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Chemistry

How Molecular Symmetry Determines Infrared and Raman Activity of Vibrations

Quick fact

In carbon dioxide (CO2), the symmetric stretch vibration (both C=O bonds lengthening and shortening together) is Raman-active but infrared-inactive, while the asymmetric stretch and bending vibrations are IR-active but Raman-inactive. This happens because of the molecule's high symmetry.

Why this is interesting

You might think that every molecular vibration appears in an infrared spectrum, but some vibrations are invisible to IR and only show up in Raman. Why does molecular symmetry decide which vibrations you can see?