Atomic Mass Dependence of the Nuclear Quantum Effect in NH4+(H2O)

The interaction between ammonium ions and water has been studied previously via molecular dynamics. However, the nuclear quantum effect phenomenon did not receive sufficient consideration therein. In this study, we report the ab initio path integral molecular dynamics (PIMD) simulations of the isotopologues of NH+4 (H2O), that is, NH+4 (H2O), ND+4 (D2O), and NT+4 (T2O), as well as the classical MD simulations. We found that the intermolecular distance does not change monotonically. Even though the nuclear quantum effect shortens the overall intermolecular distance (N-O atoms), the distance for NH+4 (H2O) is longer than that for NT+4 (T2O). Two dimensional distributions of the binding energies with the contour lines were constructed to assess this unusual quantum behavior. We find that the elongation of the covalent bond increases the binding energy, while the quantum fluctuation of the O-N-Hb bond angle decreases it. The bond length is determined by the delicate balance of such competing quantum effects.