The Role of Rehybridization

The quality of these two correlations is very good as evidenced by the average absolute deviations from the HF/6-31G CC bond lengths being only 0.003 A. The shape of curves 

Bond distances (in A) NBO s-characters (in %), Lowdin s 7r-bond orders and Lowdin atomic charges of deliberately distorted benzene and perfluorobenzene as obtained by the MP2(fc)/6-31G model. 

Molecule Bond Distance s-character 7r-bond order A 

To summarize deformed benzene and perfluorobenzene represent two clear-cut cases which underline the importance of rehybridizaton in annelated systems. They also convincingly show that reorganization of the 7r-electron density can be triggered by redistribution of the CC s-characters which takes place in the a-plane, although it is addmitedly true that a part of lengthening of the ipso bonds is due to repulsion between a pairs of two close vicinal H or F atoms. 

Another very illuminating example which illustrates the crucial role of rehybridization is provided by all-cw-tris(benzocyclobuta)-cyclohexane [38] depicted in Fig. 8. The 

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As a direct consequence of Bent s rule,56 carbon compounds with a significant amount of s-character in their bonds to an electronegative element, e.g., fluoro-alkynes, are unstable. Thus, reactions which involve change in hybridization at a carbon atom bearing such an acceptor can be promoted when the amount of s-character at the reactive atom decreases. The role of rehybridization in a variety of chemical and supramolecular processes has been extensively analyzed and reviewed recently.55,58... 

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