Tuesday 27 February 2007


I talked about the nitrogen MO scheme a while ago. Let's see what happens if you take a proton from one of the nuclei and give it to the other one.

The molecular orbitals were calculated from a minimal basis with AM1 parametrisation in Arguslab. They are pretty close to the ones of nitrogen. The difference is that lower energy MOs are rather O-centered and higher energy MOs rather C-centered. This makes sense because of the fact that the atomic orbitals of oxygen are lower in energy. It can be seen that the HOMO and the LUMO [1] are centered on the carbon. This causes the strong σ-donor and π-acceptor characteristics of CO.

The table has the LCAO coefficients for the different orbitals. The color coding shows if orbitals have a bonding or antibonding interaction [2]. It can be seen the the lowest energy AO (O-2s) has the strongest impact on the lowest energy MO, and that the highest energy AO (C-2p) has the strongest impact on the highest energy MO.










CO is usually drawn with a formal positive charge on the oxygen and a negative charge on the carbon.
Formal charge and polar bonds work in opposite ways. This results in a very small dipole moment of .1 Debye. Its direction was argued over for a while. Now theoristical chemists are pretty sure that at equilibrium distance the carbon atom is the negative end.

[1] Theorists always seem to be kind of careful when talking about unoccupied MOs. In order to sound serious I have to do that, too: One has to remember the fact that unoccupied MOs are even less real than occupied ones. For some reason they are pretty good if you use a minimal base. But as soon as you move from plain LCAO to using more basis functions you will just get a lot of "virtual" MOs close together in energy with no physical significance.

[2] For σ-MOs the color corresponds to the sign that the wave function has in between the nuclei, for π-MOs it corresponds to the sign in the positive x-direction of the atom.

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