l-Alanine methyl ester hydrochloride monohydrate

The enantiopure title compound, C4H10NO2 +·Cl−·H2O, forms a two-dimensional network by intermolecular hydrogen bonding parallel to (010). Non-merohedral twinning with a twofold rotation about the reciprocal c* axis as twin operation was taken into account during intensity integration and structure refinement. This twinning leads to alternative orientations of the stacked hydrogen-bonded layers.

The enantiopure title compound, C 4 H 10 NO 2 + ÁCl À ÁH 2 O, forms a two-dimensional network by intermolecular hydrogen bonding parallel to (010). Non-merohedral twinning with a twofold rotation about the reciprocal c* axis as twin operation was taken into account during intensity integration and structure refinement. This twinning leads to alternative orientations of the stacked hydrogen-bonded layers.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: EZ2229).

Comment
In the context of our ongoing studies of absolute structure determinations of hydrochlorides of amino acid esters, we determined the structure of the title compound (I). The related L-serine methyl ester hydrochloride (Schouten & Lutz, 2009) has an extended backbone with O-C-C-N and C-O-C-C torsion angles of -175.99 (7) and 179.72 (7)°, respectively. In (I), the O-C-C-N torsion angle is 155.83 (6)° indicating a significant deviation from an extended backbone. The C-O-C-C torsion angle of 177.22 (6)° is again close to a trans conformation (Fig. 1).
Two H atoms of the ammonium moiety are involved in hydrogen bonds with chloride anions as acceptors. This results in a one-dimensional chain in the a-direction. These two hydrogen bonds have significantly different lengths: the N1···Cl1 distance is 3.3007 (7) Å, while the N1···Cl1 (x -1, y, z) distance is 3.1665 (6) Å. The third ammonium H atom is hydrogen bonded to the co-crystallized lattice water molecule, which itself donates two hydrogen bonds to chlorides. The water thus links the one-dimensional chains into a two-dimensional network, which is parallel to the a,c-plane (Fig. 2). In the b-direction the hydrogen bonded layers of the ammonium moieties, chloride anions and lattice water molecules are alternating with the organic part of the alanine methyl ester. The O atoms of the ester functionality are not involved in strong intermolecular interactions, but there is a weak C-H···O bond with the ester O2 as acceptor (Table 1).
The crystal of (I) appeared to be twinned with a twofold rotation about hkl=(0,0,1) as twin operation. This twin relation was taken into account during the intensity integration with Eval15 (Schreurs et al., 2010) and the refinement (Herbst-Irmer & Sheldrick, 2002). As can easily be verified in Fig. 2, the twinning operation results in reversed stacking of the two-dimensional hydrogen bonded networks. At the twinning boundaries the polar and ionic groups involved in the hydrogen bonds must approach each other in the direction of the b axis and the alternation of polar and apolar moieties is broken. These stacking faults might be accompanied by shifts of the layers for a better structural fit. Such dislocations often depend on the way the twin was generated (Cahn, 1954), which has not been investigated in the present study of (I). In the macroscopic shape of the crystal of (I), faces hkl=(0,0,1) and (0,0,1) have the smallest dimensions.
For the determination of the absolute structure, reflections with inverted indices were introduced into the dataset using the TWINABS software (Sheldrick, 2008a). Thus there were in total four twin domains included in the refinement. The corresponding twin fractions refined to 0.86 (2) and 0.104 (4) for the non-merohedral domains, and 0.03 (2) and 0.005 (4) for the corresponding inverted domains. The latter values are very close to zero and we can consider the enantiopurity as proven, but it should be noted that the two twin fractions of the inverted domains are in the least-squares refinement highly correlated with each other (correlation -0.999).
Because of this correlation we also performed a single-crystal refinement only on the non-overlapping reflections of the major twin domain. This dataset has a completeness of 82% (1447 unique reflections) and the coverage of Bijvoet pairs is 80% (712 pairs). Here, the Flack parameter (Flack, 1983) refined to a value of x=0.04 (3). On these data also an analysis according to Hooft et al. (2008) was performed. Assuming a Gaussian distribution of σ(I) the absolute structure parameter was calculated as y=0.044 (9). A plot of the Bijvoet differences is shown in Fig. 3. supplementary materials sup-2 Experimental Crystalline L-alanine methyl ester (Aldrich) was dissolved in technical ethanol. Evaporation at room temperature resulted in a viscous liquid. Crystallization was initiated by adding a seed crystal of the crystalline starting material.

Refinement
The data set in HKLF-5 format (Herbst-Irmer & Sheldrick, 2002) contains non-overlapping reflections of both twin components, respectively, together with the overlapping reflections. Equivalent reflections were merged with TWINABS (Sheldrick, 2008a) prior to the least-squares refinement. The same software was used to introduce the inverted reflections for the absolute structure determination. Fig. 1. Molecular structure of (I), with atom labels and 50% probability displacement ellipsoids for non-H atoms.