Molecular crystals, polymorphism

Even though molecular crystal polymorphs contain exactly the same molecules or ions, they usually possess different chemical and physical properties such as density, diffraction pattern, solid-state spectra, melting point, stability and reactivity. 

The problem of whether an amorphous material may or may not be considered a form of a solid substance has been discussed [18a]. Many compounds yield stable noncrystalline phases either as the exclusive product of a crystallization process, or in a mixture with crystals or as a consequence of the treatment of otherwise crystalline phases [18]. The fundamental drawback when dealing with amorphous phases is the dearth of techniques for the thorough characterization of an amorphous phase or for the discrimination between different amorphous phases. In general, diffraction techniques are of little help when dealing with amorphous materials and one has to rely mainly on spectroscopic means (see below). Clearly, if one takes the fulfilment of Bragg s law as the pre-condition for the existence of a crystalline phase, and hence for the existence of a molecular crystal polymorph, amorphous materials are not to be considered. 

A (molecular) crystal polymorph is a solid crystalline phase of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state Conformational polymorphs are formed by molecules that can adopt different conformations in different crystal structures formed by coordination complexes where ligands bound in delocalized bonding modes adopt different relative orientations Concomitant polymorphs are polymorphic modifications of the same substance obtained from the same crystallisation process Pseudo-polymorphs are ... 

It has been argued above that molecular crystal polymorphism can be seen as a form of crystal isomerism, and that the change in crystal structure associated with polymorph interconversion, i.e. a solid-to-solid phase transition between ordered... 

In spite of the great interest in the phenomenon of polymorphism and of the increased research activity beyond the boundaries of organic solid-state chemistry, it is a fact that only a few molecular compounds possess several crystalline forms, whereas for many other tens of thousands of molecular compounds only one crystalline form is known. In other words, why are there so few molecular crystals polymorphs The often quoted association between number of known forms and the time and energy spent in searching for them put forward by McCrone probably contains the answer to this question. It is probable that if thorough (combinatorial ) crystallization experiments were carried out on any given molecular species or molecular salt, alternative crystalline forms would be found. It is probable but not certain. 

Hiremath, R., Basile, J.A., Varney, S.W., Swift, J.A. (2005). Controlling molecular crystal polymorphism with self assembled monolayer templates. Journal of the American Chemical Society, Vol. 127, No. 51,18321-18327... 

A Burger, R Ramberger. On the polymorphism of pharmaceuticals and other molecular crystals. I. Theory of thermodynamic rules. Mikrochim Acta (Wein) 2 259-271, 1979. 

J. Bernstein, Polymorphism in Molecular Crystals, Clarendon Press, Oxford, 2002. 

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. 

Hirata, S. Fast electron-correlation methods for molecular crystals an application to the a, /h, and /j2 polymorphs of solid formic acid. J. Chem. Phys. 2008, 129, 204104. 

Bernstein J (2002) Polymorphism in molecular crystals. Oxford University Press, Oxford... 

Figure 26. Mechanism of Preferential Enrichment characterized by homochiral molecular association, polymorphic transition, followed by crystal disintegration, and chiral discrimination in the case of NNMe,. (Reprinted with permission from ref 18. Copyright 2003 American Chemical Society.)...

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