Dipotassium carbonate sesquihydrate: rerefinement against new intensity data

The redetermination of the structure of the title compound, K₂CO₃·1.5H₂O, agrees with the results previously reported by Hunter & Jeffrey [J. Chem. Phys (1967), 47, 3297-3302], but with improved precision. The structure is described in terms of the hydrogen-bonding network and the K-O framework, and the C-O bond lengths are compared with those found in other reported alkali carbonate hydrates.

Key indicators

• Single-crystal X-ray study
• T = 298 K
• Mean [sigma](O-C) = 0.002 Å
• R factor = 0.031
• wR factor = 0.083
• Data-to-parameter ratio = 21.6

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level B:



PLAT_420  Alert B D-H Without Acceptor       O(5)   -   H(5B)             ?


 Alert Level C:



PLAT_354  Alert C Short O-H Bond (0.82A)     O(5)   -   H(5B)  =       0.69 Ang.


 0 Alert Level A = Potentially serious problem

 1 Alert Level B = Potential problem

 1 Alert Level C = Please check

---

S1. Comment

The original synthesis and structure of the title compound was reported by Hunter & Jeffrey (1967); an aqueous solution of potassium sulfide, under a CO₂ atmosphere, produced aqueous K₂CO₃ from which crystals of K₂CO₃^.1.5H₂O formed. The structure was determined using data collected on a Picker four-circle diffractometer with a 2° scan and Cu Kα radiation.

In the present work, the compound was synthesized as a by-product in a potassium sulfoarsenate synthesis performed in air, and hence is likely to have formed by a similar route to that previously described. The large clear crystals were soluble in the normal cyanoacrylate glue used to mount crystals, so a polyvinyl pyrrolidone (PVP) based glue was used instead.

The structure was solved independently to confirm the original structure determination, then atoms were numbered according to the original determination. All H atoms were located from the difference Fourier map. Bonding was analysed using both SHELXL97 (Sheldrick, 1997) and PLATON (Spek, 2000).

The structure may be described in terms of the hydrogen-bonding network. The water molecule O4 sits on a twofold axis and participates in hydrogen bonds bridging between O3 of two symmetry related CO₃^2- anions, whereas the other water O5 acts as a donor to both O2 and O3. O1 is not involved in hydrogen bonding. This leads to the formation of chains along (010), as shown in Fig. 1.

The position of the potassium ions relative to the CO₃–H₂O chains is shown in Fig. 2. K3 is located within the hydrogen-bonded chains and links the chains, so it is also possible to consider the structure as a framework of K3, carbonate and water with K1 and K2 sitting on a twofold axis within the channels (Fig. 3). Equally, the structure can be described as a K–O framework with water (O4) sitting within the channels (Fig. 4). The K–O polyhedral framework is also shown in Fig. 5; in this representation O4 is involved in the coordination to potassium, and hence the channels are small.

In the original structure determination, it was noted that the C—O2 and C—O3 bond lengths were equal, but that of C—O1 was shorter by 2.7σ. It was suggested that this was due to the fact that O1 is not involved in hydrogen bonding, but the result was perceived to be at the borderline of significance. In the current determination, whilst the bond lengths are remarkably similar to those in the original determination, the improved precision means that the C—O1 bond is shorter by 16.2σ, which is clearly significant.

Comparison with other alkali carbonates was also limited in the original report since only the structures of the bicarbonates NaHCO₃ and Na₂CO₃^.NaHCO₃ had been determined. A search on the Inorganic Crystal Structure Database (ICSD), implemented through the UK Chemical Database Service (Fletcher et al., 1996), for alkali carbonate hydrates identified six structures which have been reported since 1970: Na₂(CO₃)(H₂O) (Wu & Brown, 1975), Na₃H(CO₃)₃(H₂O)₂ (Choi & Mighell, 1982), Na₂(CO₃)(H₂O)₇ (Betzel et al., 1982), Na₅(CO₃)(HCO₃)₃ (Fernandez et al., 1990), Rb₄(HCO₃)₂(CO₃)(H₂O) and K₄(HCO₃)₂(CO₃)(H₂O) (Cirpus & Adam, 1995). Examining the carbonate bond lengths and the hydrogen-bonding network in these compounds is, again, inconclusive. In Na₃(HCO₃)₃(H₂O)₂ (Choi & Mighell, 1982), there are three different oxygen environments; O1 is a hydrogen-bond acceptor [C—O = 1.272 (1) Å], O2 is a bicarbonate hydroxyl oxygen [C—O = 1.309 (1) Å] and O3 is non-hydrogen-bonded [C—O = 1.262 (1) Å]. These values would tend to support the theory proposed by Hunter & Jeffrey (1967), in that the C—O bond length is proportional to the strength of the O—H interaction. However, for Na₂(CO₃)(H₂O) (Wu & Brown, 1975), the C—O bond length for the non-hydrogen-bonded oxygen [1.277 (4) Å] is the same, within 1su, as that for one of the hydrogen-bond acceptors [1.278 (3)]Å, and only slightly shorter than the other acceptor, 1.286 (3) Å. Finally, the C—O bond lengths for C—O with hydrogen-bond acceptors O in Rb₄(HCO₃)₂(CO₃)(H₂O) (Cirpus & Adam, 1995) show no trend. In the carbonate group, the acceptor oxygen O2 does have a significantly longer C—O bond length than the non-hydrogen-bonded O1 [1.294 (4) and 1.263 (7) Å, respectively]. However, in the bicarbonate group, whilst the hydroxyl oxygen has a long C—O bond length [1.340 (5) Å] the hydrogen-bond acceptor O5 and the non-hydrogen-bonded O4 have very similar C—O distances, 1.248 (5) and 1.255 (5) Å respectively. Thus, whilst the influence of the bicarbonate H on the C—O bond length is evident, the influence of a donor H on the carbonate group C—O bond length in these structures is much less clear.

S2. Experimental

1.10 g of KOH pellets were dissolved in 20 ml deionized water. 0.20 g of powdered realgar (later found to be a mixture of opiment, As₂S₃, and realgar, As₂S₂) was then added to the solution and heated to 1273 K for 20 min in a boiling tube on a heating mantle. The solution changed from orange to brown in colour almost immediately. After being allowed to cool, the solution was heated to 1273 K for a further 20 min then cooled again. It was then heated at 1273 K for 10 min; this was repeated three more times with cooling periods after each heating. A black–brown solution appeared during heating and during cooling a black solid settled at the bottom of the tube below a clear liquid. The solution was left unstoppered overnight. The clear liquid was poured off and the black solid was poured into a dish and left to dry. A small volume of the liquid was cooled to approximately 277 K and evaporated on a rotary evaporator. Clear colourless crystals resulted which were scraped out and allowed to dry in a desiccator. When constant mass was achieved the crystals were transferred to a sample tube and stored in the fridge.

S3. Refinement

All H atoms were located from the difference Fourier map and refined freely.

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT; program(s) used to solve structure: SHELXS86 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEX in OSCAIL (McArdle, 1994, 2000), ATOMS (Dowty, 1999), ORTEP-3 for Windows (Farrugia, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The hydrogen-bonded chains formed from water and carbonate groups, together with the numbering scheme. Non-H atoms are shown at the 50% probability level and H as small circles. Dashed lines represent hydrogen bonds.
	Fig. 2. Location of potassium ions between and within the hydrogen-bonded chains. K1 = blue, K2 = red, K3 = green. Representation otherwise as Fig. 1.
	Fig. 3. View of the structure as a framework in which K1 connects the hydrogen-bonded chains into a three-dimensional framework, with K2 and K3 located within the channels. Representation as in Fig. 2.
	Fig. 4. The K–O framework, with O4 (water) located at the centre of the channels. Representation as in Fig. 2.
	Fig. 5. Framework formed from K–O polyhedra. Polyhedra are coloured as for potassium in Fig. 2.

Potassium carbonate sesquihydrate

Crystal data

K4(CO3)2·3H2O	F(000) = 664
Mr = 330.47	Dx = 2.212 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 11.8175 (6) Å	Cell parameters from 3493 reflections
b = 13.7466 (7) Å	θ = 2.5–32.4°
c = 7.1093 (4) Å	µ = 1.82 mm^−1
β = 120.769 (1)°	T = 298 K
V = 992.34 (9) Å^3	Block, colourless
Z = 4	0.40 × 0.20 × 0.15 mm

Data collection

Bruker SMART 1000 CCD area-detector diffractometer	1790 independent reflections
Radiation source: fine-focus sealed tube	1618 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.045
ϕ and ω scans	θmax = 32.5°, θmin = 2.5°
Absorption correction: multi-scan (SADABS; Bruker, 1999)	h = −15→17
Tmin = 0.576, Tmax = 0.761	k = −14→20
4924 measured reflections	l = −10→10

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.031	All H-atom parameters refined
wR(F^2) = 0.083	w = 1/[σ^2(Fo^2) + (0.0533P)^2 + 0.0526P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max < 0.001
1790 reflections	Δρmax = 0.55 e Å^−3
83 parameters	Δρmin = −0.43 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: 0.0204 (14)

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.

H atoms were located from the difference Fourier map and refined freely.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
K1	0.0000	0.38193 (3)	0.2500	0.01984 (11)
K2	0.0000	0.68676 (3)	0.2500	0.02236 (11)
K3	0.35595 (3)	0.349981 (19)	0.45736 (4)	0.02332 (10)
C1	0.19290 (10)	0.17350 (8)	0.15183 (18)	0.01595 (19)
O1	0.10641 (8)	0.22728 (7)	0.15315 (13)	0.02284 (18)
O2	0.29732 (8)	0.15119 (6)	0.33357 (15)	0.02139 (18)
O3	0.17704 (10)	0.14186 (7)	−0.03042 (15)	0.0275 (2)
O4	0.0000	−0.00112 (14)	0.2500	0.0509 (5)
H4A	−0.057 (2)	−0.0344 (17)	0.172 (4)	0.045 (6)*
O5	0.14810 (11)	0.45886 (7)	0.0838 (2)	0.0297 (2)
H5A	0.174 (2)	0.5148 (18)	0.133 (3)	0.036 (5)*
H5B	0.189 (3)	0.445 (2)	0.046 (5)	0.054 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K1	0.01877 (16)	0.02370 (18)	0.01575 (16)	0.000	0.00789 (12)	0.000
K2	0.02508 (18)	0.02663 (19)	0.01706 (17)	0.000	0.01201 (14)	0.000
K3	0.02465 (15)	0.02638 (16)	0.01752 (15)	0.00227 (8)	0.00978 (11)	0.00148 (8)
C1	0.0165 (4)	0.0164 (4)	0.0141 (4)	−0.0008 (3)	0.0073 (3)	0.0002 (3)
O1	0.0219 (4)	0.0271 (4)	0.0212 (4)	0.0073 (3)	0.0122 (3)	0.0044 (3)
O2	0.0173 (3)	0.0237 (4)	0.0168 (4)	0.0021 (3)	0.0041 (3)	0.0018 (3)
O3	0.0314 (4)	0.0335 (5)	0.0177 (4)	0.0014 (4)	0.0127 (3)	−0.0059 (3)
O4	0.0509 (10)	0.0249 (7)	0.0442 (10)	0.000	0.0007 (8)	0.000
O5	0.0300 (5)	0.0233 (5)	0.0412 (6)	−0.0019 (3)	0.0221 (4)	−0.0034 (4)

Geometric parameters (Å, º)

K1—O2^i	2.7302 (9)	K3—O5	2.9414 (12)
K1—O2^ii	2.7302 (9)	K3—O1	3.1134 (9)
K1—O1^iii	2.7315 (9)	C1—O1	1.2653 (13)
K1—O1	2.7315 (9)	C1—O3	1.2863 (14)
K1—O5	2.7737 (11)	C1—O2	1.2864 (13)
K1—O5^iii	2.7737 (11)	O1—K2^v	2.7395 (9)
K1—O5^iv	3.0389 (12)	O1—K3^ii	2.7588 (9)
K1—O5^v	3.0389 (12)	O1—K3^i	2.7809 (9)
K2—O1^iv	2.7395 (9)	O2—K1^i	2.7301 (9)
K2—O1^v	2.7395 (9)	O2—K2^x	2.7901 (9)
K2—O2^vi	2.7901 (9)	O2—K3^i	2.8714 (10)
K2—O2^vii	2.7901 (9)	O3—K3^ix	2.8577 (10)
K2—O5^iv	2.9074 (11)	O3—K2^v	3.0037 (11)
K2—O5^v	2.9074 (11)	O3—K2^x	3.3372 (10)
K2—O3^iv	3.0037 (11)	O4—K3^i	2.8141 (14)
K2—O3^v	3.0037 (11)	O4—K3^ii	2.8141 (14)
K3—O1^viii	2.7587 (9)	O4—H4A	0.77 (2)
K3—O1^i	2.7808 (9)	O5—K2^v	2.9074 (11)
K3—O4^i	2.8141 (14)	O5—K1^v	3.0389 (12)
K3—O2	2.8457 (9)	O5—H5A	0.83 (2)
K3—O3^ix	2.8577 (10)	O5—H5B	0.69 (3)
K3—O2^i	2.8714 (10)
O2^i—K1—O2^ii	160.80 (4)	O4^i—K3—O2^i	85.679 (19)
O2^i—K1—O1^iii	81.86 (3)	O2—K3—O2^i	92.09 (3)
O2^ii—K1—O1^iii	83.22 (3)	O3^ix—K3—O2^i	140.41 (3)
O2^i—K1—O1	83.22 (3)	O1^viii—K3—O5	146.75 (3)
O2^ii—K1—O1	81.86 (3)	O1^i—K3—O5	136.69 (3)
O1^iii—K1—O1	77.79 (4)	O4^i—K3—O5	101.64 (3)
O2^i—K1—O5	97.52 (3)	O2—K3—O5	104.52 (3)
O2^ii—K1—O5	89.79 (3)	O3^ix—K3—O5	54.83 (3)
O1^iii—K1—O5	151.20 (3)	O2^i—K3—O5	90.78 (3)
O1—K1—O5	73.57 (3)	O1^viii—K3—O1	122.166 (13)
O2^i—K1—O5^iii	89.79 (3)	O1^i—K3—O1	95.56 (2)
O2^ii—K1—O5^iii	97.52 (3)	O4^i—K3—O1	155.97 (2)
O1^iii—K1—O5^iii	73.57 (3)	O2—K3—O1	43.17 (2)
O1—K1—O5^iii	151.20 (3)	O3^ix—K3—O1	73.58 (3)
O5—K1—O5^iii	135.17 (5)	O2^i—K3—O1	74.48 (2)
O2^i—K1—O5^iv	55.81 (3)	O5—K3—O1	65.91 (3)
O2^ii—K1—O5^iv	143.34 (3)	O1—C1—O3	120.20 (10)
O1^iii—K1—O5^iv	121.28 (3)	O1—C1—O2	119.69 (10)
O1—K1—O5^iv	127.00 (3)	O3—C1—O2	120.11 (10)
O5—K1—O5^iv	79.70 (2)	C1—O1—K1	159.13 (8)
O5^iii—K1—O5^iv	68.26 (4)	C1—O1—K2^v	100.63 (7)
O2^i—K1—O5^v	143.34 (3)	K1—O1—K2^v	84.48 (3)
O2^ii—K1—O5^v	55.81 (3)	C1—O1—K3^ii	115.69 (7)
O1^iii—K1—O5^v	127.00 (3)	K1—O1—K3^ii	85.03 (2)
O1—K1—O5^v	121.28 (3)	K2^v—O1—K3^ii	80.70 (2)
O5—K1—O5^v	68.26 (4)	C1—O1—K3^i	93.78 (7)
O5^iii—K1—O5^v	79.70 (2)	K1—O1—K3^i	84.61 (2)
O5^iv—K1—O5^v	87.86 (4)	K2^v—O1—K3^i	163.84 (3)
O1^iv—K2—O1^v	128.89 (4)	K3^ii—O1—K3^i	86.51 (3)
O1^iv—K2—O2^vi	108.29 (3)	C1—O1—K3	80.80 (6)
O1^v—K2—O2^vi	80.64 (3)	K1—O1—K3	78.33 (2)
O1^iv—K2—O2^vii	80.64 (3)	K2^v—O1—K3	104.94 (3)
O1^v—K2—O2^vii	108.29 (3)	K3^ii—O1—K3	161.72 (3)
O2^vi—K2—O2^vii	159.81 (4)	K3^i—O1—K3	84.44 (2)
O1^iv—K2—O5^iv	71.37 (3)	C1—O2—K1^i	170.94 (8)
O1^v—K2—O5^iv	155.98 (3)	C1—O2—K2^x	103.37 (7)
O2^vi—K2—O5^iv	80.17 (3)	K1^i—O2—K2^x	83.55 (2)
O2^vii—K2—O5^iv	85.95 (3)	C1—O2—K3	92.20 (7)
O1^iv—K2—O5^v	155.98 (3)	K1^i—O2—K3	83.40 (3)
O1^v—K2—O5^v	71.37 (3)	K2^x—O2—K3	78.34 (2)
O2^vi—K2—O5^v	85.95 (3)	C1—O2—K3^i	89.21 (6)
O2^vii—K2—O5^v	80.17 (3)	K1^i—O2—K3^i	82.73 (3)
O5^iv—K2—O5^v	92.97 (5)	K2^x—O2—K3^i	161.58 (4)
O1^iv—K2—O3^iv	45.01 (2)	K3—O2—K3^i	87.91 (3)
O1^v—K2—O3^iv	91.74 (3)	C1—O3—K3^ix	157.97 (8)
O2^vi—K2—O3^iv	78.61 (3)	C1—O3—K2^v	87.83 (7)
O2^vii—K2—O3^iv	118.19 (3)	K3^ix—O3—K2^v	74.75 (2)
O5^iv—K2—O3^iv	98.48 (3)	C1—O3—K2^x	78.64 (6)
O5^v—K2—O3^iv	158.85 (3)	K3^ix—O3—K2^x	96.90 (3)
O1^iv—K2—O3^v	91.74 (3)	K2^v—O3—K2^x	116.43 (3)
O1^v—K2—O3^v	45.01 (2)	K3^i—O4—K3^ii	84.83 (5)
O2^vi—K2—O3^v	118.19 (3)	K3^i—O4—H4A	161.6 (18)
O2^vii—K2—O3^v	78.61 (3)	K3^ii—O4—H4A	85.9 (17)
O5^iv—K2—O3^v	158.85 (3)	K1—O5—K2^v	80.66 (3)
O5^v—K2—O3^v	98.48 (3)	K1—O5—K3	80.73 (3)
O3^iv—K2—O3^v	76.69 (4)	K2^v—O5—K3	105.22 (3)
O1^viii—K3—O1^i	76.51 (3)	K1—O5—K1^v	111.74 (4)
O1^viii—K3—O4^i	79.87 (3)	K2^v—O5—K1^v	89.58 (3)
O1^i—K3—O4^i	79.49 (3)	K3—O5—K1^v	162.33 (4)
O1^viii—K3—O2	79.34 (3)	K1—O5—H5A	110.7 (13)
O1^i—K3—O2	80.26 (3)	K2^v—O5—H5A	156.0 (15)
O4^i—K3—O2	153.78 (3)	K3—O5—H5A	97.6 (15)
O1^viii—K3—O3^ix	94.54 (3)	K1^v—O5—H5A	66.7 (15)
O1^i—K3—O3^ix	159.72 (3)	K1—O5—H5B	142 (2)
O4^i—K3—O3^ix	117.29 (3)	K2^v—O5—H5B	75 (2)
O2—K3—O3^ix	80.20 (3)	K3—O5—H5B	78 (2)
O1^viii—K3—O2^i	122.33 (3)	K1^v—O5—H5B	97 (2)
O1^i—K3—O2^i	45.92 (2)	H5A—O5—H5B	103 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4A···O3^xi	0.77 (2)	1.93 (2)	2.6862 (17)	166 (2)
O5—H5A···O2^vi	0.83 (2)	1.90 (2)	2.7135 (13)	165 (2)
O5—H5B···O3^ix	0.69 (3)	2.03 (3)	2.6709 (14)	154 (3)

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