UBC Theses and Dissertations
Crystal structures of some group V compound Zobel, Tessa
The crystal and molecular structure of cacodylic acid, (CH₃)₂AsO.OH, has been determined by X-ray diffraction of a single crystal. The crystals belong to the triclinic system with a = 6.53, b = 6.82, c = 6.61 Å, α = 77°30', β = 78°45', γ = 55°9', z = 2, space group P1̅. The structure was determined from visual intensity data by Patterson and Fourier methods, and the positional and anisotropic thermal parameters were refined by least-squares. The final discrepancy factor is 0.149 for 806 observed reflections. The structure consists of centrosymmetrical, hydrogen-bonded dimers, with 0-H...0 distances of 2.57Å, the arsenic atoms having a tetrahedral configuration with bond angles in the range 106° - 115°. Crystals of antimony triiodide, SbI₃, are rhombohedral with a[subscript H] = 7.48 Å, C[subscript H] = 20.90 Å, z = 6, space group R3̅. The structure was determined from h0.ℓ Patterson, Fourier and difference projections, the final discrepancy factor being 0.131. The antimony atoms are significantly displaced from the centres of iodine octahedra, and have three near neighbour iodine atoms at 2.868 ± 0.010 Å, with I-Sb-I = 95.8 ± 0.3°, and three further off at 3.316 ± 0,010 Å, The structure is thus intermediate between that of a molecular crystal, as in Asl₃, and an ionic arrangement. Crystals of Bil₃ have a[subscript H] = 7.52 Å, c[subscript H] = 20.72 Å. Comparison of measured and calculated powder intensities suggests that the bismuth atom is situated at the centre of an iodine octahedron, so that the structure is probably largely ionic. Lone-pair steric effects, as well as changes in bonding character, are thought responsible for the differences in the crystal structures of Asl₃, Sbl₃ and Bil₃.
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