Theoretical investigations of proton transfer reactions in a hydrogen bonded complex of cytosine with water

Abstract

The potential energy functions of the electronic ground as well as the lowest ππ* and nπ* excited singlet states of the most stable hydrogen bonded complex of cytosine with water was theoretically investigated along the proton transfer (PT) reaction coordinate. The full geometry optimization was performed along the PT reaction path. In the geometry optimization, the Hartree–Fock method and the configuration interaction scheme with single excitations was used. The energy calculations at the optimized geometries were performed with the complete-active-space self-consistent-field (CASSCF) method and with second-order perturbation theory, employing the CASSCF wave function as the reference state (CASPT2), as well as with the Moller–Plesset second-order theory (MP2) for the ground state. It was found that the cytosine:water complex is stable in the amino-oxo form against the PT reaction which leads to the imino-oxo tautomeric form on the ground as well as on the excited PE surfaces. An efficient nonradiative decay channel of electronically excited cytosine resulting from nonadiabatic interactions between the ground and excited singlet states along the reaction coordinate leading to the prefulvenic form was identified. It is argued that both the above mentioned effects under the normal conditions (neutral aqueous solution, room temperature) are responsible for remarkable stability of cytosine in its ‘‘native’’ amino-oxo form against any damage that may result from exposure to UV radiation.