(RS)-Phenylsuccinic acid, C10H10O4, crystallizes with two molecules of the acid in the asymmetric unit. In the crystal structure, the carboxyl groups of each acid molecule are connected to those of adjacent molecules via hydrogen bonds; each molecule is connected to two other molecules, forming chains. Crystals of (RS)-phenylsuccinic acid are twinned. Data for one twin domain could be obtained and the structure could be solved with satisfactory results.
Supporting information
CCDC reference: 209958
Key indicators
- Single-crystal X-ray study
- T = 100 K
- Mean (C-C) = 0.005 Å
- R factor = 0.056
- wR factor = 0.133
- Data-to-parameter ratio = 13.6
checkCIF results
No syntax errors found
ADDSYM reports no extra symmetry
Alert Level A:
REFLT_03
From the CIF: _diffrn_reflns_theta_max 27.49
From the CIF: _reflns_number_total 3445
TEST2: Reflns within _diffrn_reflns_theta_max
Count of symmetry unique reflns 4310
Completeness (_total/calc) 79.93%
Alert A: < 85% complete (theta max?)
Alert Level C:
PLAT_320 Alert C Check Hybridisation of C(13) in Main Residue ?
1 Alert Level A = Potentially serious problem
0 Alert Level B = Potential problem
1 Alert Level C = Please check
Crystals were grown from aqueous solutions by dissolving the purchased material (Fluka, >99%) in pure, distilled and deionized water at room temperature. The clear solutions were evaporated to dryness under low-pressure conditions at room temperature, yielding twinned crystals of (RS)-PSA.
Reflection data for one twin domain was obtained using EVALCCD (Duisenberg, 1998). Orientation matrices for both twin domains were determined. During integration, all overlapping reflections were removed. One fifth of the observable reflections were lost due to overlap. A structural model was obtained using direct methods. All H atoms were located from a difference Fourier map. They were refined with a riding model, with Uiso equal to 1.2Ueq of the non-H atom to which they are attached.
Data collection: COLLECT (Nonius, 1999); cell refinement: EVALCCD (Duisenberg, 1998); data reduction: EVALCCD (Duisenberg, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: maXus (Mackay et al., 1998).
(
RS)-phenylsuccinic acid
top
Crystal data top
C10H10O4 | V = 944 (2) Å3 |
Mr = 194.19 | Z = 4 |
Triclinic, P1 | Dx = 1.366 Mg m−3 |
Hall symbol: -P 1 | Mo Kα radiation, λ = 0.71073 Å |
a = 5.428 (12) Å | Cell parameters from 154 reflections |
b = 10.368 (6) Å | θ = 4.2–21.4° |
c = 17.625 (9) Å | µ = 0.11 mm−1 |
α = 102.82 (5)° | T = 100 K |
β = 94.10 (14)° | Needle, colourless |
γ = 100.50 (9)° | 0.30 × 0.15 × 0.1 mm |
Data collection top
Nonius KappaCCD diffractometer | Rint = 0.075 |
Radiation source: fine-focus sealed tube | θmax = 27.5°, θmin = 4.6° |
ϕ and ω scans | h = −6→6 |
14054 measured reflections | k = −12→13 |
3445 independent reflections | l = −22→22 |
2223 reflections with I > 2σ(I) | |
Refinement top
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)] = 0.056 | H-atom parameters constrained |
wR(F2) = 0.133 | w = 1/[σ2(Fo2) + (0.0464P)2 + 0.7275P] where P = (Fo2 + 2Fc2)/3 |
S = 1.03 | (Δ/σ)max < 0.001 |
3445 reflections | Δρmax = 0.29 e Å−3 |
253 parameters | Δρmin = −0.29 e Å−3 |
0 restraints | |
Crystal data top
C10H10O4 | γ = 100.50 (9)° |
Mr = 194.19 | V = 944 (2) Å3 |
Triclinic, P1 | Z = 4 |
a = 5.428 (12) Å | Mo Kα radiation |
b = 10.368 (6) Å | µ = 0.11 mm−1 |
c = 17.625 (9) Å | T = 100 K |
α = 102.82 (5)° | 0.30 × 0.15 × 0.1 mm |
β = 94.10 (14)° | |
Data collection top
Nonius KappaCCD diffractometer | 2223 reflections with I > 2σ(I) |
14054 measured reflections | Rint = 0.075 |
3445 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.056 | 0 restraints |
wR(F2) = 0.133 | H-atom parameters constrained |
S = 1.03 | Δρmax = 0.29 e Å−3 |
3445 reflections | Δρmin = −0.29 e Å−3 |
253 parameters | |
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 | x | y | z | Uiso*/Ueq | |
C1 | 0.9239 (5) | 0.5664 (3) | 0.40993 (15) | 0.0185 (6) | |
C2 | 0.8656 (4) | 0.6240 (3) | 0.34009 (14) | 0.0191 (6) | |
C3 | 0.6931 (5) | 0.7255 (3) | 0.36170 (14) | 0.0212 (6) | |
C4 | 0.8243 (5) | 0.8446 (3) | 0.42450 (15) | 0.0197 (6) | |
C5 | 0.7569 (4) | 0.5110 (3) | 0.26821 (14) | 0.0195 (6) | |
C6 | 0.8921 (5) | 0.4912 (3) | 0.20376 (15) | 0.0228 (6) | |
C7 | 0.7971 (5) | 0.3881 (3) | 0.13808 (15) | 0.0273 (6) | |
C8 | 0.5674 (5) | 0.3039 (3) | 0.13622 (15) | 0.0266 (6) | |
C9 | 0.4297 (5) | 0.3227 (3) | 0.20025 (16) | 0.0262 (6) | |
C10 | 0.5238 (5) | 0.4258 (3) | 0.26592 (15) | 0.0229 (6) | |
C11 | 0.9419 (5) | 0.9179 (3) | 0.08494 (15) | 0.0218 (6) | |
C12 | 0.8988 (5) | 0.8579 (3) | 0.15487 (15) | 0.0219 (6) | |
C13 | 0.6222 (5) | 0.7821 (3) | 0.14725 (15) | 0.0234 (6) | |
C14 | 0.5678 (5) | 0.6596 (3) | 0.08050 (15) | 0.0229 (6) | |
C15 | 0.9685 (4) | 0.9705 (3) | 0.22895 (14) | 0.0196 (6) | |
C16 | 0.8325 (5) | 1.0741 (3) | 0.24376 (16) | 0.0257 (6) | |
C17 | 0.9015 (5) | 1.1779 (3) | 0.31043 (17) | 0.0297 (7) | |
C18 | 1.1056 (5) | 1.1794 (3) | 0.36340 (16) | 0.0273 (6) | |
C19 | 1.2413 (5) | 1.0762 (3) | 0.34896 (15) | 0.0275 (6) | |
C20 | 1.1724 (5) | 0.9727 (3) | 0.28243 (15) | 0.0238 (6) | |
O1 | 1.1525 (3) | 0.53905 (19) | 0.41591 (10) | 0.0232 (4) | |
O2 | 0.7673 (3) | 0.54591 (18) | 0.45501 (10) | 0.0226 (4) | |
O3 | 0.6901 (3) | 0.93933 (19) | 0.44148 (11) | 0.0281 (5) | |
O4 | 1.0372 (3) | 0.85106 (18) | 0.45636 (11) | 0.0251 (4) | |
O5 | 0.3420 (3) | 0.5859 (2) | 0.07813 (11) | 0.0302 (5) | |
O6 | 0.7207 (3) | 0.63323 (19) | 0.03416 (11) | 0.0272 (5) | |
O7 | 1.1799 (3) | 0.9357 (2) | 0.07038 (11) | 0.0278 (5) | |
O8 | 0.7717 (3) | 0.9502 (2) | 0.04812 (11) | 0.0257 (5) | |
H2 | 1.0152 | 0.6602 | 0.3215 | 0.023* | |
H3A | 0.6318 | 0.7525 | 0.3125 | 0.025* | |
H3B | 0.5500 | 0.6873 | 0.3797 | 0.025* | |
H6 | 1.0381 | 0.5427 | 0.2027 | 0.027* | |
H7 | 0.9080 | 0.3821 | 0.0972 | 0.033* | |
H8 | 0.5058 | 0.2285 | 0.0898 | 0.032* | |
H9 | 0.2714 | 0.2544 | 0.2023 | 0.031* | |
H10 | 0.4307 | 0.4323 | 0.3135 | 0.027* | |
H12 | 1.0270 | 0.7990 | 0.1693 | 0.026* | |
H13A | 0.6104 | 0.7364 | 0.1950 | 0.028* | |
H13B | 0.5021 | 0.8480 | 0.1322 | 0.028* | |
H16 | 0.6803 | 1.0775 | 0.2098 | 0.031* | |
H17 | 0.7923 | 1.2411 | 0.3154 | 0.036* | |
H18 | 1.1609 | 1.2651 | 0.4128 | 0.033* | |
H19 | 1.3787 | 1.0680 | 0.3898 | 0.033* | |
H20 | 1.2543 | 0.9000 | 0.2751 | 0.029* | |
H1O | 1.1953 | 0.5008 | 0.4633 | 0.028* | |
H3O | 0.7834 | 1.0230 | 0.4772 | 0.034* | |
H5O | 0.3170 | 0.5074 | 0.0449 | 0.036* | |
H7O | 1.2067 | 0.9860 | 0.0291 | 0.033* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1 | 0.0208 (14) | 0.0146 (14) | 0.0167 (13) | 0.0002 (10) | −0.0019 (10) | 0.0010 (10) |
C2 | 0.0198 (13) | 0.0221 (14) | 0.0156 (13) | 0.0026 (10) | 0.0017 (10) | 0.0064 (10) |
C3 | 0.0244 (14) | 0.0236 (15) | 0.0163 (13) | 0.0055 (11) | −0.0008 (10) | 0.0069 (11) |
C4 | 0.0242 (15) | 0.0189 (14) | 0.0178 (13) | 0.0039 (11) | 0.0026 (10) | 0.0087 (11) |
C5 | 0.0219 (14) | 0.0193 (14) | 0.0179 (13) | 0.0037 (11) | −0.0011 (10) | 0.0073 (11) |
C6 | 0.0233 (14) | 0.0246 (15) | 0.0212 (14) | 0.0026 (11) | 0.0006 (10) | 0.0092 (11) |
C7 | 0.0337 (16) | 0.0315 (17) | 0.0185 (14) | 0.0098 (13) | 0.0054 (11) | 0.0064 (12) |
C8 | 0.0354 (16) | 0.0258 (16) | 0.0175 (14) | 0.0100 (12) | −0.0034 (11) | 0.0019 (11) |
C9 | 0.0267 (15) | 0.0253 (16) | 0.0263 (15) | 0.0046 (11) | −0.0012 (11) | 0.0075 (12) |
C10 | 0.0258 (15) | 0.0240 (15) | 0.0201 (14) | 0.0066 (11) | 0.0031 (10) | 0.0067 (11) |
C11 | 0.0221 (15) | 0.0234 (15) | 0.0190 (14) | 0.0012 (11) | 0.0020 (10) | 0.0060 (11) |
C12 | 0.0235 (14) | 0.0245 (15) | 0.0199 (14) | 0.0056 (11) | 0.0016 (10) | 0.0098 (11) |
C13 | 0.0252 (15) | 0.0270 (16) | 0.0195 (14) | 0.0033 (11) | 0.0028 (10) | 0.0103 (12) |
C14 | 0.0239 (15) | 0.0292 (16) | 0.0171 (14) | 0.0037 (12) | −0.0020 (11) | 0.0111 (12) |
C15 | 0.0222 (14) | 0.0214 (14) | 0.0171 (13) | 0.0030 (11) | 0.0037 (10) | 0.0095 (11) |
C16 | 0.0251 (15) | 0.0253 (16) | 0.0288 (16) | 0.0058 (12) | 0.0012 (11) | 0.0108 (12) |
C17 | 0.0324 (16) | 0.0264 (16) | 0.0343 (17) | 0.0098 (12) | 0.0072 (12) | 0.0116 (13) |
C18 | 0.0355 (16) | 0.0215 (15) | 0.0239 (15) | 0.0024 (12) | 0.0055 (12) | 0.0059 (12) |
C19 | 0.0295 (16) | 0.0325 (17) | 0.0202 (14) | 0.0030 (12) | −0.0008 (11) | 0.0093 (12) |
C20 | 0.0247 (14) | 0.0282 (16) | 0.0217 (14) | 0.0092 (11) | 0.0036 (11) | 0.0097 (12) |
O1 | 0.0220 (10) | 0.0305 (11) | 0.0210 (10) | 0.0096 (8) | 0.0023 (7) | 0.0113 (8) |
O2 | 0.0210 (10) | 0.0298 (11) | 0.0190 (10) | 0.0050 (8) | 0.0023 (7) | 0.0100 (8) |
O3 | 0.0276 (10) | 0.0211 (11) | 0.0313 (11) | 0.0065 (8) | −0.0050 (8) | −0.0010 (8) |
O4 | 0.0251 (10) | 0.0237 (11) | 0.0255 (11) | 0.0055 (8) | −0.0034 (8) | 0.0049 (8) |
O5 | 0.0268 (11) | 0.0263 (11) | 0.0318 (12) | −0.0022 (8) | 0.0060 (8) | 0.0007 (9) |
O6 | 0.0253 (10) | 0.0307 (11) | 0.0231 (11) | 0.0006 (8) | 0.0062 (8) | 0.0042 (8) |
O7 | 0.0206 (10) | 0.0418 (13) | 0.0276 (11) | 0.0065 (8) | 0.0048 (8) | 0.0213 (9) |
O8 | 0.0223 (10) | 0.0368 (12) | 0.0230 (10) | 0.0062 (8) | 0.0033 (8) | 0.0172 (9) |
Geometric parameters (Å, º) top
C1—O2 | 1.226 (4) | C16—C17 | 1.381 (4) |
C1—O1 | 1.325 (4) | C17—C18 | 1.392 (5) |
C1—C2 | 1.521 (4) | C18—C19 | 1.394 (4) |
C2—C5 | 1.520 (4) | C19—C20 | 1.378 (4) |
C2—C3 | 1.536 (4) | C2—H2 | 0.9429 |
C3—C4 | 1.490 (4) | C3—H3A | 1.0205 |
C4—O4 | 1.232 (4) | C3—H3B | 0.9163 |
C4—O3 | 1.321 (3) | C6—H6 | 0.8753 |
C5—C6 | 1.393 (4) | C7—H7 | 0.9706 |
C5—C10 | 1.397 (4) | C8—H8 | 0.9867 |
C6—C7 | 1.383 (4) | C9—H9 | 1.0171 |
C7—C8 | 1.379 (5) | C10—H10 | 1.0059 |
C8—C9 | 1.395 (4) | C12—H12 | 1.0583 |
C9—C10 | 1.382 (4) | C13—H13A | 1.0545 |
C11—O8 | 1.229 (4) | C13—H13B | 1.0862 |
C11—O7 | 1.323 (4) | C16—H16 | 0.9960 |
C11—C12 | 1.513 (4) | C17—H17 | 0.9550 |
C12—C15 | 1.518 (4) | C18—H18 | 1.0769 |
C12—C13 | 1.544 (5) | C19—H19 | 1.0267 |
C13—C14 | 1.495 (4) | C20—H20 | 0.9326 |
C14—O6 | 1.229 (4) | O1—H1O | 1.0310 |
C14—O5 | 1.313 (4) | O3—H3O | 0.9784 |
C15—C20 | 1.395 (4) | O5—H5O | 0.8719 |
C15—C16 | 1.400 (4) | O7—H7O | 0.9886 |
| | | |
O2—C1—O1 | 124.3 (2) | C3—C2—H2 | 113.7 |
O2—C1—C2 | 121.6 (2) | C4—C3—H3A | 112.1 |
O1—C1—C2 | 114.1 (2) | C2—C3—H3A | 109.3 |
C5—C2—C1 | 110.5 (2) | C4—C3—H3B | 107.5 |
C5—C2—C3 | 113.3 (2) | C2—C3—H3B | 111.5 |
C1—C2—C3 | 109.5 (2) | H3A—C3—H3B | 105.4 |
C4—C3—C2 | 111.0 (2) | C7—C6—H6 | 117.3 |
O4—C4—O3 | 124.8 (3) | C5—C6—H6 | 122.3 |
O4—C4—C3 | 121.8 (3) | C8—C7—H7 | 126.4 |
O3—C4—C3 | 113.4 (2) | C6—C7—H7 | 113.7 |
C6—C5—C10 | 119.4 (3) | C7—C8—H8 | 118.9 |
C6—C5—C2 | 119.6 (2) | C9—C8—H8 | 120.8 |
C10—C5—C2 | 121.0 (3) | C10—C9—H9 | 118.8 |
C7—C6—C5 | 120.5 (3) | C8—C9—H9 | 120.7 |
C8—C7—C6 | 119.9 (3) | C9—C10—H10 | 118.9 |
C7—C8—C9 | 120.2 (3) | C5—C10—H10 | 120.8 |
C10—C9—C8 | 120.0 (3) | C11—C12—H12 | 117.0 |
C9—C10—C5 | 120.0 (3) | C15—C12—H12 | 96.6 |
O8—C11—O7 | 124.6 (3) | C13—C12—H12 | 111.8 |
O8—C11—C12 | 122.3 (2) | C14—C13—H13A | 100.4 |
O7—C11—C12 | 113.0 (2) | C12—C13—H13A | 105.6 |
C11—C12—C15 | 108.7 (2) | C14—C13—H13B | 105.5 |
C11—C12—C13 | 110.0 (2) | C12—C13—H13B | 107.7 |
C15—C12—C13 | 112.1 (2) | H13A—C13—H13B | 125.8 |
C14—C13—C12 | 111.3 (2) | C17—C16—H16 | 115.7 |
O6—C14—O5 | 125.2 (3) | C15—C16—H16 | 124.1 |
O6—C14—C13 | 122.6 (2) | C16—C17—H17 | 112.9 |
O5—C14—C13 | 112.2 (3) | C18—C17—H17 | 126.9 |
C20—C15—C16 | 119.3 (3) | C17—C18—H18 | 117.9 |
C20—C15—C12 | 120.1 (2) | C19—C18—H18 | 122.0 |
C16—C15—C12 | 120.6 (2) | C20—C19—H19 | 117.8 |
C17—C16—C15 | 120.1 (3) | C18—C19—H19 | 121.9 |
C16—C17—C18 | 120.1 (3) | C19—C20—H20 | 119.7 |
C17—C18—C19 | 119.9 (3) | C15—C20—H20 | 119.6 |
C20—C19—C18 | 119.9 (3) | C1—O1—H1O | 115.0 |
C19—C20—C15 | 120.6 (3) | C4—O3—H3O | 113.9 |
C5—C2—H2 | 98.4 | C14—O5—H5O | 113.3 |
C1—C2—H2 | 111.0 | C11—O7—H7O | 111.2 |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1O···O2i | 1.03 | 1.63 | 2.651 (3) | 171 |
O3—H3O···O4ii | 0.98 | 1.65 | 2.620 (4) | 171 |
O5—H5O···O6iii | 0.87 | 1.75 | 2.612 (3) | 170 |
O7—H7O···O8iv | 0.99 | 1.64 | 2.625 (3) | 172 |
Symmetry codes: (i) −x+2, −y+1, −z+1; (ii) −x+2, −y+2, −z+1; (iii) −x+1, −y+1, −z; (iv) −x+2, −y+2, −z. |
Experimental details
Crystal data |
Chemical formula | C10H10O4 |
Mr | 194.19 |
Crystal system, space group | Triclinic, P1 |
Temperature (K) | 100 |
a, b, c (Å) | 5.428 (12), 10.368 (6), 17.625 (9) |
α, β, γ (°) | 102.82 (5), 94.10 (14), 100.50 (9) |
V (Å3) | 944 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.11 |
Crystal size (mm) | 0.30 × 0.15 × 0.1 |
|
Data collection |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 14054, 3445, 2223 |
Rint | 0.075 |
(sin θ/λ)max (Å−1) | 0.649 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.056, 0.133, 1.03 |
No. of reflections | 3445 |
No. of parameters | 253 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.29, −0.29 |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1O···O2i | 1.03 | 1.63 | 2.651 (3) | 171 |
O3—H3O···O4ii | 0.98 | 1.65 | 2.620 (4) | 171 |
O5—H5O···O6iii | 0.87 | 1.75 | 2.612 (3) | 170 |
O7—H7O···O8iv | 0.99 | 1.64 | 2.625 (3) | 172 |
Symmetry codes: (i) −x+2, −y+1, −z+1; (ii) −x+2, −y+2, −z+1; (iii) −x+1, −y+1, −z; (iv) −x+2, −y+2, −z. |
Phenylsuccinic acid (PSA) is used as a resolving agent in classical resolutions of pharmaceuticals (Bayley & Vaidya, 1992; Kozma, 2002). In order to be able to study and understand the solid-state properties of PSA and its interactions with solvents or with other reagents commonly used, it is essential to know the cystal structure of both the pure enantiomer and the racemate. Generally, there are three classes of racemates in the solid state (Jacques et al., 1994). Two classes contain both enantiomers mixed in the crystal lattice: the racemic compounds that have an ordered, alternating (R)- and (S)-enantiomer distribution in the structure and the solid solutions which have both enantiomers randomly mixed in the lattice. The third class of racemates, the racemic conglomerate, has an identical crystal lattice to that of the pure enantiomer. This means that although the whole crystal mass is racemic, each crystal itself is enantiomerically pure. Most chiral substances form a racemic compound, while approximately 5–10% form racemic conglomerates. The racemic conglomerate can be separated by direct, seeded crystallization methods. The racemic compound, on the other hand, must be separated by other methods such as diastereomeric salt formation, chromatography or kinetic resolutions. The stability of racemic compounds versus conglomerates has been extensively discussed in the literature. Often, hydrogen bonds within the structures are identified as the key factor influencing the stability of the structures (Böcksei et al., 1996; Brock, 1996; Kinbara et al., 1996; Li et al., 1999). Since the structures of neither (S)- nor (RS)-PSA were known, we grew crystals of both compounds in order to determine to which class of racemates PSA belongs, and how the acid molecules are connected in respective structure. The structure of (S)-PSA was published earlier (Fischer & Profir, 2003). The structure of (RS)-PSA, (I), is presented here.
Fig. 1 shows the unit cell of (RS)-PSA in a view along the crystallographic a axis. The structure contains two molecules (R)-PSA per asymmetric unit, which are shown in Figs. 2 and 3. Since the structure is centrosymmetric, the opposite enantiomer of the respective molecule is generated. Thus, (RS)-PSA belongs to the first class of racemates mentioned above, the racemic compounds with ordered distribution of the molecules. In (RS)-PSA, each (R)-molecule is connected to two adjacent (S)-molecules via hydrogen bonds and vice versa. These hydrogen bonds are built in such a way that both carboxyl groups bind to one other carboxyl group of the other molecule. That way, chains of molecules are formed as shown in Fig. 4. There are two crystallographically different chains in this structure, formed by the two different molecules in the asymmetric unit. The geometry of the molecules is unexceptional, although one quite close H···H contact of 2.03 Å can be observed. This is, however, still slightly larger than the sum of the van der Waals radii (2.0 Å) given by Baur (1972).