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A determination of the structure of the title compound, C3H7NO2, leads to an accurate description of its molecular dimensions and crystal packing. As in the structure of the L-isomer, the mol­ecules aggregate into alternating layers, each consisting of only one type of isomer. The mol­ecules in each layer are interconnected through head-to-tail sequences generated by a cell translation and a 21 screw axis. Adjacent layers are interconnected by head-to-tail sequences generated by a glide plane.

Supporting information


Crystallographic Information File (CIF)
Contains datablocks global, I


Structure factor file (CIF format)
Contains datablock I

CCDC reference: 164668

Comment top

DL-Alanine, (I), is one of the few amino acids for which accurate X-ray crystal structure is not known. Previous work on this amino acid reports the cell dimensions (Bernal, 1931) and X-ray crystal structures derived from two-dimensional intensity data (Levy & Corey, 1941; Donohue, 1950). We report here an accurate determination of the crystal structure of DL-alanine at room temperature. This structure represents a rare case of an amino acid racemate crystallizing in a non-centrosymmetric space group. Another such structure is DL-tyrosine (Mostad & Romming, 1973). \sch

The DL-alanine molecule exists as a zwitterion. The C—O distances in the caboxylate group are unequal presumably due to the participation of one of the O atoms (O1) in one hydrogen bond and the other (O2) in two. The C—N distance, formerly thought to be unusually shorter by Levy & Corey (1941) with a value of 1.427 Å, is found to be 1.483 (3) Å in the present work. This is slightly less than the value of 1.496 Å quoted by Donohue (1950). The N atom deviates by 0.392 (5) Å from the carboxylate plane and the methyl carbon deviates by 1.356 (4) Å, in the opposite direction.

The crystal structure is stabilized by a network of characteristic head-to-tail hydrogen-bond sequences. The structure contains three types of such sequences viz. S2 (straight sequence along the c axis with O2 of the carboxylate group as acceptor), Z1 (zigzag sequence along the 21 screw axis with O1 of the carboxylate group as acceptor) and DL2 (zigzag-DL sequence among the glide related molecules with O2 of the carboxylate group as acceptor) (Suresh & Vijayan, 1983). While the sequences S2 and Z1 connect molecules in each layer, the zigzag-DL sequence connect alternating layers, each containing one isomer. The direction of the DL2 sequence is parallel to the plane of the amino acid layers. There is a striking similarity between this structure and that of its L-isomer (Simpson & Marsh, 1966; Destro et al., 1988) which is not uncommon in most other hydrophobic amino acids too (Soman & Vijayan, 1989). The cell dimensions of L– and DL– isomers are nearly identical. Both structures belong to orthorhombic system, but the space group is P212121 for the L-isomer and Pna21 for the racemate. Further, the arrangements of molecules within layers in the crystal structures of both L– and DL-alanine are identical. However, the DL2 sequence observed in the racemate is replaced by a Z2 sequence in its L-isomer. In addition, a weak C—H···O hydrogen bond, with the carboxylate oxygen O1 as acceptor, is observed among the glide-related molecules, interconnecting alternate layers, each containing one isomer.

Related literature top

For related literature, see: Bernal (1931); Destro et al. (1988); Donohue (1950); Levy & Corey (1941); Mostad & Romming (1973); Simpson & Marsh (1966); Soman & Vijayan (1989); Suresh & Vijayan (1983).

Experimental top

Colourless single crystals of the amino acid were grown as fine transparent needles from a saturated aqueous solution. The density was determined by the flotation method using a liquid mixture of carbon tetrachloride and xylene.

Refinement top

All the H atoms were generated geometrically and treated with a riding model with Uiso fixed at 1.2Ueq of the bonded atoms or 1.5Ueq for amino and methyl groups.

Computing details top

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: CAD-4 Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. The molecular structure, with atom-numbering scheme and 50% probability displacement ellipsoids.
[Figure 2] Fig. 2. Packing diagram of the molecule viewed down the b axis.
DL-Alanine top
Crystal data top
C3H7NO2Dx = 1.399 Mg m3
Dm = 1.39 Mg m3
Dm measured by floatation
Mr = 89.10Cu Kα radiation, λ = 1.54180 Å
Orthorhombic, Pna21Cell parameters from 25 reflections
a = 12.0263 (17) Åθ = 15–27°
b = 6.0321 (9) ŵ = 1.00 mm1
c = 5.829 (2) ÅT = 293 K
V = 422.88 (19) Å3Fine needles, colourless
Z = 40.42 × 0.24 × 0.18 mm
F(000) = 192
Data collection top
Enraf-Nonius CAD-4
417 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.000
Graphite monochromatorθmax = 67.6°, θmin = 7.4°
ω–2θ scansh = 140
Absorption correction: ψ scan
(North et al., 1968)
k = 07
Tmin = 0.97, Tmax = 0.99l = 07
422 measured reflections2 standard reflections every 60 min
422 independent reflections intensity decay: 2%
Refinement top
Refinement on F2Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: fullH-atom parameters constrained
R[F2 > 2σ(F2)] = 0.025 w = 1/[σ2(Fo2) + (0.0512P)2 + 0.0571P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.072(Δ/σ)max < 0.001
S = 1.09Δρmax = 0.14 e Å3
422 reflectionsΔρmin = 0.15 e Å3
56 parametersExtinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
1 restraintExtinction coefficient: 0.012 (3)
Primary atom site location: structure-invariant direct methodsAbsolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Secondary atom site location: difference Fourier map
Crystal data top
C3H7NO2V = 422.88 (19) Å3
Mr = 89.10Z = 4
Orthorhombic, Pna21Cu Kα radiation
a = 12.0263 (17) ŵ = 1.00 mm1
b = 6.0321 (9) ÅT = 293 K
c = 5.829 (2) Å0.42 × 0.24 × 0.18 mm
Data collection top
Enraf-Nonius CAD-4
417 reflections with I > 2σ(I)
Absorption correction: ψ scan
(North et al., 1968)
Rint = 0.000
Tmin = 0.97, Tmax = 0.992 standard reflections every 60 min
422 measured reflections intensity decay: 2%
422 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0251 restraint
wR(F2) = 0.072H-atom parameters constrained
S = 1.09Δρmax = 0.14 e Å3
422 reflectionsΔρmin = 0.15 e Å3
56 parametersAbsolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
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
O10.58960 (11)0.4862 (2)0.6045 (3)0.0356 (4)
O20.68614 (12)0.2009 (2)0.7409 (3)0.0328 (4)
C20.66363 (15)0.2231 (3)0.3380 (3)0.0224 (4)
C10.64429 (13)0.3133 (3)0.5818 (3)0.0222 (4)
C30.59233 (16)0.0216 (3)0.2916 (4)0.0347 (5)
N10.63948 (12)0.3976 (2)0.1659 (3)0.0258 (4)
Atomic displacement parameters (Å2) top
O10.0407 (7)0.0418 (7)0.0242 (9)0.0146 (5)0.0024 (7)0.0047 (6)
O20.0415 (7)0.0416 (8)0.0152 (7)0.0080 (6)0.0017 (6)0.0006 (6)
C20.0256 (7)0.0287 (9)0.0130 (9)0.0029 (7)0.0010 (7)0.0009 (8)
C10.0210 (8)0.0319 (8)0.0136 (9)0.0013 (6)0.0004 (6)0.0026 (8)
C30.0457 (11)0.0340 (9)0.0245 (13)0.0057 (8)0.0022 (9)0.0036 (9)
N10.0300 (7)0.0326 (8)0.0148 (8)0.0002 (6)0.0003 (6)0.0002 (6)
Geometric parameters (Å, º) top
O1—C11.240 (2)C3—H3A0.9600
O2—C11.254 (3)C3—H3B0.9600
C2—N11.483 (3)C3—H3C0.9600
C2—C31.512 (3)N1—H1A0.8900
C2—C11.540 (3)N1—H1B0.8900
N1—C2—C3109.80 (17)H3A—C3—H3B109.5
N1—C2—C1110.13 (15)C2—C3—H3C109.5
C3—C2—C1111.30 (18)H3A—C3—H3C109.5
O1—C1—O2126.08 (19)H1A—N1—H1B109.5
O1—C1—C2118.42 (19)C2—N1—H1C109.5
O2—C1—C2115.50 (15)H1A—N1—H1C109.5
N1—C2—C1—O116.3 (2)N1—C2—C1—O2163.97 (16)
C3—C2—C1—O1105.69 (18)C3—C2—C1—O274.0 (2)
Hydrogen-bond geometry (Å, º) top
N1—H1A···O2i0.891.962.817 (2)160
N1—H1B···O1ii0.892.002.865 (2)165
N1—H1C···O2iii0.891.922.804 (3)173
C2—H2···O1iv0.982.673.566 (3)153
Symmetry codes: (i) x+3/2, y+1/2, z1/2; (ii) x+1, y+1, z1/2; (iii) x, y, z1; (iv) x+3/2, y1/2, z1/2.

Experimental details

Crystal data
Chemical formulaC3H7NO2
Crystal system, space groupOrthorhombic, Pna21
Temperature (K)293
a, b, c (Å)12.0263 (17), 6.0321 (9), 5.829 (2)
V3)422.88 (19)
Radiation typeCu Kα
µ (mm1)1.00
Crystal size (mm)0.42 × 0.24 × 0.18
Data collection
DiffractometerEnraf-Nonius CAD-4
Absorption correctionψ scan
(North et al., 1968)
Tmin, Tmax0.97, 0.99
No. of measured, independent and
observed [I > 2σ(I)] reflections
422, 422, 417
(sin θ/λ)max1)0.600
R[F2 > 2σ(F2)], wR(F2), S 0.025, 0.072, 1.09
No. of reflections422
No. of parameters56
No. of restraints1
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.14, 0.15
Absolute structureFlack H D (1983), Acta Cryst. A39, 876-881

Computer programs: CAD-4 Software (Enraf-Nonius, 1989), CAD-4 Software, SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 1999), SHELXL97.

Hydrogen-bond geometry (Å, º) top
N1—H1A···O2i0.891.962.817 (2)160
N1—H1B···O1ii0.892.002.865 (2)165
N1—H1C···O2iii0.891.922.804 (3)173
C2—H2···O1iv0.982.673.566 (3)153
Symmetry codes: (i) x+3/2, y+1/2, z1/2; (ii) x+1, y+1, z1/2; (iii) x, y, z1; (iv) x+3/2, y1/2, z1/2.

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