Hydrogen transfer and hydration properties of H(n)PO4(3-n) (n=0-3) in water studied by first principles molecular dynamics simulations.

Abstract

Density functional theory Perdew-Burke-Ernzerhof [Perdew et al., Phys. Rev. Lett. 77, 3865 (1996)] molecular dynamics simulations of aqueous solutions of orthophosphate species H(n)PO(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(2)PO(4)(-), and of the orthophosphoric acid, H(3)PO(4), in explicit water show that the process of proton transfer from H(n)PO(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 H(n)PO(4)(3-n) and three water molecules. Analysis of the intermolecular H(n)PO(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(2)PO(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(2)PO(4)(-). The strength and number of hydrogen bonds of PO(4)(3-), HPO(4)(2-), and H(2)PO(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(2)PO(4)(-) species, explain the diminished effect on the structure of water with the increasing hydrogenation of the orthophosphate anions.