Interpretation of IR Spectra

Nitro compounds Compounds containing nitro groups are identified by the appearance of two strong bands near 1550 and 1380 cm-1. These absorptions appear at lower wavenumbers if the nitro group is conjugated with a benzene ring. 

Interpretation of the IR spectrum of an unknown compound is an art that requires experience and practice. The more spectra you examine, the easier it will become to recognize the absorption due to an O—H group and to differentiate between that band and one that results from an N—H group. 

Infrared spectra of compounds belonging to each of the major functional group classes are provided in the figures in this chapter. Each figure has a summary of the im- 

3400-3250 1 —N— H Weaker intensity and less broad than O—H NH2 shows two bands, NH shows one 

3000-2850 —C- H 1 O II line between this type of C—H bond and the preceding type 

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Woodruff and co-workers introduced the expert system PAIRS [67], a program that is able to analyze IR spectra in the same manner as a spectroscopist would. Chalmers and co-workers [68] used an approach for automated interpretation of Fourier Transform Raman spectra of complex polymers. Andreev and Argirov developed the expert system EXPIRS [69] for the interpretation of IR spectra. EXPIRS provides a hierarchical organization of the characteristic groups that are recognized by peak detection in discrete ames. Penchev et al. [70] recently introduced a computer system that performs searches in spectral libraries and systematic analysis of mixture spectra. It is able to classify IR spectra with the aid of linear discriminant analysis, artificial neural networks, and the method of fe-nearest neighbors. 

Adding potassium hydroxide, KOH, to a melt containing KF and a 0.1 mol fraction of K2TaF7 leads to the appearance of an additional band at 900 cm 1, as shown in Fig. 79 [342]. This band corresponds to TaO bond vibrations in TaOF63 complex ions. Interpretation of IR spectra obtained from more concentrated melts is less clear (Fig. 80). The observed absorption in the range of 900-700 cm 1 indicates the formation of oxyfluoride polyanions with oxygen bridges. ..OTaO. The appearance of a fine band structure could be related to very low concentrations of some isolated components. These isolated conditions prevent resonance interaction between components and thus also prevent expansion of the bands by a mechanism of resonance [362]. 

In this potential the energy levels are no longer equally spaced, and overtones, i.e. vibrational transitions with An > 1, become allowed. The overtone of gaseous CO at 4260 cm-1 (slightly less than 2x2143 = 4286 cm ) is an example. For small deviations of r from equilibrium, however, the Morse potential is successfully approximated by a parabola, and for the interpretation of IR spectra the harmonic oscillator description is usually sufficient. 

Elucidation of Structural Fragments by Computer-Assisted Interpretation of IR Spectra... 

Another technique which is very helpful in the identification of various systems using spectroscopic properties is the determination of the experimental polarizations of the observed transitions. This also greatly simplifies the overall interpretation of IR spectra of organic systems, particularly in the case of latter molecules. By obtaining these experimental polarizations one is able to break down a complicated spectrum into its symmetry groups and compare them each individually with the calculated frequencies and intensities of the corresponding symmetries. 

A band close to 880 cm-1, can be attributed to a P-glycosidic bond. Another band in the region of 846 cm-1 could indicate the presence of an a type involving galactose and/or mannose units. The principles of the method proposed by Barker et al. (11) proved to be a useful tool for the interpretation of IR spectra obtained from EPS from Rhizobium sp. EQ1. 

Uncertainty in such interpretations of IR spectra is more likely with longer pep-... 

Raman spectra of barbituric acid could not be obtained because of its high luminescent backround,99,100 but Willis et al. gave interpretations of Raman spectra for eight barbiturates,89 and Barnes et al. reported a detailed analysis of IR and Raman spectra for the whole frequency range for 1-methyl-, 1,3-dimethyl-, and 5,5-diethylbarbituric acids.92-94 Similar IR and Raman spectra analyses were described for 5-ethyl-5-phenylbarbituric acid and its N-mono-and N,N-disubstituted derivatives.101 A detailed interpretation of IR spectra of the 1-acyl and l-(4 -aminobutynyl) derivatives was also reported.102,103... 

We thank Patrick Buat-Menard for his comments on an early version of the manuscript and Norman Sheppard and John Hedges for discussions on the interpretation of IR spectra. During the period of this work the first author was supported by awards from the Royal Society, initially a studentship from the Rutherford Memorial Committee and subsequently a postdoctoral fellowship under the European Exchange Scheme. 

Based on extensive experience, the following scheme for interpretation of IR spectra based on group frequencies was worked out No systematic procedure of general vaHdity exists, a reasonable way to proceed is the following ... 

It should always be recalled that the aim of structural analysis based on characteristic IR frequencies is to identify structural groups, not to ascertain the total molecular structure of the analyte. Even the identification of structural groups should be based on different chemical and physical data, not just on a single IR spectrum. In general, it is impossible to deduce the total molecular structure of a molecule solely by interpretation of IR spectra by means of correlation tables. Sometimes, the absence of a particular absorption may be more informative than its presence. 

According to Tarte, in the interpretation of IR spectra of inorganic aluminates, the characteristic frequency ranges are "condensed" AIO4 tetrahedral in the 900-700 cm-i, "isolated" AIO4 tetrahedral 800-650 cm-i, "condensed" AlOe octahedral in the 680-500 cm-i. 

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