A first principles analysis of the location and affinity of protons in the secondary structure of phosphotungstic acid

Nonlocal gradient corrected density functional theory was used to determine the optimal positions of the protons and their relative affinity in the secondary structure of phosphotungstic acid. First, a body centered cubic structure associated with the hexahydrate was incorporated into the model. The desorption energies of the first and second water molecules from a bridging H ₅O ₂ ⁺ species were calculated to be 109 and 55 kJ mol ^-1, respectively. The H ₃O ⁺ or H ⁺ species remaining after dehydration are preferentially located between two oxygen atoms of adjacent Keggin units. The preferred position is between two terminal O _d atoms or between a terminal O _d atom and a bridging O _c atom, depending on the alignment of the Keggin units. The energy advantage of sharing a proton between two Keggin units is in the range of 43 to 71 kJ mol ^-1. Three-dimensional periodic calculations indicate that anhydrous phosphotungstic acid can form a regular lattice in which all protons are shared between the heteropolyanions.