Charge-transfer interaction energies

Fig. 6 Charge-transfer interaction energies between attacking H20 and CH3N2+ versus C-N. Open circle is for trajectory (a), and filled triangle for trajectory (b).

In the Born equation, the ion solvent interaction energy is determined only by one physical parameter of the solvent, i.e., the dielectric constant. However, since actual ion-solvent interactions include specific interactions such as the charge-transfer interaction or hydrogen bonds, it is natural to think that the Born equation should be insufficient. It is well known that the difference in the behavior of an ion in different solvents is not often elucidated in terms of the dielectric constant. 

This quantity represents the energy of the multiple-site charge-transfer interaction which will later play an important role in the theory of stereoselection. It is to be remarked that, although any MO may involve an arbitrary constant of which the absolute value is unity, the value of the numerator in each term of the right side of this equation is always definite. 

In recent years, direct, time-resolved methods have been extensively employed to obtain absolute kinetic data for a wide variety of alkyl radical reactions in the liquid phase, and there is presently a considerable body of data available for alkene addition reactions of a wide variety of radical types [104]. For example, rates of alkene addition reactions of the nucleophilic ferf-butyl radical (with its high-lying SOMO) have been found to correlate with alkene electron affinities (EAs), which provide a measure of the alkene s LUMO energies [105,106]. The data indicate that the reactivity of such nucleophilic radicals is best understood as deriving from a dominant SOMO-LUMO interaction, leading to charge transfer interactions which stabilize the early transition state and lower both the enthalpic and entropic barriers to reaction, with consequent rate increase. A similar recent study of the methyl radical indicated that it also had nucleophilic character, but its nucleophilic behavior is weaker than that expressed by other alkyl radicals [107]. 

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]. 

Because the precise energies of charge-transfer interactions are sensitive to small structural modifications, purely intuitive predictions often turn out to be wrong. In tetrafluorohydrazine, for instance, hyperconjugation of the type wN — should... 

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