How does InChI manage isotopes?
InChI represents isotopes as a single layer in the identifier. For each isotopically enriched atom in the structure, the InChI layer will hold that atom’s canonical number followed by the isotopic shift (i.e. +0 for chlorine-35(35-35) or +1 for carbon-13 (13-12)), followed by isotopic hydrogen (D or T) if present e.g. The only complexity arises when there are isotopically labelled hydrogens that can undergo tautomerism. In the Hydrogen sub-layer of the Main layer these hydrogen atoms are treated as non-isotopic; the number of these mobile isotopic hydrogens atoms is appended to the “exchangeable isotopic hydrogen atoms” part of the isotopic layer. The same is done to isotopic hydrogen atoms that may be subject to heterolytic dissociation in aqueous solution (for example D in RS-D). The Hydrogen sub-layer of the Main layer does not take isotopic labelling into account and thus treats the deuterium atoms as hydrogen. The layer states that the four H are shared between atoms with canonica
