13.1 : 红外（IR）光谱：概述

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.