International Chemical Identifier - Format and Layers

Format and Layers

Every InChI starts with the string "InChI=" followed by the version number, currently 1. This is followed by the letter S for standard InChIs. The remaining information is structured as a sequence of layers and sub-layers, with each layer providing one specific type of information. The layers and sub-layers are separated by the delimiter "/" and start with a characteristic prefix letter (except for the chemical formula sub-layer of the main layer). The six layers with important sublayers are:

1. Main layer
   • Chemical formula (no prefix). This is the only sublayer that must occur in every InChI.
   • Atom connections (prefix: "c"). The atoms in the chemical formula (except for hydrogens) are numbered in sequence; this sublayer describes which atoms are connected by bonds to which other ones.
   • Hydrogen atoms (prefix: "h"). Describes how many hydrogen atoms are connected to each of the other atoms.
2. Charge layer
   • proton sublayer (prefix: "p" for "protons")
   • charge sublayer (prefix: "q")
3. Stereochemical layer
   • double bonds and cumulenes (prefix: "b")
   • tetrahedral stereochemistry of atoms and allenes (prefixes: "t", "m")
   • type of stereochemistry information (prefix: "s")
4. Isotopic layer (prefixes: "i", "h", as well as "b", "t", "m", "s" for isotopic stereochemistry)
5. Fixed-H layer (prefix: "f"); contains some or all of the above types of layers except atom connections; may end with "o" sublayer; never included in standard InChI
6. Reconnected layer (prefix: "r"); contains the whole InChI of a structure with reconnected metal atoms; never included in standard InChI

The delimiter-prefix format has the advantage that a user can easily use a wildcard search to find identifiers that match only in certain layers.