Terminology Nomenclature

Figure 1(A)-(C) are schematic illustrations of different polymorphs of a compound. In (A) and (B), the molecular conformation is retained but the crystal structures are different. The occurrence of a form such as (C), in which the molecule crystallizes with a significantly different conformation gives rise to the term 'conformational polymorphism'. In addition to the possibility of forming different (but chemically identical) polymorphs, the molecules of a given substance may upon crystallization from solution incorporate solvent molecules (stoichiometrically or non-stoichiometrically) in the resulting crystal structure. These solvated crystalline forms (examples shown schematically in Figure 1(D) and (E)) are often referred to as 'pseudopolymorphs' and are no less important in practice than polymorphs of the parent substance. Thus, for a single organic compound, it is frequently possible to crystallize from solution a series of polymorphs as well as a series of pseudo-polymorphs (in the latter case species containing different solvent molecules, either individually or as mixtures). Furthermore, it is possible for a compound to yield pseudopolymorphs which are chemically identical, but structurally distinct (e.g. a monoclinic dihydrate and an orthorhombic dihydrate). Such species may be considered as polymorphic pseudo-polymorphs. Since physical properties are ultimately dependent on crystal structure, each of these species will therefore have unique properties. It is also noteworthy that the desolvation (e.g. by controlled heating) of pseudopolymorphs such as those in Figures 1(D) and (E) would yield polymorphs of the parent compound. Hence, this represents another route to isolation of polymorphs in addition to crystallization from solution, melt or vapour.

In the above descriptions of polymorphism and pseudopolymorphism, the simplest connotations of these terms have been used and a digression into semantics has deliberately been avoided. However, it is important to note that difficulties with terminology do arise due to inconsistent use and the multiplicity and overlap of synonymous terms (e.g. 'crystal form'

Figure 1 Schematic diagram illustrating polymorphism (A)-(C) and pseudopolymorphism (D), (E); Filled circles represent included solvent molecules.

and 'crystal modification' as synonyms for polymorph; 'solvate', 'inclusion compound', 'clathrate' for pseudopolymorph). Pharmaceutical chemists tend to use 'polymorph' to describe a one-component system and 'pseudopolymorph' for a multi-component system. Finally, it should be noted that amorphous (non-crystalline or 'glassy') states of matter are also relevant in the context of polymorphism, since such phases may arise during transformation of one polymorph into another or during trituration of a crystalline polymorph. Amorphous phases are thermo-dynamically unstable and will tend to undergo spontaneous crystallization to yield a stable polymorph.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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