Crystal structures of some acid salts of monobasic acids. Part XVII. Structure of sodium hydrogen diacetate, redetermined by neutron diffraction

The crystal structure of the title compound has now been studied by neutron diffraction, based on some 300 independent reflexions, and refined by anisotropic least-squares to R 8·3%. The main features determined previously by X-ray diffraction of this highly symmetrical (cubic) structure are confirmed; the positions and vibrational motions of the hydrogen atoms have been elucidated.The short hydrogen bond, lying across a two-fold axis between tow crystallographically equivalent acetate groups, has O ⋯ O 2·475(14)A, and O ⋯ H ⋯ O 176(2)°, implying that the bond is not significantly bent. There is a large amplitude of internal libration of the methyl groups, estimated at ca. 26°(root-mean-square). This is discussed in relation to the packing of the methyl groups, which is evidently loose, in spite of there being two C ⋯ C contacts much closer than twice the conventional van der Waals radius of methyl carbon (2·0 A).