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The polar O-H bond (in alcohol and carboxylic acid) usually shows strong and broad absorption bands that are easy to identify. The intensity also depends on the number of bonds responsible for the absorption, and an absorption band with more bonds involved has a higher intensity. The intensity of an absorption band depends on the polarity of the bond, and a bond with higher polarity will show a more intense absorption band. The absorption bands in IR spectra have different intensities that can usually be referred to as strong (s), medium (m), weak (w), broad and sharp. The information in Table 6.1 can be summarized in the diagram for easier identification (Figure 6.3b), in which the IR spectrum is divided into several regions, with the characteristic band of certain groups labelled.įigure 6.3b Approximate IR Absorption Range Table 6.1 Characteristic IR Frequencies of Stretching Vibrations The larger wavenumbers (shorter wavelengths) are associated with higher frequencies and higher energy. Please note that the direction of the horizontal axis (wavenumber) in IR spectra decreases from left to right. The wavenumber is defined as the reciprocal of wavelength ( Formula 6.3), and the wavenumbers of IR radiation are normally in the range of 4000 cm-1 to 600 cm-1 (approximate corresponds the wavelength range of 2.5 μm to 17 μm of IR radiation).

The horizontal axis indicates the position of an absorption band, but instead of using frequency to show the absorbed radiation, wavenumbers (, in unit of cm -1 ) are used as a conventional way in IR spectra.

A molecule has a variety of covalent bonds, and each bond has different vibration modes, so the IR spectrum of a compound usually shows multiple absorption bands. The spikes are called absorption bands in the IR spectrum. Lower values of % transmittance mean that some of the energy is absorbed by the compound and gives downward spikes. At the high end of the axis, 100% transmittance means no absorption occurred at that frequency. The solid line traces the values of % transmittance for every wavelength passed through the sample.

The vertical axis is ‘% transmittance’, which indicates how strongly light was absorbed at each frequency.

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With a basic understanding of IR theory, we will now take a look at the actual output from IR spectroscopy experiments and learn how to get structural information from the IR spectrum.

6.3 IR Spectrum and Characteristic Absorption Bands