What is InChI?

InChI (International Chemical Identifier) is a standardised, open-format identifier developed by the International Union of Pure and Applied Chemistry (IUPAC) to uniquely represent the structure of chemical substances. Unlike traditional names or formulas, InChI encodes molecular connectivity, stereochemistry, isotopic composition, and other structural features into a hierarchical, human- and machine-readable string. This enables unambiguous identification of compounds across databases, regulatory systems, and research platforms.

How does InChI ensure uniqueness and interoperability?

Each InChI string is generated algorithmically based on a molecule’s structure, ensuring that identical compounds produce identical identifiers regardless of input source. The system supports multiple layers (e.g., molecular formula, connectivity, hydrogen atoms, stereochemistry), allowing for varying levels of detail. This makes InChI ideal for cross-referencing compounds in regulatory submissions (e.g., REACH, TSCA), chemical inventory systems, and public databases like PubChem or ChEMBL. Its open nature ensures broad adoption without licensing restrictions.

What are the limitations of InChI?

While InChI excels at representing static molecular structures, it does not inherently capture dynamic properties such as tautomeric forms, conformational states, or reaction conditions. Additionally, different software tools may generate slightly varying InChI strings for the same molecule if default settings differ. To address this, InChIKey—a fixed-length, hashed version of InChI—is often used for rapid database searches, though it is not reversible.

Related concepts

InChI is frequently used alongside other identifiers such as CAS Registry Numbers, SMILES, and IUPAC names. It complements tools like HPLC, NMR, and GC-MS in compound verification and is integral to digital chemistry workflows in regulatory and procurement contexts.