Hydride transfer tunneling-ready-state structure as a function of donor-acceptor distance: A full gating coordinate for the vibrational tunneling-ready-state

摘要

The contemporary hydrogen-tunneling model involves a tunneling-ready-state (TRS) that is composed of activated structures of degenerate donor/acceptor (D/A) energies at various D-A distances (DADs). Current understanding of the DAD sampling over the gating coordinate is mainly from the observed temperature dependence of 1 degrees KIEs, which only suggests broadness/narrowness of the sampling range. To better understand the gating coordinate of the TRS, computing TRS structures as a function of DAD is needed. Such computations have, however, been quite a challenge as in a real TRS the position of the hydrogen is uncertain and DAD is fluctuating. Using the self-exchange hydride transfer reaction from 2-propanol to protonated acetone as an example, an efficient set of dihedral angle restrictions in various initial activated donor/acceptor complexes were introduced to optimize to the TRS structures. The resulting gating coordinate suggests that the TRS could sample conformations of a wide range of DADs from 2.7 to 4.1 angstrom, with two major populations centered at around 2.9 and 3.9 angstrom, respectively. Zero-point energy, HOMO/LUMO energy, as well as the imaginary frequency for TRS structures, were calculated and their changes either along the H-tunneling coordinate or with the DAD were discussed to provide insight into the nature of H-tunneling.