A path integral molecular dynamics study on the NH4+ rotation in NH4+⋯XH2 (X = Be or Mg) dihydrogen bond systems

Abstract

The nuclear quantum effects (NQEs) in dihydrogen bond (DHB) complexes, i.e., NH₄⁺⋯BeH₂ and NH₄⁺⋯MgH₂, have been investigated using multicomponent quantum mechanics (MC_QM) calculations and path integral molecular dynamics (PIMD) simulation. The MC_QM method considers the NQEs, whereas PIMD considers both the NQEs and the thermal effects. The linear C_3v structure is maintained in the optimized structures obtained by the static MP2 and MC_MP2 calculations, whereas the average structures obtained by the PIMD simulation are nonlinear. The strong DHB interaction in NH₄⁺⋯MgH₂ suppresses the fluctuation in the H^δ+NMg and H^δ−MgN angles, and hence, the NH₄⁺ rotation did not occur in the simulation of NH₄⁺⋯MgH₂. The analysis of the radius of gyration revealed that the nuclear quantum fluctuation in the perpendicular direction is suppressed by the formation of the DHB complex, whereas that in the parallel direction is slightly enhanced in both the H^δ+ and H^δ− nuclei.