Proton transfer tautomerization mechanisms of 2-thioxanthine

► Mechanisms of the intramolecular proton transfers of 2-thioxanthine were proposed. ► Sixteen reaction paths are found within the imidazole and the pyrimidine rings. ► Necessary energy barriers were reported for different products. ► Four enol-thiols species are found to be able to co-exist in the gas phase.