Molecular conformation, crystal structure-solid state

Figure 12.3 A single crystal X-ray stmcture of the Jt-expanded borazine macrocycle 12.8 (Scheme 13.4). (a) Sketch of the super-structure, viewed along the c-crystallographic axis and (b) graphical illustration of the molecular conformation in the solid state. Carbon atoms are shown in gray, nitrogens in blue, and borons in green. Hydrogen atoms and solvent molecules have been omitted for the sake of clarity.

The molecular and crystal structure of (81) is simpler than that of ferrocene as only one polymorph featuring eclipsed conformation of the cyclopentadienyl rings, has been found at ambient and low (100 K) temperature. The larger metal-carbon distance (2.186 A in (81) vs. 2.03 or 2.06 A in ferrocene) corresponds to the larger metal covalent radius see Covalent Radii) and may also be responsible for the fact that an eclipsed conformation is found for the solid-state structure of decamethylruthenocene (82), as opposed to decamethylferrocene where more closely spaced methyl groups impose the staggered Dsd conformation. 

Not all polymorphism originates from conformational requirements, and many polymorphic situations exist because of different modes of molecular packing in the solid-state structures. For example, the two polymorphs of enalapril maleate exhibit very similar molecular conformations (as evidenced by the similarity in spectral characteristics), and the observed differences in crystal structure, therefore, are attributed to different modes of crystal packing. Sufficient differences in the solid-state C-NMR spectra of the four polymorphs of sulfathiazole were observed to enable the use of this technique as an analytical tool, but these differences could not be ascribed to differences in molecular conformations among the polymorphs. ... 

Porter L J, Wong R Y, Chan B G 1985 The molecular and crystal structure of (+)-2,3-/ra 5-leuco-cyanidin [(2i ,35,4i -(+)-3,3, 4,4, 5,7-hexahydroxyflavan]dihydrate, and comparison of its heterocyclic ring conformation in solution and the solid state. J Chem Soc Perkin Trans I 1413-1418... 

X-ray crystallographic analysis is the most powerful technique for providing information not only about molecular structures, but also about the key factors controlling their conformations. Until now, solid state structures of azacalix[n]arenes where n = 3,4,5,6,8, and 10 have been reported, hi the following subsections. X-ray crystal structures of azacalixarenes will be described according to the ring sizes. 

At the end of the process, one obtains a large amount of information regarding the solid-state structure of a compound. Its three-dimensional structure and molecular conformation become known, as do the patterns of molecular packings that enable the assembly of the crystal. In addition, one obtains complete summaries of bond angles and bond lengths for the molecules in the crystal as well as detailed atomic coordinates for all of the atoms present in the solid. One generally finds... 

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. 

The differences between the cluster skeletons of the three molecules of 2 are very small with the mean values of the Ru-Ru distances being similar and the mean Ru-C(carbide) distances being identical. The most notable differences between the structures arise from the orientation of the tricarbonyl units attached to the apical Ru atoms above and below the molecular equator of the octahedral cluster (the molecular equator is defined as the Ru4 plane in which the bridging carbonyl ligand is present). The two tricarbonyl units are almost exactly staggered in the crystal obtained from benzene, whereas they approach an eclipsed conformation in the other polymorph. Although the 13C-NMR spectrum of 2 has not been recorded in solution (or in the solid state), it is not unreasonable to anticipate that... 

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