![]() ![]() The chemical constituents of the sample absorb some of the IR light at reproducible and specific wavenumbers. IR spectroscopy uses an electrically heated glowbar as the IR radiation source, and this radiation is passed through the sample to the detector. This is given by a simple law called the Beer-Lambert law that states the amount of IR absorbed is proportional to the concentration of the absorbing species and the distance the IR light has to travel through it. The final piece to the equation is how much of this radiation is absorbed. This is a more convenient way of discussing the frequency of the absorbed radiation, and is simply the number of waves in one centimeter. Chemists refer to these absorbencies as wavenumbers. It is this property that is utilized by the analytical chemist. Chemical bonds within a molecule are therefore said to exhibit characteristic IR absorptions. Therefore, for a molecule with several different kinds of bonds (for example, a C-H and a C=O), one would expect to see at least two different absorption bands. All other energies both smaller and greater have no effect. ![]() Although in this case only the exact energy required to cause vibration is absorbed. The amount of energy in the IR beam is related to its wavelength the smaller the wavelength the more energy. In this case, it is the energy of the absorbed IR and the nature of the bond between the C and H atoms. The strength of the chemical bond between the atoms, which in turn is influenced by their atomic structure, determines which part of the IR spectrum the molecules absorb.Īn analogous way of visualizing this phenomenon is demonstrated by Hooke's law of springs, where the amount of energy to start the spring oscillating is related to the strength of the spring and the mass on the end. How is IR radiation utilized by the analytical chemist? All covalent chemical bonds such as those in organic molecules (CH 4 or methane) as opposed to ionic bonds, and those found in inorganic molecules (NaCl or common salt) absorb IR radiation, causing them to vibrate by stretching and contracting. Subsequent experiments showed that this portion beyond the red was composed of a similar type of radiation to visible light, in that it could be reflected, refracted and absorbed by materials, which would reflect, refract and absorb visible light. He noticed that the temperature increased as the thermometer was moved from blue to red, but he also found that the thermometer registered even beyond the red end of the visible spectrum. He produced a solar spectrum by placing a glass prism in the path of the sun's rays and observed the changes, which took place when light of different wavelengths (different colors) fell onto the bulb of a sensitive thermometer. IR radiation was first described by William Herschel in 1800. We are unable to see this light although certain animals such as the pit viper can, enabling them to hunt at night. The infrared (IR) region of the spectrum lies to the right of the red end of the visible spectrum. ![]()
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