Imaginary Transition State (ITS) and their hierarchical organization. Superimposition of educt and product molecular graphs and subsequent removal of all atoms and bonds which do not directly participate in the chemical reaction (marked in green) yields a cyclic ITS for a chemical reaction (e.g. acidic hydrolysis of ethylacetate). Bonds which are broken/formed during the reaction are marked with a red x/o. Left: The ITS can be organized in a hierarchical structure where each tree level adds additional information to the base cycle of the ITS such as bonds or atom labels. Specific instances of reactions are found as leafs of the tree. Mono-cyclic ITS account for over 90% of all known chemical reactions .