PATH INTEGRAL SIMULATION FOR HYDROGEN BONDED SYSTEMS: PROTONIC QUANTUM NATURE AND ITS H/D ISOTOPE EFFECT

摘要

To elucidate the thermal dependency of H/D geometrical isotope effects (GIE) on H_3O_2~- anion [1], we have performed path integral molecular dynamics (PIMD) simulation [2] with 4th order Trotter expansion [3] for H_3O_2~- and its isotopomers. We have used the potential energy surface of H_3O_2~- anion by Huang [4] which has been least square fitted with 66 965 points CCSD(T)/aug-cc-pVTZ level of calculation. We have analyzed the one-dimensional distribution on H_3O_2~- with respect to the d_(OH*) at 50, 400, and 600K, where d_(OH*) is defined as the difference of R_(OH*) and R_(O H*) (H* denotes hydrogen-bonded hydrogen nucleus). At low temperatures the H* is near the center of two oxygen atoms with mostly O...X*...O geometry (where X* = H*, D*, or T*) [5], while at high temperatures above 400K hydrogen becomes more delocalized showing the coexistence between O...X*-O and O-X*...O [2]. The 00 distance tends to be shorter as the isotopomer is heavier at low temperatures [5], while this ordering becomes opposite at high temperatures [2]. It is concluded that the coupling between the 00 stretching mode and proton transfer modes is a key to understand such a temperature dependence of hydrogen-bonded structure.