Quantum chemical studies of solvent effects on cytosine tautomers

Ab initio and density functional theory methods have been employed to study the molecular properties of tautomers of nucleic acid base cytosine in different environments; gas, water, methanol and chloroform. Molecular geometries and energetics of the tautomeric forms in gaseous phase have been obtained using B3LYP and MP2 levels of theory, implementing 6-311G(2d,2p) atomic basis set. The SCRF theory has been employed by implementing the same basis set to optimize the tautomers both in polar solvent (water, methanol) and apolar solvent (chloroform) and the solvent effect has been studied. The structural and energetical parameters of all the conformers have been analyzed. Chemical hardness and chemical potential have been calculated at HF/6-311G(2d,2p) level of theory for all the optimized structures, and the principle of maximum hardness has been tested. NMR studies have been carried out on the basis of Cheeseman coworker's method at B3LYP/6-311G(2d,2p) level of theory, to analyze the molecular environment as well as the delocalization activities of electron clouds. Vibrational frequency analyses have been performed and the stationary points corresponding to local minima without imaginary frequencies have been obtained for all the optimized structures.