Infrared spectroscopy functional group identification with

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

One of the main routine uses of infrared spectroscopy is identification of specific functional groups present in an unknown molecule and, as a result, further characterization of the unknown. By far the most common example involves the carbonyl group. Location of a strong band in the infrared in the vicinity of 1730cm is almost certain proof that carbonyl functionality is present. This confidence is based on the fact that the characteristic frequency (the CO stretch in this case) is isolated, that is to say, it is sufficiently far removed from the other bands in the infrared spectrum to not be confused with them. It also assumes that carbonyl groups in different chemical environments will exhibit similar characteristic... 

An integrated GC/IR/MS instrument is a powerful tool for rapid identification of thermally generated aroma compounds. Fourier transform infrared spectroscopy (GC/IR) provides a complementary technique to mass spectrometry (MS) for the characterization of volatile flavor components in complex mixtures. Recent improvements in GC/IR instruments have made it possible to construct an integrated GC/IR/HS system in which the sensitivity of the two spectroscopic detectors is roughly equal. The combined system offers direct correlation of IR and MS chromatograms, functional group analysis, substantial time savings, and the potential for an expert systems approach to identification of flavor components. Performance of the technique is illustrated with applications to the analysis of volatile flavor components in charbroiled chicken. 

Microchemical reactions, which with care and suitably sized microscale equipment can be carried out on nanogram amounts of material, can be used to determine the presence or absence of specific functional groups, or determine the numbers, positions, and even geometries of double bonds. The application of microchemical reactions to pheromone identification has been reviewed in detail by Attygalle (1998). Coupled GC-Fourier transform infrared spectroscopy has also found occasional use in pheromone identification (Attygalle et al., 1995 review, Leal, 1998). 

Mid-infrared (IR) spectroscopy is a well-established technique for the identification and structural analysis of chemical compounds. The peaks in the IR spectrum of a sample represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. Thus, the IR spectrum of a compound is one of its most characteristic physical properties and can be regarded as its "fingerprint." Infrared spectroscopy is also a powerful tool for quantitative analysis as the amount of infrared energy absorbed by a compound is proportional to its concentration. However, until recently, IR spectroscopy has seen fairly limited application in both the qualitative and the quantitative analysis of food systems, largely owing to experimental limitations. 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

Infrared spectroscopy (IR) is also a very useful technique for characterizing stmcture. IR spectroscopy can be used for identification purposes as well as for monitoring the progress of a chemical reaction. Comparisons of the positions of absorptions in the IR spectrum of a sample with the characteristic absorption regions, leads to identification of the bonds and functional groups present in the sample. For example, the chemical stmctures of polyimide-silica hybrid films were confirmed by IR spectroscopy by the appearance of two absorptions at 1,100 and 830 cm indicating the formation of the Si-O bonds. 

Modern organic chemists rely on nuclear magnetic resonance spectroscopy for day-to-day identification of organic molecules. It is a powerful technique that allows one to count protons, identify the chemical environment (actually, the magnetic environment) of different protons, and predict how many neighbors each proton has. With this information, as well as the empirical formula and functional group information obtained from mass spectrometry and infrared spectroscopy, determination of the structure of organic molecules is usually feasible. 

An unequivocable identification of an unknown component is unlikely by the chromatographic process alone. Not the least of the reasons for this is the need for the comparisons of standards, thereby assuming reasonable prior assurance of the possible identity of the unknown. Certainly the more discrete pieces of information obtainable concerning an unknown compound, the easier it will be to obtain confident identification. Microchemical tests such as functional group classification, boiling point, elemental analysis, and derivative information, as well as infrared spectroscopy, coulometry, flame photometry, and ultraviolet (UV)-visible spectroscopy are also useful aids when used in conjunction with gas chromatographic data. 

Infrared (IR) spectra of organic compounds are characteristic of various functional groups in the molecules. IR spectral information is somewhat complementary to mass spectral information. Therefore, the combination of GC with IR spectroscopy is, after GC/mass spectrometry, the second most important structural identification tool. Since conventional IR spectroscopy is less sensitive than most GC detectors, the necessary sensitivity enhancement is achieved through the use of Fourier transform techniques. With the advent of refined optical systems and fast computational techniques, the combination of GC with Fourier-transform IR spectrometry is becoming widely used, although its sensitivity is currently less than that of mass spectrometry. Special optical cells were designed for the purposes of this combination. 

However, it is clear that vibrational spectroscopy has considerable use beyond the identification of polymorphs and solvates. The infrared spectra obtained on the polymorphs of acetohexamide and selected derivatives has been used to study the tautomerism of the drug compound [127]. It was deduced in this work that Form A existed in the enol form, stabilized by the intramolecular bonding between the O—H and groups that produces a six-membered ring. Form B was characterized by the existence of the keto form, with the urea carbonyl group being intermolecular bonding to a sulfonamide N—H functionality. This behavior can be contrasted with that noted for spironolactone, where no evidence was found for the existence of enolic tautomers in any of the four polymorphs [128]. 

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