Quantum Mechanical Studies for Structures and Energetic of Double Proton Transfer in Biologically Important Hydrogen-bonded Complexes

We have performed quantum mechanical calculations to study the geometries and binding energies of biologically important, cyclic hydrogen-bonded complexes, such as formic acid + H(2)O, formamidine + formamide + H(2)O, formic acid dimer, formamidine dimer, formamide dimer, formic acid + formamide, formic acid + formamidine, formamide + formamidine, and barrier heights for the double proton transfer in these complexes. Various ab initio, density functional theory, multilevel methods have been used. Geometries and energies depend very much on the level of theory. In particular, the transition state symmetry of the proton transfer in formamidine dimer varies greatly depending on the level of theory, so very high level of theory must be used to get any reasonable results.