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How does infrared spectroscopy reveal the vibrations of molecules?

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Infrared (IR) spectroscopy reveals molecular vibrations by measuring the interaction between infrared radiation and the internal motions of a molecule. Atoms within a molecule are not static; they engage in stretching vibrations (moving back and forth along the bond axis) and bending vibrations (rocking or bending movements).12

These vibrations correspond to specific quantized energy levels. When infrared light passes through a sample, the molecule absorbs radiation at frequencies that match the energy differences between these vibrational states. An infrared spectrophotometer records this by plotting the amount of light transmitted against the wavelength or frequency of the radiation.21

Because different functional groups and bonds possess characteristic vibrational frequencies, the resulting absorption spectrum acts as a unique "fingerprint" for a molecule. By analyzing these peaks, scientists can identify the presence of specific chemical entities, such as carboxyl groups or double bonds, making IR spectroscopy a powerful tool for structural analysis.123