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Infrared spectroscopy

Certain groups of atoms absorb characteristic frequencies of infrared radiation as the bonds between them undergo transitions between different vibrational energy levels.

Infrared spectroscopy is an analytical technique that provides information about the functional groups present in a compound.

The particular wavelengths absorbed are specific to that particular configuration of bonds and atoms (functional group).

Infrared energy is only transferred to a bond if the bond contains a dipole that changes as it vibrates. Symmetrical molecules such as O2 or H2, are therefore IR inactive.

bonds absorbIR energy

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Infrared spectra

An infrared spectrum is a plot of transmission of infrared radiation against wavenumber (1 / wavelength).

wavenumber (cm-1)

tran

smis

sion

(%)

Any wavelength that is absorbed by the sample will transmit less than the others, forming a dip in the graph.

IR absorption spectrum for chloroethane

The pattern in the fingerprint region (1500-400 cm-1) is unique to each molecule, and so can be used for identification purposes.

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IR spectra of different functional groups

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Interpreting IR spectra activity

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Uses of IR spectroscopy

Use of IR spectra to follow the progress of a reaction involving change of functional groups (e.g. in the chemical industry to determine the extent of the reaction).

Use of IR spectra to assess the purity of a compound.

Breathalyzers: modern breathalyzers calculate the percentage of ethanol in the breath by looking at the size of the absorption caused by the C–H bond stretch in the alcohol.

Some uses of infrared spectroscopy: