Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104019043/sq1160sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270104019043/sq1160Isup2.hkl |
CCDC reference: 256985
Crystals of the title compound were obtained by slow evaporation of a solution containing acidimetric grade potassium hydrogen phthalate (Aldrich, 22 g) and deionized water (Barnsted NANOpure, 18.2 M W−1 cm−1, 200 ml) in a 250 ml crystallization dish at room temperature (approx. 296 K). After approximately four days, crystals of both KAP and the title compound were produced in the same dish. Currently, the dominance of one crystal over the other appears random, though the formation of KAP is strongly favored as the title compound is only observed in approximately 1% of growth experiments conducted. Formation of the title compound does not appear to depend on temperature, exposure to light or rate of evaporation.
All H atoms were initially located in a difference Fourier map. H atoms were then placed in idealized positions and constrained to ride on their parent atoms, with C—H distances in the range 0.95–1.00 Å. Uiso(H) values were fixed at 1.2Ueq of the parent atoms for CH groups and 1.5Ueq of the parent atoms for methyl groups.
Data collection: KappaCCD Software (Nonius, 1997); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL-2000 (Otwinowski & Minor 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: maXus (MacKat et al. 1998); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).
K+·C16H11O8−·2H2O | F(000) = 840 |
Mr = 406.38 | Dx = 1.542 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 239 reflections |
a = 13.5830 (3) Å | θ = 2.4–24.4° |
b = 21.1270 (5) Å | µ = 0.36 mm−1 |
c = 6.8170 (2) Å | T = 273 K |
β = 116.5521 (11)° | Cut-block, colourless |
V = 1749.93 (8) Å3 | 0.48 × 0.31 × 0.29 mm |
Z = 4 |
Nonius KappaCCD diffractometer | 2041 independent reflections |
Radiation source: fine-focus sealed tube | 1682 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.024 |
ϕ and ω scans | θmax = 29.9°, θmin = 3.2° |
Absorption correction: multi-scan HKL-2000 (Otwinowski & Minor 1997) | h = −19→19 |
Tmin = 0.847, Tmax = 0.903 | k = −29→29 |
3680 measured reflections | l = −9→9 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.031 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.082 | H-atom parameters constrained |
S = 1.00 | w = 1/[σ2(Fo2) + (0.0522P)2] where P = (Fo2 + 2Fc2)/3 |
2041 reflections | (Δ/σ)max = 0.001 |
131 parameters | Δρmax = 0.25 e Å−3 |
0 restraints | Δρmin = −0.26 e Å−3 |
K+·C16H11O8−·2H2O | V = 1749.93 (8) Å3 |
Mr = 406.38 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 13.5830 (3) Å | µ = 0.36 mm−1 |
b = 21.1270 (5) Å | T = 273 K |
c = 6.8170 (2) Å | 0.48 × 0.31 × 0.29 mm |
β = 116.5521 (11)° |
Nonius KappaCCD diffractometer | 2041 independent reflections |
Absorption correction: multi-scan HKL-2000 (Otwinowski & Minor 1997) | 1682 reflections with I > 2σ(I) |
Tmin = 0.847, Tmax = 0.903 | Rint = 0.024 |
3680 measured reflections |
R[F2 > 2σ(F2)] = 0.031 | 0 restraints |
wR(F2) = 0.082 | H-atom parameters constrained |
S = 1.00 | Δρmax = 0.25 e Å−3 |
2041 reflections | Δρmin = −0.26 e Å−3 |
131 parameters |
Experimental. Data was collected with ω and ϕ scans in 2º increments with 30 second exposures per degree. Crystal-to-detector distance was 30 mm. 13910 full and partial reflection were integrated. |
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. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
K1 | 0.5000 | 0.550188 (16) | 0.2500 | 0.01852 (12) | |
O1 | 0.34638 (7) | 0.47208 (4) | 0.30945 (14) | 0.0208 (2) | |
O2 | 0.18625 (7) | 0.52109 (4) | 0.11436 (15) | 0.0230 (2) | |
H2 | 0.2250 | 0.5527 | 0.1596 | 0.035* | |
O3 | 0.41424 (7) | 0.34097 (4) | 0.50179 (14) | 0.0186 (2) | |
O4 | 0.39982 (7) | 0.34408 (4) | 0.16310 (14) | 0.0209 (2) | |
H4A | 0.4672 | 0.3459 | 0.2287 | 0.031* | 0.50 |
O5 | 0.70096 (8) | 0.62456 (4) | 0.27164 (16) | 0.0234 (2) | |
H5A | 0.6687 (14) | 0.6395 (7) | 0.339 (3) | 0.028* | |
H5B | 0.6704 (14) | 0.6380 (7) | 0.143 (3) | 0.028* | |
C1 | 0.24635 (10) | 0.47017 (5) | 0.19816 (19) | 0.0170 (3) | |
C2 | 0.18190 (9) | 0.41029 (5) | 0.14561 (19) | 0.0167 (3) | |
C3 | 0.06716 (10) | 0.41098 (6) | 0.0489 (2) | 0.0225 (3) | |
H3 | 0.0301 | 0.4495 | 0.0104 | 0.027* | |
C4 | 0.00777 (11) | 0.35547 (6) | 0.0094 (2) | 0.0250 (3) | |
H4 | −0.0687 | 0.3567 | −0.0543 | 0.030* | |
C5 | 0.06204 (11) | 0.29794 (6) | 0.0646 (2) | 0.0240 (3) | |
H5 | 0.0221 | 0.2605 | 0.0385 | 0.029* | |
C6 | 0.17629 (10) | 0.29612 (6) | 0.1593 (2) | 0.0211 (3) | |
H6 | 0.2127 | 0.2574 | 0.1954 | 0.025* | |
C7 | 0.23649 (10) | 0.35203 (5) | 0.20022 (19) | 0.0158 (3) | |
C8 | 0.35957 (10) | 0.34651 (5) | 0.30218 (19) | 0.0154 (2) |
U11 | U22 | U33 | U12 | U13 | U23 | |
K1 | 0.01609 (19) | 0.0186 (2) | 0.0189 (2) | 0.000 | 0.00607 (15) | 0.000 |
O1 | 0.0149 (4) | 0.0192 (5) | 0.0235 (5) | −0.0011 (3) | 0.0043 (4) | 0.0016 (3) |
O2 | 0.0166 (4) | 0.0143 (4) | 0.0332 (5) | −0.0003 (3) | 0.0066 (4) | 0.0020 (3) |
O3 | 0.0173 (4) | 0.0204 (4) | 0.0164 (4) | 0.0023 (3) | 0.0060 (4) | 0.0007 (3) |
O4 | 0.0140 (4) | 0.0325 (5) | 0.0166 (4) | 0.0011 (3) | 0.0071 (4) | −0.0003 (3) |
O5 | 0.0299 (5) | 0.0232 (5) | 0.0200 (5) | 0.0095 (4) | 0.0137 (4) | 0.0038 (3) |
C1 | 0.0155 (6) | 0.0193 (6) | 0.0160 (6) | 0.0010 (4) | 0.0070 (5) | 0.0009 (4) |
C2 | 0.0155 (6) | 0.0187 (7) | 0.0158 (6) | −0.0001 (4) | 0.0069 (5) | −0.0006 (4) |
C3 | 0.0156 (6) | 0.0213 (7) | 0.0283 (7) | 0.0030 (4) | 0.0078 (5) | −0.0005 (5) |
C4 | 0.0138 (6) | 0.0269 (8) | 0.0319 (7) | −0.0032 (5) | 0.0080 (6) | −0.0041 (5) |
C5 | 0.0217 (6) | 0.0199 (7) | 0.0311 (7) | −0.0064 (5) | 0.0123 (6) | −0.0056 (5) |
C6 | 0.0211 (6) | 0.0158 (6) | 0.0263 (7) | 0.0007 (4) | 0.0105 (5) | −0.0010 (5) |
C7 | 0.0149 (6) | 0.0186 (6) | 0.0146 (6) | −0.0001 (4) | 0.0072 (5) | −0.0009 (4) |
C8 | 0.0176 (6) | 0.0094 (6) | 0.0187 (6) | 0.0008 (4) | 0.0077 (5) | −0.0005 (4) |
K1—O3i | 2.7831 (8) | O3—K1ii | 2.7832 (9) |
K1—O3ii | 2.7831 (9) | O4—C8 | 1.2899 (14) |
K1—O1ii | 2.8270 (9) | O4—H4A | 0.8200 |
K1—O1i | 2.8270 (9) | O5—H5A | 0.827 (17) |
K1—O1 | 2.8279 (8) | O5—H5B | 0.833 (17) |
K1—O1iii | 2.8280 (8) | C1—C2 | 1.4882 (16) |
K1—O5 | 3.0954 (10) | C2—C3 | 1.3947 (17) |
K1—O5iii | 3.0954 (10) | C2—C7 | 1.3986 (16) |
K1—C8i | 3.5377 (12) | C3—C4 | 1.3800 (17) |
K1—C8ii | 3.5377 (12) | C3—H3 | 0.9300 |
K1—K1ii | 4.0143 (4) | C4—C5 | 1.3836 (18) |
K1—K1iv | 4.0144 (4) | C4—H4 | 0.9300 |
K1—H5A | 2.816 (15) | C5—C6 | 1.3895 (18) |
O1—C1 | 1.2255 (15) | C5—H5 | 0.9300 |
O1—K1ii | 2.8271 (9) | C6—C7 | 1.3921 (17) |
O2—C1 | 1.3166 (14) | C6—H6 | 0.9300 |
O2—H2 | 0.8200 | C7—C8 | 1.5010 (17) |
O3—C8 | 1.2301 (15) | C8—K1ii | 3.5376 (12) |
O3i—K1—O3ii | 68.57 (3) | O1ii—K1—K1iv | 122.27 (2) |
O3i—K1—O1ii | 133.86 (3) | O1i—K1—K1iv | 44.784 (17) |
O3ii—K1—O1ii | 65.29 (2) | O1—K1—K1iv | 95.36 (2) |
O3i—K1—O1i | 65.29 (2) | O1iii—K1—K1iv | 44.766 (18) |
O3ii—K1—O1i | 133.86 (3) | O5—K1—K1iv | 88.504 (18) |
O1ii—K1—O1i | 160.84 (4) | O5iii—K1—K1iv | 124.221 (18) |
O3i—K1—O1 | 115.73 (2) | C8i—K1—K1iv | 78.338 (19) |
O3ii—K1—O1 | 122.01 (3) | C8ii—K1—K1iv | 148.645 (19) |
O1ii—K1—O1 | 89.55 (2) | K1ii—K1—K1iv | 116.223 (17) |
O1i—K1—O1 | 79.26 (2) | O3i—K1—H5A | 69.9 (3) |
O3i—K1—O1iii | 122.01 (3) | O3ii—K1—H5A | 40.2 (4) |
O3ii—K1—O1iii | 115.73 (2) | O1ii—K1—H5A | 74.9 (3) |
O1ii—K1—O1iii | 79.26 (2) | O1i—K1—H5A | 118.9 (3) |
O1i—K1—O1iii | 89.55 (2) | O1—K1—H5A | 160.2 (3) |
O1—K1—O1iii | 108.60 (4) | O1iii—K1—H5A | 80.8 (3) |
O3i—K1—O5 | 74.34 (2) | O5—K1—H5A | 15.2 (3) |
O3ii—K1—O5 | 55.33 (3) | O5iii—K1—H5A | 106.6 (3) |
O1ii—K1—O5 | 79.48 (2) | C8i—K1—H5A | 81.1 (3) |
O1i—K1—O5 | 110.58 (3) | C8ii—K1—H5A | 47.8 (3) |
O1—K1—O5 | 168.76 (2) | K1ii—K1—H5A | 118.6 (3) |
O1iii—K1—O5 | 67.14 (2) | K1iv—K1—H5A | 103.2 (4) |
O3i—K1—O5iii | 55.33 (3) | C1—O1—K1ii | 137.71 (8) |
O3ii—K1—O5iii | 74.34 (2) | C1—O1—K1 | 127.99 (7) |
O1ii—K1—O5iii | 110.58 (3) | K1ii—O1—K1 | 90.45 (2) |
O1i—K1—O5iii | 79.48 (2) | C1—O2—H2 | 109.5 |
O1—K1—O5iii | 67.14 (2) | C8—O3—K1ii | 118.39 (7) |
O1iii—K1—O5iii | 168.76 (2) | C8—O4—H4A | 109.5 |
O5—K1—O5iii | 118.99 (3) | K1—O5—H5A | 62.9 (10) |
O3i—K1—C8i | 17.81 (2) | K1—O5—H5B | 96.3 (11) |
O3ii—K1—C8i | 86.13 (3) | H5A—O5—H5B | 108.4 (16) |
O1ii—K1—C8i | 151.23 (3) | O1—C1—O2 | 122.85 (10) |
O1i—K1—C8i | 47.84 (3) | O1—C1—C2 | 123.17 (10) |
O1—K1—C8i | 109.63 (3) | O2—C1—C2 | 113.98 (10) |
O1iii—K1—C8i | 112.59 (3) | C3—C2—C7 | 118.85 (11) |
O5—K1—C8i | 81.48 (3) | C3—C2—C1 | 121.17 (11) |
O5iii—K1—C8i | 61.44 (3) | C7—C2—C1 | 119.94 (11) |
O3i—K1—C8ii | 86.13 (3) | C4—C3—C2 | 121.03 (11) |
O3ii—K1—C8ii | 17.81 (2) | C4—C3—H3 | 119.5 |
O1ii—K1—C8ii | 47.84 (3) | C2—C3—H3 | 119.5 |
O1i—K1—C8ii | 151.23 (3) | C3—C4—C5 | 119.97 (12) |
O1—K1—C8ii | 112.59 (3) | C3—C4—H4 | 120.0 |
O1iii—K1—C8ii | 109.63 (3) | C5—C4—H4 | 120.0 |
O5—K1—C8ii | 61.44 (3) | C4—C5—C6 | 119.95 (11) |
O5iii—K1—C8ii | 81.48 (3) | C4—C5—H5 | 120.0 |
C8i—K1—C8ii | 103.81 (4) | C6—C5—H5 | 120.0 |
O3i—K1—K1ii | 142.547 (19) | C5—C6—C7 | 120.23 (11) |
O3ii—K1—K1ii | 94.538 (18) | C5—C6—H6 | 119.9 |
O1ii—K1—K1ii | 44.785 (17) | C7—C6—H6 | 119.9 |
O1i—K1—K1ii | 122.27 (2) | C6—C7—C2 | 119.95 (11) |
O1—K1—K1ii | 44.766 (18) | C6—C7—C8 | 117.37 (10) |
O1iii—K1—K1ii | 95.36 (2) | C2—C7—C8 | 122.68 (10) |
O5—K1—K1ii | 124.220 (18) | O3—C8—O4 | 124.38 (11) |
O5iii—K1—K1ii | 88.505 (18) | O3—C8—C7 | 121.08 (10) |
C8i—K1—K1ii | 148.646 (19) | O4—C8—C7 | 114.39 (10) |
C8ii—K1—K1ii | 78.338 (19) | O3—C8—K1ii | 43.80 (5) |
O3i—K1—K1iv | 94.538 (18) | O4—C8—K1ii | 114.62 (7) |
O3ii—K1—K1iv | 142.545 (19) | C7—C8—K1ii | 114.26 (7) |
O3i—K1—O1—C1 | 20.91 (10) | K1—O1—C1—C2 | 141.61 (9) |
O3ii—K1—O1—C1 | 100.72 (10) | O1—C1—C2—C3 | 169.65 (12) |
O1ii—K1—O1—C1 | 160.86 (11) | O2—C1—C2—C3 | −10.33 (16) |
O1i—K1—O1—C1 | −34.78 (9) | O1—C1—C2—C7 | −8.13 (18) |
O1iii—K1—O1—C1 | −120.63 (10) | O2—C1—C2—C7 | 171.89 (11) |
O5—K1—O1—C1 | 173.50 (12) | C7—C2—C3—C4 | 0.75 (19) |
O5iii—K1—O1—C1 | 48.23 (9) | C1—C2—C3—C4 | −177.05 (12) |
C8i—K1—O1—C1 | 2.75 (10) | C2—C3—C4—C5 | −0.4 (2) |
C8ii—K1—O1—C1 | 117.77 (9) | C3—C4—C5—C6 | −0.2 (2) |
K1ii—K1—O1—C1 | 160.86 (11) | C4—C5—C6—C7 | 0.5 (2) |
K1iv—K1—O1—C1 | −76.76 (10) | C5—C6—C7—C2 | −0.13 (19) |
O3i—K1—O1—K1ii | −139.95 (3) | C5—C6—C7—C8 | −179.84 (11) |
O3ii—K1—O1—K1ii | −60.14 (3) | C3—C2—C7—C6 | −0.47 (18) |
O1ii—K1—O1—K1ii | 0.0 | C1—C2—C7—C6 | 177.37 (11) |
O1i—K1—O1—K1ii | 164.36 (3) | C3—C2—C7—C8 | 179.23 (11) |
O1iii—K1—O1—K1ii | 78.50 (2) | C1—C2—C7—C8 | −2.94 (17) |
O5—K1—O1—K1ii | 12.64 (14) | K1ii—O3—C8—O4 | 90.91 (11) |
O5iii—K1—O1—K1ii | −112.63 (3) | K1ii—O3—C8—C7 | −93.71 (11) |
C8i—K1—O1—K1ii | −158.11 (3) | C6—C7—C8—O3 | −83.48 (14) |
C8ii—K1—O1—K1ii | −43.09 (3) | C2—C7—C8—O3 | 96.81 (14) |
K1iv—K1—O1—K1ii | 122.38 (2) | C6—C7—C8—O4 | 92.33 (13) |
K1ii—O1—C1—O2 | 112.43 (12) | C2—C7—C8—O4 | −87.37 (14) |
K1—O1—C1—O2 | −38.41 (16) | C6—C7—C8—K1ii | −132.74 (9) |
K1ii—O1—C1—C2 | −67.55 (16) | C2—C7—C8—K1ii | 47.56 (13) |
Symmetry codes: (i) x, −y+1, z−1/2; (ii) −x+1, −y+1, −z+1; (iii) −x+1, y, −z+1/2; (iv) −x+1, −y+1, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O5iii | 0.82 | 1.76 | 2.5815 (12) | 174 |
O4—H4A···O4iii | 0.82 | 1.62 | 2.4347 (17) | 173 |
O5—H5A···O3ii | 0.827 (17) | 1.925 (18) | 2.7433 (13) | 169.9 (14) |
O5—H5B···O4iv | 0.833 (17) | 1.907 (17) | 2.7330 (13) | 171.4 (15) |
Symmetry codes: (ii) −x+1, −y+1, −z+1; (iii) −x+1, y, −z+1/2; (iv) −x+1, −y+1, −z. |
Experimental details
Crystal data | |
Chemical formula | K+·C16H11O8−·2H2O |
Mr | 406.38 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 273 |
a, b, c (Å) | 13.5830 (3), 21.1270 (5), 6.8170 (2) |
β (°) | 116.5521 (11) |
V (Å3) | 1749.93 (8) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.36 |
Crystal size (mm) | 0.48 × 0.31 × 0.29 |
Data collection | |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | Multi-scan HKL-2000 (Otwinowski & Minor 1997) |
Tmin, Tmax | 0.847, 0.903 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3680, 2041, 1682 |
Rint | 0.024 |
(sin θ/λ)max (Å−1) | 0.701 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.031, 0.082, 1.00 |
No. of reflections | 2041 |
No. of parameters | 131 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.25, −0.26 |
Computer programs: KappaCCD Software (Nonius, 1997), HKL SCALEPACK (Otwinowski & Minor 1997), HKL-2000 (Otwinowski & Minor 1997), SIR97 (Altomare et al., 1994), SHELXL97 (Sheldrick, 1997), maXus (MacKat et al. 1998).
K1—O3i | 2.7831 (8) | K1—O1 | 2.8279 (8) |
K1—O1i | 2.8270 (9) | K1—O5 | 3.0954 (10) |
Symmetry code: (i) x, −y+1, z−1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O5ii | 0.82 | 1.76 | 2.5815 (12) | 174 |
O4—H4A···O4ii | 0.82 | 1.62 | 2.4347 (17) | 173 |
O5—H5A···O3iii | 0.827 (17) | 1.925 (18) | 2.7433 (13) | 169.9 (14) |
O5—H5B···O4iv | 0.833 (17) | 1.907 (17) | 2.7330 (13) | 171.4 (15) |
Symmetry codes: (ii) −x+1, y, −z+1/2; (iii) −x+1, −y+1, −z+1; (iv) −x+1, −y+1, −z. |
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The hydrated potassium salt of the proton-bound hydrogen o-phthalate dimer [space group C 2/c, Z = 4, a = 13.5830 (3) Å, b = 21.1270 (5) Å, c = 6.8170 (2) Å and β = 116.552 (1)°] was obtained in an attempt to grow crystals of potassium hydrogen o-phthalate (commonly abbreviated to KAP for potassium acid phthalate) from an aqueous solution at room temperature. Instead of precipitating the usual six-sided {010} plates characteristic of KAP, heavily etched elongated prisms were observed. As these irregular crystals lacked the perfect basal cleavage planes characteristic of KAP, the sample was indexed and found to belong to the C-centered monoclinic crystal system, space group C 2/c, as opposed to the primitive orthorhombic system of KAP.
The crystal habit is a combination of the pinacoids {010} and {110} elongated along [001], lacking other identifiable faces. The molecular formula was determined to be [K+ H+(o-C6H4COOHCOO−)2] 2H2O, a molecular complex that contains a potassium cation, a proton-bound hydrogen phthalate dimer and two water molecules (Fig. 1). The hydrogen phthalate dimer is bridged by two symmetry-related H atoms, each with half-site occupancies. The conformation of the hydrogen phthalate ion is characterized by the nearly orthogonal carboxylate groups making an angle of 84.2° to one another.
The eight-coordinate potassium cation occupies a site on the twofold axis in a distorted square-antiprism coordination geometry, approximately 82 symmetry (Fig. 2). The coordination sphere is composed of two water molecules and the hydrogen phthalate dimer in the molecular complex, and additionally two hydrogen phthalate ions from adjacent molecular complexes along the C-centered [001] staggered-cation channels (Fig. 3). The potassium ions are enveloped by spiraling proton-bound hydrogen phthalate dimers and water molecules. The dominant interchannel interactions are of the van der Waals type between aryl rings.
It is surprising that this substance has not been reported until now, given the abundant research conducted on KAP and other phthalic acid salts (Okaya, 1965; Gougoutas et al., 1980; Smith, 1975; Eremina et al., 1993; Kariuki, 1995). As KAP is frequently used in the manufacture of buffer solutions, acidimetric standards and various other products, the possible contamination arising from the crystallization of a compound with a different stoichiometry has obvious practical implications for any analytical applications utilizing KAP.
Smith (1977) reported a structure that bears a remarkable resemblance to our compound (Fig. 4), but is described under the unusual banner 'ammonium hydrogen phthalate hemihydrate(?)' and is characterized as NH4(C8H5O4)2·0.5H2O(?) [monoclinic, C 2/c, Z = 8, a = 13.564 (8) Å, b = 21.17 (1) Å, c = 6.840 (5) Å and β = 112.8 (8)°]. It was immediately evident that this was the ammonium isomorph of our potassium salt; however, it had not been described as such.
There are several questionable features contained in the report of the ammonium structure, the first and foremost being the question mark in the title. The author used flotation methods to determine the density of the crystal, and imposed a water/ammonium disorder model to account for the observed density. While the author does well to narrate the derivation of his complex solution, the disorder model employed is forced. It seems that the author's primary emphasis was to publish what he viewed as a noteworthy conformation of the phthalate anion accepting that 'the proposed formula may be wrong'. Several data sets were collected on poorly scattering crystals of the ammonium compound prepared according to the method of Smith (1977). While our proposed model was a correct solution, R1 was only 0.10.