Structural and thermodynamic properties of nine isomers of cytosine were studied in gas and aqueousphases and in micro-hydrated environment employing B3LYP and MP2 methods. Also, isomerizationsof cytosine were studied in gas phase and the activation energies (Ea) and Gibbs free energies (G#) ofthe internal O H rotations and proton transfer processes were calculated. The calculated O H rotationalbarriers were smaller than 50 kJ/mol while the activation energies of intramolecular proton transferswere in the range of 110–190 kJ/mol. Effect of mono- and di-hydration of the cytosine isomers on thetransition state structures and the energy barriers was investigated.
