The ammonium chromium(III) alum NH₄Cr(SO₄)₂·12H₂O

The title compound, ammonium chromium(III) bis­(sulfate) dodecahydrate, is composed of discrete NH₄⁺ cations, [Cr(H₂O)₆]³⁺ cations, SO₄^2- anions and water mol­ecules. The Cr and N atoms are located on special positions of site symmetry ; the S atom and one of the O atoms bonded to it are located on a threefold rotation axis. The crystal packing is stabilized by several hydrogen bonds.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](S-O) = 0.001 Å
• R factor = 0.019
• wR factor = 0.051
• Data-to-parameter ratio = 12.3

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT242_ALERT_2_C Check Low    U(eq) as Compared to Neighbors ....         S1
PLAT417_ALERT_2_C Short Inter D-H..H-D       H3B    ..  H4A     ..       2.10 Ang.

Alert level G
FORMU01_ALERT_1_G  There is a discrepancy between the atom counts in the
            _chemical_formula_sum and _chemical_formula_moiety. This is
            usually due to the moiety formula being in the wrong format.
            Atom count from _chemical_formula_sum:   H28 Cr1 N1 O20 S2
            Atom count from _chemical_formula_moiety:H16 N1 O14 S2
REFLT03_ALERT_1_G ALERT: Expected hkl max differ from CIF values
           From the CIF: _diffrn_reflns_theta_max           27.72
           From the CIF: _reflns_number_total                714
           From the CIF: _diffrn_reflns_limit_ max hkl   15.    3.   15.
           From the CIF: _diffrn_reflns_limit_ min hkl  -12.  -14.  -15.
           TEST1: Expected hkl limits for theta max
           Calculated maximum hkl   16.   16.   16.
           Calculated minimum hkl  -16.  -16.  -16.

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   4 ALERT level C = Check and explain
   2 ALERT level G = General alerts; check

   4 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion

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Computing details

Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 1991); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 1990).

(I)

Crystal data

NH4Cr(SO4)2·12H2O	Dx = 1.729 Mg m^−3
Mr = 478.35	Mo Kα radiation, λ = 0.71073 Å
Cubic, Pa3	Cell parameters from 4846 reflections
Hall symbol: -P 2ac 2ab 3	θ = 3.7–27.6°
a = 12.2491 (12) Å	µ = 0.94 mm^−1
V = 1837.9 (3) Å^3	T = 100 K
Z = 4	Block, violet
F(000) = 1004	0.24 × 0.22 × 0.18 mm

Data collection

Stoe IPDS-II two-circle diffractometer	714 independent reflections
Radiation source: fine-focus sealed tube	613 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
ω scans	θmax = 27.7°, θmin = 3.7°
Absorption correction: multi-scan (MULABS; Spek, 1990; Blessing, 1995)	h = −12→15
Tmin = 0.805, Tmax = 0.849	k = −14→3
5024 measured reflections	l = −15→15

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.019	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.051	w = 1/[σ^2(Fo^2) + (0.0348P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max < 0.001
714 reflections	Δρmax = 0.29 e Å^−3
58 parameters	Δρmin = −0.36 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.090 (3)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cr1	0.5000	0.5000	0.5000	0.00994 (16)
N1	1.0000	0.5000	0.5000	0.0323 (8)
H1	1.012 (4)	0.544 (3)	0.553 (3)	0.039*	0.667
S1	0.80864 (2)	0.69136 (2)	0.30864 (2)	0.01273 (17)
O1	0.81058 (8)	0.73618 (8)	0.41998 (8)	0.0252 (3)
O2	0.73931 (8)	0.76069 (8)	0.23931 (8)	0.0340 (5)
O3	0.48764 (7)	0.51483 (7)	0.34114 (7)	0.0148 (2)
H3A	0.4708 (16)	0.5712 (19)	0.3131 (16)	0.035 (5)*
H3B	0.4803 (16)	0.4626 (19)	0.2983 (18)	0.036 (5)*
O4	1.04676 (9)	0.63817 (8)	0.69723 (8)	0.0220 (2)
H4A	1.0075 (16)	0.6829 (17)	0.7082 (16)	0.026 (5)*
H4B	1.104 (2)	0.6663 (18)	0.7085 (18)	0.039 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr1	0.00994 (16)	0.00994 (16)	0.00994 (16)	−0.00036 (8)	−0.00036 (8)	−0.00036 (8)
N1	0.0323 (8)	0.0323 (8)	0.0323 (8)	−0.0160 (9)	0.0160 (9)	−0.0160 (9)
S1	0.01273 (17)	0.01273 (17)	0.01273 (17)	−0.00183 (9)	0.00183 (9)	−0.00183 (9)
O1	0.0219 (4)	0.0348 (5)	0.0190 (5)	−0.0111 (4)	0.0042 (3)	−0.0126 (4)
O2	0.0340 (5)	0.0340 (5)	0.0340 (5)	0.0085 (4)	−0.0085 (4)	0.0085 (4)
O3	0.0197 (4)	0.0134 (4)	0.0114 (4)	0.0013 (3)	−0.0016 (3)	−0.0003 (3)
O4	0.0205 (5)	0.0188 (4)	0.0266 (5)	0.0038 (4)	0.0060 (4)	0.0058 (4)

Geometric parameters (Å, º)

Cr1—O3^i	1.9602 (9)	S1—O1^vi	1.4704 (9)
Cr1—O3^ii	1.9602 (9)	S1—O1^vii	1.4704 (9)
Cr1—O3^iii	1.9602 (9)	S1—O2	1.4709 (18)
Cr1—O3^iv	1.9602 (9)	O3—H3A	0.80 (2)
Cr1—O3	1.9602 (9)	O3—H3B	0.83 (2)
Cr1—O3^v	1.9603 (9)	O4—H4A	0.74 (2)
N1—H1	0.85 (3)	O4—H4B	0.79 (3)
S1—O1	1.4704 (9)
O3^i—Cr1—O3^ii	180.0	O3^iv—Cr1—O3^v	88.71 (4)
O3^i—Cr1—O3^iii	88.71 (4)	O3—Cr1—O3^v	91.29 (4)
O3^ii—Cr1—O3^iii	91.29 (4)	O1—S1—O1^vi	109.72 (5)
O3^i—Cr1—O3^iv	88.71 (4)	O1—S1—O1^vii	109.72 (5)
O3^ii—Cr1—O3^iv	91.29 (4)	O1^vi—S1—O1^vii	109.72 (5)
O3^iii—Cr1—O3^iv	91.29 (4)	O1—S1—O2	109.22 (5)
O3^i—Cr1—O3	91.29 (4)	O1^vi—S1—O2	109.22 (5)
O3^ii—Cr1—O3	88.71 (4)	O1^vii—S1—O2	109.22 (5)
O3^iii—Cr1—O3	88.71 (4)	Cr1—O3—H3A	121.8 (14)
O3^iv—Cr1—O3	180.0	Cr1—O3—H3B	124.3 (14)
O3^i—Cr1—O3^v	91.29 (4)	H3A—O3—H3B	111.5 (19)
O3^ii—Cr1—O3^v	88.71 (4)	H4A—O4—H4B	103 (2)
O3^iii—Cr1—O3^v	180.0

Symmetry codes: (i) y, z, x; (ii) −y+1, −z+1, −x+1; (iii) −z+1, −x+1, −y+1; (iv) −x+1, −y+1, −z+1; (v) z, x, y; (vi) −y+3/2, −z+1, x−1/2; (vii) z+1/2, −x+3/2, −y+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O4	0.85 (3)	2.16 (3)	3.0049 (11)	173 (3)
O3—H3A···O1^viii	0.80 (2)	1.84 (2)	2.6293 (13)	172 (2)
O3—H3B···O4^ix	0.83 (2)	1.78 (2)	2.6071 (13)	173 (2)
O4—H4B···O2^x	0.79 (3)	1.99 (3)	2.7752 (15)	171 (2)
O4—H4A···O1^xi	0.74 (2)	2.02 (2)	2.7454 (14)	169 (2)

Symmetry codes: (viii) z, −x+3/2, y−1/2; (ix) −x+3/2, −y+1, z−1/2; (x) x+1/2, −y+3/2, −z+1; (xi) z+1/2, x, −y+3/2.