Molecular orbitals play a key role in spectroscopy because they determine how molecules absorb and emit light.
How Molecular Orbitals Influence Spectroscopic Transitions
- Absorption of Light
- When a molecule absorbs a photon, an electron moves from a lower-energy orbital (usually a bonding or non-bonding orbital) to a higher-energy orbital (usually an anti-bonding orbital).
- Example: HOMO → LUMO transition (Highest Occupied Molecular Orbital → Lowest Unoccupied Molecular Orbital).
- Energy Difference Determines Wavelength
- The energy gap between the orbitals determines the wavelength of light absorbed.
- Small gap → absorbs longer wavelength (red light), large gap → absorbs shorter wavelength (UV light).
- Selection Rules
- Not all orbital transitions are allowed; molecular symmetry and spin states affect whether a transition occurs.
- Allowed transitions produce strong absorption lines, while forbidden transitions are weak or absent.
- Emission of Light
- After excitation, electrons can return to lower orbitals, releasing energy as fluorescence or phosphorescence.
- The type of orbitals involved influences emission intensity and wavelength.
- Predicting Spectra
- By knowing the molecular orbitals, chemists can predict absorption spectra, colors, and photochemical behavior.
Simple Summary
- Electrons jump between molecular orbitals when they absorb light.
- The energy gap determines the color/wavelength of light absorbed or emitted.
- Molecular orbitals control which transitions are allowed and how intense they are.
In short: Molecular orbitals are like energy “levels” in a molecule, and spectroscopy observes electrons jumping between them when light is absorbed or emitted.