= 9H-xanthine
{c}
\b[9H-xanthine] is a neutral <tautomer> of <xanthine> in which the indicated <hydrogen> is attached to nitrogen9 of the <purine> framework. The oxo groups remain at positions2 and6. ChEBI identifies this particular form as CHEBI:17712, distinct from the broader tautomer-unspecified <xanthine> entry.<Xanthine in ChEBI>[CHEBI:17712, Definition and Formula; general parent CHEBI:15318]{ref}
Its <molecular formula> is $\mathrm{C_5H_4N_4O_2}$. The prefix 9H is an atom-location convention, not a count of nine hydrogens: the entire neutral molecule contains four hydrogens. As a simple inventory check, it contains $5+4+4+2=15$ atoms in all.
<7H-xanthine> has the same elemental formula and net charge but a different indicated <hydrogen> position. A mass measurement consistent with that formula therefore does not alone assign the 9H form. The distinction concerns <tautomerism>, rather than the gain of an extra proton or a change in carbon/oxygen composition.<Xanthine in ChEBI>[CHEBI:17712 and CHEBI:48517, neutral charge and reciprocal tautomer relation]{ref}
Naming 9H-xanthine specifies a structural form; it does not by itself establish its fraction in a solution. That requires evidence or a model for the relevant environment and equilibrium.
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