Physical chemistry
Infrared and Raman spectroscopy
IR absorption and Raman scattering complement one another because their selection rules depend on dipole-moment or polarizability changes.
A vibration absorbs IR when it changes dipole moment. Inelastic Raman scattering shifts photon energy when a vibration changes molecular polarizability.
Complementary selection rules
IR absorption and Raman scattering complement one another because their selection rules depend on dipole-moment or polarizability changes.
Definition: Core definition
Characteristic vibrational frequencies reflect bond stiffness and reduced mass; functional-group regions aid identification but are not absolute fingerprints.
Example: Worked example
Apply the model to a simple case: For an ideal centrosymmetric molecule, mutual exclusion applies: IR-active vibrations are often Raman-weak/inactive and vice versa. Comparing spectra helps identify symmetry and bonding groups.
Solution
For an ideal centrosymmetric molecule, mutual exclusion applies: IR-active vibrations are often Raman-weak/inactive and vice versa. Comparing spectra helps identify symmetry and bonding groups.
Characteristic vibrational frequencies reflect bond stiffness and reduced mass; functional-group regions aid identification but are not absolute fingerprints.
| Quantity | Model / rule |
|---|---|
| Key relation | IR absorption and Raman scattering complement one another because their selection rules depend on dipole-moment or polarizability changes. |
| Meaning / use | For an ideal centrosymmetric molecule, mutual exclusion applies: IR-active vibrations are often Raman-weak/inactive and vice versa. Comparing spectra helps identify symmetry and bonding groups. |
What change makes a vibration IR-active?
Which statement best matches the model described?
References
- Norman B. Colthup et al. (1990). Introduction to Infrared and Raman Spectroscopy