Spectroscopic methods, for identification

Maquelin K, Choo-Smith LP, van Vreeswijk T, Endtz HP, Smith B, Bennett R, Braining HA, Pup-pels GJ. Raman spectroscopic method for identification of clinically relevant microorganisms growing on solid culture medium. Anal Chem. 2000 72(l) 12-9... 

In the period 1910-1930 several distinguished chemists — Emil Fischer, Karl Freudenberg and Paul Karrer — made substantial contributions to early ideas on the gallotannins . But progress was severely hampered by the slow realisation that all the plant extracts consisted of mixtures of closely related metabolites and other phenols whose presence made separation and purification of the desired materials extremely difficult (97). Many of these problems have been relieved in recent years by application of various forms of chromatography and by concomitant use of spectroscopic methods for identification. 

Al-Zoubi, N., J.E. KoundoureUis, and S. Malamataris. 2002. FT-IR and Raman spectroscopic methods for identification and quantitation of orthorhombic and monocfinic paracetamol in powder mixes. J. Pharmaceut. Biomed. Anal. 29 459-467. 

Prestera T, Fahey JW, Holtzclaw WD, Abeygunawardana C, Kachinski JL, Talalay P (1996) Comprehensive chromatographic and spectroscopic methods for the separation and identification of intact glucosinolates. Anal Biochem 239 168-179... 

It should be mentioned that the described spectroscopic method for determining absolute concentrations of hydroxyl is very reliable. As spectral determinations do not interrupt the reaction course, and as identification of OH by the specific hydroxyl absorption spectrum may be carried out simultaneously, the spectroscopic method should be given preference over other methods. 

Identification of the intermediates in a multi-step reaction is the major objective of studies of reaction mechanisms. It is most useful to study intermediates present in low concentrations without chemical interference with the reacting system, i.e. by rapid spectroscopic methods. The most common methods in organic chemistry include ultraviolet-visible (UV-VIS), IR, and EPR spectroscopy. In principle, all other spectroscopic methods for the detection of reaction intermediates are also applicable provided that they are fast enough to monitor the intermediate and able to provide sufficient structural information to assist in the identification of the transient species. 

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. 

A possible solution to the above problems would be the triple-dimensional analysis by using GC x GC coupled to TOFMS. Mass spectrometric techniques improve component identification and sensitivity, especially for the limited spectral fragmentation produced by soft ionization methods, such as chemical ionization (Cl) and field ionization (FI). The use of MS to provide a unique identity for overlapping components in the chromatogram makes identification much easier. Thus MS is the most recognized spectroscopic tool for identification of GC X GC-separated components. However, quadru-pole conventional mass spectrometers are unable to reach the resolution levels required for such separations. Only TOFMS possess the necessary speed of spectral acquisition to give more than 50 spectra/sec. This area of recent development is one of the most important and promising methods to improve the analysis of essential oil components. 

Spectroscopic methods of identification and estimation of excited species are clearly of great importance, although several other techniques have been employed (mass spectrometric, chemical, calorimetric, etc., and, for radicals, esr). 

These examples show the utility of using C NMR spectroscopy, dissolved lignin, and stable isotopes as molecular tracers of photodegradation of CDOM. Other tools of mass spectroscopy and compound separation and identification should provide additional information on changes to DOM as it is photodeg-raded. Recently, several groups have presented spectroscopic and spec-trophotometric methods for identification of chromophores [47-49,131]. 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

By far the most important spectroscopic method for this purpose is IR spectroscopy. In combination with DFT or ab initio calculations matrix IR spectroscopy has become a very powerful tool for the reliable identification of reactive and unusual molecules. In addition, isotopic labeling with is frequently used to assign the IR spectra of oxidized species. However, a prerequisite for this technique is the availability of suitable photochemical or thermal precursor molecules of the reactive silicon species. During the last years, we have published details of the oxidation mechanism of alkyl-substituted silenes 2. °In this chapter, our mechanistic studies on the oxidation of silylenes 1 using the matrix-isolation technique are summarized. 

Schubert et al. (1967) have isolated cholesterol from Streptomyces olivaceus. This is a species of the order Actinomycetales. These workers used chromatography, mass spectrometric, and infrared spectroscopic methods for the isolation and identification. S. olivaceus can degrade as well as synthesize cholesterol. 

An essential requirement for all synthetic work is the rigorous characterization of all intermediates and products. In this Chapter, the application of various spectroscopic methods for the unequivocal identification of structure and stereochemistry of carotenoid products and intermediates in their total synthesis will be discussed. The examples covered are listed in Table 1. 

In general, experimenters performing recent hydrolysis studies have recognized some of the problems in earlier work (such as the need to work under an inert atmosphere in order to avoid carbonate contamination), and these studies should be more reliable, especially when coupled with direct spectroscopic methods for species determination. The major problems associated with solubility studies remain the attainment of steady-state conditions, the identification of solid phases, and control of the redox conditions. Measurements of hydrolysis reactions and solubilities at temperatures other than 25° C or ambient are almost nonexistent... 

As detailed below, the chapters of this book, based upon various SFB research projects, intend to deepen our insight into a range of catalytic transformations, to provide rational concepts for catalyst optimization, and to develop synthetic procedures employing molecular catalysis. Part I focusing on Mechanisms of elementary reactions in catalytic processes highlights both theoretical and spectroscopic methods for the investigation of the dynamics of individual reaction steps. This includes the structural identification of frequently labile and thus transient intermediates. Case histories illustrating the interplay between... 

Spectroscopic techniques are of great importance in polymer science, as they are in all branches of the chemical sciences, and these are dealt with in chapters 3 and 4. NMR is of prime importance as a method for identification and elucidation of the structure of polymer chains, and a complete chapter has been devoted to this technique. In recent years it has become an increasingly sophisticated and correspondingly expensive technique, requiring considerable skill in operating the spectrometer and in interpreting spectra, but it is still available in a simple and routine form in many laboratories. Vibrational spectroscopy encompasses infrared (IR) and Raman. These techniques, IR in... 

For the application of spectroscopic methods for the identification and structure determination of polynuclear hydrocarbons see (29), and for the application of correlation analysis (chemical shifts of protons) for the identification of polyacetylnaphthalenes in mineral oil see (30). 

Spectroscopic methods are finding increasing use in the characterization and analysis of polymers. All of the methods that are employed were developed initially for use with low-molar-mass materials and they have been extended for the analysis of polymers. The spectrum obtained for a particular polymer is often characteristic of that polymer and can therefore be used for identification purposes. Polymer spectra can be surprisingly simple given the complex nature of polymer molecules and they are often similar to the spectra obtained from their low-molar-mass counterparts. This can make the analysis of the spectra a relatively simple task allowing important spectral details to be revealed. In fact, certain information can only be obtained using spectroscopic methods. For example, nuclear magnetic resonance (n.m.r.) is the only technique that can be used to measure directly the tacticity of a polymer molecule. 

Other spectroscopic methods such as infrared (ir), and nuclear magnetic resonance (nmr), circular dichroism (cd), and mass spectrometry (ms) are invaluable tools for identification and stmcture elucidation. Nmr spectroscopy allows for geometric assignment of the carbon—carbon double bonds, as well as relative stereochemistry of ring substituents. These spectroscopic methods coupled with traditional chemical derivatization techniques provide the framework by which new carotenoids are identified and characterized (16,17). 

The presence of iminium salts can be detected by chemical means or by spectroscopic methods. The chemical means of detecting iminium salts are reactions with nucleophiles and are the subject of this review. The spectroscopic methods are more useful for rapid identification because with the large number of model compounds available now the spectroscopic methods are fast and reliable. The two methods that are used primarily are infrared and nuclear magnetic resonance spectroscopy. Some attempts have been made to determine the presence of iminium salts by ultraviolet spectroscopy, but these are not definitive as yet (14,25). 

In general however, ozonolysis is of limited synthetic importance. For quite some time ozonolysis has been an important tool for structure elucidation in organic chemistry, but has lost its importance when spectroscopic methods were fully developed for that purpose. The identification of the aldehydes and/or ketones obtained by ozonolysis of unsaturated compounds allowed for conclusions about the structure of the starting material, but has practically lost its importance since then. 

---