Phase changes and chemical reactions in molecular crystals

Abstract

Changes in the attitude of crystallographers to phase changes and chemical reactions in molecular solids over the last 50 years are sketched. Various ways of classifying polymorphic modifications and structural changes in solids are discussed, and examples are given. The methods applied include multi-temperature crystal structure analysis, analysis of anisotropic displacement parameters, differential scanning calorimetry, solid-state NMR and visual observation. The importance of defects in influencing the kinetics of solid-solid phase changes is stressed.

Concomitant Polymorphs

“…[2,5,80] In principle, enantiomerically pure crystals are different from racemic ones, but if they racemize quickly in solution and/or spontaneously resolve upon crystallization, it is still debatable whether these are to be considered polymorphic substances. Sironi and co-workers [86] have recently characterized a concomitantly crystallized system that incorporates and illustrates many of these features, and in addition, provides an example of a substance in which the synthetic approach to the material apparently plays a role in determining which polymorph is initially obtained.…”

“…One of the definitions of polymorphism [2,5,80] requires that all polymorphic structures of a material dissolve, melt, or vaporize to give the same species or equilibrium mixture of species. If this is accepted as a definition, the time necessary for equilibrium to be achieved becomes an issue, as well as the temperature of the dissolution, melting, or vaporization process.…”

Polymorphism, what it is and how to identify it: a systematic review

“…The first one is due to the different position of ligands relative to each other. Ferrocene, which has in total three polymorphs 145 is a nice example for this kind of polymorphism since only the relative position of the cyclopentadienyl anions is different ( Fig. 38 and Table 9).…”

Dimorphism of K4Ta2S11: Syntheses, Crystal Structures, and Properties of two Alkali Metal Tantalum Sulfides

“…Therefore, both the packing energy as well as the density indicate that the orthorhombic form should be the thermodynamical more stable modification at lower temperatures [19]. However, there are many exceptions, e. g. if strongly directional interactions led to an open structure with lower density [19].…”