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Normally, I think about orbitals interacting in the context of bonding. When two atomic orbitals overlap, they can do so in-phase or out-of-phase. The in-phase overlap results in a bonding molecular orbital with lower energy than the original AOs; conversely, the out-of-phase overlap results in an antibonding MO with higher energy. Valence electrons fill the bonding MOs first, resulting in a bond.

The explanation of steric effects that I often hear is that they are caused by the overlap of two electron clouds, resulting in repulsion. Since electron clouds are just composed of occupied orbitals, how is this "overlap of electron clouds" different from the picture of bonding I described above? Does the steric repulsion just come from occupied antibonding MOs? I imagine that inner shell electrons and Coulomb forces must play a role as well, right?

@matt_black answered a related question about steric vs eletronic effects by saying:

The normal distinction between "steric" and "electronic" is based on whether the effect is transmitted through space or through bonds

I guess I'm essentially asking a follow-up question on how effects are transmitted through space.

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  • $\begingroup$ The point is right that combining all atomic orbitals you already have the possible solutions. What is left out is electrostatic repulsion. I see the exclusion principle already included in the orbital itself. $\endgroup$
    – Alchimista
    Commented Mar 13, 2021 at 11:06
  • $\begingroup$ In particular thinking of conformers might help, eg rotation about a bond. $\endgroup$
    – Alchimista
    Commented Mar 13, 2021 at 11:50
  • $\begingroup$ Exact details are largely irrelevant - you just can't put one object into another. $\endgroup$
    – Mithoron
    Commented Mar 13, 2021 at 14:21

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It's all about the Pauli exclusion principle. Assume the available MOs in each of two approaching molecules are fixed, and that each molecule is in a closed shell configuration (all electrons are paired). The electrons in closed shell systems cannot accept additional electrons that share principal, azimuthal and magnetic quantum numbers, since all the spin QMs are taken up. In fact, other than filling higher energy (LUMO) orbitals there is no way to accept additional electrons while retaining the current structure. The solution is to deform the structure of the molecule so as to generate new orbitals that allow formation of novel MOs. The deformation is associated with a potential energy barrier, since it is assumed that the colliding molecules are in low energy electron configurations. Therefore the instantaneous response to the collision is repulsive.

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  • $\begingroup$ Cool, that mostly makes sense to me as a simple chemical biologist! Can you elaborate a little on why the structure has to deform to allow formation of new MOs? $\endgroup$
    – Andrew
    Commented Mar 13, 2021 at 10:26
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    $\begingroup$ @Andrew My point is that two molecules can in fact react to form a new one. That can happen in different ways, but if both reactants are initially stable closed shell molecules then it requires substantially modifying the electronic arrangement, which amounts to creating new MOs. It's not very deep. The devil is in the details. The key point is that these are closed shell stable molecules. Closed shell means no room for additional electrons within the current configuration (a bit circular of an argument perhaps). $\endgroup$
    – Buck Thorn ♦
    Commented Mar 13, 2021 at 14:35

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