Illustrated Glossary of Organic Chemistry

Nonbonding molecular orbital (nonbonding MO): A molecular orbital in which the in-phase (bonding) atomic orbital overlaps equal the number of out-of-phase (antibonding) atomic orbital overlaps. Electrons in a nonbonding molecular orbital are higher in energy than in a bonding molecular orbital, but lower in energy than electrons in an antibonding molecular orbital.

In this simplified diagram of the pi molecular orbitals of the allyl carbocation, π1 is a bonding molecular orbital because it contains two in-phase p orbital overlaps and no out-of-phase p orbital overlaps. π2 is a nonbonding molecular orbital because the number of in-phase p orbital overlaps (zero) is equal to the number of out-of-phase p orbital overlaps (also zero). π3 has more out-of-phase p orbital overlaps (two) than in-phase p orbital overlaps (zero), so π3 is an antibonding molecular orbital.