Normalisation and Business Rules

Definition of unambiguous business rules is not always straightforward and in these situations it is useful to generate an ensemble of equivalent structures.If rules to rank the structures in the ensemble can be defined, then it becomes possible to select one of these to represent the ensemble. For example, enumeration of tautomers using Leatherface creates a sorted list of canonical SMILES, the first of which is used to represent the ensemble. This effectively extends the concept of canonicalisation to an ensemble of structures, and the term canonical tautomer may be encountered in the literature.  

When a molecule is presented as more than one topological graph, the individual representations are termed mesomers or, in the context of structure registration, mesomeric duplicates. Mesomers differ only in bond order and placement of formal charges and are used to represent bond delocalisation in a molecule. 

4) to represent the delocalisation of the charge in the molecule. It is customary to link such forms by a double headed arrow to indicate that they are different representations of the same molecule, rather than distinct molecular species. 

Isomers such as tautomers that can readily inter-convert represent another type of structural equivalence that is encountered frequently in databases of chemical structures. Although the different forms are distinct molecular species, techniques used for processing mesomers are equally applicable to tautomers.  

For example, a business rule can be defined to select the typically more stable 

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