In the absorption of UV/visible light, molecules undergo electronic transitions, where electrons move from lower-energy molecular orbitals to higher-energy molecular orbitals. These transitions primarily occur between different types of molecular orbitals—such as bonding, anti-bonding, and non-bonding orbitals. The energy required for these transitions falls within the ultraviolet (UV) and visible light spectrum, and the specific wavelength of light absorbed depends on the nature of the molecule.

Occur in the absorption of UV/visible light

1. Ïƒ → σ (Sigma to Sigma Anti-Bonding Transition)*

  • Orbitals Involved: Electrons in a sigma (σ) bonding orbital transition to a sigma anti-bonding (σ*) orbital.
  • Energy: These transitions require a very high amount of energy, typically found in the vacuum ultraviolet region (below 200 nm).
  • Examples: Molecules with strong σ bonds, like alkanes, exhibit this type of transition. However, these transitions are not commonly observed in the standard UV/visible range.

2. n → σ (Non-Bonding to Sigma Anti-Bonding Transition)*

  • Orbitals Involved: Electrons from a non-bonding (n) orbital (usually a lone pair of electrons) transition to a σ* orbital.
  • Energy: These transitions occur in the ultraviolet region, often between 150-250 nm.
  • Examples: Compounds with lone pairs on oxygen, nitrogen, or sulfur atoms (like alcohols or amines) show this transition.

3. Ï€ → Ï€ (Pi to Pi Anti-Bonding Transition)*

  • Orbitals Involved: Electrons in a pi (Ï€) bonding orbital transition to a pi anti-bonding (Ï€*) orbital.
  • Energy: This transition requires less energy compared to σ → σ* transitions and usually falls within the 200-700 nm range (UV-visible region).
  • Examples: Molecules with double or triple bonds (like alkenes, alkynes, or aromatic rings) exhibit this transition. Aromatic compounds, in particular, often absorb in the visible range, making them colored.

4. n → Ï€ (Non-Bonding to Pi Anti-Bonding Transition)*

  • Orbitals Involved: Electrons from a non-bonding (n) orbital transition to a Ï€* orbital.
  • Energy: This transition requires lower energy than n → σ* transitions, often falling in the UV-visible range, typically between 200-400 nm.
  • Examples: Compounds like carbonyls (C=O) and nitro compounds exhibit this type of transition, as these molecules contain lone pairs of oxygen or nitrogen that can interact with the Ï€* orbital.

Summary of Energy Order:

The energy required for these transitions typically follows this order:

  • σ → σ* (Highest energy, far UV region)
  • n → σ*
  • Ï€ → Ï€*
  • n → Ï€* (Lowest energy, often in the visible region)

Absorption Characteristics:

  • Molecules with extensive conjugation (alternating single and double bonds) often absorb light in the visible region, giving them color.
  • More conjugation lowers the energy gap between the ground and excited states, shifting the absorption towards longer wavelengths (lower energy).

These electronic transitions are key to understanding molecular absorption spectra, which can be used to identify and analyze compounds in UV/visible spectroscopy.