Dilithium (citrate) crystals and their relatives

New compounds of the type LiN/HC6H5O7 (M = Li, Na, K, Rb) have been prepared from the metal carbonates and citric acid in solution. The crystal structures have been solved and refined using laboratory powder X-ray diffraction data, and optimized using density functional techniques. The compounds crystallize in the triclinic space group P (1) over bar and are nearly isostructural. The structures are lamellar, with the layers in the ab plane. The boundaries of the layers consist of hydrophobic methylene groups and very strong intermolecular O-H center dot center dot center dot O hydrogen bonds. The O center dot center dot center dot O distances range from 2.666 angstrom for M = Li to 2.465 angstrom for M = Rb. The Li-O bonds exhibit significant covalent character, while the heavier M-O bonds are ionic. The Li atoms are four-, five-, or six-coordinate, while the coordination numbers of the larger cations are higher, i.e. eight for Na and nine for K and Rb. The citrate anion occurs in the trans,trans conformation, one of the two low-energy conformations of an isolated citrate anion. The crystal structure of LiRbHC6H5O7 center dot H2O was also solved and refined. It consists of the same layers as in the anhydrous M = Rb compound, with interlayer water molecules and a different hydrogen-bonding pattern.