Infrared Spectroscopy
From The Science of Spectroscopy
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Typically when a molecule is exposed to infra-red (IR) radiation, it absorbs specific frequencies of radiation IR Absorption Table. The frequencies which are absorbed are dependent upon the functional groups within the molecule and the symmetry of the molecule. IR radiation can only be absorbed by bonds within a molecule, if the radiation has exactly the right energy to induce a vibration of the bond. This is the reason only specific frequencies are absorbed.
Infrared spectroscopy focuses on electromagnetic radiation in the frequency range 400-4000cm-1, where cm-1 is known as wavenumber (1/wavelength), which is a unit of measure for the frequency. To generate the infrared spectrum, radiation containing all frequencies in the IR region is passed through the sample. Those frequencies which are absorbed appear as a decrease in the detected signal. This information is displayed as a spectrum of % transmitted radiation plotted against wavenumber.
Infrared spectroscopy is very useful for qualitative analysis (identification) of organic compounds because a unique spectrum is produced by every organic substance with peaks corresponding to distinct structural features. Also, each functional group absorbs infrared light at a unique frequency. For example, a carbonyl group, C=O, always absorbs infrared light at 1670-1780 cm-1, which causes the carbonyl bond to stretch.
A carbonyl group always absorbs infrared radiation in this frequency range because the bond between the carbon atoms is constantly stretching and contracting within a range of bond lengths. This "vibration" occurs as if the bond was a spring connecting the two atoms and it always occurs within a certain frequency range, 1670-1780 cm-1. When a molecule is irradiated with infrared radiation, a vibrating bond will absorb energy of the same frequency as its vibration, increasing the amplitude of the oscillation.
In addition to identifying the molecule using IR spectroscopy, other information can be obtained. In particular the frequency of the stretching is related to the ratio of the strength of the bond and the reduced mass of the atoms involved. If the reduced mass is known then the strength of the bond within the molecule can be estimated. For a given value of the reduced mass, a vibration of long wavelength (small frequency) corresponds to a long bond (weak bond) and one of short wavelength (high frequency) corresponds to a short bond (strong bond).
The technique is also useful for quantitative analysis. For example the concentration of a solution can be estimated if the specific absorption of the solute is known in a spectral region where the solvent is transparent.
