HD isotope effect on the dihydrogen bond of NH+4...BeH2 by ab initio path integral molecular dynamics simulation

In order to investigate the HD isotope effect on a dihydrogen bonded cation system, we have studied NH+4...BeH2 and its isotopomers by ab initio path integral molecular dynamics. It is found that the dihydrogen bond can be exchanged by NH+(4) rotation. The deuterated isotopomer (ND+(4)...BeD(2); DD) can exchange the dihydrogen bond more easily than other isotopomers such as (NH+4...BeH2; HH). This unusual isotope effect is ascribed to the "quantum localization" which occurs when the effective energy barrier for the rotational mode becomes higher by the zero point energy of other modes. We also found that the binding energy of dihydrogen bonds for DD species is the smallest among the isotopomers.