= 7H-xanthine
{c}
\b[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> $\mathrm{C_5H_4N_4O_2}$.<Xanthine in ChEBI>[CHEBI:48517, Definition, Formula and tautomer relation]{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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