Ab initio MP2 and DFT studies on the tautomers of cytosine and the related hydrated tautomers have been carried out. The ground-state structures of four tautomers of cytosine and related transition states were fully optimized. The vibrational frequency analysis was performed on all the optimized structures. Detailed intrinsic reaction coordinate （IRC） calculations were carded out to guarantee the optimized transition-state structures being connected to the related tautomers. We obtained the relative stability order for the tautomers of cytosine and the related hydrated tautomers. In the isolated and hydrated condition, the bond types of C（2）--O（7） and C（4）--N（8） greatly affect the stability of the cytosine tautomers. Moreover, we have explored the influence of the water molecules on the intramolecular proton transfer between the keto and enol forms of the cytosine tautomers. The first water molecule obviously decreases the isomerization activation energy for the monohydrated cytosine tautomers. It is shown that the isomerization energy barrier changes only a little when the second and third water molecules are added in the reaction loop. The solvent effects have an obvious influence on the proton-transfer barrier of the isolated cytosine. However, the solvent effects seem to be insignificant for the isomerization energy barriers of the monohydrated, dihydrated and trihydrated cytosine. The water molecule in these complexes can be looked on as the explicit water. Therefore, the explicit water model may be more credible to explore the intramolecular proton transfer, in comparison with the PCM which is the implicit water model.
