Intermolecular forces-crystal packing

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. 

Symposium "Intermolecular Forces and Packing in Crystals", Abstract N 5. 

Be38 Bernstein, J.L., Abrahams, S.C. Am. Ciyst Assoc., Winter Meeting, March 1-5, 1970, New Orleans Symposium "Intermolecular Forces and Packing in Crystals", Abstract PI. 

Figure 5.2 (a) Electron density contour map of the CI2 molecule (see Chapter 6) showing that the chlorine atoms in a CI2 molecule are not portions of spheres rather, the atoms are slightly flattened at the ends of the molecule. So the molecule has two van der Waals radii a smaller van der Waals radius, r2 = 190 pm, in the direction of the bond axis and a larger radius, r =215 pm, in the perpendicular direction, (b) Portion of the crystal structure of solid chlorine showing the packing of CI2 molecules in the (100) plane. In the solid the two contact distances ry + ry and ry + r2 have the values 342 pm and 328 pm, so the two radii are r 1 = 171 pm and r2 = 157, pm which are appreciably smaller than the radii for the free CI2 molecule showing that the molecule is compressed by the intermolecular 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. 

NDI 5 is an eloquent example of the delicate balance between the intermolecular forces that determine crystal packing. Analysis of the X-ray structures of 5 crystallised from three different solvents - acetone, acetonitrile and benzene - reveals that the solid state arrangement of this L-phenylalanine methyl ester derivative of NDI is... 

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