Theoretical Studies Of Hydrated Clusters Of Ammonium And Hydrated Clusters Of Sulfate

By means of quantum chemistry and the ABEEM/MM fluctuating charge force field, an accurate interaction potential function for NH₄⁺-H₂O system was constructed, and then it was applied to study the structures and stabilities of hydrated ammonium clusters NH₄⁺(H₂O)_n(n=1⁹). Analysis on the binding energies and charge distributions of the clusters shows that when n≤4, as the number of water molecules increases, NH₄⁺ tends to form linear hydrogen bonds with water molecules as many as possible until its first hydration shell is completely constructed. The clusters for 5-9 water molecules tend to form a ring or a cage around the central NH₄⁺ through the network of hydrogen bonds. The strength of H-bonds between NH₄⁺ and water molecules in its 1st hydration shell is stronger than that of the H-bonds between water molecules. Analysis of the Vibrational spectrum show that O-H stretching vibration of the water molecules is reduced, which is because the polarization addition of the protonated NH₄⁺. Furthermore, the dynamics simulation of the aqueous ammonium shows that the first peak of N-O radial distribution function is located at 0.305 nm, the hydration of the first layer is 11.9, the boundary of the first hydration layer is located at 0.365 nm. The peak ofθ(ONO) angular distribution function is between 30o to 50o, which corresponds to a coordination structure of more than 8 water moleculars of the first layer. The results of the ABEEM/MM model are well consistent with those from quantum chemistry calculations and experiment data.The MP2/6-31G (d, p) level was used to study the hydrated sulfate clusters, SO₄^2-(H₂O)_n(n=1⁸). The low-energy structures were optimized. It is found that the polarization of SO₄^2- made the angle of water molecules changed much, and the maximum is 13°. As the number of water molecules increases, the distance between S atom and the O atoms of water molecules gradually tends larger, the hydrogen bond tends weaker, the charge of S atoms increases, the charge of atom O of SO₄^2- and water decreases, and the charge of H atoms gradually increases. It can be seen from the spectral analysis, the strong hydrogen bonds interaction between SO₄^2- and water molecules, makes a red shift for the S-O stretching of the sulfate, which a blue shift for the O-H stretching of water.