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Chemistry

How Molecular Symmetry Determines IR and Raman Activity

Quick fact

The water molecule is a classic example: its symmetric stretch is Raman-active but not IR-active, while the asymmetric stretch is IR-active but not Raman-active. This inversion arises from the molecule's C2v symmetry and how different vibrations change its dipole moment versus molecular polarizability.

Why this is interesting

You might think that every molecular vibration shows up in both infrared and Raman spectra, but that's not true. Why do some vibrations appear in one and not the other?