key term - σ → σ*

Definition

The notation σ → σ* describes an electronic transition where an electron is promoted from a bonding molecular orbital (σ) to an antibonding molecular orbital (σ*). This transition is significant in understanding how molecules absorb light and can lead to changes in their energy states, contributing to various phenomena such as fluorescence and phosphorescence.

5 Must Know Facts For Your Next Test

1. The σ → σ* transition usually requires higher energy UV or visible light to promote electrons due to the energy gap between the bonding and antibonding orbitals.
2. These transitions are typically less intense than π → π* transitions because σ bonds are more stable and less polarizable than π bonds.
3. In many organic compounds, σ → σ* transitions can indicate the presence of functional groups like alkanes and saturated compounds.
4. The energy difference for σ → σ* transitions is generally larger compared to π → π* transitions, influencing the absorption properties of different substances.
5. The Franck-Condon factors play a crucial role in determining which vibrational states contribute to the intensity of a σ → σ* transition in electronic spectroscopy.

Review Questions

• How does the σ → σ* transition impact the electronic properties of a molecule?
  • The σ → σ* transition impacts a molecule's electronic properties by promoting electrons from stable bonding orbitals to higher energy antibonding orbitals. This can lead to increased reactivity as the molecule may undergo various photochemical processes after absorbing UV or visible light. Consequently, understanding these transitions helps predict how molecules interact with light, which is essential for applications in photochemistry and materials science.
• Discuss the relationship between the Franck-Condon Principle and the intensity of σ → σ* transitions in spectroscopy.
  • The Franck-Condon Principle states that electronic transitions occur much faster than nuclear movements, resulting in specific vibrational states being favored during such transitions. For σ → σ* transitions, this means that certain vibrational levels will dominate the absorption spectrum based on their overlap with the initial and final electronic states. Consequently, this principle helps explain why some transitions are more intense than others, influencing how we interpret spectral data in relation to molecular structure.
• Evaluate the significance of σ → σ* transitions in understanding molecular behavior under UV light exposure.
  • Evaluating the significance of σ → σ* transitions highlights their crucial role in understanding how molecules behave when exposed to UV light. These transitions can trigger various chemical reactions such as photodissociation or induce fluorescence and phosphorescence, affecting everything from biological processes like photosynthesis to technological applications such as phototherapy. By studying these transitions, scientists can manipulate molecular responses to light, paving the way for advancements in materials science and photochemistry.

Related terms

Molecular Orbital Theory:

A theory that explains the behavior of electrons in molecules by describing the formation of molecular orbitals through the combination of atomic orbitals.

Franck-Condon Principle:

A principle that explains the intensity of vibronic transitions in spectroscopy, stating that electronic transitions are much faster than nuclear movements, leading to specific vibrational states being favored.

Absorption Spectrum: A spectrum that displays the wavelengths of light absorbed by a substance, providing insight into its electronic structure and energy transitions.