Phototautomerism in uracil: A quantum chemical investigation

A detailed study of the molecular geometry and electronic spectra of uracil tautomers, anions, and their hydrated complexes was performed. The geometries were optimized both in the ground and lowest singlet excited states without any symmetry restriction. Ground-state geometries were optimized at the Hartree-Fock level of theory, while the excited states were generated by 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 via an harmonic vibration frequency analysis. All geometries were found to be minima at their corresponding potential energy surfaces. The 6-311G(d,p) basis set was used for neutral species, and the 6-311++G(d,p) basis set was used for anionic structures. It has been found that, in the gas phase and in aqueous solution at neutral pH, a normal uracil tautomer will phototautomerize to its monoenol tautomer, the fluorescence of which would be appreciably red-shifted compared to the normal fluorescence. At appreciable alkaline pH in aqueous solution, uracil would coexist as a neutral and monoanionic form obtained by deprotonation of the N1H site. While ground-state geometries are found to be planar, the corresponding excited-state geometries were predicted to be highly nonplanar.