# How can I distinguish between n, pi, pi* orbitals when looking for excited states?

I recently asked about how to distinguish between transition types. I got an answer there about determining the character or type of the orbitals.

My problem now is that I'm not sure how to determine the type of an orbital. How can you distinguish, either by inspection or some algorithm, whether an orbital is $$\pi$$, $$\pi^*$$ or $$n$$ on a system that contains double bonds and heteroatoms? For example what are the types of the HOMO and LUMO shown below.

LUMO orbital

HOMO orbital

• In my opinion, distinction based on appearance may make only sense if you can find the pairs of bonding - bonding orbitals. In low symmetry / large molecules those are often not possible and not very informative either.
– Greg
Jun 18, 2022 at 5:15

Both orbitals look like $$\pi$$ orbitals, as both have nodes at the plane of the molecule, which is characteristic for $$\pi$$ systems. Occupied valence orbitals are usually considered to be bonding while excited unoccupied orbitals are often antibonding. In this particular case I woudn't say that the distinction between bonding and antibonding is 100% clear, but I'd call the LUMO $$\pi^*$$ in this case since the nodal structure can clearly be seen. I would also call this a charge transfer transition since you have a clear transfer of density from the left benzodioxole group to the central indene(?) group and the right benzene group.