supplementary materials


Acta Cryst. (2012). E68, i82    [ doi:10.1107/S1600536812043565 ]

Redetermination of (NH4)2HAsO4

M. Weil

Abstract top

In comparison with the original determination based on Weissenberg film data [Khan et al. (1970). Acta Cryst. B26, 1889-1892], the current redetermination of diammonium hydrogenarsenate(V) reveals all atoms with anisotropic displacement parameters and all H atoms localized. This allowed an unambiguous assignment of the hydrogen-bonding pattern, which is similar to that of the isotypic phosphate analogue (NH4)2HPO4. The structure of the title compound consists of slightly distorted AsO3(OH) and NH4 tetrahedra, linked into a three-dimensional structure by an extensive network of O-H...O and N-H...O hydrogen bonds.

Comment top

(NH4)2HAsO4 is a frequently used precursor material for preparation of arsenate(V) compounds, starting either from (aqueous) solutions or via ceramic routes. The crystal structure of (NH4)2HAsO4 has originally been determined by Khan et al. (1970) based on Weissenberg photographs. In the original study all atoms were refined with isotropic displacement parameters. Since H atoms could not be localized, the authors could make only assumptions with respect to the resulting hydrogen bonding pattern, deduced from N···O and O···O distances. These assumptions included three models: i) the NH4 ions exhibit rotatory oscillations; ii) the NH4 ions are in static disorder; iii) each of the two N atoms forms a bifurcated bond in addition to three normal hydrogen bonds (Khan et al., 1970). Somewhat later Khan et al. (1972) showed for the isotypic phosphate analogue (NH4)2HPO4 that dynamic or static disorder can be ruled out for the NH4 groups and that the ammonium tetrahedra form four classical hydrogen bonds to the PO3(OH) groups. The current redetermination of the structure of (NH4)2HAsO4 using modern CCD-based data was intended to shed some light on its hydrogen bonding pattern and to compare the results with the phosphate analogue.

The redetermination confirmed the basic features of the original study, however with the unambiguous localization of all H atoms and, as expected, with higher precision and accuracy. Like in (NH4)2HPO4, the ammonium groups show no static or dynamic disorder and four normal N—H···O hydrogen bonds are formed between the constituents. The largest difference between the two determinations pertains to the O···O distance of the O—H···O (O1···O4) hydrogen bond. In the original study this distance was determined as 2.669 (13) Å, whereas it is 2.613 (2) Å in this study. The latter matches very well with 2.615 (1) Å for the phosphate analogue for which all H atoms could be localized (Khan et al., 1972). In the latter study it was suggested that the difference between these O···O distances of the phosphate (2.615 (1) Å) and the arsenate (2.669 (13) Å) structure is a consequence of the different size of the P5+ and the As5+ ions. However, the current redetermination of (NH4)2HAsO4 shows that the influence of the different sizes for the phosphate (average P—O distance 1.54 Å) and the arsenate (average As—O distance 1.68 Å) tetrahedra can in fact be neglected.

Fig. 1 shows the structural set-up of the two different NH4 and the AsO3(OH) tetrahedra. All tetrahedra show slight angular distortions; the difference in As—O bond lengths for the three As—O bonds (average 1.674 Å) and the longer As—OH bond (1.7291 (15) Å) is normal. The ammonium cations and hydrogenarsenate anions are linked into a three-dimensional network by classical O—H···O and N—H···O hydrogen bonds, the numerical details of which are given in Table 1. The latter are very similar to those of the isotypic phosphate (NH4)2HPO4 (Khan et al., 1972).

For (NH4)2HPO4 another crystalline polymorph has been described, resulting from hydrolysis of the educt, viz. ammonium hexafluoridophosphate (Kunz et al., 2010). It would be interesting to know whether an arsenate polymorph isotypic with the phosphate analogue or another polymorph (NH4)2HAsO4 exist as well.

Related literature top

For the previous determination of (NH4)2HAsO4, see: Khan et al. (1970). The arsenate compound is isotypic with the phosphate analogue (NH4)2HPO4 (Khan et al., 1972), for which another modification with Z' = 2 has also recently been described (Kunz et al., 2010).

Experimental top

Crystals of the title compound were grown from an aqueous solution containing diluted arsenic acid (20%wt) mixed with a concentrated aqueous solution of ammonia in excess. The solution was kept in a desiccator with CaCl2 as drying agent. The first crystals, mostly with a plate-like form, appeared approximately after one week.

Refinement top

For better comparison, the same unit cell setting as in the previous determination of (NH4)2HAsO4 (Khan et al., 1970) was used. The current setting is not reduced and can be transformed to the reduced setting by application of the matrix (101, 010, 001). For refinement, the atomic coordinates of the As, O and N atoms (Khan et al., 1972) were used as starting parameters. All H atoms were clearly discernible from difference Fourier maps and were refined freely.

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. Projection of the crystal structure along [010]. AsO4 tetrahedra are red, NH4 tetrahedra are blue, O atoms are white, and H atoms are grey. Atoms are displayed with displacement ellipsoids at the 50% probability level. H···O hydrogen bonds are displayed with black lines.
Diammonium hydrogenarsenate(V) top
Crystal data top
(NH4)2HAsO4F(000) = 352
Mr = 176.01Dx = 2.026 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 2800 reflections
a = 11.3426 (4) Åθ = 3.6–30.0°
b = 6.8512 (3) ŵ = 5.82 mm1
c = 8.1130 (3) ÅT = 293 K
β = 113.784 (4)°Plate, colourless
V = 576.92 (4) Å30.14 × 0.12 × 0.02 mm
Z = 4
Data collection top
Bruker APEXII CCD
diffractometer
1674 independent reflections
Radiation source: fine-focus sealed tube1413 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.033
ω– and φ–scansθmax = 30.0°, θmin = 2.0°
Absorption correction: multi-scan
(SADABS; Bruker, 2009)
h = 1515
Tmin = 0.496, Tmax = 0.873k = 69
6318 measured reflectionsl = 1110
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.022Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.055All H-atom parameters refined
S = 1.04 w = 1/[σ2(Fo2) + (0.0304P)2]
where P = (Fo2 + 2Fc2)/3
1676 reflections(Δ/σ)max = 0.001
101 parametersΔρmax = 0.59 e Å3
0 restraintsΔρmin = 0.84 e Å3
Crystal data top
(NH4)2HAsO4V = 576.92 (4) Å3
Mr = 176.01Z = 4
Monoclinic, P21/cMo Kα radiation
a = 11.3426 (4) ŵ = 5.82 mm1
b = 6.8512 (3) ÅT = 293 K
c = 8.1130 (3) Å0.14 × 0.12 × 0.02 mm
β = 113.784 (4)°
Data collection top
Bruker APEXII CCD
diffractometer
1674 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2009)
1413 reflections with I > 2σ(I)
Tmin = 0.496, Tmax = 0.873Rint = 0.033
6318 measured reflectionsθmax = 30.0°
Refinement top
R[F2 > 2σ(F2)] = 0.022All H-atom parameters refined
wR(F2) = 0.055Δρmax = 0.59 e Å3
S = 1.04Δρmin = 0.84 e Å3
1676 reflectionsAbsolute structure: ?
101 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
As10.249593 (18)0.89261 (3)0.42786 (3)0.00648 (7)
O10.20879 (15)0.9789 (2)0.2115 (2)0.0126 (3)
O20.25894 (13)0.0977 (2)0.5442 (2)0.0098 (3)
O30.38858 (13)0.7692 (2)0.4982 (2)0.0114 (3)
O40.13017 (13)0.7484 (2)0.4289 (2)0.0098 (3)
N10.44933 (18)0.1231 (3)0.1532 (3)0.0111 (3)
N20.12140 (18)0.3798 (3)0.2643 (3)0.0100 (3)
H1N10.494 (3)0.131 (3)0.275 (4)0.017 (7)*
H2N10.422 (2)0.003 (4)0.114 (4)0.013 (6)*
H3N10.388 (3)0.209 (4)0.117 (4)0.022 (7)*
H4N10.509 (3)0.167 (4)0.111 (4)0.016 (7)*
H1N20.118 (3)0.491 (5)0.302 (4)0.020 (7)*
H2N20.169 (3)0.392 (3)0.201 (4)0.013 (7)*
H3N20.046 (3)0.336 (4)0.209 (4)0.022 (7)*
H4N20.164 (2)0.297 (4)0.358 (4)0.013 (6)*
H1O0.186 (3)0.907 (4)0.139 (5)0.032 (10)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
As10.00865 (10)0.00455 (11)0.00669 (11)0.00021 (7)0.00355 (7)0.00027 (8)
O10.0228 (7)0.0084 (8)0.0071 (7)0.0016 (6)0.0064 (6)0.0004 (6)
O20.0134 (7)0.0062 (7)0.0105 (7)0.0002 (5)0.0056 (6)0.0026 (6)
O30.0102 (6)0.0113 (7)0.0131 (7)0.0031 (5)0.0051 (6)0.0011 (6)
O40.0111 (6)0.0068 (7)0.0119 (7)0.0018 (5)0.0051 (5)0.0003 (6)
N10.0117 (8)0.0096 (9)0.0123 (9)0.0008 (7)0.0052 (7)0.0006 (7)
N20.0121 (8)0.0071 (9)0.0120 (9)0.0003 (7)0.0059 (7)0.0005 (7)
Geometric parameters (Å, º) top
As1—O2i1.6718 (14)N1—H2N10.89 (3)
As1—O31.6732 (14)N1—H3N10.87 (3)
As1—O41.6793 (14)N1—H4N10.92 (3)
As1—O11.7293 (15)N2—H1N20.83 (3)
O1—H1O0.73 (3)N2—H2N20.89 (3)
O2—As1ii1.6718 (14)N2—H3N20.85 (3)
N1—H1N10.91 (3)N2—H4N20.92 (3)
O2i—As1—O3113.29 (7)H1N1—N1—H4N1103 (2)
O2i—As1—O4111.04 (7)H2N1—N1—H4N1112 (2)
O3—As1—O4110.62 (7)H3N1—N1—H4N1105 (2)
O2i—As1—O1102.46 (7)H1N2—N2—H2N2105 (2)
O3—As1—O1110.40 (7)H1N2—N2—H3N2110 (3)
O4—As1—O1108.67 (7)H2N2—N2—H3N2116 (3)
As1—O1—H1O117 (3)H1N2—N2—H4N2111 (3)
H1N1—N1—H2N1114 (2)H2N2—N2—H4N2107 (2)
H1N1—N1—H3N1110 (2)H3N2—N2—H4N2108 (2)
H2N1—N1—H3N1113 (3)
Symmetry codes: (i) x, y+1, z; (ii) x, y1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H3N1···O2iii0.87 (3)1.88 (3)2.750 (2)178 (3)
N1—H1N1···O3iv0.91 (3)1.91 (3)2.780 (3)158 (2)
N1—H2N1···O3iii0.89 (3)2.06 (3)2.930 (3)167 (2)
N1—H4N1···O3v0.92 (3)1.86 (3)2.777 (2)173 (3)
N2—H4N2···O20.92 (3)2.00 (3)2.910 (2)174 (2)
N2—H2N2···O2iii0.89 (3)1.93 (3)2.809 (2)174 (2)
N2—H1N2···O40.83 (3)2.02 (3)2.840 (2)171 (3)
N2—H3N2···O4vi0.85 (3)1.95 (3)2.793 (2)176 (3)
O1—H1O···O4vii0.73 (3)1.89 (3)2.613 (2)171 (4)
Symmetry codes: (iii) x, y+1/2, z1/2; (iv) x+1, y+1, z+1; (v) x+1, y1/2, z+1/2; (vi) x, y1/2, z+1/2; (vii) x, y+3/2, z1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H3N1···O2i0.87 (3)1.88 (3)2.750 (2)178 (3)
N1—H1N1···O3ii0.91 (3)1.91 (3)2.780 (3)158 (2)
N1—H2N1···O3i0.89 (3)2.06 (3)2.930 (3)167 (2)
N1—H4N1···O3iii0.92 (3)1.86 (3)2.777 (2)173 (3)
N2—H4N2···O20.92 (3)2.00 (3)2.910 (2)174 (2)
N2—H2N2···O2i0.89 (3)1.93 (3)2.809 (2)174 (2)
N2—H1N2···O40.83 (3)2.02 (3)2.840 (2)171 (3)
N2—H3N2···O4iv0.85 (3)1.95 (3)2.793 (2)176 (3)
O1—H1O···O4v0.73 (3)1.89 (3)2.613 (2)171 (4)
Symmetry codes: (i) x, y+1/2, z1/2; (ii) x+1, y+1, z+1; (iii) x+1, y1/2, z+1/2; (iv) x, y1/2, z+1/2; (v) x, y+3/2, z1/2.
Acknowledgements top

The X-ray centre of the Vienna University of Technology is acknowledged for providing access to the single-crystal diffractometer.

references
References top

Bruker (2009). SMART and SAINT. Siemens Analytical X-ray Instruments Inc., Madison, Wisconsin, USA.

Dowty, E. (2006). ATOMS. Shape Software, Kingsport, Tennessee, USA.

Khan, A. A., Roux, J. P. & James, W. J. (1972). Acta Cryst. B28, 2065–2069.

Khan, A. A., Straumanis, E. & James, W. J. (1970). Acta Cryst. B26, 1889–1892.

Kunz, P. C., Wetzel, C. & Spingler, B. (2010). Acta Cryst. E66, i26–i27.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925.