Electron delocalization distortion

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

Recently, work has been reported on the 7-boranorbornene (89) and 7-boranorboma-diene (90) systems related to 84 and 85. Schleyer and colleagues have examined the parent molecules 87 and 88 theoretically [MP2(FU)/6-3 lG(d)] and concluded that they have very similar structures and electron delocalizations to the related cations (Table 8)203 204. X-ray structures of substituted derivatives of 89 and 90 have been determined and these have very similar distorted conformations to those of 84, 85 and 86205. Calculated (IGLO) NMR chemical shifts of 87 and 88 correspond well with those observed experimentally. 

Before moving to the next topic, it is important to recall that ultimately all the distortions discussed above are rather small and the annealated benzenes have UV, NMR, and other properties which still typify an aromatic mode of electron delocalization.36-171... 

The variable nature of the X3 species in the isoelectronic series, form a general model for electronic delocalization, enabling one to classify the species either as distortive or as stable ones. Using the isoelectronic analogy, one might naturally ask about the isoelectronic Tr-species in allyl radical does it behave by itself like H3 or like Li3 Moreover, the same two extreme VBSCDs for X3 in Fig. 6.19 can be shown for X3 +, X3, X4 and X6 species (64). Likewise one might wonder about the status of the corresponding isoelectronic -n-components in allyl cation, anion, cyclobutadiene, or benzene. These questions were answered in detail elsewhere and the reader is advised to consult a recent review (64). Below we briefly discuss the problem of one of our molecular icons, benzene. 

Figure 16.8.2(g) shows the structure of Teg+ this 44 valence electron cluster takes a cube shape with two cleaved edges, the positive charges being located on four three-coordinate Te atoms. The Teg+ cation can be viewed as two planar Te2+ ions that have dimerized via the formation of a pair of Te-Te bonds with simultaneous loss of electronic delocalization and distortion from planarity. 

The revision of characteristic frequencies of nuclear modes is a general result of electronic delocalization holding for both the intramolecular vibrational modes and the solvent modes. The fact that this effect shows up already in the harmonic expansion term makes it much stronger compared to nonlinear solvation in respect to nonparabolic distortion of the free energy surfaces. 

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