Conformations, crystal packing

Klebe, G., Graser, F., Hadicke, E. and Berndt, J. (1989). Crystallochromy as a solid state effect correlation of molecular conformation, crystal packing and colour in perylene-3,4 9,10-bis(carboximide) pigments. Acta Crystallogr. B, 45, 69-77. [263, 264]... 

Diphenyl-2,5-cyclohexadienone 3 exists as four polymorphs, labeled A-D, whose crystallographic details are summarized in Table 3-2 [14]. Polymorphs A-D are conformational polymorphs since they have different molecular conformers in their crystal structures. They are also concomitant polymorphs because they crystallized simultaneously from the same flask and under identical crystal nucleation and growth conditions. Forms B-D with multiple molecules in the asymmetric unit Z > ) may also be classified as conformational isomorphs. Such polymorph clusters with different molecular conformations, crystal packing and C—H - O... 

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. 

Certain monomers crystallize in a conformation such that they can be zipped together without changing the symmetry of the crystal lattice. In the crystalline state, the arrangement of monomers is strictly determined by crystal packing. Polymerization is usually initialed by irradiation with UV, X- or y-rays and is assumed to proceed by a radical mechanism. For example, muconic acid esters (25, 27, 29) and ammonium salts (26,28, 30) can be stcrcospccifically polymerized in the crystalline state to high conversion.224-227228 This form of... 

Let us consider a structural limiting model, in which the polymer molecules, presenting a periodic conformation, are packed in a crystal lattice with a perfect three-dimensional order. Besides this limiting ordered model, it is possible to consider models of disordered structures having a substantially identical lattice geometry. 

Keywords Liquid crystals. Crystal structures. Crystal packing. Conformation... 

Baumeister et al. [116] described the crystal structure of 4-(2-cyanoethyl)-cyclohexyl 4-n-pentylcyclohexanecarboxylate. The almost fully stretched molecule is only distorted by the gauche conformation of the cyanoethyl group. The crystal packing is characterised by a discrete layered arrangement with an antiparallel orientation of neighbouring molecules. With respect to intermolecular interactions, no remarkable contacts between the cyano groups can be observed. 

In 1990, Baumeister et al. [127] described the crystal and molecular structure of 4-ethoxy-3 -(4-ethoxyphenyliminomethyl)-4 -(4-methoxy-benzoy-loxy)azobenzene. The molecules have a bifurcated shape. The phenyliminom-ethyl branch is bent markedly from the nearly linear three ring fragment, but is almost coplanar with the azobenzene moiety. They found that the molecular conformation is affected by an intramolecular interaction of the carboxylic and azomethine groups. The crystal packing was described in terms of a sheet structure with interdigitating rows of molecules. 

The structure of a second polymorph of 4,5-diphenyl- lH-imidazole has been discussed, with the new form exhibiting significantly different phenyl/imidazole dihedral angles and mode of crystal packing relative to the known form [53], A new triclinic polymorph of 1,4-dibenzoyl-butane was found, differing from the monoclinic form in the torsional angles of the central chain [54], Two polymorphs of diphenyl-(4-pyridyl)methyl methacrylate have been found, where the molecules in the two forms contain weak C—H— n and C—H O/N contacts that lead to the existence of different conformations [55]. 

In the case of DuP747 [24], XRD, DSC, and thermomicroscopic studies determined the polymorphic system to be monotropic. Distinct diffuse reflectance IR, Raman, and solid state 13C NMR spectra existed for each physical form. The complementary nature of IR and Raman gave evidence that the polymorphic pair were roughly equivalent in conformation. It was concluded that the polymorphic character of DuP 747 resulted from different modes of packing. Further crystallographic information is required in order to determine the crystal packing and molecular confirmation of this polymorphic system. 

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