Molecular dynamics simulation study of NH4+ and NH2- in liquid ammonia: interaction potentials, structural and dynamical properties.

We provide tailor-made GAFF2-type interaction potentials for modeling ammonium and amide ions in ammonia. Based on harmonic approximation of intra-molecular bond stretching and bending, our force fields nicely reproduce the vibrational modes of NH4+ and NH2-, respectively. Moreover, quantum calculations of pair-wise NH4+/NH2--NH3 interactions were used for inter-molecular force field parameterization, while (NH3)n, [(NH4)(NH3)n]+, and [(NH2)(NH3)n]- complexes with n > 2, respectively, were reserved for benchmarking in terms of both structure and formation energy. Despite the limited reliability of molecular mechanics models for describing dimer complexes (n = 1), we find that GAFF2 reasonably reproduces [(NH4)(NH3)n]+ species for n = 2-4. For the assessment of [(NH2)(NH3)n]- complexes with n = 2-4, we however suggest the introduction of specific van der Waals parameters for amide-ammonia interactions. The application of the (extended) GAFF2 models is demonstrated for the study of ammonium and amide solvation in liquid ammonia at 240 K and 1 atm, respectively. On this basis, we suggest the applicability of our model for both gas phase and liquid states of ammonia.