3.2.9 (HL)—Infrared spectroscopy

IR absorptions identify bond types through characteristic wavenumbers. Use the functional-group region and the supplied data-booklet values to match absorptions with structural features.

Treat an absorption as evidence for a bond or functional group, then check whether the proposed structure accounts for all decisive peaks.

A broad O–H absorption and a strong C=O absorption together support a carboxylic acid more strongly than either peak alone. Use the data-booklet range, then check both presence and absence of decisive absorptions; IR identifies bonds and groups, not a unique whole structure by itself.

Greenhouse-gas link: an IR-active vibration must change the molecule's dipole moment, allowing it to absorb matching outgoing infrared radiation. A molecule can be non-polar overall yet have IR-active vibrations; $\ce{CO2}$ is the key example. Absorption at characteristic wavenumbers supports the presence of particular vibrating bonds, but greenhouse effect also depends on concentration, absorption bands and atmospheric lifetime, not one peak alone.