Infrared Spectroscopy

Infrared (IR) spectroscopy is an analytical technique used to identify the bonds, and so the functional groups, present in an organic molecule.A beam of infrared radiation covering a range of frequencies is passed through a sample. Covalent bonds absorb IR radiation at particular frequencies. A detector measures how much radiation passes through at each frequency, and a computer plots the IR spectrum.An IR spectrum plots percentage transmittance (vertical axis) against wavenumber (horizontal axis). Wavenumber is 1 ÷ wavelength, measured in cm⁻¹, and is proportional to frequency: a higher wavenumber means higher-frequency, higher-energy radiation. Spectra usually run from 4000 cm⁻¹ on the left down to about 500 cm⁻¹ on the right.Where radiation is absorbed, less passes through, so absorptions appear as downward dips called troughs (often loosely called peaks). The position of a trough shows which bond absorbed; its shape (sharp or broad) gives extra clues.The region below about 1500 cm⁻¹ is the fingerprint region. It contains many overlapping absorptions and is unique to each compound, so comparing it with a database of known spectra can confirm the identity of a compound.Worked example: radiation of wavenumber 1700 cm⁻¹ has a wavelength of1 ÷ 1700 = 5.9 × 10⁻⁴ cmwhich is about 5.9 μm, in the infrared region of the electromagnetic spectrum.