Crystal structures of two isostructural compounds: a second polymorph of dipotassium hydrogen citrate, K₂HC₆H₅O₇, and potassium rubidium hydrogen citrate, KRbHC₆H₅O₇

The crystal structures of a new polymorph of dipotassium hydrogen citrate, 2K⁺·HC₆H₅O₇ ²⁻, and potassium rubidium hydrogen citrate, K⁺·Rb⁺·HC₆H₅O₇ ²⁻, have been solved and refined using laboratory powder X-ray diffraction and optimized using density functional techniques. In the new polymorph of the dipotassium salt, KO₇ and KO₈ coordination polyhedra share corners and edges to form a three-dimensional framework with channels parallel to the a axis and [111]. The hydrophobic methylene groups face each other in the channels. The un-ionized carboxylic acid group forms a strong charge-assisted hydrogen bond to the central ionized carboxylate group. The hydroxy group forms an intermolecular hydrogen bond to a different central carboxylate group. In the potassium rubidium salt, the K⁺ and Rb⁺ cations are disordered over two sites, in approximately 0.72:0.28 and 0.28:0.72 ratios. KO₈ and RbO₉ coordination polyhedra share corners and edges to form a three-dimensional framework with channels parallel to the a axis. The un-ionized carboxylic acid group forms a strong charge-assisted hydrogen bond to an ionized carboxylate group. The hydroxy group forms an intermolecular hydrogen bond to the central carboxylate group. Density functional theory (DFT) calculations on the ordered cation structures suggest that interchange of K⁺ and Rb⁺ at the two cation sites changes the energy insignificantly.