A- and B-type hydrogen bond modifications of potassium hydrogen tartronate
ACCORDING to Speakman1 two types of intermolecular hydrogen bonds between carboxyl groups can be discerned in the crystal structures of acid salts of carboxylic acids. The first, type B, occurs when two carboxyl groups are not symmetry related, and have structural differences sufficient to characterise them, separately, as unionised (−CO2H) and ionised (−CO2−). Thus, any hydrogen bond between these groups must be asymmetric. The second, type A, occurs when two carboxyl groups are related by a symmetry element of the crystal, so that they are effectively equivalent. Thus any hydrogen bond between the groups must be, at least formally, symmetrical.
Crystal structure of the acid salts of some dibasic acids. Part IX. Potassium hydrogen meso-tartrate: a neutron diffraction study
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 A 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)A neutrons]. The other short hydrogen bond [2.452(2) and 2.440(8)A] 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.
Analytic constants for atomic scattering factors
New and revised values of the analytical constants A, a, B, b, and C for both CuK alpha and MoK alpha radiation are presented. Analytical constants for atomic scattering factors derived from self-consistent or variational wave functions and from Thomas-Fermi-Dirac statistics are tabulated. (P.C.H.)