title: Describing hydrogen-bonded structures; topology graphs, nodal symbols and connectivity tables, exemplified by five polymorphs of each of sulfathiazole and sulfapyridine.
abstract: BACKGROUND
  Structural systematics is the comparison of sets of chemically related crystal structures with the aim to establish and describe relevant similarities and relationships. An important topic in this context is the comparison of hydrogen-bonded structures (HBSs) and their representation by suitable descriptors.
  
  
  RESULTS
  Three different description methods for HBSs are proposed, a graphical representation, a symbolic representation and connectivity tables. The most comprehensive description is provided by a modified graph of the underlying net topology of an HBS which contains information on the multiplicity of links, the directionality and chemical connectivity of hydrogen bonds and on symmetry relations. By contrast, the alternative symbolic representation is restricted to essential properties of an HBS, i.e. its dimensionality, topology type and selected connectivity characteristics of nodes. A comparison of their connectivity tables readily identifies differences and similarities between crystal structures with respect to the intermolecular interaction modes adopted by their functional groups. The application of these methods to the known polymorphs of sulfathiazole and sulfapyridine is demonstrated and it is shown that they enable the rationalisation of previously reported and intricate relationships.
  
  
  CONCLUSIONS
  The proposed methods facilitate the comprehensive description of the most important relevant aspects of an HBS, including its chemical connectivity, net topology and symmetry characteristics, and they represent a new way to recognise similarities and relationships in organic crystal structures. Graphical AbstractGraphical Representation of mixing of structures StzIV and StzV to give structure StzIII.
authors: Chemistry, Faculty of Natural and Environmental Sciences, University of Southampton, Southampton, SO17 1BJ UK ; Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, 21588 Saudi Arabia.
authors: Michael B
authors: MB
authors: Hursthouse
authors: Chemistry, Faculty of Natural and Environmental Sciences, University of Southampton, Southampton, SO17 1BJ UK.
authors: David S
authors: DS
authors: Hughes
authors: Institute of Pharmacy, University of Innsbruck, Innrain 52, Innsbruck, 6020 Austria.
authors: Thomas
authors: T
authors: Gelbrich
authors: Chemistry, Faculty of Natural and Environmental Sciences, University of Southampton, Southampton, SO17 1BJ UK.
authors: Terence L
authors: TL
authors: Threlfall
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