Ab initio based exploration of the potential energy surface for the double proton transfer in the first excited singlet electronic state of the 7-azaindole dimer

In this paper ab initio full geometry optimizations are carried out for the ground and first excited singlet electronic states of the 7-azaindole dimer, a well-known prototype of the DNA base pairs. Results indicate that the C-2h symmetry Of the ground-state minimum energy is not maintained in the excited state that has to be described as a dimer between an excited base unit and another one in the ground state. Given this asymmetry, the double proton transfer in the excited state is found to be stepwise in nature, passing through a very shallow zwitterionic intermediate. Inclusion of the zero point energy and the rest of the thermodynamic corrections points to the nonexistence of bound states for the intermediate well. Our theoretical calculations have also confirmed the presence of another intermediate where the transfer of a single proton is compensated by a charge-transfer electronic excitation. This neutral intermediate is found lower in energy than the zwitterionic one and could be responsible for the stepwise reaction observed in several recent experiments done at the femtosecond time scale. A nonadiabatic transition should occur between the initially accessed electronic state (involving an excitation localized in one base unit) and the one possessing the neutral intermediate that involves a charge-transfer transition. When including the bulk effect of a polar solvent it is observed that the energies of the intermediates are lowered so that the zwitterionic intermediate may exist in solution. The transition state energies are also lower, a result that points to an increase in the rate constant of the process in condensed phase.