Data reference—Infrared absorption frequencies

An infrared spectrum records bonds absorbing characteristic frequencies. A strong absorption in a functional-group range can support the presence of that bond, while a broad O–H or sharp C=O band helps narrow the possibilities.

Read the spectrum by matching significant bands to the reference ranges, then combine them with formula, mass spectrum and NMR evidence. The fingerprint region is most useful for comparison with a known sample.

A broad band around the O–H region together with a strong carbonyl absorption supports a carboxylic acid; an aldehyde or ketone would need other evidence to distinguish it.

An absent or weak band is not automatically proof that a group is absent, and one IR peak rarely identifies a complete molecule on its own.