Infrared spectroscopy characteristic absorption frequencies

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. 

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... 

The structure of the protonated enamines has been investigated by infrared spectroscopy. On protonation there is a characteristic shift of the band in the double-bond stretching region to higher frequencies by 20 to 50 cm with an increased intensity of absorption (6,13,14a). Protonated enamines also show absorption in the ultraviolet at 220-225 m/x due to the iminium structure (14b). This confirms structure 5 for these protonated enamines, because a compound having structure 4 would be expected to have only end absorption as the electrons on nitrogen would not be available for interaction with the n electrons of the double bond. 

Fourier-transform infrared (FTIR) spectroscopy Spectroscopy based on excitation of vibrational modes of chemical bonds in a molecule. The energy of the infrared radiation absorbed is expressed in inverse centimeters (cm ), which represents a frequency unit. For transition-metal complexes, the ligands -C N and -C=0 have characteristic absorption bands at unusually high frequencies, so that they are easily distinguished from other bonds. The position of these bonds depends on the distribution of electron density between the metal and the ligand an increase of charge density at the metal results in a shift of the bands to lower frequencies. 

The wavelengths of IR absorption bands are characteristic of specific types of chemical bonds. In the past infrared had little application in protein analysis due to instrumentation and interpretation limitations. The development of Fourier transform infrared spectroscopy (FUR) makes it possible to characterize proteins using IR techniques (Surewicz et al. 1993). Several IR absorption regions are important for protein analysis. The amide I groups in proteins have a vibration absorption frequency of 1630-1670 cm. Secondary structures of proteins such as alpha(a)-helix and beta(P)-sheet have amide absorptions of 1645-1660 cm-1 and 1665-1680 cm, respectively. Random coil has absorptions in the range of 1660-1670 cm These characterization criteria come from studies of model polypeptides with known secondary structures. Thus, FTIR is useful in conformational analysis of peptides and proteins (Arrondo et al. 1993). 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

It may seem there are too many numbers to memorize in infrared spectroscopy. Hundreds of characteristic absorptions for different kinds of compounds are listed in Appendix 2. Please glance at Appendix 2, and note that Appendix 2A is organized visually, while Appendix 2B is organized by functional groups. For everyday use, we can get by with only a few stretching frequencies, shown in Table 12-2. When using this table, remember... 

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm-1 for simple ketones and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

Fig. 3.30 Schematic representation of infrared and Raman experiments. In infrared spectroscopy the excitations are detected by absorption of characteristic frequencies, in Raman spectroscopy the excitations are detected by characteristic s/ti/is in frequencies of the scattered light, [rrom Harris, D. C Bertolucci, M. D. Symmetry and Spectroscopy, Dover New York, 1989. Reproduced with permission.]...

Section 13.19 Infrared spectroscopy probes molecular structure by examining transitions between vibrational energy levels using electromagnetic radiation in the 625-4000-cm range. The presence or absence of a peak at a characteristic frequency tells us whether a certain functional group is present. Table 13.4 lists IR absorption frequencies for common structural units. 

Thus, if a transition exists which is related to the frequency of the incident radiation by Planck s constant (h = 6.626-1 O 34), then the radiation can be absorbed. Conversely, if the frequency (v) does not satisfy Planck s expression, then the radiation will be transmitted. A plot of the frequency of the incident radiation against some measure of the percent radiation absorbed by the sample provides the absorption spectrum of the compound or component. The absorption spectrum is characteristic for the compound and this spectrum is often called the fingerprint of the compound. Infrared spectroscopy is based on the measurement of the absorption of electromagnetic radiation that arises from the altering of the vibration level of the component s molecule. An example of the adsorption and transmission of the infrared radiation is shown in Figure 2.30. 

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, that is the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrate. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and the associated vibronic coupling. 

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