Geometry optimization of zirconium sulfophenylphosphonate layers by molecular simulation methods.

Classical molecular simulation methods were used for a detailed structural description of zirconium 4-sulfophenylphosphonate and zirconium phenylphosphonate 4-sulfophenylphosphonates with general formula Zr(HO3 SC6 H4 PO3 ) x (C6 H5 PO3 )2-x ·yH2 O (x = 0.7-2; y = 0 or 2). First, models describing the structure of zirconium 4-sulfophenylphosphonate (x = 2) were calculated for the hydrated (y = 2) and dehydrated (y = 0) compounds. Subsequently, models for two mixed zirconium phenylphosphonate 4-sulfophenylphosphonates (x = 1.3 and 0.7) were calculated. Optimized models suggest that the presence of water molecules between sulfo groups creates a water-sulfonate layer with a system of hydrogen bonds. We suppose that this arrangement is the reason for a higher proton conductivity of the hydrated samples compared to dehydrated samples. When the water molecules are removed, a small decrease in the basal spacing (around 0.06 Å) is observed. This behavior is confirmed by the simulated models, where no significant changes in the structure on dehydration were observed except the absence of the water molecules and a lower number of hydrogen bonds between two adjacent sulfonate sheets. Due to the good crystallinity of the samples and the presence of sharp non-basal peaks in their X-ray diffraction patterns, Miller indices of the non-basal peaks in the diffraction patterns calculated from the models can be compared with those found in the experimental data. This allowed us to precisely describe for example (15 5-2) planes, from which mutual distances of the phenyl rings were determined to be 2.62 Å. Graphical Abstract Detailed ball and stick view into the interlayer structure of ZrSPhP1.3.