One-Electron, Two-Orbital Interaction

Figure 3.10. The one-electron, two-orbital interaction half-filled orbital is stabilized.

Figure 3.9. Two-electron, two-orbital interaction (a) both electrons in lower orbital, dative bond formation and electron transfer from to A (b) one electron in each orbital, large stabilization and covalent bond formation.

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

At bonding separations 3 (Figure 3.12), the four-electron, two-orbital interactions are strongly dominant, leading to the observed r n behavior at distances within the van der Waals separation. For two molecules in their ground states to undergo chemical reaction, there must be at least one exceptionally strong two-electron, two-orbital interaction which will permit close approach of the molecules. This interaction is necessarily accompanied by partial electron transfer. 

Substituent Effects and Reactivity. If the SOMO is relatively low in energy, the principal interaction with other molecules will be with the occupied MOs (three-electron, two-orbital type, Figure 3.8). In this case the radical is described as electrophilic. If the SOMO is relatively high in energy, the principal interaction with other molecules may be with the unoccupied MOs (one-electron, two-orbital type, Figure 3.10). In this case the radical is described as nucleophilic. Substituents on the radical center will affect the electrophilicity or nucleophilicity of free radicals, as shown below. 

As in the case for alkene additions, if the SOMO of the radical is relatively high in energy, such as is the case for alkyl radicals, the principal interaction with the abstractable X-H bond will be with its unoccupied a MO (one-electron-two-orbital type), and such a radical would be considered nucleophilic. If the SOMO is relatively low in energy, such as is the case for perfluoroalkyl radicals, the principal interaction with the abstractable X-H bond will be with its occupied a MO (three-electron-two-orbital type), and the radical is considered electrophilic. Either way, a good match-up in polarities in an H-atom transition state will give rise to beneficial transition state charge-transfer interaction [130,136,137]. 

Sometimes one faces the situation where none of the explanations presented in Sections III.B.1-III.B.8 is sufficient to account for the observed conformational preference. Then, additional interactions need to be taken into account. They are usually discussed in terms of stabilizing two electron-two orbital interactions. Some of these, which may be useful in discussions of the anomeric effect, are presented below. 

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