Mobile hydrogens and tautomersStep 9 of 17 of the InChI walkthrough. A hydrogen that moves between several atoms is not placed on any of them. Acetic acid’s acidic hydrogen belongs to both carboxyl oxygens at once, written (H,3,4). This is what makes tautomers share an identifier — and it is the single hardest thing to do by hand, because deciding whether a hydrogen is mobile is a chemical judgement, not a bookkeeping rule.It opens on CC(=O)O, drawn live, so the layer under discussion changes as you change the structure.Convert a structure to its InChI and InChIKey and back, convert a whole CSV, XLSX or SDF file, learn how the identifier is derived, and practise writing its layers. The InChI engine is WebAssembly running in your browser, so the tool needs JavaScript — and nothing you open ever leaves your machine.

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