Crystal forces

Compare the flexibility of the geometry of hydrogen-bonding to the non-bonding electron-pairs of neutral oxygen elicited by the demands of crystal forces (Donohue, 1968)... 

All the crystal forces that we treat in this section can be considered in terms of the recognition between a given, reference molecule and the cavity it is to occupy in the crystal. In chiral systems the cavity is clearly of different shape in the d and / crystals, and this generally results in differential incorporation of R and S molecules. However, in molecules containing the sec-butyl group, discrimination is often ineffective. This is because the two enantiomers can assume different conformations with very similar external shapes, and they can then interchangeably enter the same cavity in the crystal. This effect was recognized some time ago (55), and recently its consequences have been studied in detail (56). In the case where two enantiomers may readily replace one another in the crystal, it follows that there is a tendency to conformational disorder (see biphenyl, above), and in many cases, the resolved enantiomers and the racemates are isostructural. 

Dauber, P. and Hagler, A.T. Crystal packing, hydrogen bonding, and the effect of crystal forces on molecular conformation. Acc. Chem. Res. 1980, 13, 105-112. 

Section 6.2, the crystal forces are of magnitude comparable to those inducing complex formation. Thus a complex in the solid state can have a different structure from that in solution. The complex of p-nitrophenol 74 with hexakis(2,6-di-OMe)-a-cyclodextrin 75 illustrates this point, exhibiting strikingly different modes of entrance of the guest into the host cavity in these two states (Figure 3.4) [39]. Nevertheless, numerous solid state studies ofthe complexes... 

The consequences of intermolecular interactions are of importance when considering the results of X-ray analysis of crystals the apparent deviations from planarity of molecules having no obviously overcrowded atoms may be wholly caused by crystal forces. Anthracene is an interesting example in this category. Cruickshank (1956a) showed that... 

There are competing advantages of tetrahedral and octahedral holes for housing impurities or second components of an alloy. If the crystal forces, whatever their nature, depend mostly on interactions between nearest-neighbors, the octahedral hole has the advantage of having more nearest-neighbors with which it interacts (6 instead of 4). 

Introduction Crystals Crystal forces Ionic radii Forces in liquids... 

N(9)H tautomer has been established in the crystal of 8-azaguanine monohydrate,38 42 the existence of the N(8)H tautomer has been advocated159 for the crystal of 8-azaxanthine monohydrate. This situation, if confirmed,160 could then be attributed, following the general discussion of Section XI, to the influence of crystal forces, which would then be particularly interesting to compute. 

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