Hydrogen-Bonded Structure in Concentrated Aqueous Phosphoric Acid Solutions

Polarized Raman scattering, X-ray, and time-of-flight (TOF) neutron-diffraction measurements were carried out for highly concentrated aqueous phosphoric acid solutions, in order to deduce detailed structural information on both the PO4-intramolecular geometry and intermolecular hydrogen bonds in the solution. Isotropic Raman spectra observed for (H3PO4)x(H2O)1-x with x = 0.1—0.6 exhibited a single band assigned to the symmetrical stretching vibrational mode of the PO4 structural unit, which suggests that all P-O bonds in the unit are spectroscopically equivalent. The results of X-ray and neutron-diffraction measurements for the 53 mol% phosphoric acid solution indicated that the structure of the PO4 unit is a regular tetrahedron with a P-O bond-length of 1.540(3) Å. From a least-squares fit to the observed X-ray and neutron intermolecular interference terms, the nearest-neighbor intermolecular distances, reflecting intermolecular hydrogen-bonded interactions in the solution, were determined to be r(O…D) = 1.77(1) Å and r(O…O) = 2.73(2) Å, respectively, which are both ca. 0.1 Å shorter than those observed for pure liquid water. Therefore, it has been clarified that a strongly hydrogen-bonded network exists in such concentrated aqueous phosphoric acid solutions.