infrared spectroscopy
/ IN-fruh-red /
Every bond in a molecule behaves a bit like a tiny spring joining two weights, and like any spring it can stretch and bounce at its own natural rhythm. Infrared light happens to vibrate at just about those same rhythms. So when you shine infrared through a sample, bonds soak up exactly the frequencies that match their wobble, and the spectrum that results is a tally of which springs are present — a chemical fingerprint built from molecular vibrations.
Infrared spectroscopy measures the absorption of infrared radiation by a substance, which excites the stretching and bending vibrations of its chemical bonds. Each type of bond and group — an O–H, a C=O, a C–H — absorbs in a characteristic range, so peaks at particular positions reveal which functional groups the molecule contains. The crowded low-energy end of the spectrum, called the fingerprint region, is so detailed that it can distinguish one compound from a very similar one.
It matters because it is fast, needs only a tiny sample, and is one of the surest everyday ways to identify what functional groups a molecule has. The key restriction is that a vibration only absorbs infrared if it changes the molecule's dipole moment as it moves; perfectly symmetric vibrations stay invisible to infrared, which is exactly where the complementary technique of Raman spectroscopy steps in.
An unknown liquid gives a strong, sharp absorption near 1715 cm⁻¹ and a broad one stretching across 3000–3500 cm⁻¹. A chemist reads these instantly: the first says there is a carbonyl (C=O) group, the second a hydroxyl (O–H). Together they point straight at a carboxylic acid, narrowing the identity in seconds.
Each bond absorbs at its own frequency, so peaks name the functional groups.
Infrared positions are usually reported in wavenumbers (cm⁻¹), not wavelength. A higher wavenumber means higher energy and a stiffer or lighter-atom bond — which is why O–H and C–H stretches sit high and heavy single bonds sit low.