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Calcium tartrate esahydrate, CaC4H4O6.6H2O: A structural and spectroscopic study Article in Acta Crystallographica Section B: Structural Science · February 2015 DOI: 10.1107/S2052520614027516
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research papers Acta Crystallographica Section B
Structural Science, Crystal Engineering and Materials
Calcium tartrate esahydrate, CaC4H4O66H2O: a structural and spectroscopic study
ISSN 2052-5206
Gennaro Ventruti,a* Fernando Scordari,a Fabio Bellatreccia,b Giancarlo Della Venturab and Armida Sodob a
Dipartimento di Scienze della Terra e Geoambientali, Universita` di Bari, Via Orabona, 4, 70125 Bari, Italy, and bDipartimento di Scienze, Universita` di Roma Tre, Largo San Leonardo Murialdo 1, 00146 Roma, Italy
Correspondence e-mail:
[email protected]
The crystal structure of calcium tartrate esahydrate, CaC4H4O66H2O, has been solved by the charge-flipping method from single-crystal X-ray diffraction data and refined to R = 0.021, based on 1700 unique observed diffractions. Salient crystallographic data are: a = 7.7390 (1), b = ˚ , Z = 2, and space group P21212. 12.8030 (2), c = 5.8290 (1) A During the refinement step it was possible to locate all H atoms by difference Fourier synthesis. The tartrate molecule has a ()-gauche conformation and is coordinated to two calcium ions to form infinite chains along the a axis which alternate Ca polyhedra with tartrate molecules. The chains are interlinked by a three-dimensional network of hydrogen bonds from four water molecules surrounding the Ca ion, reinforced by hydrogen bonds from one interstitial water molecule. Micro-Raman and FT–IR spectroscopic data are provided.
Received 8 October 2014 Accepted 17 December 2014
1. Introduction
# 2015 International Union of Crystallography
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doi:10.1107/S2052520614027516
During the study of some rock specimens containing galena (PbS), baryte (BaSO4), siderite (FeCO3), calcite (CaCO3) and iron oxides, well formed, transparent, light-yellow dipyramidal crystals with a perfect basal cleavage were observed (Fig. 1a). As these crystals did not match the characteristics of any of the minerals known to be present in the samples, we decided to perform some preliminary analyses to identify this phase. SEM–EDS (scanning electron microscopy–energy dispersive spectroscopy) showed only Ca and C (Fig. 1b), and the X-ray powder pattern did not match any reported in the ICCD database for minerals and organic compounds. Considering, however, that the studied rocks containing these crystals (provided to us by a mineral collector, Mr Simone Ferrero) were from very different occurrences and from very different geological settings, it was immediately clear that the origin of these crystals was not natural, but very likely connected with the method used for cleaning the specimens. Indeed, as later explained by Mr Ferrero, all samples had been treated with HCl to remove calcite (CaCO3), and with tartaric acid (C4H6O6) to reduce iron oxide coatings. This treatment produced a solution containing Ca2+ and C4H4O62 and the consequent incidental crystallization of what resulted to be a new organic compound, i.e. calcium tartrate esahydrate, with composition CaC4H4O66H2O. The crystal structure of this compound was solved by combining single-crystal X-ray diffraction (SCXRD), FT–IR and micro-Raman spectroscopies. Acta Cryst. (2015). B71, 68–73
research papers 0.5 scan width and exposure times from 10 to 30 s per frame. Data were reduced using the Bruker program SAINT (Bruker, Crystal data 2008), and were corrected for Lorentz, polarization and Chemical formula C4H16CaO12 background effects. The final unit-cell parameters were Mr 296.25 obtained from the xyz centroids of the measured reflections Crystal system, space group Orthorhombic, P21212 Temperature (K) 293 after integration and are reported in Table 1. A semi-empirical ˚) a, b, c (A 7.7390 (1), 12.8030 (2), 5.8290 (1) absorption correction was applied using the SADABS ˚ 3) V (A 577.55 (2) program (Sheldrick, 2005). Subsequent analysis of the intenZ 2 Radiation type Mo K sity data, by XPREP (Sheldrick, 2003), proved the ortho (mm1) 0.60 rhombic symmetry, allowing the assignment of the unique Crystal size (mm) 0.22 0.20 0.07 space group P21212. No violations of the systematic absences Data collection of space group P21212 were observed. The structure was Diffractometer Bruker APEX II solved by the charge-flipping method using the program Absorption correction None SUPERFLIP (Palatinus & Chapuis, 2007). The structure No. of measured, independent and 8303, 1759, 1700 observed [I > 2(I)] reflections solution confirmed the space group P21212. The atomic posiRint 0.025 tions, as obtained by SUPERFLIP, were used for the subse1 ˚ ) 0.715 (sin /)max (A quent refinement utilizing SHELXL software (Sheldrick, Refinement 2008, 2014). The H atoms were located from the difference 0.021, 0.075, 0.68 R[F 2 > 2(F 2)], wR(F 2), S Fourier maps. The isotropic atomic displacement parameters No. of reflections 1759 of H atoms were evaluated as 1.2Ueq of the parent atom. All No. of parameters 102 No. of restraints 8 atoms except hydrogen were refined anisotropically, and the H-atom treatment Only H-atom coordinates refined final R index converged to 0.021. A difference-Fourier map 3 ˚ ) max, min (e A 0.29, 0.19 computed at this stage was featureless. The full structure Absolute structure Flack x determined using 679 quotients [(I+) (I)]/[(I+)+(I)] solution of this compound is consistent with the following (Parsons & Flack, 2004) overall composition Ca2+C4H4O626H2O. The main crystalAbsolute structure parameter 0.173 (15) lographic features concerning the study compound and the Computer programs: SHELXL (Sheldrick, 2014). working conditions are given in Table 1. The X-ray powder diffraction pattern of CTE was collected with a Scintag X1 powder diffractometer using Cu K ( = ˚ ) radiation, fixed divergence slits and a Peltier-cooled 1.5418 A 2. Experimental Si(Li) detector (resolution < 200 eV). A divergent slit width of Diffraction data on calcium tartrate esahydrate (hereafter 2 mm and a scatter slit width of 4 mm were employed for the CTE) were collected with graphite-monochromated Mo K beam source; a receiving slit width of 0.5 mm and scatter-slit ˚ ), using a Bruker APEX II diffractradiation ( = 0.71073 A width of 0.2 mm were used for the detector. Data were ometer equipped with a 2 K CCD detector. Operating collected in step-scan mode: 2–70 2 range, step size 0.05 2, conditions were: 50 kV, 30 mA. Three sets of 12 frames were counting time 3 s per step. The pattern was indexed on the acquired with 0.5 ’ rotation and the results were used for the basis of the powder spectrum calculated from the crystal initial unit-cell determinations. The intensities of reflections structure. The unit-cell parameters, refined with the leastwere recorded by a combination of ! and ’ rotation sets, squares program UNITCELL (Holland & Redfern, 1997), are ˚, V = optimized by the APEX program suite (Bruker, 2008), with a a = 7.7342 (6), b = 12.8086 (10), c = 5.8341 (4) A 3 ˚ 577.95 (6) A . The slight differences in the cell parameters determined by powder versus singlecrystal X-ray diffraction are absolutely compatible with differences typically observed when comparing data from these two techniques (Pan et al., 2012). The mid-IR (MIR) spectrum of CTE was collected on a Nicolet iS50 FT–IR spectrophotometer equipped with a KBr beam-splitter and a DTGS detector. The spectra were acquired between 4000 and 400 cm1 with nominal resolution Figure 1 4 cm1 averaging 64 scans. The (a) SEM image of a CTE crystal with the (b) SEM-EDS spectrum acquired with a FEI SEM XL30 sample was prepared as KBr pellets electron microscope equipped with an EDAX Si(Li) detector. Table 1
Experimental details.
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research papers by mixing 0.8 mg of powdered mineral with 150 mg of KBr. The unpolarized single-crystal near-IR (NIR) spectrum was collected at LNF-INFN (Frascati, Rome, Italy), with a Bruker Hyperion 3000 microscope equipped with a liquid nitrogencooled MCT (mercury cadmium telluride) detector. The nominal resolution was 4 cm1 and 128 scans were averaged for both background and sample. Raman measurements were performed by using a Labram Micro-Raman spectrometer by Horiba, equipped with a He–Ne laser source at 632.8 nm (nominal output power 18 mW). The illumination and collecting optics of the system consists of a microscope in confocal configuration. The system achieves the high contrast required for the rejection of the elastically scattered component by the use of an edge filter. The backscattered light is dispersed by a 1800 line per mm grating and the Raman signal is detected by a Peltier cooled (203 K) 1024 256 pixel CCD detector. Nominal spectral resolution was 1 cm1. Spectral acquisitions (3 accumulations, 30 s each, in the range from 100 to 3800 cm1) were performed with a long distance 20 objective (N.A. = 0.35).
3. Results and discussion 3.1. Crystal structure
The unit cell of CTE showing the chain arrangement of Ca polyhedra is presented in Fig. 2, whereas a perspective view of the tartrate anion is shown in Fig. 3. The calcium ion is located on a twofold axis at the centre of a distorted trigonal dodecahedron built by four water molecules and four O atoms from two tartrate anions. The observed Ca—O bond distances are ˚ with a mean distance between 2.386 (1) and 2.542 (1) A ˚ [2.453 (1) A] consistent with the values reported in other Ca-
based compounds (MacLennan & Beevers, 1955, 1956; Tazzoli & Domeneghetti, 1980; Ambady, 1968; de Vries & Kroon, 1984; Hawthorne et al., 1982). Each calcium ion links two symmetrically related tartrate molecules to build up infinite chains parallel to the a axis. An additional water molecule, O6, participates only in an interstitial hydrogen-bonding network to reinforce the net and stabilize the structure. The tartrate molecule consists of two halves symmetrically related by a twofold axis (Fig. 3). Each asymmetric part is composed of a planar carboxyl group, a four-coordinated carbon and a hydroxyl group. The two halves are oriented such that the carbon backbone (C1—C2—C20 —C10 ) adopts a ()-gauche conformation (G) which is slightly non-planar (torsion angle C1—C2—C20 —C10 = 41 ). All the bond distances observed are consistent with those reported for various crystals with tartaric ions (Ambady, 1968; de Vries & Kroon, 1984; Hawthorne et al., 1982, Pe´rez, 1977). In the structure each infinite chain develops along the a screw axis (21) passing through calcium ions. Parallel chains are interlinked by a three-dimensional network of hydrogen bonds (Fig. 4). Both the water molecules coordinated to the calcium ion (O1 and O5) and the interstitial water molecule (O6) act as hydrogen donors. From the combined analysis of suitable interatomic O O distances and O—H O angles between the water molecules acting as hydrogen donors, it has been possible to deduce the possible neighbour acceptors leading to a reliable hydrogen-bonding system as illustrated in Fig. 4. Hydrogen bonding O1—H2 to an oxygen O2 provides the only linkage between adjacent chains in the (010) plane, and thus a good cleavage plane ensues. Hydrogen bondings, O6—H5 and O6—H6, from the interstitial water molecule towards O3 and O1, respectively, and O5—H3 and O5—H4 towards O6 and O3 provide the interlink between chains along the b direction.
Figure 2
Figure 3
The unit cell of CTE showing the chain arrangement of Ca polyhedra. Atom-numbering scheme of the carbon backbone of the tartrate anion and the interstitial water are also outlined.
Perspective view of the tartrate anion showing the atom-numbering scheme. Atoms symmetrically related by the twofold axis have primed labels. Displacement ellipsoids are drawn at the 50% probability level.
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research papers Table 2 IR and Raman frequencies (cm1) of calcium tartrate esahydrate, CaC4H4O66H2O. v = very; s = strong; m = medium; w = weak; sh = shoulder; b = broad. Raman
IR
– – – – 3556 3464 3406 3252 2978 2884 – – 1618 1566
– – – – vvw vvw wb vvwb ms ms – – vvwb vvwb
1436 1398 1377 – – 1311 1279
m m m – – wb mw
1232
mw
5226 5048 4809 4650 3585 3550 3422 3277 2988 2927 2895 2596 1647 1589 1482 1438 1405 1386 1335 1327 1315 1283 1268 1245 1239
vvsb vvs vvsb vvsb sh sb vvsb vsb vvw vvw vvw wb msh vvs vw m ms s ms ms mwsh w vw vw vw
Assignment
Raman
H2O (3 + 2)
1141 – 1057 1011 951 914 881 843 807 – 707 – 602 531 517 452 380 277 244 167 139 110 – – –
OH (3 + 2) OH stretching/H2O (3, 22)
CH stretching
Combinations 2 (H2O) CO2 antisymmetric stretching CO2 symmetric stretching OH in-plane bending CH bending
Band assignment from Kaneko et al. (1984). CO2 refers to carboxylate group O
IR mw – mw mw vwb mw s m s – vwb – bmw w wb vvwb vvwb vvwb vvw s vs mw – – –
1148 1070 1062 1011 963 923 884 846 816 775 713 628 606 535 478 – – – – – – – – – –
Assignment ms msh ms mw mw vw vw vw m w sm msh w sm msh – – – – – – – – – –
CO stretching
CC stretching CC stretching CO2 deformation (H2O) libration CO2 deformation
Skeletal deformation (H2O) libration CO2 twisting CC torsion – – –
C—O.
3.2. IR and Raman spectroscopy
The MIR spectrum of CTE from 400 to 4000 cm1 (Fig. 5a) shows several sharp bands and some very broad, multicomponent absorptions. For the sake of simplicity, it can be divided into two main regions, from 4000 to 2000 cm1 and from 2000 to 400 cm1, respectively. Bands positions, estimated intensities and assignment, essentially based on previous literature data, are reported in Table 2. The higherfrequency region is characterized by a broad multi-component band extending from 3000 to 3700 cm1 which is due to the overlapping of H2O (3, 1, 22) and OH stretching modes. In
Figure 4
Figure 5
Hydrogen-bonding scheme of calcium tartrate esahydrate.
(a) MIR spectrum of CTE; (b) NIR region from 4400 to 6000 cm1.
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research papers Table 3 Hydrogen-bonding geometry from FT–IR data (Libowitzky, 1999). Frequencies (cm1)
˚) d(O O) (A
3585 3550 3422 3277
3.24 3.00 2.81 2.73
d(O O) = 0.1321ln[(3592 )/304 109].
the lower-frequency region several sharp absorptions due to the skeletal vibrations are present. In detail, from 400 to 930 cm1 there is a rather broad absorption, with the most intense components at 535, 713 and 816 cm1, while from 930 to 1800 cm1 several well resolved and intense peaks (Table 2) occur. From a general point of view, the MIR spectrum of CTE is very similar to that reported for calcium tartrate tetrahydrate (CaC4H4O64H2O, hereafter CTT) by Kaneko et al. (1984), with two notable differences: (i) the spectrum of CTE shows a very intense H2O 2 band at 1647 cm1 which is missing in the spectrum of CTT, and (ii) the spectrum of CTT shows an intense band at 1365 cm1, assigned to a CH stretching mode (Kaneko et al., 1984), which is absent in the CTE pattern. To differentiate between the H2O and OH absorptions we collected a single-crystal spectrum in the NIR (4000– 7000 cm1) range where the combination (3 + 2) modes occur at different wavenumbers (e.g. Della Ventura et al., 2009). The resulting spectrum (Fig. 5b) is relatively complex and is the result of several overlapping bands. Two components can be recognized at 4809 and 4650 cm1, respectively; these can be assigned to the combination mode of the OH groups and considering that the bending occurs at 1386 cm1 (see Table 4 in Kaneko et al., 1984, and Table 2, this work) their frequency suggests that the corresponding stretching should be found around 3400 cm1. The lower frequency band in the broad absorption observed in the MIR spectrum can thus be related to OH. The two bands at 5226 and 5048 cm1 can be assigned to the combinations of the H2O molecules (e.g. Della Ventura et al., 2007; Gatta et al., 2007); the same type of argument discussed above suggests that the higher frequency bands in the 3500–3600 cm1 MIR range can be related to H2O. The Raman spectrum of CTE (Fig. 6, Table 2) shows many strong and sharp bands (e.g. 2978, 2884, 881, 807 and 167 cm1), which can be assigned to the internal vibrations of the tartrate ions. There are also some weaker and broader bands (e.g. 3406 and 1618 cm1), which can be assigned to the vibration of the water of crystallization. The CTE Raman pattern, except small differences, is similar to the Raman spectra reported in Kaneko et al. (1984) for metal tartrates and particularly for the CTT compound.
hydrate (CTT), both in its orthorhombic form (I) (Hawthorne et al., 1982) and the triclinic form (II) (Le Bail et al., 2009), and the calcium tartrate trihydrate (de Vries & Kroon, 1984), leads the following conclusions: (i) The calcium ion exhibits eightfold coordination in both esahydrate and tetrahydrate compounds, while it is sevenfold coordinated in calcium tartrate trihydrate. (ii) The tartrate anion assumes in the compound described here a ()G conformation, while in calcium tartrate tetrahydrate, both form (I) and (II) of CTT show a trans (T) conformation with the carbon backbone exhibiting a zigzag configuration. (iii) All the six O atoms of the tartrate anion in form (I) are shared with four distinct calcium ions, whereas five O atoms of the trihydrate compound are shared with three distinct calcium ions. Four O atoms are shared with two different calcium ions both in form (II) and in the compound investigated here. (iv) The structure of CTT form (I) assumes a denser packing (D = 1.84 g cm3) as a consequence of the stronger linkage between tartrate molecules and calcium ions thus giving rise to a three-dimensional framework. The structure of calcium tartrate esahydrate is less dense (D = 1.70 g cm3) consisting of parallel chains cross-linked by hydrogen bonds from Ca-coordinated waters and interstitial water molecules. (v) The H2O and the OH stretching bands of the tartrate ions occur, in the MIR and Raman spectra, at frequencies very close to those reported by Kaneko et al. (1984) and Rajagopal et al. (1989) for other metal tartrates. Finally, the range of H2O frequencies calculated using the correlations of Libowitzky (1999) (Table 3) are in excellent agreement with the range of oxygen donor–oxygen acceptor [d(O O)] distances observed by X-ray refinement.
4. Concluding remarks Comparison of the crystal structures of calcium tartrate esahydrate studied here with those of calcium tartrate tetra-
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Figure 6 Raman spectrum of CTE. Acta Cryst. (2015). B71, 68–73
research papers Thanks are due to Mr Simone Ferrero (Borgo San Dalmazzo, CN, Italy) for providing the studied specimens. The authors thank two anonymous referees for helpful suggestions and comments.
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