Molecular Spectroscopy and Radiative Transitions
Spectroscopy as a Probe of Structure
Transition Moments
A spectroscopic transition is governed by the matrix element . If this quantity vanishes by symmetry, the transition is forbidden in the electric dipole approximation. Real spectra also include magnetic dipole, quadrupole, vibronic, and collisional contributions.
Major Spectroscopic Regimes
Rotational spectroscopy
- Probes molecular moments of inertia
- Best for gas-phase polar molecules
- Yields precise structure information
Vibrational spectroscopy
- Probes normal modes and force constants
- Infrared and Raman complementary
- Sensitive to bonding and functional groups
What determines whether a vibrational mode is IR active?
IR activity requires a change in dipole moment during the vibration.
Correct answer: Whether it changes the dipole moment
Franck–Condon Principle
Electronic transitions are much faster than nuclear motion, so transitions occur approximately at fixed nuclear geometry. Vibrational intensities arise from overlap between initial and final vibrational wavefunctions, encoded in Franck–Condon factors.
Vibronic coupling
When electronic and nuclear motion are not separable, symmetry-forbidden transitions can borrow intensity through vibronic coupling, especially in polyatomic chromophores and transition-metal complexes.
State the physical reason for rotational fine structure in molecular spectra.
Quantized rotational spacings create fine structure superimposed on vibrational or electronic bands.
Correct answer: Rotational energy levels are quantized, so transitions between them produce closely spaced spectral lines.
Spectroscopic Selection Rules at a Glance
| Technique | Primary condition | Typical information |
|---|---|---|
| IR | Change in dipole moment | Bond strengths and functional groups |
| Raman | Change in polarizability | Symmetric stretches and lattice modes |
| Microwave | Nonzero dipole moment | Moments of inertia and structure |
| UV-Vis | Allowed electronic transition moment | Electronic states and chromophores |
Which statement best describes the Franck–Condon principle?
Because nuclei move slowly relative to electrons, electronic transitions occur at nearly fixed nuclear geometry.
Correct answer: Electronic transitions are vertical on the nuclear coordinate scale
Why are Raman and IR spectroscopy often complementary?
IR depends on dipole changes; Raman depends on polarizability changes.
Correct answer: They obey different selection rules, so vibrational modes that are weak or inactive in one technique may be active in the other.