What I want to show here are the orbitals because I think they look kind of cool. What you see are the two highest occupied orbitals (n, π) and the lowest unoccupied orbital (π*). The two important excitations are just between these orbitals leading to the ππ* and nπ* states respectively.
Related to the proton transfer it can be seen that the π* LUMO has more density on the N which increases its basicity to help it catch the proton.
Something else that is interesting is that a non-bonding orbital (coming from MO theory) really looks a lot like the free electron pair you would talk about in VB theory. I was arguing with an analytical chemistry professor once that he should not interchange the terms non-bonding orbital and free electron pair because they come from different approaches. Now I see that he was right but I did not believe him then because he was from analytical chemistry.
|Enol (ground state)||Keto (ππ* state)|