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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.^[ref]

Its molecular formula is ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$. 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.^[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.

A tautomer is one of the readily interconvertible constitutional isomers related by tautomerism. In a proton-transfer example, the forms retain the same atom inventory and net charge but differ in where a hydrogen is attached and in associated bond orders. A molecular formula alone therefore cannot identify which tautomer is meant.^[ref]

Xanthine supplies a concrete example. ChEBI distinguishes 7H-xanthine, with the indicated hydrogen on nitrogen7, from 9H-xanthine, with it on nitrogen9. Both neutral forms have formula ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$, and the curated records explicitly identify them as tautomers. The atom inventory is identical while the indicated hydrogen placement differs.^[ref]

Interconvertibility does not mean equal abundance. For an invented dilute two-tautomer equilibrium $A\rightleftharpoons B$ with $[B]/[A]=0.01$, the fraction in form $B$ is $0.01/(1+0.01)=1/101$, about0.99%. This is a bookkeeping example, not a measured xanthine ratio. Solvent, temperature and interactions with other molecules can affect which form is favored.

A tautomer is not merely a resonance drawing: atom attachments change between tautomers. It is also not automatically a different protonation state, because gaining or losing a proton changes the atom inventory or charge rather than just relocating an existing hydrogen.