Intermolecular packing forces

In the crystalline phase, intermolecular packing forces (Viattice) may further modify the conformation of the molecule from the gas (or liquid) phase, forcing the molecule to be planar or quasi-planar. 

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. 

A priori, this situation could be ascribed to two principal factors (a) the greater intrinsic stability of the form N(7)H (b) the higher value of the crystal packing forces in the case of the N(7)H form, a complex problem which may perhaps be simplified by comparing the intermolecular interaction energies in the dimer (86) with those in the hypothetical dimer (87), which differs from 86 only in the shift of the proton from N-7 to N-9 of the bases involved. 

The intermolecular assembly of crystals is ruled by packing forces. Here the molecules have lost most of the unique mobility and conformational variability of biological or synkinetic tissues. Nevertheless, the preferences and orientations of binding interactions between amino acids can nowhere be studied and understood in more detail than in crystal structures. 

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