[nate]

Can someone explain to me the antibonding interaction when aligning 3 orbitals.  We covered it in class and I was confused when it appears that you have 1 bonding interaction and 1 antibonding interaction.  I guess the molecular orbital that I am concerned with is the phi B orbital.  There is a large node in the middle resulting from ortho?? interaction.  He said that with turning the orbital, they become the same as something.  I thought that you weren't allowed to turn things.

Please help.

THanks

[movies]

Well, I'm not sure what your professor was getting at when he was talking about turning things.  Maybe he was referring to changing the sign of the p-orbitals which is effectively like flipping them over 180 degrees.  This is what the three molecular orbitals of an allyl fragment (can be a cation, anion, or radical) should look like.  On the left are the atomic orbitals and on the right the molecular orbitals that they form.  As you can see, the lowest one has all bonding interactions between the atomic orbitals and therefore leads to a bonding MO.  The middle one has one bonding interaction and one anti-bonding interaction and therefore they cancel each other out to make a non-bonding MO with a node at the center carbon atom.  The highest orbital has two anti-bonding interactions and non bonding interaction, therefore an antibonding MO.

[nate]

Thanks for the help.  I think your pictures really helped to answer the question.  It does bring up another question though.

Do all Phi B orbitals behave this way?  Will all Phi A orbitals be bonding and Phi C antibonding?

[movies]

The Greek letter they use should be psi, not phi.  Also, most texts use numbers instead of letters for the different MOs.  At any rate...

The assignment of these MOs will be different for each system because different systems will have different numbers of orbitals and possibly more or less energy levels.  For example, there are 6 MOs for benzene, 3 bonding and 3 anti-bonding, and a total of 4 distinct energy levels.  Using typical notation these would be psi(1)-psi(3) and psi(4)-psi(6), respectively, where psi(2) and psi(3) have identical energy and psi(4) and psi(5) also have identical energy.  Also note that in this case there are no non-bonding orbitals.

Specifically, I can't think of any examples where psi(1) would be anti-bonding or even non-bonding, although someone else might know an example where this is the case.