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⁺·HC₆H₅O₇²⁻, and potassium rubidium hydrogen citrate, K⁺·Rb⁺·HC₆H₅O₇²⁻, 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.