Computational study of hydrated phosphate anions

The interaction of PO43− with water has been investigated by ab initio calculations. Geometry optimizations of the orthophosphate anion complex with one or two water molecules (PO43−/H2O) show that the most stable conformations have C2v symmetry and symmetrical hydrogen bonds as part of one or two six-membered rings. Intermolecular potential functions and an intramolecular force field were derived from ab initio calculated energies. They were used in molecular dynamics simulations of aqueous solutions of K3PO4, K2HPO4 and KH2PO4. The simulations were performed in the NVT ensemble with systems consisting of one anion and about 500 water molecules. The main results of the simulations are a pronounced first hydration shell consisting of about 16 water molecules for orthophosphate and about 20 water molecules for mono- and dihydrogenphosphate and a partially ordered second hydration shell. The results are in good agreement with recent experimental data.