Time-dependent density functional theory (TD-DFT) study of the excited state proton transfer in hypoxanthine

Theoretical investigations on the ground- and excited-state proton transfer in the isolated and monohydrated forms of hypoxanthine have been performed. Ground- and transition-state geometries were optimized at the Hartree-Fock (HF)/6-311 + +G(d,p) and B3LYP/6-311 + +G(dp) levels. The geometries of tautomers including the transition states were also optimized in the lowest singlet pi pi* excited state at the CIS/6-311 + +G(d,p) level. The time-dependent density function theory augmented with B3LYP functional (TD-B3LYP) and the 6-311 + + G(dp) basis set were used to compute vertical transition energies using the B3LYP geometries. The TD-B3LYP/6-311 + +G(dp) calculations were also performed using the CIS/6-311 + +G(dp) geometries to predict the adiabatic transition energies of tautomers and the excited-state proton transfer barrier heights. The effect of aqueous environments was considered using the polarizable continuum model (PCM). The harmonic vibrational frequency calculations were performed to ascertain the nature of potential energy surfaces. It was found that proton transfer is characterized by a high barrier height both in the gas phase and in aqueous solution. The explicit inclusion of a water molecule in the proton transfer path reduces the barrier height drastically. The excited-state barrier height was found to be increased as compared with that in the ground state. The transition states corresponding to the proton transfer from the keto to the enol form for the monohydrated forms were found to have a zwitterionic structure. The zwitterionic nature of the hydrated transition-state structure is increased in the excited state. The transition-state geometries are more easily expressed in the form of H3O(+)center dot center dot center dot HX- for the monohydrated complex of the molecule. The excited-state geometries, including that of transition states, were found to be largely nonplanar. The nonplanar fragment was localized mostly in the six-membered ring. (c) 2005 Wiley Periodicals, Inc. Int j Quantum Chem 105: 387-395,2005.