Why is there a Fixed-H layer if tautomerism is represented in the Main layer?
The Fixed-H layer is useful if you wish represent one particular tautomer of a given structure. If we create InChIs for the tautomers below with ‘Mobile H Perception’ checked, then the normalisation performed by the generation program will make their InChIs identical. However, if we uncheck ‘Mobile H Perception’ then the extra Fixed-H layer will be appended. This layer is essentially an InChI for the whole structure without normalization of the mobile hydrogens, thus giving an InChI that specifies a single tautomeric form of the structure.
