Hydrogen transfer and hydration properties of HnPO_4 ~(3-n) (n=0–3)in water studied by first principles molecular dynamics simulations

摘要

Density functional theory Perdew–Burke–Ernzerhof Perdew et al., Phys. Rev. Lett. 77, 3865 (1996) molecular dynamics simulations of aqueous solutions of orthophosphate species HnPO_4~(3-n) (n=0–3) provide new insights into hydrogen transfer and intermolecular and hydration properties of these important aqueous species. Extensive Car–Parrinello molecular dynamics simulations of the orthophosphate ion PO_4~(3-), of the hydrogen phosphate anions, HPO_4~(2-) and H_2PO~(4-), and of the orthophosphoric acid, H_3PO_4, in explicit water show that the process of proton transfer from HnPO_4~(3-n) to the surrounding water molecules is very fast, less than 1 ps, and indicate that the dehydrogenation occurs through a concerted proton hopping mechanism, which involves HnPO_4~(3-n) and three water molecules. Analysis of the intermolecular HnPO_4~(3-n)-water structure shows that the PO_4~(3-) anions have a significant effect on the H-bonding network of bulk water and the presence of P–O- moieties induce the formation of new types of H–H interactions around this orthophosphate. Calculated probability distributions of the coordination numbers of the first hydration shell of PO_4~(3-), HPO_4~(2-), and H_2PO~(4-) show that these phosphate species display a flexible first coordination shell (between 7 and 13 water molecules) and that the flexibility increases on going from PO_4~(3-) to H_2PO~(4-). The strength and number of hydrogen bonds of PO_4~(3-), HPO_4~(2-), and H_2PO~(4-) are determined through a detailed analysis of the structural correlation functions. In particular, the H-bond interactions between the oxygen atoms of the phosphates and the surrounding water molecules, which decrease on going from PO_4~(3-) to the hydrogenated H_2PO~(4-) species, explain the diminished effect on the structure of water with the increasing hydrogenation of the orthophosphate anions.