Electronic transitions

Definition of Electronic transitions

• 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

1. Ground State:

   • The lower energy state of the molecule, where electrons are in their stable, unexcited state.

2. Excited State:

   • The higher energy state reached after electrons absorb energy and move to higher orbitals.

Selection Rules

Not all electronic transitions are permitted. The likelihood of a transition depends on certain selection rules:

1. Allowed Transitions:

   • Transitions that typically involve a change in dipole moment, e.g., π → π* and n → π*.

2. 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.

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