Aromaticity and Hypervalence
Special bonding motifs arise as molecules become more complex and do involve more than one bond. Examples are hypervalence or the multifaceted phenomenon of aromaticity. Why, for example, does carbon strongly resist violating the octet rule, i.e., why is it very difficult to form stable, electron-rich five-coordinate carbon? An example would be D3h-symmetric Cl-CH3-Cl]– but, as we all know, that is a labile structure, located at the saddle-point (or transition state) of the energy profile of an SN2 reaction. Silicon, on the other hand, easily forms five- (and higher-) coordinate species. Interestingly, we have shown that benzene undergoes C–C bond-length equalization not because but despite the π-electron system. And there are many more examples of textbook wisdom that awaits to be verified, or falsified, with modern quantum theory. The results can be paradigm shifting!
Aromaticity. Molecular Orbital Picture of an Intuitive Concept
S. C. A. H. Pierrefixe, F. M. Bickelhaupt
Hypervalent Silicon versus Carbon: Ball-in-a-Box Model
S. C. A. H. Pierrefixe, C. Fonseca Guerra, F. M. Bickelhaupt
Hypervalent Carbon Atom. "Freezing" the SN2 Transition State
S. C. A. H. Pierrefixe, S. J. M. van Stralen, J. N. P. van Stralen, C. Fonseca Guerra, F. M. Bickelhaupt