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Indicate the transitions responsible for peak(s) in the UV-spectrum
Accepted Answer
UV-Vis spectroscopy involves electronic transitions, specifically those between molecular orbitals. Peaks in the UV spectrum arise from the absorption of UV radiation by molecules, causing the promotion of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The types of transitions responsible depend on the specific molecule and its structure. Common transitions include:
π → π
transitions: These involve the promotion of an electron from a π bonding orbital to a π
antibonding orbital. They are typically observed in molecules containing double or triple bonds.
n → π
transitions: These transitions involve the promotion of an electron from a non-bonding orbital (n) to a π
antibonding orbital. They are commonly observed in molecules containing heteroatoms with lone pairs, such as oxygen or nitrogen.
σ → σ
transitions: These transitions involve the promotion of an electron from a σ bonding orbital to a σ
antibonding orbital. They are usually found at higher energies in the UV spectrum.
The specific energy of these transitions, and thus the wavelength at which the UV peak appears, is influenced by factors such as the nature of the chromophore (the part of the molecule responsible for absorbing UV light), the presence of substituents, and conjugation within the molecule.
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