Interaction of Water Molecules with Cytosine Tautomers: An Excited-State Quantum Chemical Investigation

Abstract

A detailed theoretical study of molecular geometry and electronic spectra was performed for four cytosine tautomers (amino−oxo (N1H and N3H), amino−hydroxy, and imino−oxo) and their hydrated forms with three water molecules. Geometries were optimized both in the ground and lowest singlet ππ* and nπ* excited states without any symmetry restriction. Ground-state geometries were optimized at the Hartree−Fock level of theory, whereas excited states were generated employing the configuration interaction technique involving singly excited configurations (CIS method). This was followed by excited-state geometry optimization. The nature of the corresponding potential energy surfaces was ascertained with the help of harmonic vibrational frequency analysis. All geometries were found to be minima at the respective potential energy surfaces. For the N1H tautomer, the ground-state geometry was also optimized at the B3LYP level, and the optimized geometry was used for the excitation energy calculation from the time-dependent density functional theory (TDDFT) method. The 6-311G(d,p), 6-311++G(d,p), and aug-cc-pVTZ basis sets were used in the study. However, in some singlet ππ* excited-state geometry optimizations, the 4-31G basis set was used for all atoms except the amino nitrogen, for which the 6-311+G(d) basis set was used. Although, ground-state geometries of all tautomers were found to be planar except for the amino group, corresponding excited-state geometries were found to be appreciably nonplanar. The mode of interaction of water molecules with cytosine tautomers was found to be different in the singlet nπ* excited state as compared to the ground and singlet ππ* excited states. Furthermore, in the gas phase and under hydration, cytosine is suggested to phototautomerize to the N3H tautomeric form.