infrared spectroscopy

Hold a guitar string and it vibrates at a pitch set by its length, tension, and weight. The bonds inside a molecule are like tiny springs connecting balls of different mass, and they too vibrate — stretching and bending at characteristic frequencies. Infrared spectroscopy shines invisible infrared light through a sample, and each bond absorbs exactly the frequency that matches its own natural vibration, leaving a dip in the transmitted light. The pattern of dips tells you which bonds, and therefore which functional groups, the molecule contains.

Here is how to read it. A heavier atom on a bond, like a heavier weight on a spring, vibrates slower (lower frequency); a stiffer bond, like a triple bond, vibrates faster (higher frequency). So each functional group absorbs in a characteristic region, measured in wavenumbers (cm-1). The most diagnostic stretches live in the high-frequency region above about 1500 cm-1: a broad O-H stretch of an alcohol around 3300, a strong sharp C=O of a carbonyl near 1700, an N-H of an amine around 3400, the sharp C#N of a nitrile near 2250, and the weaker C=C of an alkene near 1650. Spotting a strong band near 1700 cm-1 shouts 'there is a carbonyl here.'

IR is fast, cheap, and needs only a tiny sample, so it is often the first spectrum a chemist runs to ask the simplest question: which functional groups are present? It will not draw the whole carbon skeleton — that is NMR's job — but it confirms the presence or absence of the tell-tale groups, which is half the battle in identifying an unknown.