Asymmetric hydrogen bonds in centrosymmetric environment: neutron study ofvery short hydrogen bonds in potassium hydrogen dichloromaleate

The structure of the title compound has been studied by neutron diffraction at 30 and 295 K, with the emphasis on the location of the protons. There a re two crystallographically independent H atoms in two very short hydrogen bonds, 2.437 (2) and 2.442 (2) Angstrom at 30 K. The structure could be ref ined successfully in the centrosymmetric space group P (1) over bar 1, with the H atoms located at the centres of symmetry. However, the form of the t hermal ellipsoids of hydrogen indicated either asymmetric hydrogen bonds or overlap of two closely spaced, partially occupied positions around the cen tres of symmetry. Several different types of refinements have then been app lied, including unconventional models; with all atoms except hydrogen const rained in P (1) over bar 1, but with hydrogen allowed to refine without any constraints in P1, anisotropic refinement of all atoms resulted in clearly off-centred hydrogen positions. Significance tests clearly showed that the results from this constrained refinement give the most satisfactory descri ption of the structure. This structure may be described as 'pseudo-centrosy mmetric with non-centred protons'. The results demonstrate that it is very important to also include refinement models with non-centrosymmetric hydrog en in a centrosymmetric environment when studying very short hydrogen bonds . The shifts of the two H atoms from the centres of symmetry are 0.15 (1) a nd 0.12 (1) Angstrom, respectively, at 30 K, and 0.15 (1) Angstrom for both H atoms at room temperature. At 30 K: R(F) = 0.036 for 1485 reflections; a t 295 K: R(F) = 0.035 for 1349 reflections.