Fortes, AD; (2015) Crystal structures of deuterated sodium molybdate dihydrate and sodium tungstate dihydrate from time-of-flight neutron powder diffraction. Acta Crystallographica Section E: Structure Reports Online , 71 (7) pp. 799-806. 10.1107/S2056989015011354.

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Abstract

Time-of-flight neutron powder diffraction data have been measured from ~90 mol. % deuterated isotopologues of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O at 295 K to a resolution of sin(θ)/λ = 0.77 / Å. The use of neutrons has allowed refinement of structural parameters with a precision that varies by a factor of two from the heaviest to the lightest atoms; this contrasts with the X-ray refinements where precision may be > 20x poorer for O in the presence of atoms such as Mo and W. The accuracy and precision of interatomic distances and angles are in excellent agreement with recent X-ray single-crystal structure refinements whilst also completing our view of the hydrogen-bond geometry to the same degree of statistical certainty. The two structures are isotypic, space-group Pbca, with all atoms occupying general positions, being comprised of edge- and corner-sharing NaO₅ and NaO₆ polyhedra that form layers parallel with (010), interleaved with planes of XO₄ (X = Mo, W) tetrahedral that are linked by chains of water molecules along [100] and [001]. The complete structure is identical with the previously described molybdate [Capitelli et al. (2006) Asian J. Chem. 18, 2856-2860] but shows that the purported three-centred interaction involving one of the water molecules in the tungstate [Farrugia (2007) Acta Cryst. E 63, i142] is in fact an ordinary two-centred 'linear' hydrogen bond.

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