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

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.