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.

Graphical Abstract

Graphical Representation of mixing of structures StzIV and StzV to give structure StzIII.

Figure 1

Definition of D and A sites in the molecules of sulfathiazole (Stz; broken line: torsion angle C − N − S − C) and sulfapyridine (Spn).

Figure 2

Connectivity tables for the D − H∙∙∙A interactions in polymorphic forms of sulfathiazole (Stz) and sulfapyridine (Spn).

Figure 3

Topology graphs for the hydrogen bonded layer structures of the forms a) IV, b) V, c) III and d) II of sulfathiazole (Stz) and e) different arrow types used for the representation of one-point and two-point connections.

Figure 4

Topology graphs for the hydrogen bonded structures of form I of sulfathiazole (Stz), showing separately a) the nov framework formed by molecules of type A, b) the hcb net formed by molecules of type B and then c) the framework of connected A and B molecules.

Figure 5

Matching geometrical arrangements in the isostructural forms Stz-I and Spn-VI. (a) Stz-I: A- and B-type molecules, connected by an H2'∙∙∙A4 bond, which serves as the only link between the nov (A) and hcb (B) nets. (b) Spn-VI: A larger separation between A and B molecules results in the absence of an H2'∙∙∙A4 connection between the nov and hcb nets. Symmetry operation (i) 3/2 - x, 1/2 + y, 1/2 - z.

Figure 6

Topology graphs for the hydrogen bonded structures of four polymorphs of sulfapyridine (Spn): a) the sqp framework of form II, b) the tts net of form III, c) the SnS-type net of form IV, d) interpenetration of the nov-type framework (A molecules) by an hcb-type layer (B molecules) and e) the framework of type V.

Figure 7

Connectivity tables for the D − H∙∙∙A interactions in polymorphic forms of sulfathiazole (Stz) and sulfapyridine (Spn), with the symbols + and − indicating connections between molecules of the same or of the opposite handedness, respectively.