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Infrared Spectroscopy |
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Infrared spectroscopy is the subset of spectroscopy that deals with the infrared region of electromagnetic spectrum. It can be used to identify compounds or investigate sample composition and the most common is absorption spectroscopy. In the infrared field when we look at infrared spectroscopy, other types of spectroscopy are NMR spectroscopy, Spectroscopy, Raman spectroscopy, UV spectroscopy, Mass spectroscopy and Infrared Absorption Frequencies
The infrared part of electromagnetic spectrum is in three regions; the near, mid and far infrared, vis a vis the visible spectrum.
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The far-infrared lies adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The mid-infrared, may be used to study the fundamental vibrations and associated rotational-vibrational structure by infrared spectroscopy. The higher energy can excite overtone or harmonic vibrations. The names and classifications of these subregions are conventions not based on exact molecular or electromagnetic properties.
Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels. These resonant frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, by the associated vibrionic coupling. The resonant frequencies can be in a first approach related to the strength of the bond, and the mass of the atoms at either end of it. Thus, the frequency of the vibrations can be associated with a particular bond type. Simple diatomic molecules have only one bond, which may stretch. More complex molecules have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. For example, the atoms in a CH2 group, commonly found in organic compounds can vibrate in six different ways: symmetrical and antisymmetrical stretching, scissoring, rocking, wagging and twisting:
In infrared spectroscopy, the infrared spectra of a sample is collected by passing a beam of infrared light through the sample. Examination of the transmitted light reveals how much energy was absorbed at each wavelength. This can be done with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all the wavelengths at once. From this, an absorbance or transmittance spectrum can be produced, showing at which IR wavelengths the sample has absorbed. Analysis of these absorption characteristics reveals details about the molecular structure of the sample. The technique has been used for the characterization of very complex mixtures. This elucidates the many strengths of infrared spectroscopy.
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