Abstract
The analysis of atom-to-atom and/or residue-to-residue contacts remains a favoured mode of analysing the mol-ecular packing in crystals. In this contribution, additional tools are highlighted as methods for analysis in order to complement the 'crystallographer's tool', PLATON [Spek (2009). Acta Cryst. D65, 148-155]. Thus, a brief outline of the procedures and what can be learned by using Crystal Explorer [Spackman & Jayatilaka (2009). CrystEngComm 11, 19-23] is presented. Attention is then directed towards evaluating the nature, i.e. attractive/weakly attractive/repulsive, of specific contacts employing NCIPLOT [Johnson et al. (2010). J. Am. Chem. Soc. 132, 6498-6506]. This is complemented by a discussion of the calculation of energy frameworks utilizing the latest version of Crystal Explorer. All the mentioned programs are free of charge and straightforward to use. More importantly, they complement each other to give a more complete picture of how mol-ecules assemble in mol-ecular crystals.
Highlights
A widely employed approach to describe the packing of molecular compounds in their crystals is based on describing specific atom-to-atom contacts, such as in conventional A— HÁ Á ÁD hydrogen bonding
Far from being mere curiosities, interactions involving metals, chelate rings and secondary bonding interactions impart stabilization energies to molecular packing akin to conventional hydrogen bonding (Tiekink, 2017)
A caveat: the popularity and importance of identifying atom-to-atom/residue-to-residue contacts notwithstanding, the perennial question facing those trying to understand how and why molecular crystals form is nicely summarized by the ‘egg causality dilemma’ – what came first, the chicken or the egg? In the present context, are the identified intermolecular interactions responsible for directing the way molecules assemble in crystals or are the identified intermolecular interactions the result of the formation of crystals? Developing this last point further, in a global molecular packing approach, molecules assemble to minimize free space so that protrusions (‘bumps’) in a molecule are accommodated by impressions (‘craters’) of symmetry-related molecules
Summary
A widely employed approach to describe the packing of molecular compounds in their crystals is based on describing specific atom-to-atom contacts, such as in conventional A— HÁ Á ÁD hydrogen bonding. Two in particular form the focus of this contribution, namely Hirshfeld surface analysis and noncovalent interaction plots As indicated below, these are easyto-use programs and can provide complementary information useful for the study of molecular packing. Whatever the origin of the intermolecular interactions revealed in crystals, their identification and analysis, especially in a systematic and thorough manner, is surely a worthwhile enterprise
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