2.3. p-Orbitals and p-Overlap

The interaction diagrams in Sects. 2.1 and 2.2 dealt with s-AOs. Because they are spherically symmetric, s-orbitals have no directional preferences. In contrast, p-AOs interact very differently depending on their relative directions. Two p-AOs can, for instance, form s- and s*-MOs, as shown in Figure 2.6:

Figure 2.6 The interaction of two p-AOs to give s- and s*-MOs.

Note that if they approach each other too closely, two p-orbitals in a s-MO can actually begin to overlap less well than at larger distances, as shown in Figure 2.7:

Figure 2.7 As two p-orbitals approach each other in a s-fashion (top) they reach a position of optimum overlap (centre) and, on approaching closer, one in which the total overlap is decreased by antibonding interactions outside the nodal planes of the two AOs (bottom).

Another important aspect of p-orbital overlap is its angle-dependence. Figure 2.8 shows the progression from a pure s-overlap to a "banana" bond by rotation of the two orbitals away from each other. Such bonds are found in strained compounds such as cyclopropanes (see Sect. 3.2). Further rotation of the two orbitals away from each other results in pure p-overlap, in which the two p-AOs are parallel to each other.

Figure 2.8 The transformation of a s- into a p-MO by rotation of the two p-orbitals away from each other. The intermediate situation is a bent, or "banana" bond, as found in strained compounds.

Note that the overlap between the two orbitals (assuming that the distance between their centres remains constant) is largest in the s-situation and becomes smaller as the orbitals are twisted away from each other. The pure p-MO represents a minimum in the overlap. Thus, "banana" and p-orbitals lie higher in energy than their s-counterparts. This energy difference is manifested as strain in organic compounds. Demonstration 3 shows the effect of distance and angle on the overlap between p-orbitals.

H₂ and He₂ -The Simplest Examples
The Effect of Electronegativity
p-Orbitals and p-Overlap
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Combining AOs to Build MOs
The AH₂ Walsh Diagram