What Are the Applications of Quantum Mechanics in Chemistry?

[DrDu]

I've read the page you linked on the Born-Oppenheimer approximation. However, I'm still confused as to what you mean by "molecular structure and isomers have no meaning at a fully quantum mechanical level" - surely there must be something to distinguish isomers from each other at the most fundamental level, since they are indeed different species and we can isolate them separately from one another in the lab?

Isomers can isomerize. On a most fundamental level, the eigenstates of the full hamiltonian are superpositions of different isomers.
The question why we really observe isomers is far from trivial. Probably it is due to interactions with neighbouring molecules, so it is a colligative effect. You may want to google for Hund's paradox.

 

[Einstein Mcfly]

Isomers can isomerize. On a most fundamental level, the eigenstates of the full hamiltonian are superpositions of different isomers.
The question why we really observe isomers is far from trivial. Probably it is due to interactions with neighbouring molecules, so it is a colligative effect. You may want to google for Hund's paradox.

Doesn't Hund's paradox just apply to chiral systems? I thought his question was more about non-degenerate isomers (boat versus chair and all that).

 

[DrDu]

Doesn't Hund's paradox just apply to chiral systems? I thought his question was more about non-degenerate isomers (boat versus chair and all that).

Yes, but as always, for degenerate states the consideration of superpositions is most relevant.