Hydrogen transfer and hydration properties of $HnPO43−n$ $(n=–3)$ in water studied by first principles molecular dynamics simulations

Density functional theory Perdew–Burke–Ernzerhof [Perdew et al, Phys. Rev. Lett. 77, 3865 (1996)] molecular dynamics simulations of aqueous solutions of orthophosphate species $HnPO43−n$ $(n=0–3)$ 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 $PO43−$⁠, of the hydrogen phosphate anions, $HPO42−$ and $H2PO4−$⁠, and of the orthophosphoric acid, $H3PO4$⁠, in explicit water show that the process of proton transfer from $HnPO43−n$ 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 $HnPO43−n$ and three water molecules. Analysis of the intermolecular $HnPO43−n$-water structure shows that the $PO43−$ 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 $PO43−$⁠, $HPO42−$⁠, and $H2PO4−$ 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 $PO43−$ to $H2PO4−$⁠. The strength and number of hydrogen bonds of $PO43−$⁠, $HPO42−$⁠, and $H2PO4−$ 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 $PO43−$ to the hydrogenated $H2PO4−$ species, explain the diminished effect on the structure of water with the increasing hydrogenation of the orthophosphate anions.