A theoretical investigation on the structures of (NH3)•(H2SO4)•(H2O)0-14 clusters

Ternary clusters (NH3)center dot(H2SO4)center dot(H2O)(n) have been widely studied. However, the structures and binding energies of relatively larger cluster (n > 6) remain unclear, which hinders the study of other interesting properties. Ternary clusters of (NH3)center dot(H2SO4)center dot(H2O)(n), n = 0-14, were investigated using MD simulations and quantum chemical calculations. For n = 1, a proton was transferred from H2SO4 to NH3. For n = 10, both protons of H2SO4 were transferred to NH3 and H2O, respectively. The NH4+ and HSO4- formed a contact ion-pair [NH4+-HSO4-] for n = 1-6 and a solvent separated ion-pair [NH4+-H2O-HSO4-] for n = 7-9. Therefore, we observed two obvious transitions from neutral to single protonation (from H2SO4 to NH3) to double protonation (from H2SO4 to NH3 and H2O) with increasing n. In general, the structures with single protonation and solvated ion-pair were higher in entropy than those with double protonation and contact ion-pair of single protonation and were thus preferred at higher temperature. As a result, the inversion between single and double protonated clusters was postponed until n = 12 according to the average binding Gibbs free energy at the normal condition. These results can serve as a good start point for studies of the other properties of these clusters and as a model for the solvation of the [H2SO4-NH3] complex in bulk water.