Tautomer

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.