Re-refinement of sodium ammonium sulfate dihydrate at 170 K

The crystal structure of sodium ammonium sulfate dihydrate has been redetermined at 170 K on the basis of single-crystal X-ray data.


data reports
As shown in Fig. 1, the sodium cation is hexa-coordinated with a considerably distorted octahedral coordination sphere formed by four water molecules in the equatorial plane and two sulfate O atoms in the apical positions. Selected bond lengths and angles are listed in Table 1. Each of the ligands links two sodium cations in a -coordination mode, resulting in chains along the [100] direction with the Na cations located near to a 2 1 screw axis. Na1Á Á ÁNa1 i and Na1Á Á ÁNa1 ii are separated by 3.1317 (2) and 3.1316 (2) Å , respectively [symmetry codes: (i) x À 1 2 , Ày + 3 2 , Àz + 1; (ii) x + 1 2 , Ày + 3 2 , Àz + 1]. The chains can be described as consisting of NaO 6 octahedra sharing one face (Fig. 2) defined by atoms O1, O2 and O4. The sulfate anion exhibits the typical tetrahedral shape with an r.m.s. deviation from exact T d symmetry of only 0.0092 Å , as calculated with MOLSYM in PLATON (Spek, 2020). In the chains, the SO 4 tetrahedra have one O atom in common with a pair of NaO 6 octahedra. Chain motifs are encountered in the structures of many other sulfates (Gorogotskaya & Bokii, 1973).
The crystal structure features hydrogen bonds of the O-HÁ Á ÁO and N-HÁ Á ÁO type ( Table 2). The water molecules form medium-strong and nearly linear intra-and interchain hydrogen bonds to sulfate oxygen atoms. The interstices between the [Na(-SO 4 )(-H 2 O) 2 ] n À chains accommodate the ammonium cations, which form hydrogen bonds to sulfate oxygen atoms, thus establishing a three-dimensional network. The positions of the ammonium hydrogen atoms determined in the current study appear to be more accurate than those in the room-temperature structure reported by Arzt & Glazer (1994). Note that details of hydrogen bonding were not discussed in the latter report; based on the reported structure data (Arzt & Glazer, 1994), N-H distances range between 0.73 and 0.99 Å . Corazza et al. (1967) did not refine hydrogen- Table 1 Selected geometric parameters (Å , ).  (3) O3-S1-O6 109.31 (9) O2 i -Na1-O4 i 81.28 (5) Symmetry code: (i) x À 1 2 ; Ày þ 3 2 ; Àz þ 1.

Figure 1
Section of the crystal structure of SASD, viewed approximately along the c-axis direction towards the origin. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are represented by small spheres of arbitrary radius. Thick dashed lines represent hydrogen bonds.
[Symmetry codes: (i) x + 1 2 , Ày + 3 2 , Àz + 1; (ii) x À 1 2 , Ày + 3 2 , Àz + 1.] atom parameters in the original room-temperature structure determination but included their presumed positions in the structure-factor calculation for the final refinement of the nonhydrogen atoms. In the current study, semi-free refinement applying only similarity restraints on the 1,2-distances involving hydrogen atoms resulted in reasonable hydrogen-atom parameters and a sensible hydrogen-bonding scheme. The absolute structure of the crystal was established by anomalous-dispersion effects in the diffraction data, as indicated by a Flack x parameter close to zero with a reasonably small standard uncertainty (Table 3). The Hooft y parameter (Hooft et al., 2008), as calculated with PLATON, is 0.07 (2). Interestingly, the structure exhibits chirality opposite to the previously reported room temperature structures (Corazza et al., 1967;Arzt & Glazer, 1994).

Synthesis and crystallization
A crystal of the title compound suitable for single-crystal X-ray analysis was obtained unintentionally from a solution in an acetonitrile/water mixture after synthesis of an organic compound. Ammonium ions and sodium sulfate in this mixture originated from an employed reagent and the drying agent, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. diffractometer Radiation source: sealed X-ray tube, Incoatec Iµs Detector resolution: 6.67 pixels mm -1 ω scans Absorption correction: multi-scan [MULABS (Blessing, 1995) in PLATON (Spek, 2020)] Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

data-2
IUCrData (2020). 5, x201275 Refinement. Hydrogen-atom positions were located in a difference Fourier map, and their parameters were refined with standard similarity restraints on 1,2-distances for O-H and and N-H bonds (with a standard uncertainty of 0.02 Å). U iso (H) values were refined freely.