Crystallization methods and conditions

There is ample evidence that this also is a genuine case of polymorphism. The blue, square-planar form can be dissolved to obtain the green tetrahedral one indeed the 

The effect of temperature is demonstrated in the polymorphic behaviour of the diphenylcarbamide 3-V, one of the systems originally cited by Groth (1906a), and more recently studied by Etter et al. (1990a,b) and Huang el al. (1995). 

Concomitant crystallization is by no means limited to crystallization from solution, nor to preservation of constant molecular conformation. As noted in Section 2.2.5 the classic pressure vs temperature phase diagram for two solid phases (Fig. 2.6) of one material exhibits two lines corresponding to the solid/vapour equilibrium for each of two polymorphs. At any one temperature one would expect the two polymorphs to have different vapour pressures. This, in fact, is the basis for purification of solids by sublimation. Nevertheless there are examples where the two have nearly equal vapour pressures at a particular temperature and thus cosublime. This could be near the transition temperature or simply because the two curves are similar over a large range of temperatures or in close proximity at the temperature at which the sublimation is carried out. For instance, the compounds 3-VI and 3-Vn both yield two phases upon 

For 3-VI slow sublimation at 140 °C and O.lTorr led to a mixture of a few feathery needles (mp 192-193 °C) among the main product of lustrous coppery blocks (mp 220-223 °C), which could be separated manually for further characterization. 3-VII was also purified/crystallized by vacuum sublimation (120 °C/10 Torr) yielding manually separable deep red needles a phase, mp 157-160 °C) and blocks (/3 phase, mp 165-168 °C). The crystal structure determinations of the two phases of 3-VII indicated significantly different molecular geometry, a cofacial dimer in the a phase, and a trans antarafacial dimer in the phase. 

Perhaps most prominent of these materials are the organic conducting and superconducting salts based on the so-called ET compounds, in which BEDT-TTF 3-X is the donor (cation in the salt), generally in a 2 1 ratio with the acceptor (anion in the salt) (Williams et al. 1991). One of the most widely studied of these salts is (ET)2l3, for which at least 14 different phases have been reported (Carlson et al. 1990 Williams et al. 1991), although the a and phases tend to dominate (Carlson et al. 1990 Shibaeva et al. 1990). It has been shown that a is the kinetically favoured product ( 90 per cent) under conditions of high current density and a small amount of water or oxidant added to the crystallization medium. Under more nearly equilibrium conditions (i.e. much lower current density) and dry solvent (tetrahydrofuran), pure phase can be obtained, suggesting that it is the thermodynamically preferred form. Intermediate conditions apparently lead to concomitant crystallization of the two forms (Carlson et al. 1990 Williams et al. 1991). 

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