2,4-Dithiouracil tautomers: Structures and energies

Six tautomers of 2,4-dithiouracil have been studied by using ab initio theory. All correspond to energy minima at the HF/3-21G(*) level. Their stability order has been determined by using the MP2/6-31G*//3-21G(*) approximation with inclusion of zero-point energy corrections. The dithione tautomer 1 has the global minimum energy. Its energy difference with the second most stable structure, the dithiol tautomer 2, amounts to only 28 kJ mol^-1. Substitution of oxygens by sulfur atoms in uracil may enhance the probability of its biologically important process of spontaneous mutations by a factor of 10³.