Crystal structures of alkali metal (Group 1) citrate salts

Abstract

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−1and 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.