When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.
Different compounds display unique properties due to their functional groups, which allows IR spectroscopy to be used to determine the present functional groups. Vibrational IR spectroscopy is performed in the 2.5 to 25 µm wavelength range. A molecule is considered IR active if it exhibits a change in dipole moment during vibration when absorbing IR radiation. For a molecule to absorb IR radiation effectively, its vibrations must fluctuate its dipole moments, allowing it to interact with the electromagnetic field of the IR light.
Since different functional groups absorb IR radiation at varying frequencies, the IR spectrum is similar to the "fingerprint" for each molecule. IR spectroscopy is primarily employed in qualitative analysis to identify the functional groups in organic and inorganic compounds by comparing their vibrational frequencies with known compounds. Additionally, It can determine the concentration of a substance by applying the Beer-Lambert law, where absorbance is proportional to concentration.
On passing IR radiation through molecules, their covalently bonded atoms absorb radiation between 2.5 to 25 micrometers in wavelength as they transition from lower to higher vibrational energy levels.
Molecular vibrations that produce a substantial change in bond dipole moment are termed IR active.
IR radiation is expressed in inverse centimeters which represent the spectroscopic wavenumber calculated as the reciprocal of wavelength in centimeters.
Using the formula for the speed of light, wavelength, λ, is equal to the speed of light, c divided by its frequency, v.
Substituting for wavelength λ shows that IR radiation is proportional to frequency divided by the speed of light in that medium.
The frequencies from 4000 to 400 cm−1 correspond to the vibrational region of the IR spectrum.
IR spectroscopy is used to identify the functional groups present in a molecule.
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