Geminal overlap

From Reference 51. In cyclopropane, bonding overlap corresponds to overlap between hybrid orbitals 1 and 2, geminal overlap to overlap between 2 and 3, vicinal through-ring overlap to overlap between 1 and 3, vicinal rc-type oveerlap between 1 and 4. Compare with Figure 17. 

Geminal overlap is 10-20% larger in 1 than in the two other compounds. 

Calculations place G substantially below C, with the latter being more stable than trans-1,2-difluoroethylene for reasons discussed before. Both G and C isomers have an identical C core. However, if we separate the ligands into two sets, one containing the two F s and the other containing the two H s, we can see that the G and C isomers differ in one important way In the former, the F and H sets are segregated, i.e., there is only vicinal overlap of F and H AO s, while in the latter case, the two sets are brought into proximity, i.e., there is now appreciable geminal overlap of F and H AO s as they are both attached on the same carbon atom. 

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. 

Figure 3.81 Geminal interactions in cyclopropane and propane, showing bent cyclopropane (a) bond

Scheme 1.5 represents case f, that is, an anion-radical belonging to the borderline between moderately and completely delocalized species. Its optical spectra, along with frontier orbital analysis, testifies that in this anion-radical there is a positive overlap between C-N bonds and the psendo-geminal carbons of the opposite rings, as shown by the dashed lines in the strnctnre of Scheme 1.5 (Nelsen et al. 2005). Taken together, the experimental results considered provide direct evidence for the throngh-bond mechanism of electron transfer in these paracyclophane systems.

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

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 ... 

Destabilizing (antibonding) overlap repulsions between geminal bonds increase (i.e. IBP becomes more negative) in the order open-chain compound < four-membered ring < three-membered ring. 

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