Theoretical study for solvent effect on the potential energy surface for the double proton transfer in formic acid dimer and formamidine dimer

Double proton transfers in formic acid dimer and formamidine dimer were studied as prototypes of multiple proton transfer. The potential energy surface (PFS) for the double proton transfer was studied using ab initio quantum mechanical methods. The solvent effect on the PES was also included using the Onsager self-consistent reaction field model. In the gas phase, the transition state for the double proton transfer in the formic acid dimer complex has D-2h symmetry, but in water it is changed to a C-2v structure, when the Hartree-Fock (HF) level of theory is used. When the density functional theory is used, the transition state has D-2h symmetry with and without solvent. However the barrier height depends very much on the electron correlation. The double proton transfer occurs synchronously in all the cases. For the formamidine dimer complex, the transition state has C-2v symmetry in the gas phase, and it changes to C-s symmetry in water at the HF level of theory. The C-2v structure becomes an intermediate in water, which means that double proton transfer occurs asynchronously. In the density functional theory for the gas phase, the transition state has D-2h symmetry, and it changes to C-2v structure in solution. However the double proton transfer occurs synchronously in both cases. These results suggest that the correlation is very important to the PES for double proton transfer, not only in the gas phase but also in solution.