Infrared Spectroscopy: Principles

Infrared (IR) spectroscopy is one of the most widely used techniques for identifying functional groups in organic molecules. While mass spectrometry tells you the molecular mass and gives clues about fragmentation, IR spectroscopy tells you which types of bonds are present. Together, they begin to build a detailed picture of molecular structure.

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The Basis of IR Spectroscopy

All covalent bonds in molecules are not static — they vibrate continuously. These vibrations are quantised, meaning bonds can only vibrate at specific frequencies that correspond to specific energy levels. When infrared radiation passes through a sample, bonds absorb radiation at frequencies that match their natural vibrational frequencies. The radiation that is not absorbed passes through to the detector.

By measuring which frequencies of IR radiation are absorbed by the sample, we can identify which types of bonds are present.

Types of Bond Vibration

There are two main categories of vibration:

Stretching vibrations — the bond length changes (gets longer and shorter) while the atoms remain on the same bond axis. These can be symmetric (both atoms move in and out together) or asymmetric (one atom moves in while the other moves out).