7H-xanthine is the neutral xanthine tautomer with the indicated hydrogen attached to nitrogen7, while the two oxo groups occupy positions2 and6. ChEBI describes this form in CHEBI:48517 and relates it to 9H-xanthine. Both forms have molecular formula .[ref]
The change from the 9H description to the 7H description relocates an existing hydrogen; it does not add another hydrogen or produce a 7+ ion. Thus an atom inventory, elemental analysis or formula-based molecular-mass calculation cannot distinguish the two forms. Their bonding and hydrogen placement must be specified as well.
A population example shows why a tautomer-specific name is more information than a generic formula. In an invented sample containing 1 millimole total xanthine, suppose an independently determined equilibrium assigned 30% to the 7H form and 70% to the 9H form. The amounts would be 0.3 and 0.7 millimole, respectively, but every molecule in this two-form model would still have the same elemental formula. These fractions are illustrative assumptions, not reported xanthine measurements.
The indicated hydrogen position also differs from protonation state: removing a proton would change the hydrogen inventory and net charge. Keeping these descriptions separate prevents an interconversion between tautomers from being mistaken for simple acid dissociation.
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