Acta Crystallographica Section B

Structural Science

Volume 58, Part 4 (August 2002)

research papers

Acta Cryst. (2002). B58, 627-631 [ doi:10.1107/S0108768102004858 ]

Asymmetric hydrogen bonds in centrosymmetric environment. II. Neutron study of very short hydrogen bonds in potassium hydrogen dichloromaleate at 90 K and 170 K

I. Olovsson, H. Ptasiewicz-Bak, T. Gustafsson and I. Majerz

Abstract: In our earlier neutron diffraction study of the title compound at 30 K and 295 K an unconventional strategy in the refinement of hydrogen was applied and the same procedure has now been followed in the present investigation at 170 K and 90 K. There are two short OHO hydrogen bonds [2.437 (2) Å and 2.442 (2) Å at 30 K] and the `heavy-atom' structure is centrosymmetric (P $\bar1$ ) with centres of symmetry in the middle of the OO bonds. However, statistical significance tests clearly show that an asymmetric location of both H atoms gives the most satisfactory description of the structure at all temperatures. The shift of hydrogen from the centre of symmetry is 0.15, 0.14, 0.15 and 0.15 Å for H2 at 30, 90, 170 and 295 K, respectively, and 0.15, 0.15, 0.15 and 0.12 Å for H4 ( [bold sigma] = 0.01 Å). Furthermore, the behaviour of H2 is very interesting: at 295 K and 170 K it is located on one side of the symmetry centre but at 90 K and 30 K it is located on the other side. A detailed determination of the unit-cell parameters by X-ray diffraction in the whole temperature range from 30 K to 295 K has revealed that the data points of the cell parameters as a function of temperature fall on two different straight lines with a sudden change in the slope around 135 K. It appears likely that the change in the location of H2 as the temperature is lowered is related to this behaviour. At 170 K, R(F) = 0.029 for 1236 reflections; at 90 K, R(F) = 0.030 for 1457 reflections.

Keywords: asymmetric hydrogen bonds; hydrogen bonds; potassium hydrogen dichloromaleate.


	Structure factor file (SHELXL table format) (52.8 kbytes) Supplementary material

	Structure factor file (SHELXL table format) (61.9 kbytes) Supplementary material

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