Non-planarity and solvent effects on structural and polarizability properties of cytosine tautomers

We report equilibrium geometries, relative stabilities, electronic and vibrational polarizabilities of cytosine tautomers and of reference compounds phenol, aniline and pyrimidine, in the gas phase and in solution, calculated by conventional correlated ab initio (CCSD, CCSD(T) and MP2) and density functional (B97-1) methods, using a variety of basis sets. Gas phase ionization potentials, electron affinities and torsional potentials for the internal rotation of the -OH and -NH2 groups, were obtained at MP2 and B97-1 levels of theory. The results show that the structures of the tautomers are non-planar in the gas phase and become planar as the polarity of the solvent increases, in contrast with aniline where the amino group remains non-planar also in the polar CH3CN solvent, what has a correspondence in the NH2 inversion barrier and NH2 wagging vibration lower in cytosine than in aniline. To constrain NH2 to be planar does not produce important changes in the relative energies, polarizabilities, ionization potential and electron affinities of the tautomers in vacuo. Polarizabilities are consistently influenced by solvent effects. The -OH and -NH2 groups show much higher π interaction with the heterocyclic ring than with the phenyl ring, what determines barriers for internal rotation of the groups much higher in cytosine than in phenol and aniline. Hardness and polarizability profiles for the -OH and -NH2 rotation do not follow, however, the maximum hardness and the minimum polarizability principles, respectively.