Delocalization interactions stabilization energy

Of the 17 kcal mol-1 total error, about half is estimated to arise from the single hp— h[ j sigma-type interaction shown in Fig. 2.8, while the remainder arises from weaker pi-type interactions (2-3 kcal mol-1 each). For example, we can carry out a partially localized variational calculation, similar to that described above but with only h prevented from delocalizing into tip (hisleads to a stabilization energy (at 7 = 1.6 A)... 

The unusual form of the C=C NBOs in Fig. 3.50 is clearly related to hypercon-jugative interactions with the nN lone pair. In (he 4> = 80° geometry, the nN has comparable delocalizations into both banana antibonds (stabilization energies of 8.41 and 7.06 kcal mol-1). The C=C NBOs revert rather abruptly to their usual sigma/pi form as nN is twisted to somewhat smaller

[Pg.220]

Investigation of environmental effects. As has been stressed in this chapter, homoaromaticity is just a matter of a few kcal mol-1 stabilization energy in most cases, and therefore environmental effects may have a large impact on structure, stability and other properties of a homoaromatic compound. Future work in theory (as well as in experiment) has to clarify how environmental effects can influence electron delocalization, through-space interactions and bonding in homoaromatic molecules. The theoretical methods are now available to calculate solvent and counter ion effects (for homoaromatic ions in solution) or to study intermolecular and crystal packing forces in the solid state. 

The balance of electrostatic and delocalization interactions in an isolated molecule may be perturbed by the influence of the solvent. In calculations based on Eq. 7, the analysis of solvation-energy terms suggested that the electrostatic contribution stabilizing the ap orientation of the acetal s ment is the conformationally dominant term. For example, in 2-methoxyoxane, the difference in energy of the (ap, ap) and (ap, sc) conformers in water, compared to that in the isolated molecule, caused by solute-solvent electrostatic interactions alone, amounts to 4 kJ.mor. Accordingly, the inter-and intra-molecular, electrostatic interactions operate in reverse directions in acetals. Whereas the intramolecular, electrostatic interactions are responsible, together with delocalization interactions, for the aiq)earance of the anomeric, reverse anomeric, and exo-anomeric effects, the solute-solvent electrostatic interactions lessen their im nitude, and may even cancel them. Of course, the solvent may also influence the electron distribution and energy of MO s in a molecule. In this way, the orbital interactions of lone-pairs and delocalization contributions to the anomeric effect may be scaled by the solvent, but this mechanism of the environmental effect is, in most cases, of only minor importance. 

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