Abstract
The crystal structure of 'aquabis-(di-hydrogen citrato)calcium hydrate', better formulated as aqua-(citric acid)(hydrogen citrato)calcium monohydrate, (I), has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. The CaO8 coordination polyhedra are isolated, but occur in layers parallel to the ab plane. Both the Rietveld-refined and DFT-optimized structures indicate that one citrate is doubly ionized and that the other is citric acid. All of the active hydrogen atoms participate in strong (11-16 kcal mol-1) hydrogen bonds. Hydrogen atoms were added to the existing calcium hydrogen citrate trihydrate structure [Sheldrick (1974 ▸). Acta Cryst. B30, 2056-2057; CSD refcode: CAHCIT], (II), and a DFT calculation was carried out to assess the hydrogen bonding and compare it to this optimized structure.
Highlights
A systematic study of the crystal structures of Group 1 citrate salts has been reported in Rammohan & Kaduk (2018)
The crystal structures of anhydrous calcium citrate, a new polymorph of calcium citrate tetrahydrate, and calcium citrate hexahydrate have been reported recently (Kaduk, 2018)
The carboxylic acid group O7— H40 in the hydrogen citrate anion acts as a donor to the coordinating water molecule O39
Summary
A systematic study of the crystal structures of Group 1 (alkali metal) citrate salts has been reported in Rammohan & Kaduk (2018). Calcium citrate (as the tetrahydrate) is a common dietary supplement. Reported calcium citrate structures include calcium hydrogen citrate trihydrate (Sheldrick, 1974; CSD refcode: CAHCIT) and calcium citrate tetrahydrate (Herdtweck et al, 2011; ISEQIH). The ISEQIH structure was derived from a hydrothermally synthesized pseudomerohedrally twinned crystal at 123 K, and in this study a Rietveld refinement using roomtemperature powder data was reported. The crystal structures of anhydrous calcium citrate, a new (and commercially relevant, for it is the phase that occurs in dietary supplements) polymorph of calcium citrate tetrahydrate, and calcium citrate hexahydrate have been reported recently (Kaduk, 2018)
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