Ab initio molecular dynamics studies on the growth of ammonium chloride clusters

The growth of small ionic ammonium chloride clusters, (NH4Cl)n + HCl + NH3 → (NH4Cl)n+1, for n = 1–8, has been investigated by means of ab initio molecular dynamics. It is found in our studies that the proton transfers from HCl to NH3 and structural transformations are two important processes for the cluster growth, and rotations of NH4+ ions play critical roles in structural transformation. In addition, a series of molecule-contained ammonium chloride clusters, (NH3)(NH4Cl)n, (HCl)(NH4Cl)n, (NH3)(HCl)(NH4Cl)n, n = 1–8, are predicted using DFT method, and they show obvious alternations in stability between even and odd number sizes. Furthermore, for the reactions in which molecule-contained ammonium chloride clusters are generated, enthalpies and free energies of each size are examined, showing that molecule-contained clusters may exist as competitive products during the growth of ammonium chloride clusters.