Structural investigations of methylenediphosphonic acids. 2. The molecular and crystal structure of propane-1,3-diphosphonic acid

Propane-1,3-diphosphonic acid, (CH/sub 2/)/sub 3/(PO/sub 3/H/sub 2/)/sub 2/, PC/sub 3/P, crystallizes in the noncentrosymmetric, monoclinic, space group Pc (C/sub s//sup 2/, no. 7) with unit cell constants a = 9.540(2), b = 9.964(2), c = 9.705(2) A, ..beta.. = 118.90(1)/sup 0/, and Z = 4. The asymmetric unit of PC/sub 3/P consists of two crystallographically unique molecules. The structure was solved by a combination of direct methods, Fourier, and least-squares refinement techniques; the final R/sub F/ = 0.024 for 1412 independent unique reflections. The two PC/sub 3/P molecules within the asymmetric unit have quite different conformations. Hydrogen-bonded rings of differing size shape the P-C-C-C-P backbone of one PC/sub 3/P molecule into an almost planar conformation, while the backbone of the other molecule is distorted into a distinctive nonplanar conformation. Eight intermolecular hydrogen bonds of lengths varying from 2.579(8) to 2.711(5) A are involved in four rings of 10, 12, 14 and 16 members. The hydrogen bond length appears to be correlated with the size of the hydrogen bonded ring in which it participates.