Theoretical study of the acidity and basicity of the cytosine tautomers and their 1:1 complexes with water

This work deals with a theoretical study of the acidity and basicity of the amino-oxo, amino-hydroxy, and imino-oxo tautomers (including their rotamers) along with their interaction with one water molecule. The calculations are carried out using the DFT/B3LYP functional combined with the 6-31++G(d,p) or 6-311++G(d,p) basis sets. The proton affinities (PA) of the O and N atoms and the deprotonation enthalpies (DPE) of the OH and NH bonds of the cytosine tautomers are calculated as well. The results suggest that the amino-oxo tautomer may be the most stable form in the gas phase. The optimized geometries, binding energies, and harmonic vibrational frequencies of the cyclic structures of the monohydrated cytosine tautomers are calculated. Complex formation results in a moderate change of the pyramidal character of the amino group. For the cyclic C=O...HO...HN structures, the binding energies depend on the PA and DPE of the sites involved in the interaction. The perturbations of selected vibrational modes such as the stretching vibrations and the inversion mode of the amino group along with the blue shift of the NH stretching vibration inn the imino-oxo complexes are discussed. The natural bond orbital analysis shows that there is an increase of the occupancy of the sigma* antibonding orbitals of the proton donor groups involved in the interaction.