[Omega Glory]

For a long time now, I have had poor understanding of MO theory, I think because it was never properly taught to me in the first place, and every time I've gone back to try to study up on it I've been mystified by one thing or another.  I've gotten by with only a rudimentary knowledge, mostly because MO theory comes up so infrequently.  But when it comes to absorption/emission, it seems to be of paramount importance.

I understand Jablonski -- electrons in the ground state being promoted to higher energy levels (orbitals) upon absorption of electromagnetic radiation, followed by vibrational relaxation / internal conversion / external conversion / intersystem crossing / fluorescence / phosphorescence / what have you.  What I DON'T understand is where MO theory and the HOMO/LUMO energy gap comes into it.  Jablonski makes no mention of different MO transitions like pi to pi star or n to pi star.

Nor, for that matter, do I really have a concrete understanding of what such transitions actually *mean*.  I think I grasp the fundamentals of bonding/anti-bonding, but if pi electrons are promoted to an anti-bonding state, shouldn't there be bond breakage due to the sudden appearance of a node where before there was a covalent bond?  What is the actual physical meaning of an n -> pi star transition?  Is the physical location of the electrons in the molecule being altered, or is something subtler afoot?  And how does all of this relate back to the Jablonski diagram?

[iolzizlyi]

When electrons are promoted to an antibonding MO, the bond could break, but this depends on the bond order of the molecule.  Take nitrogen: it has a bond order of three (a triple bond) because all of its valence electrons are in bonding MO's.  Promote one of the bonding electrons to an antibonding orbital and you get a reduced bond order of 2.5.  The bond between nitrogen atoms weakens but doesn't break.

An n to pi* transition is a transition from a nonbonding MO to an antibonding MO.