High-Pressure Polymorphs of Molecular Solids:  When Are They Formed, and When Are They Not? Some Examples of the Role of Kinetic Control

Abstract

A few examples of the kinetic control of the formation of new polymorphs at high pressure are discussed. Crystallization of liquids, crystallization from solutions, and solid-state polymorphic transformations are considered.

High-pressure diffraction studies of molecular organic solids. A personal view

“…In many cases, the sample is recycled many times, combining compression with slight heating and subsequent cooling, in order to get a single-crystalline sample, for which the structure can be solved more easily. In a recent paper (Boldyreva, 2007b), we have supposed that for some highly polymorphic compounds a procedure involving only 'pure' compression without temperature variation at all would give forms other than those given by compression combined with temperature variations (either cooling or heating). The experimental evidence to substantiate this hypothesis was reported recently .…”

“…These chains are remarkably robust and can mimic peptide chains (Vinogradov, 1979;Suresh & Vijayan, 1983). The compressibility of shorter NHÁ Á ÁO hydrogen bonds linking zwitterions along the head-to-tail chains is usually smaller than that of other hydrogen bonds in the structure (Boldyreva, Drebushchak, Shakhtshneider et al, 2004;Boldyreva, Ivashevsyaya et al, 2005; Moggach, Marshall & Parsons, 2006;Boldyreva, 2007b). It is only slightly affected even by jumpwise structural rearrangements in the course of phase transitions.…”

“…More often than not, the transformations give metastable forms and not the thermodynamically preferable one. The facts which can indicate kinetically and not thermodynamically controlled transformations were discussed in a recent review (Boldyreva, 2007b). Different forms can be obtained on compression and on decompression, as well as with the same conditions from different starting polymorphs.…”

“…Even if the solid is not soluble in this liquid, one cannot exclude the possibility of an interaction between the solid surface and the liquid, which can affect the occurrence of a phase transition, its kinetics and the structure of the highpressure phase (Boldyreva, 2007b). Examples are known from the literature where pressure-induced transitions could be observed when selected liquids were used and did not occur with other liquids or in dry samples (Boldyreva, A schematic presentation of the pressure-induced transitions between the polymorphs of glycine; notations for the polymorphs are as in the publications, where the polymorphs were described first (Iitaka, 1960(Iitaka, , 1961Jö nsson & Kvick, 1972;Boldyreva et al, 2003;Boldyreva, Drebushchak, Paukov et al, 2004;Boldyreva, Drebushchak, Shakhtshneider et al, 2004;Boldyreva, Kolesnik et al, 2005 2000; Boldyreva, Dmitriev & Hancock, 2006).…”

Crystallization of high-pressure polymorphs. Examples of kinetic control

“…The short halogen···halogen contacts are considered to evidence attractive forces, considerably stronger than van der Waals interactions, and therefore vital for the molecular arrangement in crystals [1,2]. According to other theories, the close inter-halogen distances result from molecular packing [3].…”