New analysis of the neutron diffraction data for anhydrous orthophosphoric acid and the structure of H3PO4 molecules in crystals.

Abstract

Two neutron data sets, which had been analyzed separately to determine the H-atom positions in H3PO4 crystals [Cole (1966). PhD Thesis, Univ. of Washington, Pullman, USA], have been re-analyzed in a joint refinement, fitting separate scale and extinction parameters for each data set, in order to obtain more precise positional and vibrational parameters. The new refinement gave R(F) = 0.036 for the combined 743 data. For the new results, and for six other H3PO4 molecules from four other, different crystal structures, thermal vibration analyses have been performed, and the molecular structures, thermal vibrations and hydrogen-bonding effects are compared. The rigid-body model is found to be better than the riding model for the PO4 groups. The P-OH bond lengths are markedly affected by hydrogen bonding, but seem to be independent of O = P-O-H conformation. These effects are interpreted in terms of the P-O partial double-bond character. Crystal data (Cole, 1966): anhydrous orthophosphoric acid, H3PO4, Mr = 98.00, room temperature, P2(1)/c, a = 5.779(9), b = 4.826 (4), c = 11.606 (40) A, beta = 95.26 (18) degrees, V = 322.3 (20) A3, Z = 4, Dx = 2.019 mg mm-3, mu = 0.1687 mm-1 for neutrons with lambda = 1.450 A.