Crystal-packing forces

The parameters in the original parameterization are adjusted in order to reproduce the correct results. These results are generally molecular geometries and energy differences. They may be obtained from various types of experimental results or ah initio calculations. The sources of these correct results can also be a source of error. Ah initio results are only correct to some degree of accuracy. Likewise, crystal structures are influenced by crystal-packing forces. 

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in entries 4—6. Single-crystal X-ray dilfiaction studies have unambiguously established that all the chlorine atoms of trans, cis, ira j-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal. Each chlorine in die molecule is bonded to an anomeric carbon and is subject to the anomeric effect. Equally striking is the observation that all the substituents of the tri-0-acetyl-/ -D-xylopyranosyl chloride shown in entry 5 are in the axial orientation in solution. Here, no special crystal packing forces can be invoked to rationalize the preferred conformation. The anomeric effect of a single chlorine is sufficient to drive the equilibrium in favor of the conformation that puts the three acetoxy groups in axial positions. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

In this connection, in order to judge the level of these molecular rearrangements, the solid state X-ray structures of ferrocene and ferrocenium ion could be compared. Unfortunately, the molecular disorder caused by the rotation of the cyclopentadienyl rings in ferrocene means that the comparison procedure is far from simple and, in fact, the first results were interpreted in terms of a staggered conformation of the two cyclopentadienyl rings. It is now believed that the eclipsed conformation is the more stable (with a rotation angle of about 10°).2 However, as the rotational barrier is notably low (about 4 kJ mol-1), the conformation that one observes is probably that imposed by crystal packing forces. 

A similar situation occurred for the ferrocenium ion. Until 1983, on the basis of the available structural data, it was thought that the cyclopentadienyl rings assumed an eclipsed conformation. However, more recent data demonstrate that it assumes a staggered conformation, even if once again the crystal packing forces are considered to be the determining factor.3... 

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