The Role of Steric Repulsion in Bonding Models

So far, we have looked at different modes of bonding and how Pauli repulsive orbital interactions may either influence them (2c-2e bond) or be an essential part (2c-3e bond). In this section, we examine a different role of Pauli repulsion, namely, the one it plays in the absence of bonding interactions between groups. Here, it is responsible for the fact that such groups, A and B say, repel each other. Or, to put it in another way, steric repulsion between A and B is a pure quantum effect, caused by the Pauli repulsion between same-spin electrons of the different fragments, such as the well-known two-center, four-electron (2c-4e) repulsion (19) or the two-center, two-same-spin-electron (2c-2sse) repulsive component (20) of the three-electron bond. 

As pointed out earlier, the physical basis of this repulsion is the increase in kinetic energy of the electrons due to the Pauli exclusion principle, which is most easily seen from the large gradients induced in the wavefunctions by the orthogonality requirement. 

A correct description of Pauli repulsive interactions between valence, subvalence, and core electrons as well as of electrostatic interactions is an essential requirement for accurate quantum chemical predictions. We now show that a proper analysis of steric repulsion—Pauli repulsion in nonbonded 

This qualitative model, based on semiempirical MO theory, focuses entirely on the so-called electronic effects, as the A—H bonding orbital interactions are often called. However, steric repulsion (i.e., the destabilizing orbital interactions) between the hydrogen substituents in AH3 is just as important in the interplay of mechanisms that determine whether the molecule adopts a planar or a pyramidal shape. In fact, as will become clear from the following discussion, which is based on a Kohn-Sham DFT study at the BP86/TZ2P level,107 108 steric repulsion turns out to be the decisive factor in determining the pucker of our example.133 

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