organic compounds
rac-Ammonium cis-2-carboxycyclohexane-1-carboxylate
aFaculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia
*Correspondence e-mail: g.smith@qut.edu.au
In the structure of the title compound, NH4+·C8H11O4−, the carboxyl and carboxylate groups of the cation adopt C—C—C—O torsion angles of 174.9 (2) and −145.4 (2)°, respectively, with the alicyclic ring. The ammonium H atoms of the cations give a total of five hydrogen-bonding associations with carboxylate O-atom acceptors of the anion which, together with a carboxyl O—H⋯Ocarboxylate interaction give sheet structures which lie in the (101) planes.
Related literature
For the structure of the isomeric racemic ammonium salt of trans-cyclohexane-1,2-dicarboxylic acid (TCDA), see: Stibrany et al. (2004). For the structures of rac-cis-CDA, rac-trans-CDA and (+)-trans-CDA, see: Benedetti et al. (1970); Benedetti, Corradini, Pedone & Post (1969); Benedetti, Corradini & Pedone (1969); Rizal & Ng (2008). The cis,trans-isomer exists as an essentially unresolvable racemate, see: Eliel (1962). For hydrogen-bond motifs, see: Etter et al. (1990).
Experimental
Crystal data
|
Refinement
|
Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON.
Supporting information
https://doi.org/10.1107/S1600536810051883/ng5083sup1.cif
contains datablocks global, I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536810051883/ng5083Isup2.hkl
The title compound was synthesized by reacting 1 mmol of cyclohexane-1,2-dicarboxylic anhydride with 50 ml of an 5M ammoniacal 1:1 ethanol–water solution. The solution was allowed evaporate to moist dryness at room temperature over several months, finally giving colourless poorly formed plates of (I) from which a specimen was cleaved for the X-ray analysis.
Hydrogen atoms involved in hydrogen-bonding interactions were located by difference methods and their positional and isotropic displacement parameters were refined. Other H-atoms were included in the
at calculated positions [C–H = 0.96–0.97 Å and with Uiso(H) = 1.2Ueq(C), using a riding-model approximation.Cyclohexane-1,2-dicarboxylic acid (CDA) is of interest conformationally since the cis,cis- (or trans,trans)- configurational isomers (the trans form) may be resolved while the cis,trans-isomer exists as an essentially unresolvable racemate (Eliel, 1962). The structures of both racemic-trans-CDA (TCDA) (Benedetti, Corradini, Pedrone & Post, 1969; Rizal & Ng, 2008), and (+)-trans-CDA (Benedetti, Corradini, Pedrone & Post, 1969) are known as well as that of racemic-cis-CDA (CCDA) (Benedetti et al., 1970). Our reaction of cyclohexane-1,2-dicarboxylic anhydride in 50% ethanol/water with an ammoniacal solution gave, after evaporation, crystals which were found to have a monoclinic β = 104.96 (4)°, V = 975.5 (8) Å3, Z = 4, P21/c], suggesting either a crystal polymorph or the configurational cis-isomeric salt. The compound has been confirmed as the racemic cis-salt of CDA, NH4+ C8H11O4- (I) and the structure is reported here.
which was very similar to that previously reported for the roon-temperature structure of ammonium trans-2-carboxycyclohexanecarboxylate (Stibrany et al., 2004) [a = 15.712 (7), b = 6.141 (3), c = 10.464 (5) Å,With (I) (Fig. 1) the ammonium cations give five hydrogen-bonding interactions with carboxylate O-atom acceptors of the anion (Table 1), including a three-centre asymmetric cyclic N—H···O,O' association [graph set R21(4) (Etter et al., (1990)]. The two-dimensional sheet structures generated extend along the (101) planes in the
(Fig. 2) with the ammonium ions lying close to these planes and providing the linkages within the sheets (Fig. 3), together with strong carboxylic acid O—H···Ocarboxyl hydrogen bonds. This and all other features of the hydrogen bonding in (I), including the centrosymmetric cyclic R24(8) heteromolecular motifs, are similar to those of the trans-CDA ammonium salt (Stibrany et al., 2004) but conformationally, the anions differ although not in a major way. Comparative carboxylic acid and carboxylate groups defined by torsion angles C1–C2–C21–O22 [174.9 (2)°] and C2–C1–C11–O11 [-145.4 (2)°] in (I) compare with -166.66 (19) and 137.3 (2)° respectively for the trans salt but are more comparable with -178.8 (5) and 152.9 (2)° for the rac-cis-CDA acid (Benedetti et al., 1970).For the structure of the isomeric racemic ammonium salt of trans-cyclohexane-1,2-dicarboxylic acid (TCDA), see: Stibrany et al. (2004). For the structures of rac-cis-CDA, rac-trans-CDA and (+)-trans-CDA, see: Benedetti et al. (1970); Benedetti, Corradini, Pedone & Post (1969); Benedetti, Corradini & Pedone (1969); Rizal & Ng (2008). The cis,trans-isomer exists as an essentially unresolvable racemate, see: Eliel (1962). For hydrogen-bond motifs, see: Etter et al. (1990) .
Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell
CrysAlis PRO (Oxford Diffraction, 2010); data reduction: CrysAlis PRO (Oxford Diffraction, 2010); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON (Spek, 2009).Fig. 1. Molecular configuration and atom naming scheme for the ammonium cation the CDA anion in (I). Inter-species hydrogen bonds are shown as dashed lines and displacement ellipsoids are drawn at the 40% probability level. | |
Fig. 2. The two-dimensional hydrogen-bonded sheet structures in (I) which extend down the (101) planes in the unit cell, showing hydrogen-bonding interactions as dashed lines. Non-associative H atoms are omitted. For symmetry codes, see Table 1. | |
Fig. 3. A portion of the sheet structure in (I) viewed down the a axis of the unit cell. |
NH4+·C8H11O4− | F(000) = 408 |
Mr = 189.21 | Dx = 1.325 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 2294 reflections |
a = 15.4908 (13) Å | θ = 3.4–28.6° |
b = 5.3475 (3) Å | µ = 0.11 mm−1 |
c = 12.1716 (9) Å | T = 200 K |
β = 109.795 (9)° | Plate, colourless |
V = 948.68 (13) Å3 | 0.30 × 0.22 × 0.10 mm |
Z = 4 |
Oxford Diffraction Gemini-S CCD-detector diffractometer | 1862 independent reflections |
Radiation source: Enhance (Mo) X-ray source | 1313 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.046 |
Detector resolution: 16.077 pixels mm-1 | θmax = 26.0°, θmin = 3.4° |
ω scans | h = −19→12 |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2010) | k = −6→6 |
Tmin = 0.86, Tmax = 0.98 | l = −15→15 |
5997 measured reflections |
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)] = 0.058 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.141 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.99 | w = 1/[σ2(Fo2) + (0.0838P)2] where P = (Fo2 + 2Fc2)/3 |
1862 reflections | (Δ/σ)max < 0.001 |
138 parameters | Δρmax = 0.44 e Å−3 |
0 restraints | Δρmin = −0.22 e Å−3 |
NH4+·C8H11O4− | V = 948.68 (13) Å3 |
Mr = 189.21 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 15.4908 (13) Å | µ = 0.11 mm−1 |
b = 5.3475 (3) Å | T = 200 K |
c = 12.1716 (9) Å | 0.30 × 0.22 × 0.10 mm |
β = 109.795 (9)° |
Oxford Diffraction Gemini-S CCD-detector diffractometer | 1862 independent reflections |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2010) | 1313 reflections with I > 2σ(I) |
Tmin = 0.86, Tmax = 0.98 | Rint = 0.046 |
5997 measured reflections |
R[F2 > 2σ(F2)] = 0.058 | 0 restraints |
wR(F2) = 0.141 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.99 | Δρmax = 0.44 e Å−3 |
1862 reflections | Δρmin = −0.22 e Å−3 |
138 parameters |
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles |
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 | ||
O11 | 0.13732 (12) | 0.9964 (3) | 0.33214 (13) | 0.0306 (5) | |
O12 | 0.08448 (12) | 1.2330 (3) | 0.44276 (14) | 0.0329 (6) | |
O21 | 0.16061 (13) | 0.7624 (3) | 0.59854 (14) | 0.0368 (6) | |
O22 | 0.20622 (15) | 0.9149 (4) | 0.77916 (15) | 0.0450 (7) | |
C1 | 0.24584 (16) | 1.1450 (4) | 0.51309 (18) | 0.0243 (7) | |
C2 | 0.24599 (17) | 1.1538 (4) | 0.63967 (18) | 0.0242 (7) | |
C3 | 0.34336 (18) | 1.1894 (4) | 0.7268 (2) | 0.0323 (8) | |
C4 | 0.40930 (18) | 0.9929 (5) | 0.7113 (2) | 0.0345 (8) | |
C5 | 0.41077 (18) | 0.9927 (5) | 0.5871 (2) | 0.0383 (9) | |
C6 | 0.31483 (17) | 0.9528 (5) | 0.4989 (2) | 0.0302 (8) | |
C11 | 0.14981 (17) | 1.1222 (4) | 0.42428 (19) | 0.0253 (7) | |
C21 | 0.20048 (16) | 0.9222 (4) | 0.66831 (19) | 0.0245 (7) | |
N1 | −0.01140 (18) | 0.6903 (5) | 0.3749 (2) | 0.0320 (8) | |
H1 | 0.26870 | 1.30850 | 0.49890 | 0.0290* | |
H2 | 0.20960 | 1.29920 | 0.64640 | 0.0290* | |
H22 | 0.192 (3) | 0.764 (8) | 0.796 (3) | 0.088 (13)* | |
H31 | 0.36540 | 1.35430 | 0.71610 | 0.0390* | |
H32 | 0.34180 | 1.18000 | 0.80560 | 0.0390* | |
H41 | 0.47050 | 1.02650 | 0.76520 | 0.0410* | |
H42 | 0.39080 | 0.82920 | 0.72960 | 0.0410* | |
H51 | 0.45100 | 0.86070 | 0.57860 | 0.0460* | |
H52 | 0.43490 | 1.15090 | 0.57130 | 0.0460* | |
H61 | 0.29390 | 0.78600 | 0.50860 | 0.0360* | |
H62 | 0.31740 | 0.96480 | 0.42050 | 0.0360* | |
H1A | 0.016 (2) | 0.832 (6) | 0.363 (3) | 0.057 (10)* | |
H1B | 0.031 (3) | 0.566 (7) | 0.402 (3) | 0.071 (11)* | |
H1C | −0.057 (2) | 0.647 (5) | 0.300 (2) | 0.044 (8)* | |
H1D | −0.038 (2) | 0.728 (6) | 0.436 (3) | 0.072 (11)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O11 | 0.0415 (11) | 0.0289 (9) | 0.0191 (8) | 0.0057 (8) | 0.0071 (7) | −0.0059 (7) |
O12 | 0.0338 (11) | 0.0378 (10) | 0.0264 (9) | 0.0104 (8) | 0.0093 (7) | −0.0021 (7) |
O21 | 0.0556 (13) | 0.0292 (9) | 0.0254 (9) | −0.0167 (9) | 0.0134 (8) | −0.0038 (7) |
O22 | 0.0814 (16) | 0.0360 (11) | 0.0207 (9) | −0.0190 (11) | 0.0215 (9) | −0.0004 (8) |
C1 | 0.0336 (14) | 0.0186 (11) | 0.0232 (12) | −0.0030 (10) | 0.0130 (10) | 0.0004 (9) |
C2 | 0.0328 (14) | 0.0171 (11) | 0.0227 (11) | 0.0002 (10) | 0.0094 (10) | −0.0009 (9) |
C3 | 0.0398 (16) | 0.0259 (13) | 0.0288 (13) | −0.0043 (11) | 0.0086 (11) | −0.0033 (10) |
C4 | 0.0258 (14) | 0.0362 (14) | 0.0349 (14) | 0.0010 (12) | 0.0017 (11) | −0.0002 (11) |
C5 | 0.0302 (15) | 0.0431 (16) | 0.0434 (16) | 0.0057 (13) | 0.0148 (12) | 0.0011 (12) |
C6 | 0.0367 (15) | 0.0315 (14) | 0.0260 (13) | 0.0026 (11) | 0.0153 (11) | 0.0003 (10) |
C11 | 0.0386 (15) | 0.0173 (11) | 0.0225 (12) | 0.0017 (11) | 0.0137 (10) | 0.0037 (9) |
C21 | 0.0301 (13) | 0.0216 (11) | 0.0212 (12) | 0.0020 (10) | 0.0081 (10) | 0.0016 (9) |
N1 | 0.0373 (14) | 0.0327 (13) | 0.0244 (12) | −0.0003 (11) | 0.0085 (10) | 0.0038 (10) |
O11—C11 | 1.265 (3) | C3—C4 | 1.521 (4) |
O12—C11 | 1.257 (3) | C4—C5 | 1.520 (3) |
O21—C21 | 1.216 (3) | C5—C6 | 1.525 (4) |
O22—C21 | 1.322 (3) | C1—H1 | 0.9800 |
O22—H22 | 0.88 (4) | C2—H2 | 0.9800 |
N1—H1D | 0.99 (3) | C3—H31 | 0.9700 |
N1—H1B | 0.91 (4) | C3—H32 | 0.9700 |
N1—H1C | 0.97 (2) | C4—H41 | 0.9700 |
N1—H1A | 0.90 (3) | C4—H42 | 0.9700 |
C1—C11 | 1.519 (3) | C5—H52 | 0.9700 |
C1—C2 | 1.541 (3) | C5—H51 | 0.9700 |
C1—C6 | 1.534 (4) | C6—H61 | 0.9700 |
C2—C3 | 1.534 (4) | C6—H62 | 0.9700 |
C2—C21 | 1.523 (3) | ||
C21—O22—H22 | 109 (2) | C6—C1—H1 | 106.00 |
H1B—N1—H1C | 112 (3) | C1—C2—H2 | 108.00 |
H1C—N1—H1D | 114 (3) | C3—C2—H2 | 108.00 |
H1B—N1—H1D | 108 (3) | C21—C2—H2 | 108.00 |
H1A—N1—H1D | 107 (3) | H31—C3—H32 | 108.00 |
H1A—N1—H1B | 110 (3) | C2—C3—H31 | 109.00 |
H1A—N1—H1C | 106 (3) | C2—C3—H32 | 109.00 |
C6—C1—C11 | 114.70 (18) | C4—C3—H31 | 109.00 |
C2—C1—C6 | 111.59 (18) | C4—C3—H32 | 109.00 |
C2—C1—C11 | 112.6 (2) | C3—C4—H42 | 109.00 |
C1—C2—C3 | 111.3 (2) | C5—C4—H41 | 109.00 |
C1—C2—C21 | 111.10 (18) | C3—C4—H41 | 109.00 |
C3—C2—C21 | 111.46 (18) | H41—C4—H42 | 108.00 |
C2—C3—C4 | 111.90 (19) | C5—C4—H42 | 109.00 |
C3—C4—C5 | 111.2 (2) | C6—C5—H51 | 109.00 |
C4—C5—C6 | 111.2 (2) | C4—C5—H52 | 109.00 |
C1—C6—C5 | 112.2 (2) | C4—C5—H51 | 109.00 |
O11—C11—O12 | 121.2 (2) | C6—C5—H52 | 109.00 |
O11—C11—C1 | 119.5 (2) | H51—C5—H52 | 108.00 |
O12—C11—C1 | 119.25 (19) | H61—C6—H62 | 108.00 |
O21—C21—O22 | 122.3 (2) | C1—C6—H61 | 109.00 |
O21—C21—C2 | 125.2 (2) | C1—C6—H62 | 109.00 |
O22—C21—C2 | 112.41 (19) | C5—C6—H61 | 109.00 |
C11—C1—H1 | 106.00 | C5—C6—H62 | 109.00 |
C2—C1—H1 | 106.00 | ||
C6—C1—C2—C3 | 52.1 (2) | C1—C2—C3—C4 | −54.1 (2) |
C6—C1—C2—C21 | −72.7 (3) | C21—C2—C3—C4 | 70.6 (3) |
C11—C1—C2—C3 | −177.29 (17) | C1—C2—C21—O21 | −7.1 (4) |
C11—C1—C2—C21 | 57.9 (2) | C1—C2—C21—O22 | 174.9 (2) |
C2—C1—C6—C5 | −53.0 (3) | C3—C2—C21—O21 | −131.9 (3) |
C11—C1—C6—C5 | 177.5 (2) | C3—C2—C21—O22 | 50.1 (3) |
C2—C1—C11—O11 | −145.4 (2) | C2—C3—C4—C5 | 56.3 (3) |
C2—C1—C11—O12 | 36.0 (3) | C3—C4—C5—C6 | −56.4 (3) |
C6—C1—C11—O11 | −16.4 (3) | C4—C5—C6—C1 | 55.1 (3) |
C6—C1—C11—O12 | 165.0 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1A···O11 | 0.90 (3) | 2.22 (3) | 3.012 (3) | 146 (3) |
N1—H1A···O12 | 0.90 (3) | 2.44 (3) | 3.237 (3) | 147 (3) |
N1—H1B···O12i | 0.91 (4) | 1.96 (4) | 2.835 (3) | 161 (4) |
N1—H1C···O11ii | 0.97 (2) | 1.85 (3) | 2.811 (3) | 168 (2) |
N1—H1D···O12iii | 0.99 (3) | 1.86 (3) | 2.842 (3) | 174 (3) |
O22—H22···O11iv | 0.88 (4) | 1.76 (4) | 2.619 (3) | 165 (5) |
C2—H2···O21v | 0.98 | 2.60 | 3.485 (3) | 150 |
C3—H32···O22 | 0.97 | 2.46 | 2.827 (4) | 102 |
Symmetry codes: (i) x, y−1, z; (ii) −x, y−1/2, −z+1/2; (iii) −x, −y+2, −z+1; (iv) x, −y+3/2, z+1/2; (v) x, y+1, z. |
Experimental details
Crystal data | |
Chemical formula | NH4+·C8H11O4− |
Mr | 189.21 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 200 |
a, b, c (Å) | 15.4908 (13), 5.3475 (3), 12.1716 (9) |
β (°) | 109.795 (9) |
V (Å3) | 948.68 (13) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.11 |
Crystal size (mm) | 0.30 × 0.22 × 0.10 |
Data collection | |
Diffractometer | Oxford Diffraction Gemini-S CCD-detector |
Absorption correction | Multi-scan (CrysAlis PRO; Oxford Diffraction, 2010) |
Tmin, Tmax | 0.86, 0.98 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 5997, 1862, 1313 |
Rint | 0.046 |
(sin θ/λ)max (Å−1) | 0.617 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.058, 0.141, 0.99 |
No. of reflections | 1862 |
No. of parameters | 138 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.44, −0.22 |
Computer programs: CrysAlis PRO (Oxford Diffraction, 2010), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), PLATON (Spek, 2009).
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1A···O11 | 0.90 (3) | 2.22 (3) | 3.012 (3) | 146 (3) |
N1—H1A···O12 | 0.90 (3) | 2.44 (3) | 3.237 (3) | 147 (3) |
N1—H1B···O12i | 0.91 (4) | 1.96 (4) | 2.835 (3) | 161 (4) |
N1—H1C···O11ii | 0.97 (2) | 1.85 (3) | 2.811 (3) | 168 (2) |
N1—H1D···O12iii | 0.99 (3) | 1.86 (3) | 2.842 (3) | 174 (3) |
O22—H22···O11iv | 0.88 (4) | 1.76 (4) | 2.619 (3) | 165 (5) |
Symmetry codes: (i) x, y−1, z; (ii) −x, y−1/2, −z+1/2; (iii) −x, −y+2, −z+1; (iv) x, −y+3/2, z+1/2. |
Acknowledgements
The authors acknowledge financial support from the Australian Research Council, the Faculty of Science and Technology and the University Library, Queensland University of Technology and the School of Biomolecular and Physical Sciences, Griffith University.
References
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Cyclohexane-1,2-dicarboxylic acid (CDA) is of interest conformationally since the cis,cis- (or trans,trans)- configurational isomers (the trans form) may be resolved while the cis,trans-isomer exists as an essentially unresolvable racemate (Eliel, 1962). The structures of both racemic-trans-CDA (TCDA) (Benedetti, Corradini, Pedrone & Post, 1969; Rizal & Ng, 2008), and (+)-trans-CDA (Benedetti, Corradini, Pedrone & Post, 1969) are known as well as that of racemic-cis-CDA (CCDA) (Benedetti et al., 1970). Our reaction of cyclohexane-1,2-dicarboxylic anhydride in 50% ethanol/water with an ammoniacal solution gave, after evaporation, crystals which were found to have a monoclinic unit cell which was very similar to that previously reported for the roon-temperature structure of ammonium trans-2-carboxycyclohexanecarboxylate (Stibrany et al., 2004) [a = 15.712 (7), b = 6.141 (3), c = 10.464 (5) Å, β = 104.96 (4)°, V = 975.5 (8) Å3, Z = 4, space group P21/c], suggesting either a crystal polymorph or the configurational cis-isomeric salt. The compound has been confirmed as the racemic cis-salt of CDA, NH4+ C8H11O4- (I) and the structure is reported here.
With (I) (Fig. 1) the ammonium cations give five hydrogen-bonding interactions with carboxylate O-atom acceptors of the anion (Table 1), including a three-centre asymmetric cyclic N—H···O,O' association [graph set R21(4) (Etter et al., (1990)]. The two-dimensional sheet structures generated extend along the (101) planes in the unit cell (Fig. 2) with the ammonium ions lying close to these planes and providing the linkages within the sheets (Fig. 3), together with strong carboxylic acid O—H···Ocarboxyl hydrogen bonds. This and all other features of the hydrogen bonding in (I), including the centrosymmetric cyclic R24(8) heteromolecular motifs, are similar to those of the trans-CDA ammonium salt (Stibrany et al., 2004) but conformationally, the anions differ although not in a major way. Comparative carboxylic acid and carboxylate groups defined by torsion angles C1–C2–C21–O22 [174.9 (2)°] and C2–C1–C11–O11 [-145.4 (2)°] in (I) compare with -166.66 (19) and 137.3 (2)° respectively for the trans salt but are more comparable with -178.8 (5) and 152.9 (2)° for the rac-cis-CDA acid (Benedetti et al., 1970).