Geminal delocalization

The cross-bonded terms cxCy/ ax and caCaFay result from interaction of X and Y with the wrong bonding hybrids on A. The magnitudes of these terms can usually be judged from simple overlap considerations. Unless X and Y are of quite dissimilar electronic character, the two cross-bonded terms are inherently of similar magnitude and therefore tend to cancel one another out. Thus, cross-bonded terms tend to make only minor contributions to geminal delocalization. 

Thus, other possible contributions to TSE appear to be less important than the geminal delocalization effects noted above. 

Finally, we briefly compare the NBO donor-acceptor picture of geminal delocalization with Dewar s sigma-aromaticity concept and the associated pi-complex... 

TABLE 10. Conventional strain energies (CSE), hybridizations, s-character, overlap values, overlap repulsions and geminal delocalizations of propane, cyclobutane, cyclopropane and their heterologues with X = NH, O, SiH2, PH, S from Reference 47 ... 

Appropriate interactions leading to the three types of delocalization were estimated by Inagaki et al. (226, 267 in a perturbational approach. Whereas in the conventional orbital interaction geminal delocalization b- c (G-Ti) is independent of conformation, their approximation showed (when higher order terms of the orbital interactions were not neglected) that G-Tj is under the influence of the orientation of the donor bond A, that is, dependent on conformation. The G-Ti interaction involves the occupied orbital a of the donor bond, in addition to b and c. ... 

It is also apparent that geminal delocalizations (i.e., of (Tax ( ay fortn, involving X—A—Y bonding pattern) are generally much weaker than vicinal delocalizations (i.e., of a AX ct by form, involving X—A—B—Y bonding pattern). Only... 

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