Rapid Review

Infrared (IR) Spectroscopy

Covalent bonds behave like tiny springs connecting atoms. They vibrate in a number of ways including stretching and bending.

Infrared electromagnetic radiation (commonly referred to as heat waves) causes atoms and groups of atoms to vibrate with greater amplitude. When this happens, the radiation is absorbed by the molecule and converted into kinetic energy. The increase in kinetic energy of the molecules is observed as an increase in temperature.

Different types of bonds absorb specific frequencies of IR. Hence, by studying the IR absorption spectrum of a compound, much can be learned about the types of bonds and groups present.

Certain features of IR spectra are listed below. Instead of using Hz as a measure of frequency, usually the wave number is used, which is calculated as 1 / wavelength in cm. The units of wave number are thus cm^-1.

Atoms or groups with smaller masses vibrate at a higher frequency and, therefore, absorb IR in the higher frequency part of the spectrum.
E.g. C-H absorbs frequencies around 3,000 cm^-1, while C-O absorbs frequencies around 1,000 cm^-1.

Stronger bonds are stiffer and so vibrate at higher frequencies.
E.g. C=O absorbs frequencies around 1,700 cm^-1, while C-O absorbs frequencies around 1,000 cm^-1.

The frequencies at which bonds stretch are higher than the frequencies at which they bend.
E.g. C-H stretching absorbs frequencies around 3,000 cm^-1, while C-H bending absorbs frequencies around 1,000 cm^-1.

Bonds with strong dipole moments absorb more intensely than non-polar bonds.
In particular, O-H bonds absorb intensely at around 3,500 cm^-1 and C=O bonds at around 1,700 cm^-1.