Which one of the following electronic transitions requires the least energy?

CONCEPT:

Electronic Transitions and Energy Requirements

• In electronic transitions, electrons move from one molecular orbital to another due to the absorption of energy.
• The energy required for a transition depends on the energy gap between the molecular orbitals involved.
• Common electronic transitions in molecules include:
  • σ → σ*: Transition between bonding and antibonding sigma orbitals. This requires the highest energy because sigma bonds are the strongest.
  • n → σ*: Transition from a non-bonding orbital to an antibonding sigma orbital. This requires less energy than σ → σ*.
  • π → π*: Transition between bonding and antibonding pi orbitals. This requires less energy than the above two transitions.
  • n → π*: Transition from a non-bonding orbital to an antibonding pi orbital. This requires the least energy among the listed transitions.

EXPLANATION:

• σ → σ* involves the largest energy gap, so it requires the most energy.
• n → σ* involves a smaller energy gap than σ → σ*, so it requires less energy.
• π → π* involves an even smaller energy gap than the above two transitions.
• n → π* involves the smallest energy gap, so it requires the least energy.

Therefore, the electronic transition that requires the least energy is n → π*.