Question

Describe different types of electronic transitions in a molecule. Which type of transition is responsible for a band at max = 279 nm in absorption spectrum of acetone.

Answer 1

σ → σ *Transitions
An electron in a bonding σ orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo σ → σ *transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to σ → σ *transitions are not seen in typical UV-Vis. spectra (200 - 700 nm)
n → σ * Transitions
Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n → σ *transitions. These transitions usually need less energy than σ → σ * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n → σ *peaks in the UV region is small.
n → π* and π → π* Transitions
Most absorption spectroscopy of organic compounds is based on transitions of n or π electrons to the π * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the π electrons.
The type of transition is responsible for a band at λmax = 279 nm in absorption spectrum of acetone. In case of n → π* transitions, the polar solvents form hydrogen bonds with the ground state of polar molecules more readily than with their excited states. Therefore, in polar solvents the energies of electronic transitions are increased.