Infrared absorptions/spectra characteristic frequencies

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. 

Glycogen shows the characteristic infrared absorption spectrum of starch-type polysaccharides.—The infrared spectrum of glycogen, in the frequency range 730-960 cm., has three absorption peaks, at 928 3, 838 3, and 760 2 cm.- the absorption peak at 838 cm.- is displayed by all carbohydrates containing a-D-glucopyranose units, whilst the peaks... 

The infrared spectrum of diltiazem hydrochloride dispersed in potassium bromide was recorded at 4 cm 1 resolution on a Nicolet Model 740 FTIR (11). Figure 3 shows the diltiazem HCI vibrational features in the 4000 to 400 cm-1 region after spectral subtraction of the absorptions (3430 and 1629 cm 1) due to adsorbed water. Table I lists characteristic frequencies, relative intensities, and vibrational assignments for the primary diltiazem HCI absorption bands. 

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. 

Absorption in the infrared region of the spectrum by organic compounds arises as a result of bending and stretching of covalent bonds at different characteristic frequencies. The frequency of vibration is related to both bond order and the mass of the atom attached to the bond. Many organic compounds have a large number of relatively narrow absorption bands in the mid-infrared region (1600-900 cm ). These... 

The dipole correlation time of the system is a valuable quantity to calculate as it is related to the sample s absorption spectrum. Liquids usually absorb in the infrared region of the electromagnetic spectrum, a typical spectrum being shown in Figure 7.12. As can be seen, the spectrum is very broad with none of the sharp peaks characteristic of a well-resolved spectrum for a species in the gas phase. This is because the overall dipole of a liquid does not change at a constant rate but, rather, there is a distribution of frequencies. The intensity of absorption at any frequency depends upon the relative contribution of that frequency to the overall distribution. If, on average, the overall dipole changes very rapidly (i.e. the relaxation time is short) then the maximum in the absorption spectrum will occur at a... 

In practice, the absorption spectrum in the infrared region is shown as a plot of absorption (or percent transmission) versus the wavelength in microns (or the frequency). The presence or absence of characteristic absorption bands permits the species within the molecules, and thus the type of polymer, to be identified. 

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