Sodium potassium hydrogen citrate, NaKHC6H5O7

The crystal structure of sodium potassium hydrogen citrate has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The most prominent feature of the structure is the chain along [111] of very short, very strong hydrogen bonds; the O⋯O distances are 2.414 and 2.400 Å.

The crystal structure of sodium potassium hydrogen citrate has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional theory techniques. The Na + cation is six-coordinate, with a bond-valence sum of 1.17. The K + cation is also six-coordinate, with a bondvalence sum of 1.08. The distorted [NaO 6 ] octahedra share edges, forming chains along the a axis. The likewise distorted [KO 6 ] octahedra share edges with the [NaO 6 ] octahedra on either side of the chain, and share corners with other [KO 6 ] octahedra, resulting in triple chains along the a axis. The most prominent feature of the structure is the chain along [111] of very short, very strong hydrogen bonds; the OÁ Á ÁO distances are 2.414 and 2.400 Å . The Mulliken overlap populations in these hydrogen bonds are 0.138 and 0.142 e, which correspond to hydrogen-bond energies of 20.3 and 20.6 kcal mol À1 .

Chemical context
We have carried out a systematic study of the crystal structures of Group 1 (alkali metal) citrate salts to understand the anion's conformational flexibility, ionization, coordination tendencies, and hydrogen bonding. Most of the new structures were solved using powder diffraction data (laboratory and/or synchrotron), but single crystals were used where available. The general trends and conclusions about the 16 new compounds and 12 previously characterized structures are being reported separately (Rammohan & Kaduk, 2015). The initial study considered salts containing one type of Group 1 cation. The title compound ( Fig. 1) represents the beginning of an extension of the study to salts containing more than one alkali metal cation.

Structural commentary
The root-mean-square deviation of the non-hydrogen atoms in the refined and optimized structures is only 0.088 Å . A comparison of the refined and optimized structures is given in Fig. 2. The excellent agreement between the structures is strong evidence that the experimental structure is correct (van de Streek & Neumann, 2014). This discussion uses the DFT-optimized structure. Most of the bond lengths, and all of the bond angles and torsion angles fall within the normal ranges indicated by a Mercury Mogul Geometry Check (Macrae et al., 2008). Only the C6-O15 [observed = 1.281 (4), optimized = 1.268, normal = 1.20 (2) Å , Z-score = 2.7] and C1-O11 [observed = 1.260 (4), optimized = 1.318, normal = 1.330 (3) Å , Z-score = 3.9] bonds are flagged as unusual. The citrate anion occurs in the trans,trans-conformation (about C2-C3 and C3-C4), which is one of the two low-energy conformations of an isolated citrate. The central carboxylate group and the hydroxyl group occur in the normal planar arrangement. The citrate chelates to Na19 through the terminal carboxylate oxygen O11 and the central carboxylate oxygen O16. The Na + cation is six-coordinate, with a bondvalence sum of 1.17. The K + cation is also six-coordinate, with a bond-valence sum of 1.08. Both cations are thus slightly crowded. The metal-oxygen bonding is ionic, based on the Mulliken overlap populations.
The Bravais-Friedel-Donnay-Harker (Bravais, 1866;Friedel, 1907;Donnay & Harker, 1937) morphology suggests that we might expect platy morphology for sodium potassium hydrogen citrate, with {001} as the principal faces. A 4th-order spherical harmonic preferred orientation model was included in the refinement; the texture index was only 1.013, indicating that preferred orientation was not significant in this rotated flat-plate specimen. The powder pattern is included in the Powder Diffraction File as entry 00-065-1255.

Supramolecular features
In the crystal structure (Fig. 3), distorted [NaO 6 ] octahedra share edges to form chains along the a axis. The likewise distorted [KO 6 ] octahedra share edges with the [NaO 6 ] octahedra on either side of the chain, and share corners with other [KO 6 ] octahedra, resulting in triple chains along the a axis. The most prominent feature of the structure is the chain along [111] of very short, very strong O-HÁ Á ÁO hydrogen bonds (Table 1); the refined OÁ Á ÁO distances are 2.385 (15) and 2.346 (14) Å , and the optimized OÁ Á ÁO distances are 2.414 and 2.400 Å . The Mulliken overlap populations in these hydrogen bonds are 0.138 and 0.142 e, which correspond to hydrogen bond energies of 20.3 and 20.6 kcal mol À1 . The distances indicate that these are among the shortest O-HÁ Á ÁO hydrogen bonds ever reported. H18 forms bifurcated hydrogen bonds; one is intramolecular to O15, and the other intermolecular to O11.

Database survey
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2015). A reduced cell search in the Cambridge Structural Database (Groom & Allen, 2014) (increasing the default tolerance from 1.5 to 2.0%, to account for the differences between ambient and low-temperature lattice parameters) yielded 35 hits, but limiting the chemistry to C, H, O, Na, and K only resulted in no hits. The powder pattern matched no entry in the Powder Diffraction File (ICDD, 2015). Comparison of the refined and optimized structures of sodium potassium hydrogen citrate. The refined structure is in red, and the DFT-optimized structure is in blue.

Figure 3
Crystal structure of NaKHC 6 H 5 O 7 , viewed approximately down the a axis.

Synthesis and crystallization
2.0832 g (10.0 mmol) H 3 C 6 H 5 O 7 (H 2 O) was dissolved in 10 mL deionized water. 0.5282 g Na 2 CO 3 (10.0 mmol Na, Sigma-Aldrich) and 0.6913 g K 2 CO 3 (10.0 mmol, Sigma-Aldrich) were added to the citric acid solution slowly with stirring. The resulting clear colourless solution was evaporated to dryness in a 393 K oven.

Refinement details
The powder pattern (Fig. 4) was indexed using Jade 9.5 (MDI, 2012). Pseudovoigt profile coefficients were as parameterized in Thompson et al. (1987) and the asymmetry correction of Finger et al. (1994) was applied and microstrain broadening by Stephens (1999). The structure was solved with FOX (Favre-Nicolin & Č erný, 2002) using a citrate, Na, and K as fragments. Two of the 10 solutions yielded much lower cost functions than the others. Centrosymmetric pairs of close OÁ Á ÁO contacts made it clear that H21 and H22 were located on centers of symmetry between these oxygen atoms, forming very strong hydrogen bonds. The hydrogen atoms were included at fixed positions, which were re-calculated during the course of the refinement. Crystal data, data collection and structure refinement details are summarized in Table 2. The U iso of C2, C3, and C4 were constrained to be equal, and those of H7, H8, H9, and H10 were constrained to be 1.3Â that of these carbon atoms. The U iso of C1, C5, C6, and the oxygen atoms were constrained to be equal, and that of H18 was constrained to be 1.3Â this value. The U iso of H21 and H22 were fixed.

Density functional geometry optimization
A density functional geometry optimization (fixed experimental unit cell) was carried out using CRYSTAL09 (Dovesi et al., 2005). The basis sets for the H, C, and O atoms were those of Gatti et al. (1994), the basis sets for Na and K were those of Dovesi et al. (1991). The calculation used 8 k-points and the B3LYP functional, and took about 42 h on a 2.8 GHz PC. The observed U iso were assigned to the refined values.

Figure 4
Rietveld plot for the refinement of NaKHC 6 H 5 O 7 . The red crosses represent the observed data points, and the green line is the calculated pattern. The magenta curve is the difference pattern, plotted at the same scale as the other patterns. The vertical scale has been multiplied by a factor of 6 for 2 > 41.0 , and by a factor of 20 for 2 > 63.0 . The row of black tick marks indicates the reflection positions for the phase. with 27 terms Pseudovoigt profile coefficients as parameterized in Thompson et al. (1987). Asymmetry correction of Finger et al. (1994). Microstrain broadening by Stephens (1999). Only H-atom displacement parameters refined Weighting scheme based on measured s.u.'s (Δ/σ) max = 0.04