Crystal structure of the acid salts of some dibasic acids. Part IX. Potassium hydrogen meso-tartrate: a neutron diffraction study

Abstract

Kroon and Kanters (1972) determined the crystal structure of the title compound with X-rays at 113 K: there are four KH(C₄H₄O₆) units in the triclinic (P) cell. The structure has now been studied by neutron diffraction at room temperature, and refined to R 0.063 for 1511 reflexions. In general the geometry found by X-rays is confirmed, though the hydrogen atoms are more precisely located. Thermal motion of the meso-tartrate residues is much enhanced at room temperature, and some of the longer K⁺⋯ O contacts are significantly lengthened; these findings may be connected.

The structure contains two very short O ⋯ H ⋯ O bonds, between carboxy-groups, which lie across centres of symmetry. One of them had been found to be anomalous in several structural details, and an electron-density ‘difference’ synthesis had revealed twin peaks which might be interpreted in terms of a hydrogen atom disordered between sites ca. 0.4 Å on either side of the bond centre. The neutron analysis does not support this possibility, though there is probably disorder of a subtler kind affecting this particular bond [O ⋯ O 2.483(2)(X-rays) and 2.474(8)Å neutrons]. The other short hydrogen bond [2.452(2) and 2.440(8)Å] is an ordinary Type A O ⋯ H ⋯ O bond, and its proton probably vibrates in a flattened, single-minimum potential-energy well at the centre.