Layer 2: Basic Tautomeric Layer (BT)
In order to represent classes of compounds that rapidly isomerize by well-known varieties of H-atom migration, a separate tautomeric layer is defined. This is, in effect, generated from the Basic Connectivity Layer by locating and labeling mobile H-atom donor and acceptor sites. Included here are the classes of tautomerism treated by the Chemical Abstracts Service (CAS).