![]() The intensity also depends on the number of bonds responsible for the absorption, the absorption band with more bonds involved has higher intensity. The intensity of a absorption band depends on the polarity of the bond, the bond with higher polarity will show more intense absorption band. The absorption bands in IR spectra have different intensity, that can usually be referred to as strong (s), medium (m), weak (w), broad and sharp. Figure 6.3b Approximate IR Absorption Range The information in Table 6.1 can be summarized in the diagram that is easier to be identified ( Figure 6.3b), in which the IR spectrum is divided in several regions, with the characteristic band of certain groups labelled. Table 6.1 Characteristic IR Frequencies of Stretching Vibrations FormulaĬharacteristic IR Frequency range (cm -1) The characteristics stretching vibration bands associated with the bonds in some common functional groups are summarized in Table 6.1. Generally, stretching vibrations the stretching vibrations require more energy and show absorption bands in the higher wavenumber/frequency region. Within that range, carboxylic acids, esters, ketones and aldehydes tend to absorb in the higher wavenumber/frequency end (1700-1750 cm -1), while conjugated unsaturated ketones and amides tend to absorb on the lower wavenumber/frequency end (1650-1700 cm -1). A strong absorbance band in the 1650-1750 cm -1 region indicate that a carbonyl group (C=O) is present. It is a very strong band comparing to the others on the spectrum. The technique is therefore very useful as a means of identifying which functional groups are present in a molecule of interest.įor example, the most characteristics absorption band in the spectrum of 2-hexanone ( Figure 6.3a) is that from the stretching vibration of carbonyl double bond C=O, at 1716 cm -1. The power of infrared spectroscopy arises from the observation that the covalent bonds characterizing different functional groups have different characteristic absorption frequencies (in wavenumber, Table 6.1 ). The larger wavenumbers (shorter wavelengths) are associated with higher frequencies and higher energy. Please note the direction of the horizontal axis (wavenumber) in IR spectra decrease from left to right. The wavenumber is defined as the reciprocal of wavelength ( Formula 6.3), and the wavenumbers of infrared 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). 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. The horizontal axis indicates the position of an absorption band.A molecule have a variety of covalent bonds, and each bond have different vibration modes, so the IR spectrum of a compound usually show multiple absorption bands. The spikes are called absorption bands in an IR spectrum. Lower values of % transmittance mean that some of the energy is absorbed by the compound, and gives the 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 tells how strongly light was absorbed at each frequency. ![]() With the basic understanding of the IR theory, we will take a look at the actual output from IR spectroscopy experiments, and learn how to get structural information from IR spectrum. These complex vibrations can be broken down mathematically into individual vibrational modes, a few of which are illustrated below.\) At room temperature, organic molecules are always in motion, as their bonds stretch, bend, and twist. \)Ĭovalent bonds in organic molecules are not rigid sticks – rather, they behave more like springs.
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