Decoding hydrogen-bond patterns. The case of iminodiacetic acid

The hydrogen-bond patterns of three polymorphs of iminodiacetic acid are evaluated and compared using a recently developed form of analysis based on graph theory. The results of the analysis are compared with several previous analyses of these structures which are noted for their hydrogenbond complexities. Graph-set analysis provides a way of abstracting hydrogen-bond patterns as recognizable motifs which are easily compared and contrasted among related structures. It is shown that there are five hydrogen-bond motifs present in different combinations in these polymorphs. Two structures are indistinguishable on the basis of these characteristic motifs alone, and higher-order networks involving multiple hydrogen-bond types are used to define their differences. The two metastable polymorphs are shown to lack a particular cyclic pattern involving hydrogen bonds between two carboxylate and two iminium ions found only in the stable form.