Polymorphism molecular lattices

Generally speaking, the concepts of monotropy and enantiotropy in phase theory appear to coincide with the structural concepts of unrelated and related lattices. Nevertheless, one must avoid equating the two, for it is certainly possible that one of two related lattices of the same substance is less stable than the other under all conditions of temperature and pressure. This would indicate the existenced of monotropy in spite of the existence of related lattices. This situation becomes especially important for polymorphic organic compounds, which form molecular lattices. 

The survey given above cannot possibly represent a systematization of polymorphic substances, but the examples chosen from the range of inorganic structural possibilities serve to provide an insight into the underlying causes of polymorphism. The systems become more complex when one considers the structural causes of polymorphism among molecular lattices, where it becomes essential to develop the structural considerations by a closer examination of the states of bonding of the atoms in the different lattices. 

Bar and Bernstein continued their work on conformational polymorphism, using lattice energy minimization techniques to characterize the influence of crystal forces (as well as orientational and positional disorder) on the molecular conformation of/>-methyl-A -(p-methylbenzylidine)aniline [36], This compound has been obtained in three polymorphic forms, in which the title molecule was found to adopt different conformations in each form. A summary of the reported crystallographic data is found in Table 13. 

Polymorphs are common in organic chemistiy and are prevalent when hydrogen bonding is involved in the crystal lattice. 2,6 di-hydroxybenzoic acid [13] is a well documented example. Polymorphism is more probable when molecules are large and possess conformational flexibility, which increases the number of feasible molecular arrangements that can produce a stable ciystal... 

Subsequent to the discovery of the crystal polymorphism of bmimCl, the crystal structures of bmimCl and bmimBr were determined. We determined the crystal structures of bmimCl Crystal (1) and bmimBr at room temperature [11, 12]. Independently, Holbrey et al. [9] reported the crystal structures of bmimCl Crystal (1) and Crystal (2), as well as that of bmimBr at — 100°C. The two sets of structures determined at different temperatures agree well with each other except for the lattice constants that vary with temperature. They also show that the molecular structure of the bmim cation in bmimCl Crystal (2) is different from that in (1) but that it is the same as that in bmimBr, as already indicated by the Raman spectra. In the following, we discuss the crystal structures of bmimCl Crystal (1) and bmimBr as the two representative stmctures at room temperature. 

The rarity of the phenomenon of conformational polymorphism (one of the rare examples of conformational polymorphism is D,L-methionine (30)) is an indication that it is unusual for the crystal field forces to exercise a dominant influence on molecular conformation. It appears that the differences in the lattice sums of the crystal field forces for different conformations are in most cases an order of magnitude too small to induce conformational changes—i.e., tenths of kilocalorie per interaction rather than kilocalories. 

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