A Molecular Orbital Description of Stability

So far in our discussion we have used resonance contributors to explain why compounds are stabilized by electron delocalization. Molecular orbital (MO) theory can also explain why electron delocalization stabilizes compounds. 

We saw in Section 1.6 that the two lobes of ap orbital have opposite phases. We also saw that when two in-phase p orbitals overlap, a covalent bond is formed, and when two out-of-phase p orbitals overlap, they cancel each other and produce a node between the two nuclei. 

Let s start by reviewing how the MOs of ethene are constructed. The two p atomic orbitals can be either in-phase or out-of-phase. (The different phases are indicated by different colors.) Notice that the number of orbitals is conserved— the number of molecular 

The overlap of in-phase orbitals holds atoms together it is a bonding interaction. 

The overlap of out-of-phase orbitals pulls atoms apart it is an antibonding interaction. 

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The stability order of alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tr bond and adjacent C-H a bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom C=C—< -H grouping, as shown in Figure 6.6. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

Figure 5. Thorn and Hoffmann molecular orbital description of olefin insertion. The organic radical/anion HOMO is stabilized by a Pt d orbital of proper shape.

In Summary Allylic radicals, cations, and anions are unusually stable. In Lewis terms, this stabilization is readily explained by electron delocalization. In a molecular-orbital description, the three interacting p orbitals form three new molecular orbitals One is considerably lower in energy than the p level, another one stays the same, and a third is higher in energy. Because only the first two are populated with electrons, the total it energy of the system is lowered. 

Pyridine, symmetry group C2v, has six electrons in a system delocalized around the ring, and two lone-pair electrons in an orbital localized at the Nitrogen atom. The Is electrons, as well as the electrons in orbitals describing the a bonds, need not be considered explicitly in describing the resonance stabilization and low-lying excited states of pyridine. The simple molecular orbital description has the following characteristic assumptions ... 

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