Definition
Electronic transitions refer to the movement of electrons between different energy levels within a molecule upon absorption of light.
In UV-Visible spectroscopy, these transitions typically include:
1) π → π*:
Electrons in pi bonds (π bonds) are excited to anti-bonding pi-star (π*) orbitals.
2) n → π*:
Non-bonding electrons (n electrons), often lone pairs on heteroatoms like oxygen or nitrogen, are excited to anti-bonding pi-star (π*) orbitals.
3) σ → σ*:
Electrons in sigma bonds (σ bonds) are excited to anti-bonding sigma-star (σ*) orbitals.
These transitions generally require higher energies (shorter wavelengths) compared to π → π* or n → π* transitions.
Energy Levels
Ground State:
The lower energy state of the molecule, where electrons are in their stable, unexcited state.
Excited State:
The higher energy state reached after electrons absorb energy and move to higher orbitals.
Selection Rules of Electronic transitions
Not all electronic transitions are permitted. The likelihood of a transition depends on certain selection rules:
Allowed Transitions:
Transitions that typically involve a change in dipole moment, e.g., π → π* and n → π*.
Forbidden Transitions:
Transitions that do not involve a change in dipole moment; for example, those occurring between orbitals of the same symmetry.
These transitions are less likely or weaker in intensity.