framework of a compound. Of these instrumental techniques, mass spectrometry is the only one that does not involve electromagnetic radiation. Thus, it is called spectrometry, whereas the others are called spectroscopy. 

The application of nuclear magnetic resonance (NMR) spectroscopy to the study of metal-ligand equilibria is growing rapidly. This technique can be used not only to determine stability constants but also to provide information on the structures of the complexes formed as well as the location of the binding sites on the ligand of the hydrogen and metal ions. ... 

According to the majority of the theoretical and experimental studies available in the literature, the simpler reasonable description of a proton in solution is the hydronium (HjO ) ion. The shape and size of the HjO" ion is fairly well established from nuclear magnetic resonance (NMR) data on solid acid hydrates (Bockris and Reddy [90]). The HjO ion exhibits a rather flat trigonal pyramidal structure with the hydrogen atoms at the corners of the pyramid and the oxygen at the center (Fig. 7.3). But no reliable experimental geometric information is available for the hydronium ion in solution. 

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