Characteristic Absorption Frequencies

TABLE 13.4 Infrared Absorption Frequencies of Some Common Structural Units  

Structural unit Frequency, cm Structural unit 1 Frequency, cm 

In later chapters, when families of compounds are discussed in detail, the IR frequencies associated with each type of functional group will be revisited. 

The C=0 stretching frequencies of aldehydes are similar to those of ketones. 

The C—H stretch of the CH=0 group in aldehydes appears as a pair of bands in the range 2700-2900 cm  

All of the spectra In this and the next section are displayed on a common %T scale to better show how peak Intensities differ among various groups. 

Very few other groups absorb in this region, the most notable being C=C triple bonds (2100-2200 cm ). 

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Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

Fairly good agreement exists between the calculated value of 1682 cm-1 and the experimental value of 1650 cm-1. Based upon the Hooke s law approximation, numerous correlation tables have been generated that allow one to estimate the characteristic absorption frequency of a specific functionality [3], It becomes readily apparent how IR spectroscopy can be used to identify a molecular entity, and subsequently to physically characterize a sample or to perform quantitative analysis. 

Infrared has also been used to assess the type of acidity present on a catalyst. The method involves measuring the spectrum of adsorbed pyridine on the catalyst certain characteristic absorption frequencies are assigned to Lewis acid centers (coordinately bound pyridine) and others to Brqnsted sites (pyridine adsorbed as pyridinium ion). 

The characteristic absorption frequencies of this varied group of sulphur compounds may be inferred by reference to the data given in Appendices 2, 3 and 4. 

It is usual in calculations using Eq. (17.3) for the common characteristic absorption frequency ve to be assigned a value of 3 x 1015 s 1 (see, for example, Israelachvili, 1992 French, 2000). Horn and Israelachvili (1981) have derived a slightly more complex form of Eq. (17.3) for the situation where materials 1 and 2 have the same absorption frequency but material 3 has a different value of absorption frequency, and Prieve and Russel (1988) have derived a form of Ai32 for the most general situation where the three materials have different absorption frequencies q, v2 and v3. 

As a result, specific kinds of vibration modes always appear at similar frequencies for different molecules then, it is feasible to construct a table of characteristic absorption frequencies. Typical absorption band tables for significant bonds included in polyatomic molecules are reported in ref. [58], Subsequently, using the typical absorption frequencies, it is relatively simple to recognize the occurrence of functional groups in unidentified compounds, and use this information to get structural information about these substances in order to identify them. 

FIGURE 3.4 Illustration of the process of acquiring a one-dimensional NMR spectrum. The steps involved in obtaining an NMR spectrum are shown. The sample is a tetra-peptide (Val-Ala-Ser-Ala). A short (10 asec) intense RF pulse is applied to the sample. This pulse excites all of the nuclei and they emit energy at their characteristic absorption frequencies. This signal is called the free induction decay (FID) and is collected as a function of time. This time domain signal is converted to spectrum by Fourier transformation. Note the characteristic chemical shifts for amide protons (H v), a-protons, (i-protons, and methyl protons. Also note that the two alanine residues, although chemically equivalent, have different chemical shifts because they experience different local environments. 

Fricke H (1920) X-characteristic absorption frequencies for the chemical elements magnesium to chromium. Phys Rev 16 202-215... 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

For a more comprehensive compilation of data you are referred to Meites (1965). We can use such tables of Ei data in much the same way as characteristic absorption frequency tables are used in infrared spectroscopy they indicate the likely content of the solution. Fig. 1.6e shows the polarogram expected for a solution containing lead, cadmium and zinc in aqueous 0.1 mol dm " KCl. 

Table 19.15. Characteristic Absorption Frequencies" of the Tocopherols, cm (Pennock, 1965)...

UV (characteristic absorption frequencies) nm 20°C in solution (broad, maximum, depends on diene type) <200 (2-4)... 

IR (characteristic absorption frequencies) cm D2238 See table below (17-23)... 

Spectra by the Thousands 575 Infrared Spectra 576 Characteristic Absorption Frequencies Ultraviolet-Visible (UV-VIS) Spectroscopy Mass Spectrometry 584 Molecular Formula as a Clue to Structure 589 Summary 590 Problems 593... 

Table 13.4 lists the characteristic absorption frequencies (in wavenumbers) for a variety of structural units found in organic compounds. Generally, absorptions above 1500 cm for functional groups such as OH, C=0, and C=N are the easiest to assign and provide the most useful information. 

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