Direct dynamics calculation for the double proton transfer in formic acid dimer

The dynamics of the double proton transfer in formic acid dimer (FAD) complex has been studied by the direct dynamics approach with variational transition state theory using multidimensional semiclassical tunneling approximations. High-level ab initio quantum mechanical calculations were performed to estimate the energetics of the double proton transfer. Dimerization energies and the barrier height have been calculated at the G2* level of theory, which yields -14.2 and 8.94 kcal mol(-1), respectively. A quantum mechanical potential energy surface has been constructed using the AM1 Hamiltonian with specific reaction parameters (AM1-SRP) which are obtained by adjusting the standard AM1 parameters to reproduce the energetics by high-level ab initio quantum mechanical calculation. The minimum energy path has been calculated on this potential energy surface and other characteristics of the surface were calculated as needed. The two protons are transferred synchronously, so the transition state possesses D-2h symmetry. The reaction path curvature is very large, so the tunneling coefficient is also very large as calculated by the large-curvature ground-state tunneling approximation (LCG3). The distance which the proton hops during tunneling is about 0.429 Angstrom. This is a very long distance compared with the normal single proton transfer. Before the tunneling the hydrogenic motion is minimal. Mostly the heavy atoms move to bring the two formic acid molecules closer. The kinetic isotope effect (KIE) was also calculated. The tunneling contribution to the KIE is not extremely large since not only two protons but two deuterium atoms tunnel well. The quasiclassical contribution to the KIE is quite large due to the synchronous motion of the two protons.