Spectroscopic Methods of Structure Elucidation

In exeeptional eases, terpenes erystallize after ehromatographie purifieation, thus enabling determination of flieir three-dimensional straemre in the solid state by X- 

Copyright 2006 WILEY-VCH ferlag GmbH Co. KGaA, Wfeinheim ISBN 3-527-31786-4 

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Identification of the constituents of complex materials such as coal may proceed in a variety of ways but generally can be classified into three methods (1) spectroscopic techniques, (2) chemical techniques, and (3) physical property methods whereby various structural parameters are derived from a particular property by a sequence of mathematical manipulations. It is difficult to completely separate these three methods of structural elucidation, and there must, by virtue of need and relationship, be some overlap. Thus, although this review is more concerned with the use of spectroscopic methods applied to the issues of coal structure, there will also be reference to the other two related methods. 

You should already be familiar with approximately half of the reactions listed in Table 9.2 from your introductory class. Moreover, you have probably tried to prepare an oxime, a phenylhydrazone, a 2,4-dinitrophenylhydrazone, or a semicarbazone. These compounds serve as crystalline derivatives with sharp and characteristic melting points for identifying aldehydes and ketones and for distinguishing them. When spectroscopic methods for structure elucidation were not available, such a means of identification was very important. 

Nuclear magnetic resonance (NMR) spectroscopy is the most powerful spectroscopic method for structural elucidation of organic molecules and is routinely used by organic chemists. Summarised below are common NMR active nuclei chemical shift data for NMR solvents, common impurities, and functional groups coupling constants and details of common NMR experiments used to determine the connectivity and stereochemistry of small organic molecules. 

From the practical standpoint one of the most appealing features of mass spectrometry is the requirement of minute sample quantities. Yet a great deal of information can be retrieved from a mass spectrum, even by the nonexpert with only a qualitative knowledge of mass spectrometry. Because the mass spectrum shows the mass of the molecule (molecular ion, Mt) and the masses of the fragments derived from it, the identification of relatively simple molecules is easier than by other spectroscopic methods. The combination of mass spectrometry with other spectral techniques provides a powerful and indispensable complement to the existing and constantly improving chemical methods of structure elucidation. 

Particular aspects of carotenoids have been reviewed before in this series Geometrical isomerism was treated by Zechmeister (181) in 1960 and the application of spectroscopic methods in structural elucidation by Weedon (172) in 1969. 

Weedon, B. C. L. Spectroscopic methods for structural elucidation of carotenoids. Fortschr. Chem. Org. Naturstoffe 27, 81 (1969). 

In principle all elements of the periodic table are accessible by NMR spectroscopic methods for structure elucidation usually the following isotopes are the most important H, C, N, O, and P. The following discussion will be limited to these nuclei, but it should be noted that the methods described here can be applied in principle to all other nuclei too. 

Coordination of reactive and/or unstable molecules to metal centers is a useful approach for their stabilization,1 and it presents unique opportunities for their characterization by spectroscopic methods and for elucidation of their structure. Moreover, under appropriate conditions the coordinated species can be chemically modified. In addition, displacement of the coordinated compound from the metal and its trapping in solution by reactions with suitable substrates can form the basis for useful synthetic methodology. 

The final section, on analytical chemistry, is a combination of structure-elucidation techniques and instrumental optimizations. Instrumental analysis can be broken into several steps method development, instrumental optimization, data collection, and data analysis. The trend today in analytical instrumentation is computerization. Data collection and analysis are the main reasons for this. The chapters in this section cover all aspects of the process except data collection. Organic structure elucidation is really an extension of data analysis. These packages use spectroscopic data to determine what structural fragments are present and then try to determine... 

Before the development and widespread application of spectroscopic methods for the elucidation of structure, confirmation of the class type of an unknown organic compound was completed by the preparation of two or more crystalline functional derivatives. If the compounds had been previously reported in the literature, agreement between the published physical constants of the derivatives with those prepared by the worker was accepted as proof of identity. In many cases, and particularly in natural product chemistry, functional group recognition led to oxidative, reductive, or hydrolytic breakdown into smaller carbon-containing fragments. These were, if necessary, separated, characterised and identified by derivative preparation. The reassembly of the jig-saw of fragments inferred by the identity of the fission products, then led to postulated structures. 

The complex process of structure elucidation usually includes chemical degradation, spectroscopic methods, and correlation analysis with known, related compounds. Occasionally, however, the feasibility of a mechanism that accounts for the formation of the problem compound structure—if its precursor is structurally clear—can also be used as an additional criterion. During the course of a reinvestigation of the reaction described here, serious difficulties were found with structural assignments of the isolated compound. To a number of spectroscopic incompatibilities was added the authors inability to find reasonable mechanistic routes that could explain the generation of a structure suggested by earlier researchers. 

Preparative chromatography is the process of using liquid chromatography to isolate a sufficient amount of material for other experimental or functional purposes. This section describes the use of preparative HPLC to isolate tens of milligrams of pure unknown compound(s) for the purpose of structure elucidation by spectroscopic techniques, which is often referred to as semipreparative HPLC. This section will focus primarily on preparative HPLC methods with the following parameters ... 

Weinges, K., Mattauch, H., Wilkins, C., and Frost, D. 1971. Oxidative coupling of phenols. V. Spectroscopic and chemical methods for structure elucidation of dehy-drodicatechin A. Justus Liebigs Ann. Chem. 754 124-36. 

Diverse spectroscopic methods have been employed to characterise triterpenes. Ultraviolet (UV) and infrared (IR) spectroscopy are not very useful techniques in elucidating the structure of triterpenes, but the former gives information about compounds with conjugated double bonds and the latter may provide some information about substituents like the hydroxyl group, ester carbonyl group or a,p-unsaturate carbonyl. Other physical data may be of interest to characterise new compounds, but the use of modem spectroscopic methods of nuclear magnetic resonance (NMR) and mass spectroscopy (MS) are essential for the structural determination. 

The application of spectroscopic methods for the elucidation of the structures of quinoline alkaloids has developed to the point where the constitutions of many new alkaloids have been established solely by UV, IR, NMR, and mass spectrometry. The NMR spectra of quinoline alkaloids have been discussed (Volume IX of this treatise) (4, 5), and the application of mass spectrometry has also been reviewed (6). Schemes 1 and 22 summarize NMR data for a selection of typical quinoline alkaloids. In discussing structure work spectroscopic data will be given only when they are of special significance. 

In many areas of chemistry, chemical methods of structure proof have given way to spectroscopic methods (Chapter 14) and analysis by mass spectrometry. In the field of carbohydrate chemistry, however, chemical reactions play an important part in the elucidation of the structures of unknown carbohydrates. The reason for this is that many proteins and lipids contain carbohydrates that have very complex structures, and these do not yield easily to spectroscopic methods. Also, since it is notoriously difficult to crystallize many carbohydrates, proof of structure by X-ray crystallography is often not possible. In this section we will discuss methods for determining determine the structure of a monosaccharide by chemical methods. Several synthetic reactions can be used to convert one monosaccharide into another. If the structure of a related monosaccharide is known, and the mechanism of the reaction that interconverts them is well understood, then the structure of the unknown monosaccharide can be established. 

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

Spectroscopic methods have been successfully applied to the elucidation of some details of the fine structure of isoxazole derivatives. Thus IR spectra revealed steric hindrance in the case of some 3,4,5-trisubstituted isoxazoles for phenylisoxazoles this results in the nonplanarity of the benzene and isoxazole rings and decreasing mutual interaction. 

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