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?
Read the full explanation
Understanding How Molecular Symmetry Determines IR and Raman Activity
Imagine a diatomic molecule like HCl. It has one vibrating bond. Because the molecule is polar (unequal sharing of electrons), the vibration changes the dipole moment, so it's IR-active. However, because the electron cloud is easily distorted, it also changes the polarizability, so it's Raman-active. Now consider carbon dioxide (O=C=O), a linear molecule with two bonds. Its symmetric stretch (both oxygens moving outward and inward together) does not change the dipole moment (the molecule remains nonpolar throughout), so it's IR-inactive. But it does change the polarizability (the electron distribution becomes more or less extended), so it's Raman-active. Conversely, the asymmetric stretch (one oxygen moving toward the carbon while the other moves away) changes the dipole moment, so it's IR-active, but it does not change the polarizability, so it's Raman-inactive. Thus, symmetry determines whether a vibration changes the dipole moment (IR) or the polarizability (Raman).
A deeper explanation
For a vibration to be IR-active, it must cause a net change in the molecule's dipole moment. For a vibration to be Raman-active, it must cause a net change in the molecule's polarizability (its ability to be distorted by an electric field). Molecular symmetry dictates which modes satisfy these conditions. In a molecule belonging to a point group (like C2v for water or D∞h for CO2), each vibrational mode transforms as a symmetry species (irreducible representation). The selection rules state: a mode is IR-active if it transforms as the x, y, or z coordinates (i.e., as a dipole moment component), and it is Raman-active if it transforms as a component of the polarizability tensor (xy, yz, xz, x2-y2, etc.). By inspecting the character table of the point group, chemists can predict exactly which modes are IR or Raman active. For example, water's symmetric stretch (ν1) transforms as the A1 species, which corresponds to z (dipole) and to x2-y2 and z2 (polarizability), making it both IR and Raman active. But its asymmetric stretch (ν3) transforms as B2, which corresponds to y (dipole) and xy (polarizability), so it's also both. However, in tetrahedral molecules like CH4, the symmetric stretch (A1) is only Raman-active, while the antisymmetric stretch (T2) is both IR and Raman active. This symmetry-based analysis is fundamental for interpreting vibrational spectra and assigning modes, which is crucial for structural elucidation.