[Iamu]

I'm a physics major trying to get my bearings in an organic chemistry course I'm taking this semester, and my professor didn't satisfactorily answer a question I had about resonance structures.

He said that we shouldn't think of resonance structures as being seperate configurations that a molecule could have, but that we should instead think of compounds with resonance structures as existing in a combination of all these states at once.

I read in another string on these forums that resonance hybrids are a superposition of resonance structures.  So are resonance hybrids in a state of quantum superposition between their resonance structures?  Or do they actually exist in a singular state that's a low-energy "compromise" between their resonance structures?

This question may be confusing, because it's possible to think of a specific quantum superposition of states as a being a single relatively well-defined state, too, but I'm not sure how to clarify my question.  I'm new to quantum mechanics, too.

Any help is much appreciated.

[Yggdrasil]

No, the resonance structures do not represent a superposition of states.  They are eigenstates of the hamiltonian (so they have a defined energy).  They basically represent chemical bonds (or more precisely, molecular orbitals) that span more than two atoms, something which conventional organic chemists' drawings have trouble representing.

A good example of this is the molecule benzene (http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch11/ch11-4-1.html), a six carbon ring drawn with alternating double and single bonds.  One can draw two resonance structures where the double and single bonds switch places.  In reality, the lowest energy molecular orbital spans all six atoms.

[Iamu]

Ok, this makes sense.  Thanks a lot, you've made this significantly easier to understand for me.