Polymorphism

Polymorphism, the existence of the same chemical substance in more than one crystalline modification, was an enigma at first, then became a curiosity, and more recently a subject of systematic studies. Extensive studies have classified them into conformational polymorphs, synthon polymorphs, packing polymorphs, isotopomorphism, etc. and their energy profiles have been quantified by theoretical calculations. This phenomenon is immensely important in pharmaceuticals, where different polymorphs of the same compound can exhibit different physicochemical properties and oral tablet behavior. The present themed issue of CrystEngComm on Polymorphism is hence timely and appropriate to fill a need for those working in the areas of polymorph discovery, crystal nucleation and growth, controlling crystallization, characterization of polymorphs and their stability, and pharmaceutical development. The 20 articles of this special issue from specialists in the area may be classified for the convenience of readers under the following sub-categories:

Techniques and characterization (2 articles):

Schurko et al. (DOI:10.1039/C5CE00060B ) and Burley and Nanubolu (DOI:10.1039/C5CE00008D ).

Hydrogen bonding interactions (4 articles):

Chopra et. al. (DOI:10.1039/C5CE00124B ), Jones et al. (DOI:10.1039/C4CE02477J), Wilson et. al. (DOI:10.1039/C5CE00123D) and Parsons et al. (DOI:10.1039/C5CE00327J).

Pharmaceutical compounds (8 articles):

Höpfl et al. (DOI:10.1039/C4CE01934B), Nangia et al. (DOI:10.1039/C5CE00116A), Li et al. (DOI:10.1039/C4CE02290D), Martins, Ellena et al. (DOI:10.1039/C4CE02373K), Abramov (DOI:10.1039/C4CE02523G), Bond and Upadhyay (DOI:10.1039/C5CE00050E), Lemmerer et al. (DOI:10.1039/C5CE00275C) and Kenis, Gong et al. (DOI:10.1039/C5CE00355E.

Lattice energies and phase transformations (4 articles):

Bobrovs, Seton and Dempster (DOI:10.1039/C4CE02484B), Báthori et al. (DOI:10.1039/C4CE02334J), Davey et al. (DOI:10.1039/C5CE00353A) and Day and Nyman (DOI:10.1039/C5CE00045A).

Host–guest materials (2 articles):

Barbour et al. (DOI:10.1039/C5CE00362H) and Moorthy, Venugopalan et al. (DOI:10.1039/C5CE00243E). The papers by Höpfl et al., Kenis, Gong et al. and Wilson et al. deal with polymorphism in multi-component crystals and their hydrogen bonding and packing motifs, phase transformation, and H/D exchange. The role of weak and strong hydrogen bonding as well as cohesive forces and molecular conformations in the crystal structures of polymorphs are discussed in articles by Chopra et. al., Parsons et. al. and Jones et. al., while polymorph hunting for popular drugs form the theme of the articles by Nangia et al., Martins, Ellena et al., Bobrovs, Seton and Dempster, Bond and Upadhyay, and Lemmerer et. al. Lattice energies, polymorph stability, Ostwald's rule, and hydration are discussed in papers by Davey et al., Day and Nyman, Li et al., Bourne, Báthori et al., and Abramov. Lastly, pseudopolymorphism or host–guest structures complete the full scope of Polymorphism (including pseudopolymorphs) (Barbour et al. and Moorthy, Venugopalan et al.). Apart from X-ray diffraction, which is the technique de rigueur for the characterization of polymorphs, Raman mapping and ss-NMR spectroscopy complement the structure elucidation of crystal forms (Burley and Nanubolu, and Schurko et al.). The selected themed issue articles cover almost all the scientific aspects on polymorphism and its prominent role in pharmaceuticals in the present day age.

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