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The structure of the title compound, C
3H
8NO
2+·C
2HO
4−·H
2O, is formed by two chiral counterparts (
L- and
D-alaninium cations), semi-oxalate anions and water molecules, with a 1:1:1 cation–anion–water ratio. The structure is compared with that of the previously known anhydrous
DL-alaninium semi-oxalate [Subha Nandhini, Krishnakumar & Natarajan (2001).
Acta Cryst. E
57, o666–o668] in order to investigate the role of water molecules in the crystal packing. The structure of the hydrate resembles that of anhydrous alaninium semi-oxalate, with the water molecule incorporated into the general three-dimensional network of hydrogen bonds where it forms four hydrogen bonds with neighbours disposed tetrahedrally about it. Although the main structural motifs in the hydrate and in the anhydrous form are topologically similar, the incorporation of water molecules in the network results in significant geometric distortion. There are several types of hydrogen bond in the crystal structure of the hydrate, two of which (O—H
O bonds between the semi-oxalate anions and O—H
O hydrogen bonds between water and alaninium cations) are very short. Such hydrogen bonds between semi-oxalate anions are also present in the anhydrous form of this compound. Short distances between semi-oxalate anions in neighbouring chains in the hydrate alternate with longer ones, whereas in the anhydrous structure they are equidistant. Despite the similarity of these compounds, dehydration of the hydrate on storage is not of a single-crystal to single-crystal type, but gives a polycrystalline pseudomorph, preserving the crystal habit. This transformation proceeds through the formation of an intermediate compound, presumably a hemihydrate.
Supporting information
CCDC reference: 815000
Crystals of DL-alaninium semi-oxalate hydrate, (I), were obtained by
slow cooling of an aqueous solution of DL-alanine and oxalic acid in a
1:1 stoichiometric ratio saturated at 323 K. To avoid decomposition during the
diffraction experiment, the sample was protected with cryo-oil.
All H atoms were located in a difference Fourier map. The positions of the H
atoms bonded to methyl C, amine N and carboxylic O atoms in the alaninium
cation were refined using distance restraints, with target values of O—H =
0.82 (2) Å, N—H = 0.89 (2) Å and C—H = 0.96 (2) Å, and with
Uiso(H) = 1.2Ueq(O) or 1.5Ueq(C,N). The torsion
angles were allowed to refine while keeping the X—H distance and
Y—X—H angle fixed. The positions and isotropic displacement
parameters of all other H atoms were refined freely.
Data collection: X-AREA (Stoe & Cie, 2006); cell refinement: X-AREA (Stoe & Cie, 2006); data reduction: X-RED (Stoe & Cie, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and X-STEP32 (Stoe & Cie, 2000); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: Mercury (Macrae et al., 2006), PLATON (Spek, 2009),
publCIF (Westrip, 2010)
and enCIFer (Allen et al., 2004).
DL-Alaninium semioxalate monohydrate
top
Crystal data top
C3H8NO2+·C2HO4−·H2O | F(000) = 416 |
Mr = 197.15 | Dx = 1.523 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 4226 reflections |
a = 12.4987 (19) Å | θ = 1.8–29.2° |
b = 6.6017 (6) Å | µ = 0.15 mm−1 |
c = 11.3732 (18) Å | T = 300 K |
β = 113.585 (12)° | Prism, colourless |
V = 860.0 (2) Å3 | 0.40 × 0.30 × 0.15 mm |
Z = 4 | |
Data collection top
Stoe IPDS II diffractometer | 1771 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.051 |
Plane graphite monochromator | θmax = 29.2°, θmin = 1.8° |
Detector resolution: 6.67 pixels mm-1 | h = −17→16 |
rotation method scans | k = −8→9 |
7933 measured reflections | l = −15→15 |
2303 independent reflections | |
Refinement top
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.037 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.104 | w = 1/[σ2(Fo2) + (0.0525P)2 + 0.083P] where P = (Fo2 + 2Fc2)/3 |
S = 1.04 | (Δ/σ)max < 0.001 |
2303 reflections | Δρmax = 0.24 e Å−3 |
145 parameters | Δρmin = −0.18 e Å−3 |
0 restraints | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.045 (6) |
Crystal data top
C3H8NO2+·C2HO4−·H2O | V = 860.0 (2) Å3 |
Mr = 197.15 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 12.4987 (19) Å | µ = 0.15 mm−1 |
b = 6.6017 (6) Å | T = 300 K |
c = 11.3732 (18) Å | 0.40 × 0.30 × 0.15 mm |
β = 113.585 (12)° | |
Data collection top
Stoe IPDS II diffractometer | 1771 reflections with I > 2σ(I) |
7933 measured reflections | Rint = 0.051 |
2303 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.037 | 0 restraints |
wR(F2) = 0.104 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.04 | Δρmax = 0.24 e Å−3 |
2303 reflections | Δρmin = −0.18 e Å−3 |
145 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 | |
O3 | 0.07909 (8) | 0.70838 (17) | 0.44183 (8) | 0.0393 (2) | |
H3 | 0.047 (2) | 0.712 (4) | 0.346 (3) | 0.109 (9)* | |
O6 | −0.00664 (8) | 0.78300 (17) | 0.69506 (8) | 0.0381 (2) | |
O4 | −0.09365 (8) | 0.81046 (16) | 0.43677 (8) | 0.0386 (2) | |
O5 | 0.16786 (8) | 0.70784 (17) | 0.69763 (8) | 0.0404 (3) | |
C5 | 0.00674 (10) | 0.75948 (18) | 0.49414 (10) | 0.0264 (2) | |
C4 | 0.06276 (10) | 0.74937 (18) | 0.64295 (10) | 0.0271 (2) | |
O2 | 0.42083 (9) | 0.52827 (17) | 0.86114 (11) | 0.0497 (3) | |
O1 | 0.45614 (9) | 0.20394 (16) | 0.83206 (11) | 0.0460 (3) | |
H1 | 0.5151 | 0.2508 | 0.8275 | 0.086 (8)* | |
N1 | 0.23091 (9) | 0.44368 (17) | 0.90592 (10) | 0.0333 (2) | |
H1A | 0.2183 | 0.5408 | 0.8476 | 0.047 (4)* | |
H1B | 0.2756 | 0.4917 | 0.9832 | 0.050 (5)* | |
H1C | 0.1630 | 0.4025 | 0.9057 | 0.061 (5)* | |
C1 | 0.39582 (10) | 0.3521 (2) | 0.85593 (11) | 0.0326 (3) | |
C2 | 0.29039 (11) | 0.2708 (2) | 0.87486 (11) | 0.0321 (3) | |
H2 | 0.2359 (14) | 0.214 (2) | 0.7917 (15) | 0.041 (4)* | |
C3 | 0.32156 (15) | 0.1097 (3) | 0.97869 (16) | 0.0499 (4) | |
H3A | 0.2529 | 0.0722 | 0.9917 | 0.057 (5)* | |
H3B | 0.3791 | 0.1623 | 1.0572 | 0.079 (7)* | |
H3C | 0.3524 | −0.0072 | 0.9527 | 0.071 (6)* | |
O7 | 0.64997 (9) | 0.37273 (18) | 0.84316 (10) | 0.0404 (2) | |
H7A | 0.7082 (19) | 0.313 (3) | 0.8303 (19) | 0.064 (6)* | |
H7B | 0.625 (2) | 0.470 (4) | 0.784 (3) | 0.092 (8)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O3 | 0.0368 (5) | 0.0588 (6) | 0.0252 (4) | 0.0092 (4) | 0.0155 (4) | 0.0003 (4) |
O6 | 0.0319 (4) | 0.0610 (7) | 0.0235 (4) | 0.0071 (4) | 0.0133 (3) | 0.0025 (4) |
O4 | 0.0285 (4) | 0.0578 (6) | 0.0265 (4) | 0.0041 (4) | 0.0078 (3) | 0.0017 (4) |
O5 | 0.0280 (4) | 0.0631 (7) | 0.0290 (4) | 0.0084 (4) | 0.0103 (3) | 0.0123 (4) |
C5 | 0.0288 (5) | 0.0281 (6) | 0.0230 (5) | −0.0005 (4) | 0.0111 (4) | 0.0009 (4) |
C4 | 0.0271 (5) | 0.0315 (6) | 0.0227 (5) | 0.0004 (4) | 0.0100 (4) | 0.0028 (4) |
O2 | 0.0384 (5) | 0.0422 (6) | 0.0751 (7) | 0.0020 (5) | 0.0295 (5) | 0.0119 (5) |
O1 | 0.0358 (5) | 0.0484 (6) | 0.0616 (6) | −0.0014 (4) | 0.0279 (5) | −0.0074 (5) |
N1 | 0.0261 (5) | 0.0407 (6) | 0.0342 (5) | 0.0018 (4) | 0.0133 (4) | 0.0055 (4) |
C1 | 0.0266 (5) | 0.0408 (7) | 0.0288 (5) | 0.0016 (5) | 0.0094 (4) | 0.0045 (5) |
C2 | 0.0268 (5) | 0.0379 (7) | 0.0305 (5) | −0.0018 (5) | 0.0101 (4) | −0.0016 (5) |
C3 | 0.0511 (8) | 0.0444 (9) | 0.0626 (9) | 0.0051 (7) | 0.0316 (8) | 0.0158 (7) |
O7 | 0.0330 (5) | 0.0476 (6) | 0.0445 (5) | 0.0012 (4) | 0.0195 (4) | −0.0028 (4) |
Geometric parameters (Å, º) top
O3—C5 | 1.3100 (13) | N1—H1B | 0.8900 |
O3—H3 | 1.00 (3) | N1—H1C | 0.8900 |
O6—C4 | 1.2507 (13) | C1—C2 | 1.5158 (17) |
O4—C5 | 1.2075 (14) | C2—C3 | 1.5199 (19) |
O5—C4 | 1.2385 (14) | C2—H2 | 0.992 (16) |
C5—C4 | 1.5527 (15) | C3—H3A | 0.9600 |
O2—C1 | 1.1994 (17) | C3—H3B | 0.9600 |
O1—C1 | 1.3278 (16) | C3—H3C | 0.9600 |
O1—H1 | 0.8200 | O7—H7A | 0.89 (2) |
N1—C2 | 1.4799 (17) | O7—H7B | 0.89 (3) |
N1—H1A | 0.8900 | | |
| | | |
C5—O3—H3 | 116.3 (15) | O2—C1—C2 | 124.09 (12) |
O4—C5—O3 | 125.67 (10) | O1—C1—C2 | 111.41 (11) |
O4—C5—C4 | 121.26 (10) | N1—C2—C1 | 108.01 (10) |
O3—C5—C4 | 113.07 (10) | N1—C2—C3 | 110.57 (10) |
O5—C4—O6 | 126.78 (10) | C1—C2—C3 | 113.02 (11) |
O5—C4—C5 | 118.82 (10) | N1—C2—H2 | 107.5 (9) |
O6—C4—C5 | 114.40 (10) | C1—C2—H2 | 107.7 (9) |
C1—O1—H1 | 109.5 | C3—C2—H2 | 109.9 (10) |
C2—N1—H1A | 109.5 | C2—C3—H3A | 109.5 |
C2—N1—H1B | 109.5 | C2—C3—H3B | 109.5 |
H1A—N1—H1B | 109.5 | H3A—C3—H3B | 109.5 |
C2—N1—H1C | 109.5 | C2—C3—H3C | 109.5 |
H1A—N1—H1C | 109.5 | H3A—C3—H3C | 109.5 |
H1B—N1—H1C | 109.5 | H3B—C3—H3C | 109.5 |
O2—C1—O1 | 124.49 (12) | H7A—O7—H7B | 105 (2) |
| | | |
O4—C5—C4—O5 | −175.78 (12) | O2—C1—C2—N1 | −2.64 (17) |
O3—C5—C4—O5 | 4.11 (17) | O1—C1—C2—N1 | 177.80 (10) |
O4—C5—C4—O6 | 5.08 (18) | O2—C1—C2—C3 | −125.29 (15) |
O3—C5—C4—O6 | −175.03 (11) | O1—C1—C2—C3 | 55.14 (15) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1C···O6i | 0.89 | 1.99 | 2.7794 (14) | 147 |
N1—H1C···O4i | 0.89 | 2.36 | 3.0616 (13) | 136 |
N1—H1B···O7ii | 0.89 | 2.02 | 2.9034 (15) | 169 |
N1—H1A···O5 | 0.89 | 1.91 | 2.7893 (14) | 168 |
O7—H7A···O5iii | 0.89 (2) | 1.84 (2) | 2.7257 (14) | 175.7 (19) |
O7—H7B···O1iv | 0.89 (3) | 2.02 (3) | 2.8976 (16) | 167 (2) |
O3—H3···O6v | 1.00 (3) | 1.57 (3) | 2.5732 (13) | 178 (3) |
O1—H1···O7 | 0.82 | 1.81 | 2.6234 (14) | 171 |
Symmetry codes: (i) −x, y−1/2, −z+3/2; (ii) −x+1, −y+1, −z+2; (iii) −x+1, y−1/2, −z+3/2; (iv) −x+1, y+1/2, −z+3/2; (v) x, −y+3/2, z−1/2. |
Experimental details
Crystal data |
Chemical formula | C3H8NO2+·C2HO4−·H2O |
Mr | 197.15 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 300 |
a, b, c (Å) | 12.4987 (19), 6.6017 (6), 11.3732 (18) |
β (°) | 113.585 (12) |
V (Å3) | 860.0 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.15 |
Crystal size (mm) | 0.40 × 0.30 × 0.15 |
|
Data collection |
Diffractometer | Stoe IPDS II diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7933, 2303, 1771 |
Rint | 0.051 |
(sin θ/λ)max (Å−1) | 0.686 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.037, 0.104, 1.04 |
No. of reflections | 2303 |
No. of parameters | 145 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.24, −0.18 |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1C···O6i | 0.89 | 1.99 | 2.7794 (14) | 146.9 |
N1—H1C···O4i | 0.89 | 2.36 | 3.0616 (13) | 135.7 |
N1—H1B···O7ii | 0.89 | 2.02 | 2.9034 (15) | 169.4 |
N1—H1A···O5 | 0.89 | 1.91 | 2.7893 (14) | 167.5 |
O7—H7A···O5iii | 0.89 (2) | 1.84 (2) | 2.7257 (14) | 175.7 (19) |
O7—H7B···O1iv | 0.89 (3) | 2.02 (3) | 2.8976 (16) | 167 (2) |
O3—H3···O6v | 1.00 (3) | 1.57 (3) | 2.5732 (13) | 178 (3) |
O1—H1···O7 | 0.82 | 1.81 | 2.6234 (14) | 170.5 |
Symmetry codes: (i) −x, y−1/2, −z+3/2; (ii) −x+1, −y+1, −z+2; (iii) −x+1, y−1/2, −z+3/2; (iv) −x+1, y+1/2, −z+3/2; (v) x, −y+3/2, z−1/2. |
Torsion angles for hydrated and anhydrous forms of DL-alaninium semi-oxalate
(°) topAngle | Hydrated form | Anhydrous form* |
O4—C5—C4—O5 | 175.78 (12) | 176.9 (3) |
O3—C5—C4—O5 | -4.11 (17) | -4.0 (3) |
O4—C5—C4—O6 | -5.08 (18) | -2.1 (4) |
O3—C5—C4—O6 | 175.03 (11) | 177.1 (2) |
O2—C1—C2—N1 | 2.64 (17) | 6.2 (3) |
O1—C1—C2—N1 | -177.80 (10) | 174.0 (2) |
O2—C1—C2—C3 | 125.29 (15) | 129.1 (3) |
O1—C1—C2—C3 | -55.14 (15) | -51.0 (3) |
Reference: *Subha Nandhini et al. (2001a). |
Hydrogen-bond geometry for DL-alaninium semi-oxalate (Å, °)* topD—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O5i | 0.82 | 1.80 | 2.591 (2) | 160.6 |
O3—H1A···O6ii | 0.82 | 1.77 | 2.587 (2) | 174.0 |
N1—H1A···O5iii | 0.89 | 1.98 | 2.834 (3) | 161.1 |
N1—H1B···O2iv | 0.89 | 2.03 | 2.863 (3) | 154.2 |
N1—H1C···O6 | 0.89 | 1.96 | 2.818 (2) | 162.0 |
Symmetry codes:
(i) 1/2 + x, 3/2 - y, -1/2 + z;
(ii) -1 + x, y, z;
(iii) 3/2 - x, -1/2 + y, 3/2 - z;
(iv) 1 - x, 1 - y, 1 - z.
Reference: *Subha Nandhini et al. (2001a). |
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Amino acids and their salts attract attention as drugs, biomimetics and molecular materials (Boldyreva, 2007). They are interesting for crystal engineering as individual components and together with other compounds, such as carboxylic acids. The amino and carboxyl groups, and in many cases also the side chains, of amino acids are capable of forming hydrogen bonds, giving rise to a variety of crystal structures. Of special interest is the comparison of the crystal structures of amino acids and their salts with those of the corresponding solvates, to determine whether the solvate molecules can compete for the formation of hydrogen bonds with the amino or carboxyl groups of the amino acid molecules. Even for the simplest amino and dicarboxylic acids (glycine and oxalic acid), several crystal structures with different stoichiometries of the anhydrous forms, as well as a methanol solvate, have been reported (Chitra & Choudhury, 2007; Chitra et al., 2006; Tumanov et al., 2010). Alanine is the smallest and simplest chiral amino acid. The crystal structures of the salts formed by alanine and oxalic acid have been described, namely DL-alaninium semi-oxalate and L-alaninium semi-oxalate (Subha Nandhini et al., 2001b). The present contribution reports the structure of a new member of the same family, the title monohydrate of DL-alaninium semi-oxalate, (I).
The asymmetric unit of (I) contains one alaninium cation, C3H8NO2+, one semi-oxalate anion, C2HO4-, and one molecule of water (Fig. 1). The crystal structure of the hydrate resembles that of the anhydrous alaninium semi-oxalate, with water molecules incorporated into the general three-dimensional hydrogen-bond network: each water molecule forms four tetrahedral hydrogen bonds with its neighbours in the structure (Fig. 2). The semi-oxalate anion in (I) is not fully flat, whereas the same torsion angle in the anhydrous form (O6—C4—C5—O3) is closer to 180° (Table 2). The torsion angles in the alaninium cations, O1—C1—C2—N1 and O1—C1—C2—C3, are also different in the hydrate and in the anhydrous form. These intramolecular changes are a response to the distortion of the intermolecular hydrogen-bond network.
The main structure-forming motif in the crystal structures of individual amino acids is a head-to-tail chain, in which amino groups are linked to carboxyl groups (Boldyreva, 2007; Görbitz, 2010). It is in these chains that proton transfer results in the formation of zwitterions, NH3+–CH(R)–COO-, instead of less polar NH2–CH(R)–COOH molecules. In the salts of amino acids, these motifs are often not observed.
A structural motif that seems to be common for oxalates of amino acids is a chain formed by the semi-oxalate anions. This motif, C22(5) according to the classification suggested by Bernstein (2002), is also present in the crystal structures of alaninium semi-oxalate and its hydrate (Fig. 3). Although the chains in the hydrate and in the anhydrous forms are topologically similar, incorporation of water molecules into the network, linking the chains with each other, results in significant geometric distortion. In the anhydrous form, all neighbouring semi-oxalate anions in the chains are practically in the same plane, whereas in the hydrate the angle between two neighbouring semi-oxalate anions in the chain is close to 30°.
In the hydrate, the O—H···O hydrogen bond linking anions along the [001] direction is shorter than the corresponding distance in the anhydrous form (respective O3···O6 distances in Tables 1 and 3). Another difference between the anhydrous form and the hydrate is related to the distances between the semi-oxalate anions in neighbouring chains: in the anhydrous form, all these distances l are 3.719 (3) Å at 293 K, whereas in the hydrate, the `normal' distances [l1 = 3.435 (2) Å at 300 K] alternate with shorter contacts [l2 = 3.186 (2) Å at 300 K] (Fig. 3). To the best of our knowledge, such short contacts between semi-oxalate chains have not been reported before, and they must arise because of the way the chains are linked to each other via alaninium cations and water molecules (Fig. 4). A similar effect, although with alternating normal and unusually long distances, has been observed previously in the DL-lysinium hydrogen oxalate dihydrate (Venkatraman et al., 1997), where semi-oxalate chains are linked alternately by water molecules and lysine molecules, resulting in normal contacts of 3.713 (5) Å between semi-oxalate chains in the structure alternating with very long distances of 7.092 (5) Å.
The semi-oxalate chains in the structure of the hydrate are linked to each other in the [010] direction via bridges between water molecules and alaninium cations, forming an R66(16) ring motif. They are further connected to each other in the [120] and [210] directions via a cluster of two water molecules and two chiral alaninium cations to give an R24(14) motif, thereby completing the three-dimensional network (Fig. 5). The R66(16) motif is formed by weak N—H···O and O—H···O hydrogen bonds, where the amino group of alanine is connected to the water molecule and the semi-oxalate anion (N1···O7ii and N1···O6i hydrogen bonds, respectively; symmetry codes as in Table 1), and the water molecule is linked to another semi-oxalate anion by another hydrogen bond (O7···O5iii). In the R24(14) ring, the same hydrogen-bond parameters are observed, but the water molecule is connected to the carboxyl group of alanine instead of the semi-oxalate anion, resulting in a strong O1···O7 hydrogen bond (Table 1).
A comparison of the hydrate structure with that of the anhydrous form reveals that, in the anhydrous form, the semi-oxalate chains are linked to each other in the [011] direction via rings formed by two alaninium cations, which are very similar to those present in the hydrate (Fig. 2). In the structure of the anhydrous form, the semi-oxalate chains are linked with each other via alaninium dimers in the [010] direction, forming weak N1—H···O6 and strong O1—H···O5i hydrogen bonds with the deprotonated O atoms of the carboxyl groups (Table 3, Fig. 2). The water molecule embedded in the structure of the hydrate finds the most favourable position to form strong hydrogen bonds: the O1···O7 distance between the water O atom and the carboxyl group of the alaninium cation is 2.6234 (14) Å at 300 K. The water molecule also occupies the most suitable position to achieve tetrahedral coordination via four hydrogen bonds to three alaninium cations and one semi-oxalate anion, while, remarkably, the main structural framework of the anhydrous form is to a large extent preserved.
A comparison of the proton-donating and proton-accepting groups in the hydrogen-bond network shows the following details. In the anhydrous form, the alaninium cations are linked to each other via N—H···O bonds; the amino group of a cation receives an H atom from the oxalic acid. In the hydrate, there are no hydrogen bonds directly linking the alaninium cations with each other, but rather they are connected via water molecules or, in an even more complicated way, via water + semi-oxalate bridges (Fig. 5). A water molecule acts as a proton donor with respect to a carboxyl group of one alaninium cation, but at the same time also acts as a proton acceptor for the carboxyl and amino groups of other neighbouring alaninium cations. It also acts as a proton donor in the O—H···O hydrogen bond to a carboxyl group of a neighbouring semi-oxalate anion (Fig. 2).
The inclusion of water molecules into the structural framework of (I) also has a noticeable effect on the weak N—H···O hydrogen bonds. Thus, the N—H···O bonds between alaninium cations and semi-oxalate anions (N1···O5 and N1···O6i, respectively) are somewhat shorter than those in the anhydrous form, while the N1···O7ii bonds formed by the water molecules are weak (Tables 1 and 3).
When stored in air, (I) transforms into an anhydrous form of DL-alaninium semi-oxalate within 8–12 h at room temperature, as has been confirmed by X-ray powder diffraction analysis. Despite the similarity of the two structures, the dehydration of the hydrate on storage is not of a single-crystal to single-crystal type, but gives a polycrystalline pseudomorph, preserving the crystal habit. This transformation proceeds through the formation of an intermediate compound, presumably a hemihydrate, as has been shown by thermogravimetry analysis. Unfortunately, we could not collect either single-crystal or powder X-ray diffraction data, and could not measure vibrational spectra of this intermediate compound because of its low stability.
It is interesting to compare the structures of DL-alaninium semi-oxalate and its hydrate with the third structure from the same family, L-alaninium semi-oxalate. This compound crystallizes in space group P212121, i.e. in another crystal system. The chains of the semi-oxalate anions are also present in the structure of L-alaninium semi-oxalate. The O3···O6 distance in the O3—H···O6(1 + x, y, z) bond is 2.545 (2) Å at 293 K. In both hydrate and anhydrous forms of DL-alaninium semi-oxalate, the corresponding distances are somewhat longer, but these chains are linked to each other by bridges consisting of only one alaninum cation, and no ring motifs are found. This difference in the crystal packing also affects the N—H···O hydrogen bonds between L-alaninium cations and semi-oxalate anions in the chains: at 293 K in L-alaninium semi-oxalate the N···O distances in these bonds are 2.728 (2) Å [N1···O6(-x, 1/2 + y, 1/2 - z) and 2.896 (2) Å [N1···O5]. Thus, for L-alaninium semi-oxalate, the lengths of the N—H···O hydrogen bonds are not equal, whereas in the DL-alaninium semi-oxalates they are.
Finally, one can compare the crystal structures of the L- and DL-alaninium semi-oxalates (both hydrate and anhydrous forms) with those of glycinium semi-oxalate and its methanol solvate (Tumanov et al., 2010). Increasing the size of the amino acid by introducing a bulky –CH3 group has an effect on the way the dimers of the amino acid cations link the semi-oxalate chains in the crystal structure, as well as on the structure of the semi-oxalate chains themselves and their mutual orientation. In the structure of the glycinium semi-oxalate, individual amino acid molecules link the semi-oxalate chains to one another, while the glycinium cation dimers link stacks of semi-oxalate chains. Such semi-oxalate chains are not found in the structure of the methanol solvate of bis-glycinium oxalate. In the glycinium semi-oxalate, the oxalate chains are undulating, while they are flat in both the anhydrous DL-alaninium semi-oxalate and the L-alaninium semi-oxalate structures. In the hydrated form of DL-alaninium semi-oxalate, neighbouring semi-oxalate chains lie in different planes. The incorporation of solvent molecules into the crystal structures of DL-alaninium semi-oxalate hydrate and the methanol solvate of bis-glycinium oxalate is similar: the main structural framework is preserved, but the solvent molecules act as bridges between amino acid cations, thus preventing the cations from forming hydrogen bonds with one another. It appears that the coordination shell of a solvated amino acid cation existing in solution is partly preserved in the crystal structure.