E2 Band

Definition

The E2 band is a characteristic absorption band observed in the ultraviolet (UV) spectrum of conjugated organic molecules. It arises due to the electronic transitions between the highest occupied molecular orbital (HOMO) and the second lowest unoccupied molecular orbital (LUMO+1) of the conjugated system.

5 Must Know Facts For Your Next Test

1. The E2 band is typically observed at higher energy (shorter wavelength) compared to the E1 band, which arises from the HOMO to LUMO transition.
2. The position and intensity of the E2 band are sensitive to the degree of conjugation in the molecule, with more extensive conjugation leading to a bathochromic (red) shift and increased intensity.
3. The E2 band is often used as a diagnostic tool to determine the presence and extent of conjugation in organic compounds.
4. The energy difference between the HOMO and LUMO+1 orbitals, which determines the E2 band position, is influenced by factors such as the nature and arrangement of the conjugated system.
5. The intensity of the E2 band is related to the degree of electronic delocalization and the symmetry of the molecular orbitals involved in the transition.

Review Questions

• Explain the origin of the E2 band in the UV spectrum of conjugated organic molecules.
  • The E2 band in the UV spectrum of conjugated organic molecules arises from the electronic transition between the highest occupied molecular orbital (HOMO) and the second lowest unoccupied molecular orbital (LUMO+1) of the conjugated system. This transition occurs at higher energy (shorter wavelength) compared to the HOMO to LUMO transition, which gives rise to the E1 band. The position and intensity of the E2 band are sensitive to the degree of conjugation in the molecule, with more extensive conjugation leading to a bathochromic (red) shift and increased intensity.
• Describe how the E2 band can be used as a diagnostic tool to determine the presence and extent of conjugation in organic compounds.
  • The E2 band is often used as a diagnostic tool to determine the presence and extent of conjugation in organic compounds. The position and intensity of the E2 band are directly related to the degree of conjugation in the molecule. Compounds with more extensive conjugation will exhibit a bathochromic (red) shift and increased intensity of the E2 band, while compounds with less conjugation will show a hypsochromic (blue) shift and decreased intensity. By analyzing the characteristics of the E2 band, chemists can infer the presence and extent of conjugation in the molecule, which is crucial for understanding its structural and electronic properties.
• Analyze how the factors that influence the energy difference between the HOMO and LUMO+1 orbitals can affect the position and intensity of the E2 band in the UV spectrum.
  • The energy difference between the HOMO and LUMO+1 orbitals, which determines the position of the E2 band, is influenced by various factors related to the conjugated system. The nature and arrangement of the conjugated moieties, the presence of substituents, and the overall symmetry of the molecule can all affect the electronic structure and the energy gap between the HOMO and LUMO+1. As this energy gap changes, the position of the E2 band will shift accordingly, with a smaller gap leading to a bathochromic (red) shift and a larger gap resulting in a hypsochromic (blue) shift. Additionally, the intensity of the E2 band is related to the degree of electronic delocalization and the symmetry of the molecular orbitals involved in the transition, which can also be influenced by the structural features of the conjugated system.

Related terms

Conjugation: The arrangement of alternating single and double bonds in a molecule, which allows for the delocalization of electrons and extended π-electron systems.

HOMO: The highest occupied molecular orbital, which is the most energetic filled orbital in a molecule.

LUMO: The lowest unoccupied molecular orbital, which is the least energetic empty orbital in a molecule.