Crystal structures of alkali metal (Group 1) citrate salts.

The crystal structures of 16 new alkali metal citrates were determined using powder and/or single crystal techniques. These structures and 12 previously determined citrate structures were optimized using density functional techniques. The central portion of a citrate ion is fairly rigid, while the conformations of the terminal carboxylate groups exhibit no preferences. The citrate-metal bonding is ionic. Trends in metal-citrate coordination are noted. The energy of an O-H...O hydrogen bond is proportional to the square root of the H...acceptor Mulliken overlap population, and a correlation between the hydrogen bond energy and the H...acceptor distance was developed: E (kJ mol-1) = 137.5 (5) - 45.7 (8) (H...A, Å). The hydrogen bond contribution to the crystal energy ranges from 62.815 to 627.6 kJ mol-1 citrate-1 and comprises ∼5 to 30% of the crystal energy. The general order of ionization of the three carboxylic acid groups of citric acid is: central, terminal, terminal, although there are a few exceptions. Comparisons of the refined and DFT-optimized structures indicate that crystal structures determined using powder diffraction data may not be as accurate as single-crystal structures.