InChI

InChI, or International Chemical Identifier, is a non-proprietary textual identifier for chemical substances. It was developed by IUPAC and collaborators to provide a unique, machine-readable representation that can be used across databases and software. The standard was released in 2005 and is maintained by the InChI Trust and IUPAC, with ongoing updates to refine layers and parsing rules.

InChI can represent chemical substances including salts, isotopologues, tautomers, and stereochemical variants. It is not limited to a single salt form; for salts or mixtures, separate InChI strings may be used, or a fixed salt layer can be included within the InChI. An InChI string begins with InChI=Version, followed by the chemical type and layers separated by slashes. The layers encode connectivity (the constitution), tautomeric state, isotopes, stereochemistry, electronic charge, and protonation state. This layered approach yields a unique string intended to be invertible to a canonical structural representation by software. However, due to tautomerism and stereoisomerism, some substances may have multiple valid representations, which canonicalization within InChI strives to address.

To facilitate indexing and searching, the InChIKey is a fixed-length, 27-character representation derived from the InChI that is easier to include in databases and web searches. It comprises three blocks separated by hyphens (for example, 14-character connectivity block–10-character stereochemical block–a final single-character checksum block). InChI strings and InChIKeys are widely used in chemical databases such as PubChem, ChemSpider, and ChEMBL, as well as in scholarly publications.

Tools such as OpenBabel, RDKit, and the official InChI software can generate InChIs from chemical structures and vice versa.