Mutual Exclusion Rule
Definition
For a centrosymmetric molecule (one with a centre of inversion), a vibration is either IR active or Raman active, never both. This rule is what makes IR and Raman strictly complementary for symmetric molecules like CO2 and benzene.
- Applies to
- Centrosymmetric molecules with a centre of inversion
- Rule
- A vibration is IR active or Raman active, never both
- Example molecules
- CO2, benzene
- Field
- Vibrational spectroscopy
Common questions
Why does the mutual exclusion rule make IR and Raman complementary rather than redundant?+
Because a centrosymmetric molecule's vibrations split cleanly between the two techniques, running both IR and Raman on such a sample recovers the full set of vibrational modes, whereas either technique alone would miss half of them.
Does the mutual exclusion rule apply to most forensic trace samples?+
No, most real-world materials such as polymers, fibres and drugs lack a centre of inversion, so their vibrational modes commonly show up in both IR and Raman spectra at once, and the strict either-or rule only holds for the specific symmetric molecules it was derived for.
Related terms
- Polarisability
- The ease with which the electron cloud of a molecule deforms in an external electric field. A vibration is Raman active only...
- Raman Effect
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- SERS
- Surface-enhanced Raman scattering. A nanorough gold or silver surface enhances the Raman signal of molecules adsorbed on it by factors of 10^6...
- SORS
- Spatially offset Raman spectroscopy. The collection optics are offset from the laser spot so the detector reads photons that have travelled through...
- Stokes and Anti-Stokes Lines
- Stokes lines (longer wavelength, lower energy than the laser) come from molecules starting in the ground vibrational state; anti-Stokes lines (shorter wavelength,...