Sodium rubidium hydrogen citrate, NaRbHCHO, and sodium caesium hydrogen citrate, NaCsHCHO: crystal structures and DFT comparisons.

The crystal structure of sodium rubidium hydrogen citrate, NaRbHCHO or [NaRb(CHO)] , has been solved and refined using laboratory powder X-ray diffraction data, and optimized using density functional techniques. This compound is isostructural to NaKHCHO. The Na atom is six-coordinate, with a bond-valence sum of 1.16. The Rb atom is eight-coordinate, with a bond-valence sum of 1.17. The distorted [NaO] octa-hedra share edges to form chains along the -axis direction. The irregular [RbO] coordination polyhedra share edges with the [NaO] octa-hedra on either side of the chain, and share corners with other Rb atoms, resulting in triple chains along the -axis direction. The most prominent feature of the structure is the chain along [111] of very short, very strong hydrogen bonds; the O⋯O distances are 2.426 and 2.398 Å. The Mulliken overlap populations in these hydrogen bonds are 0.140 and 0.143 electrons, which correspond to hydrogen-bond energies of about 20.3 kcal mol. The crystal structure of sodium caesium hydrogen citrate, NaCsHCHO or [NaCs(CHO)] , has also been solved and refined using laboratory powder X-ray diffraction data, and optimized using density functional techniques. The Na atom is six-coordinate, with a bond-valence sum of 1.15. The Cs atom is eight-coordinate, with a bond-valence sum of 0.97. The distorted trigonal-prismatic [NaO] coordination polyhedra share edges to form zigzag chains along the axis direction. The irregular [CsO] coordination polyhedra share edges with the [NaO] polyhedra to form layers parallel to the (101) plane, unlike the isolated chains in NaKHCHO and NaRbHCHO. A prominent feature of the structure is the chain along [100] of very short, very strong O-H⋯O hydrogen bonds; the refined O⋯O distances are 2.398 and 2.159 Å, and the optimized distances are 2.398 and 2.347 Å. The Mulliken overlap populations in these hydrogen bonds are 0.143 and 0.133 electrons, which correspond to hydrogen-bond energies about 20.3 kcal mol.