The crystal and molecular structure of nitrilotrimethylene triphosphonic acid

Abstract

A three-dimensional X-ray diffraction analysis of the structure of nitrilotrimethylene triphosphonic acid N(CH₂PO₃H₂)₃, has been carried out. The crystals are monoclinic, space group P2₁/c, with a= 12·050, b= 7·474, c= 11·868 Å, β= 104° 10′, Z= 4. The solution of the crystal structure presented considerable difficulties since all three phosphorus atoms lie close to special positions. However, once a reasonable trial structure had been obtained, refinement proceeded smoothly until the final R factor for 2236 independent reflexions measured with a Hilger–Watts linear diffractometer was 5·76%. All twelve hydrogen atoms were located from a difference synthesis, and their disposition shows that the molecules exist as zwitterions in the crystal. A true representation of the molecular structure is thus NH⁺(CH₂PO₃H₂)₂(CH₂PO₃H)^–. All except the methylene hydrogen atoms are involved in an efficient and complex system of intermolecular hydrogen bonds. The bonds and angles round the nitrogen atom show no deviations from each other and are close to the expected values, but the angles at all three phosphorus atoms diverge very significantly from a regular tetrahedral value. The P–O distances split into three recognisable groups. The shortest, with a mean value of 1·477 Å, correspond to PO bonds. The longest (1·534 Å) are associated with P–OH bonds. Intermediate between these two extremes (1·501 Å) are two bonds which may be described as P–O^– bonds, each oxygen atom carrying a fractional negative charge.