Organic compounds, characterization using spectroscopy

Compared with IR and Raman spectroscopies, ultraviolet-visible (UV-Vis) spectroscopy has had only limited use in heterogeneous catalysis. Nevertheless, this spectroscopy can provide information on concentration changes of organic compounds dissolved in a liquid phase in contact with a solid catalyst, be used to characterize adsorbates on catalytic surfaces, provide information on the... 

Dendrimers are regarded as macromolecules with a structural precision comparable to proteins or organic compounds. Accurate analysis and quantitative identification of side products are required to optimize and adjust the reaction conditions for the synthesis of DAB-dendr-(NH2)n and DAB-dendr-(CN)n. Therefore, it is a prerequisite to characterize the products obtained unambiguously. To achieve complete molecular characterization of the polypropylene imine) dendrimers and the possible side-products, NMR- and IR-spectroscopy, HPLC, GPC and electrospray mass spectrometry are used. 

For species of higher than singlet multiplicity, matrix isolation coupled with ESR spectroscopy is often used for detection and characterization purposes. This is how the first examples of exotic organic compounds with quintet ground states were obtained. Since then, this kind of spectroscopy has been closely linked to the developments in the area of carbenes and nitrenes. 

Normal physicoorganic methods used for the formal identification of organic compounds are not applicable to organic astatine chemistry. The mass quantities required for the characterization of compounds by UV, NMR, and IR spectroscopy are in the region 10 -10" g molar concentrations of 10 preclude the application of such techniques. Mass spectrometry has not yet been developed to operate at such a concentration, except under special laboratory conditions (4). 

Nuclear magnetic resonance spectroscopy (NMR) is one of the most powerful analytical methods for identification and structure elucidation of organic compounds. Since NMR spectra are recorded in solution, no phase transfer like in MS is necessary when coupled with LC techniques. Additionally, NMR is a non-destructive detection technique, allowing the analyte to be transferred for characterization using additional methods. As of today, LC—NMR coupling was used in a wide range of applications [65,66,67,68,69,70,71],... 

Many applications have been reported in the field of biomolecular NMR spectroscopy which use RDCs for the refinement of three-dimensional structures. The approach is quite powerful and can also be applied to smaller molecules whenever the conformation of a molecule is important, as for example in the case of rational drug design. Traditionally, NMR in liquid crystals is applied on a multitude of small organic compounds to obtain their fully characterized structure. Most examples are measured on all kinds of aromatic systems as reported in refs. 204—212 other recent examples deal with substituted alkanes, aldehydes216,217 or bridged systems like norbomadiene.218 In general, these very detailed studies can be applied to molecules with up to 12 protons. 

The identification and quantitative determination of specific organic compounds in very complex samples is an area of intense current research activity in analytical chemistry Optical spectroscopy (particularly UV-visible and infrared absorption and molecular fluorescence and phosphorescence techniques) has been used widely in organic analysis. Any optical spectroscopic technique to be used for characterization of a very complex sample, such as a coal-derived material, should exhibit very high sensitivity (so that trace constituents can be determined) and extremely great selectivity (so that fractionation and separation steps prior to the actual analysis can be held to the minimum number and complexity). To achieve high analytical selectivity, an analytical spectroscopic technique should produce highly structured and specific spectra useful for "fingerprinting purposes," as well as to minimize the extent of overlap of spectral bands due to different constituents of complex samples. 

Tetrahydrocannabinol (THC), first isolated from Indian hemp, is the primary active constituent of marijuana. Although recreational use of cannabis is illegal in the United States, the FDA has approved THC in capsule form as an anti-nausea agent for chemotherapy patients and as an appetite stimulant for AIDS-related anorexia. Like other controlled substances,THC can be detected in minute amounts by modern instrumental methods. In Chapter 13, we learn about mass spectrometry and infrared spectroscopy, two techniques used for characterizing organic compounds. 

Another approach has been developed for the prediction chemical shifts in H-NMR spectroscopy. In this case, special proton descriptors were applied to characterize the chemical environment of protons. It can be shown that 3D proton descriptors in combination with geometric descriptors can successfully be used for the fast and accurate prediction of H-NMR chemical shifts of organic compounds. The results indicate that a neural network can make predictions of at least the same quality as those of commercial packages, especially with rigid structures where 3D effects are strong. The performance of the method is remarkable considering a relatively small data set that is required for training. A particularly useful feature of the neural network approach is that the system can be easily dynamically trained for specific types of compounds. 

Common methods used to characterize drugs and excipients are infrared (IR) and Raman spectroscopy. These techniques are sensitive to the structure, conformation, and environment of organic compounds. Because of this sensitivity, they are useful characterization tools for pharmaceutical crystal forms. Qualitative as well as quantitative analysis can be performed with both techniques. 

The ultraviolet-visible spectra of most compounds are of limited value for qualitative analysis and have been largely superseded by the more definitive infrared and mass spectroscopies. Qualitative analytical use of ultraviolet-visible spectra has largely involved describing compounds in terms of the positions and molar absorptivities of their absorption maxima, occasionally including their absorption minima. Indeed, some organic compounds are still characterized in terms of the number of peaks in the UV-visible spectrum and their absorbance ratios. This is usually the case in phytochemistry and photodiode array chromatography and when the analyst has a limited range of compounds to work with whose spectra are known to differ. In the pharmacopeias, however, absorbance ratios have found use in identity tests, and are referred to as Q-values in the U.S. Pharmacopia (USP). 

In spite of the fact that NMR spectroscopy has been used routinely in characterization of organic compounds, no systematic studies appear on the spectral properties of the parent tricyclic rings. This is mainly due to the lack of these compounds and to the difficulty of unambiguous assignments of the resonance signals in earlier times. The H NMR spectral data of some pyrrolonaphthyridine derivatives are shown below, assignments being made on the basis of H- H correlation spectroscopy (COSY) and HETCOR spectra <93(H)(36)1945,93(H)(36)2513>. 

Quantitative and qualitative analyses of inorganic and organic compounds can be performed by Raman spectroscopy. Raman spectroscopy is used for bulk material characterization, online process analysis, remote sensing, microscopic analysis, and chemical... 

The use of NMR spectroscopy, as an analytical technique to complement IR and UV-visible spectroscopy, mass spectrometry, purity, and elemental ( CHN ) analysis, completes the suite of methods that are available for the characterization of novel organic compounds, and many of the applications that follow involve NMR spectroscopy and at least one other technique. The coverage of the compounds and themes below is an overall selection of what is deemed to be significant in organophosphorus chemistry research involving multinuclear, especially NMR spectroscopy. NMR spectroscopy (including H, and NMR) experimental data, as mentioned earlier can be predicted accurately using ab mitio/DFT calculations. 

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