A theoretical study of hydration of 4-thiouracil in the electronic singlet excited state

A comprehensive theoretical investigation was performed to study the interaction of water molecules with 4-thiouracil (4TU) in the ground and electronic lowest singlet nπ* excited state. The studied system included the isolated 4TU and the hydrated complexes of 4TU with one, two and three water molecules. The interacting water molecules were placed in between different hydrogen bond donating and accepting sites of 4TU. The ground state geometries were optimized at the HF level using the 6-311++G(d,p) basis set. The electronic excited state geometries of 4TU and different hydrated complexes were optimized at the CI-singles (CIS) level using the 6-311++G(d,p) basis set. The harmonic vibrational frequency calculations were performed to ascertained stationary points at the respective potential energy surfaces; all geometries were found minima. The lowest singlet nπ* transition of 4TU was assigned to the excitation of the lone-pair electron of the thiocarbonyl group to π*-antibonding orbitals of the molecule. It was found that the nπ* state provides a repulsive potential for water binding in the excited state. Therefore, the structures of the hydrated complexes, where water molecule was bonded at the thiocarbonyl group, were remarkably modified in the electronic singlet nπ* excited state as compared to the corresponding structure in the ground state. Further, in going from the ground state to the excited state significant changes in the features of molecular electrostatic potentials are also revealed.