Crystal structure and vibrational spectroscopy of the new acidic diphosphate (NH4)(2)Zn(H2P2O7)(2).2 H2O

The hydrated ammonium hemizinc dihydrogen diphosphate (NH4)(2)Zn(H2P2O7)(2) - 2 H2O was synthesized. It crystallizes in the triclinic system, space group P1, Z = 1, with the following unit-cell-parameters: a = 7.0026(2) angstrom, b 7.3297(2) angstrom, c = 7.7885(3) angstrom, a = 81.229(1)degrees, beta = 71.064(1)degrees, gamma = 88.172(1)degrees, V = 373.64(2) angstrom(3). The structure was obtained by single-crystal X-ray diffractometry, and a full-matrix least-squares refinement based on F-2 gave a final R-1 index of = 0.0447 (wR(2) = 0.1336), utilizing 2055 observed reflections with I > 2 sigma(I). The crystal packing consists in a three-dimensional network made by layers parallel to ab plane of ZnO6 octahedra sharing four vertices with H2P2O7 double tetrahedra: (NH4)(+) cations fill up the cavities between tetrahedra and octahedra; strong hydrogen bonds stabilize the framework. The dihydrogendiphosphate anion (H2P2O7)(2-) shows bent eclipsed conformation and the Zn2+ ion lies on inversion centre. Raman and infrared spectra of (NH4)(2)Zn(H2P2O7)(2) - 2 H2O have been collected and interpreted using factor group analysis. No coincidences observed between most of Raman and IR spectral bands confirmed the centrosymmetric structure of the title compound; the vibrational spectra point to a bent POP bridge angle