Group frequencies, correlation chart

The value of infrared spectrometry as a means of identification of unknown compounds and to investigate structural features is immense. Spectra are used in an empirical manner by comparison of samples with known materials and by reference to charts of group frequencies. A simplified correlation chart is shown in Table 9.8. The interpretation of infrared spectra is best considered by discussing the prominent features of a representative series of compounds. 

For an unknown compound without a reference standard, important structural information can be obtained from the IR spectrum. Fig. 9 is a simplified illustration of the correlation between the absorption frequency incm and the functional groups (a more comprehensive description of this type of correlation chart is given in Ref. ). By observing the presence or absence of certain group frequencies, related to common functional groups such as -OH, -NH2, -CH3, -C=0, -CN, -C-O-C, -COOH, etc., the gross structural features of an unknown compound can be quickly determined. 

NH2), etc. As a result, correlation charts tabulating the vibrational frequencies or absorption bands of the various functional groups are used. 

Limitations to the Use of Correlation Charts The unambiguous establishment of the identity or the structure of a compound is seldom possible from correlation charts alone. Uncertainties frequently arise from overlapping group frequencies, spectral variations as a function of the physical stale of the sample (that is. whether it is a solution, a mull, in a pelleted form, and so forth), and instrumental limitations. 

To extract structural information from infrared spectra, you must be familiar with the frequencies at which various functional groups absorb. You may consult infrared correlation tables, which provide as much infonnation as is known about where the various functional groups absorb. The references listed at the end of this chapter contain extensive series of correlation tables. Sometimes the absorption information is presented in the form of a chart called a correlation chart. Table 2.3 is a simplified correlation table a more detailed chart appears in Appendix 1. 

Figure 8.10. ( Next two pages). Correlation chart of group frequencies. Courtesy of Dow Chemical Company.

The interpretation of infrared spectra requires practice, but the task is eased with the help of correlation charts of group frequencies [2, 3]. Such a chart is shown in Figure 8.10, pp. 118-19. Other charts are available that document group frequencies in the near infrared and the far infrared. 

Not all vibrations exhibit characteristic frequencies. For instance, vibrational frequencies of the various C—C bonds of the carbon backbone in ahphatic molecules are very much coupled to each other (so-called skeleton modes), and they depend very much on the chemical groups coimected to the aliphatic chain. This behavior can also be used for spectrum interpretation. A short list of group frequencies of some chemical groups is given in Tab. 6.1. A more comprehensive list of characteristic bands can be found in spectral correlation tables and charts, for example in [5,6]. 

Calculate the absorption frequency corresponding to the —C—H stretching vibration treating the group as a simple diatomic C—H molecule with a force constant of k = 5.0 X 10 N/m. Compare the calculated value with the range found in correlation charts (such as the one shown in Figure 17-6). Repeat the calculation for the deuterated bond. 

More recently, the fundamental frequencies of all types of phosphate ions, such as metaphosphates, pyrophosphates, hypo-phosphites, etc., have been studied by several groups of workers, notably by Corbridge [31,32], by Lecomte [33,34] and by Tsuboi [42]. Their findings have been condensed in the form of a correlation chart for phosphorus oxyacids by Corbridge [32], and sufficient data are now available to enable most of the individual types to be differentiated. These absorptions Jiave already been discussed in Chapter 18, to which further reference should be made. The spectra of some phosphate high polymers in molten, glaceous and crystalline states have been reported by Dues and Gehrke [35]. 

The high wavenumber limit for fundamental vibration absorption is 4000 cm (HF absorbs at 3958 cm ). The low wavenumber limit is more variable, as it changes with instrumental and cell transmission limitations. If sodium chloride windows are used, the low wavenumber limit is about 650 cm The correlation charts for this region (4000-600 cm ) are shown in Fig. 13.3(a-f). Major group frequencies below 600 cm" such as C—Br and C—I stretch and aromatic out-of-plane ring bend, to name some, have been covered in the text. 

The charts are arranged in order of increasing element content. Correlations given in one chart (e.g. those for Cl I2 and CH3) are not repeated in subsequent charts. The frequency limits within which the band of a particular grouping is usually found are indicated by the black strips and extensions of the range to include unusual examples are shown as thin lines, e g. relative intensities are given in a very approximate fashion (see below). Both the position and the intensity of some absorptions are dependent on state, solvent, etc., and the actual frequency quoted is that most commonly observed. 

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