HYDROGEN BRIDGES .1. MOLECULAR AND CRYSTAL-STRUCTURE OF PHOSPHONIC ACID H3PO3 - X-RAY AND NEUTRON-DIFFRACTION STUDIES OF THE HYDROGEN AND DEUTERIUM COMPOUNDS

The structure of phosphonic acid H3PO3 has been redetermined by single crystal neutron diffraction (lambda = 104.22 pm) at 15.0 +/- 0.1 K yielding lattice parameters {Pna2(1); Z = 8; a = 716.6(3); b = 1201.3(5); c = 674.3(3) pm} and bond lengths {mean values from two crystallographically independent molecules: P-O 155; P = O 150; P-H 139; O-H 101 pm} of of high reliability (R = 0.053). Each molecule is involved in four asymmetric hydrogen bonds (O...H 155 to 160 pm; O-H...O 168 to 177-degrees) with either hydroxyl group donating and the phosphoryl fragment acting as a twofold acceptor. Thus a complex, three-dimensional net, consisting of four- and eight-point circuits in a 1:2 ratio, is put up although the molecules are packed in a comparatively simple way to form an almost cubic closest arrangement. An X-ray crystal structure determination (R = 0.032) carried out at 173 +/- 3 K for comparison revealed no significant differences as to the distances and angles between phosphorus and oxygen atoms; and additional comparing neutron diffraction study at 15.0 +/- 0.1 K (lambda = 131.68 pm; isotropic atomic displacement parameters) of the hydrogen (R = 0.044) and deuterium compounds (R = 0.041) resulted in nearly identical structural models for the two isotopomers.