The crystal structure of L-aspartic acid, C4H7NO4, has been determined using two types of refinement, viz. the standard independent atom model (IAM) and the experimental library multipolar atom model (ELMAM). The ELMAM refinement shows a good improvement of the statistical indices compared with the IAM model, notably in terms of thermal displacement parameters and bond distances involving H atoms.
Supporting information
CCDC references: 652520; 652521
Crystals of L-aspartic acid were grown by slow evaporation of an aqueous
solution when attempts were made to grow single crystals of a complex of
L-aspartic acid with phosphoric acid. Crystals of (I) suitable for
single-crystal X-ray diffraction were selected directly from the sample as
prepared.
The crystal structure of L-aspartic acid (Fig. 1) was solved in the
non-centrosymmetric space group P21 by direct methods using the
program SHELXS97 (Sheldrick, 1997). Least-squares refinement, based on
|F|, was carried out using the program MOPRO (Guillot et
al., 2001; Jelsch et al., 2005) using the conventional spherical
neutral-atom model. The reflection weights were set equal to
1/σ2(Fo). All H atoms were located in difference Fourier maps.
Their Uiso(H) values were restrained to be 1.2Ueq of the
attached atom with a standard deviation of 0.01 Å. Observations with
significantly large ΔF values (>10) were carefully excluded at every
stage in order to avoid their undue influence on the course of refinement. In
the absence of suitable anomalous scattering, refinement of the Flack (1983)
parameter led to inconclusive values, so a definite conclusion on the absolute
structure and chirality of the molecule cannot be drawn (Flack &
Bernardinelli, 2000). Therefore, Friedel equivalents were merged prior to the
final refinements, and the absolute structure was set by reference to the
known chirality of the enantiopure acid used in the crystallization
experiment.
For both compounds, data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: MOPRO6 (Jelsch et al., 2005). Molecular graphics: ORTEPIII (Burnett & Johnson, 1996) for I_ELMAM; PLATON (Spek, 2003) for I_IAM. For both compounds, software used to prepare material for publication: enCIFer (Allen et al., 2004).
(I_ELMAM)
L-Aspartic acid
top
Crystal data top
C4H7NO4 | F(000) = 140 |
Mr = 133.11 | Dx = 1.678 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2yb | Cell parameters from 7129 reflections |
a = 5.1135 (2) Å | θ = 2.7–32.9° |
b = 6.9059 (3) Å | µ = 0.15 mm−1 |
c = 7.5925 (3) Å | T = 100 K |
β = 100.662 (4)° | Prism, colourless |
V = 263.48 (3) Å3 | 0.2 × 0.15 × 0.1 mm |
Z = 2 | |
Data collection top
Oxford Diffraction Xcalibur-Sapphire2 diffractometer | 995 independent reflections |
Radiation source: Enhance (Mo) X-ray Source | 968 reflections with I > 0σ(I) |
Graphite monochromator | Rint = 0.045 |
Detector resolution: 16.0009 pixels mm-1 | θmax = 32.9°, θmin = 2.7° |
ω scans | h = −7→7 |
Absorption correction: integration (ABSORB; DeTitta, 1985) | k = 0→10 |
Tmin = 0.971, Tmax = 0.982 | l = 0→11 |
7129 measured reflections | |
Refinement top
Refinement on F | 7 restraints |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.043 | w = 1/[σ2(Fo)] |
wR(F2) = 0.019 | (Δ/σ)max < 0.001 |
S = 1.47 | Δρmax = 0.30 e Å−3 |
968 reflections | Δρmin = −0.20 e Å−3 |
110 parameters | |
Crystal data top
C4H7NO4 | V = 263.48 (3) Å3 |
Mr = 133.11 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 5.1135 (2) Å | µ = 0.15 mm−1 |
b = 6.9059 (3) Å | T = 100 K |
c = 7.5925 (3) Å | 0.2 × 0.15 × 0.1 mm |
β = 100.662 (4)° | |
Data collection top
Oxford Diffraction Xcalibur-Sapphire2 diffractometer | 995 independent reflections |
Absorption correction: integration (ABSORB; DeTitta, 1985) | 968 reflections with I > 0σ(I) |
Tmin = 0.971, Tmax = 0.982 | Rint = 0.045 |
7129 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.043 | 7 restraints |
wR(F2) = 0.019 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.47 | Δρmax = 0.30 e Å−3 |
968 reflections | Δρmin = −0.20 e Å−3 |
110 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 F against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F, 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 | |
NT | 0.8583 (3) | 0.978 (9) | 0.3162 (2) | 0.0102 (3) | |
H1 | 1.029 (5) | 0.914 (10) | 0.388 (3) | 0.0121 (2)* | |
H2 | 0.810 (4) | 1.100 (10) | 0.389 (3) | 0.0121 (2)* | |
H3 | 0.900 (4) | 1.022 (10) | 0.188 (3) | 0.0121 (2)* | |
CA | 0.6348 (3) | 0.836 (9) | 0.2952 (2) | 0.0083 (4) | |
CT | 0.5605 (3) | 0.796 (9) | 0.4786 (2) | 0.0088 (4) | |
CB | 0.7156 (3) | 0.644 (9) | 0.2198 (2) | 0.0116 (4) | |
CG | 0.7930 (3) | 0.659 (9) | 0.0378 (2) | 0.0097 (4) | |
OT1 | 0.7497 (2) | 0.784 (9) | 0.6095 (1) | 0.0123 (3) | |
OT2 | 0.3184 (2) | 0.771 (9) | 0.4817 (1) | 0.0124 (3) | |
OD1 | 1.0020 (2) | 0.593 (9) | 0.0085 (1) | 0.0199 (3) | |
OD2 | 0.6125 (2) | 0.744 (9) | −0.0839 (1) | 0.0125 (3) | |
HA | 0.468 (3) | 0.898 (10) | 0.207 (2) | 0.0099 (1)* | |
HB1 | 0.881 (4) | 0.578 (10) | 0.313 (3) | 0.0139 (2)* | |
HB2 | 0.541 (3) | 0.556 (10) | 0.207 (2) | 0.0139 (2)* | |
HD | 0.679 (4) | 0.750 (10) | −0.203 (3) | 0.0149 (2)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
NT | 0.0102 (6) | 0.0105 (7) | 0.0096 (6) | −0.0037 (6) | 0.0025 (5) | −0.0016 (6) |
CA | 0.0088 (7) | 0.0083 (7) | 0.0077 (7) | −0.0018 (6) | 0.0033 (5) | 0.0006 (5) |
CT | 0.0103 (7) | 0.0102 (7) | 0.0059 (6) | 0.0017 (6) | 0.0029 (5) | 0.0018 (6) |
CB | 0.0179 (8) | 0.0092 (7) | 0.0076 (7) | −0.0002 (6) | 0.0051 (6) | −0.0016 (6) |
CG | 0.0153 (7) | 0.0083 (7) | 0.0055 (7) | 0.0013 (6) | 0.0045 (6) | 0.0001 (6) |
OT1 | 0.0103 (5) | 0.0191 (6) | 0.0071 (5) | 0.0008 (5) | 0.0008 (4) | 0.0013 (5) |
OT2 | 0.0091 (5) | 0.0157 (5) | 0.0124 (5) | −0.0011 (5) | 0.0035 (4) | 0.0021 (5) |
OD1 | 0.0205 (6) | 0.0273 (7) | 0.0119 (5) | 0.0124 (6) | 0.0064 (5) | 0.0020 (5) |
OD2 | 0.0126 (6) | 0.0166 (6) | 0.0082 (5) | 0.0015 (5) | 0.0024 (4) | 0.0022 (5) |
Geometric parameters (Å, º) top
NT—H1 | 1.04 (4) | CT—OT2 | 1.254 (1) |
NT—H2 | 1.07 (3) | CB—CG | 1.510 (2) |
NT—H3 | 1.08 (3) | CB—HB1 | 1.09 (3) |
NT—CA | 1.492 (3) | CB—HB2 | 1.07 (2) |
CA—CT | 1.535 (2) | CG—OD1 | 1.220 (2) |
CA—CB | 1.532 (3) | CG—OD2 | 1.317 (3) |
CA—HA | 1.07 (3) | OD2—HD | 1.03 (2) |
CT—OT1 | 1.255 (2) | | |
| | | |
NT—CA—CT | 109.7 (3) | CA—CB—HB2 | 104 (2) |
NT—CA—CB | 110.7 (4) | CT—CA—CB | 108.0 (4) |
NT—CA—HA | 108 (2) | CT—CA—HA | 110 (3) |
H1—NT—H2 | 109 (5) | CB—CA—HA | 111 (3) |
H1—NT—H3 | 108 (5) | CB—CG—OD1 | 121.8 (5) |
H1—NT—CA | 109 (2) | CB—CG—OD2 | 113.9 (4) |
H2—NT—H3 | 111 (5) | CG—CB—HB1 | 109 (3) |
H2—NT—CA | 109 (2) | CG—CB—HB2 | 108 (2) |
H3—NT—CA | 111 (2) | CG—OD2—HD | 110 (3) |
CA—CT—OT1 | 116.5 (3) | OT1—CT—OT2 | 126.5 (4) |
CA—CT—OT2 | 116.9 (3) | OD1—CG—OD2 | 124.2 (5) |
CA—CB—CG | 114.6 (4) | HB1—CB—HB2 | 111 (4) |
CA—CB—HB1 | 110 (3) | | |
| | | |
NT—CA—CT—OT1 | −39.0 (4) | H3—NT—CA—CT | −174 (1) |
NT—CA—CT—OT2 | 143.3 (4) | H3—NT—CA—CB | 67 (3) |
NT—CA—CB—CG | −60.3 (6) | CA—CB—CG—OD1 | 128.3 (8) |
H1—NT—CA—CT | 67 (4) | CA—CB—CG—OD2 | −53.5 (7) |
H1—NT—CA—CB | −52 (5) | CT—CA—CB—CG | 179.6 (2) |
H2—NT—CA—CT | −51 (4) | CB—CA—CT—OT1 | 81.8 (5) |
H2—NT—CA—CB | −170 (1) | CB—CA—CT—OT2 | −95.9 (4) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
OD2—HD···OT1i | 1.03 | 1.55 | 2.567 (1) | 171 |
NT—H2···OT2ii | 1.07 | 1.74 | 2.792 (2) | 169 |
NT—H3···OD1iii | 1.08 | 1.73 | 2.806 (2) | 175 |
NT—H1···OT2iv | 1.04 | 1.81 | 2.838 (3) | 170 |
Symmetry codes: (i) x, y, z−1; (ii) −x+1, y+1/2, −z+1; (iii) −x+2, y+1/2, −z; (iv) x+1, y, z. |
(I_IAM)
L-Aspartic acid
top
Crystal data top
C4H7NO4 | F(000) = 140 |
Mr = 133.11 | Dx = 1.678 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2yb | Cell parameters from 7129 reflections |
a = 5.1135 (2) Å | θ = 2.7–32.9° |
b = 6.9059 (3) Å | µ = 0.15 mm−1 |
c = 7.5925 (3) Å | T = 100 K |
β = 100.662 (4)° | Prism, colourless |
V = 263.48 (3) Å3 | 0.2 × 0.15 × 0.1 mm |
Z = 2 | |
Data collection top
Oxford Diffraction Xcalibur-Sapphire2 diffractometer | 995 independent reflections |
Radiation source: Enhance (Mo) X-ray Source | 968 reflections with I > 0σ(I) |
Graphite monochromator | Rint = 0.045 |
Detector resolution: 16.0009 pixels mm-1 | θmax = 32.9°, θmin = 2.7° |
ω scans | h = −7→7 |
Absorption correction: integration (ABSORB; DeTitta, 1985) | k = 0→10 |
Tmin = 0.971, Tmax = 0.982 | l = 0→11 |
7129 measured reflections | |
Refinement top
Refinement on F | 7 restraints |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.055 | w = 1/[σ2(Fo)] |
wR(F2) = 0.026 | (Δ/σ)max = 0.000012 |
S = 1.88 | Δρmax = 0.43 e Å−3 |
968 reflections | Δρmin = −0.26 e Å−3 |
110 parameters | |
Crystal data top
C4H7NO4 | V = 263.48 (3) Å3 |
Mr = 133.11 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 5.1135 (2) Å | µ = 0.15 mm−1 |
b = 6.9059 (3) Å | T = 100 K |
c = 7.5925 (3) Å | 0.2 × 0.15 × 0.1 mm |
β = 100.662 (4)° | |
Data collection top
Oxford Diffraction Xcalibur-Sapphire2 diffractometer | 995 independent reflections |
Absorption correction: integration (ABSORB; DeTitta, 1985) | 968 reflections with I > 0σ(I) |
Tmin = 0.971, Tmax = 0.982 | Rint = 0.045 |
7129 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.055 | 7 restraints |
wR(F2) = 0.026 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.88 | Δρmax = 0.43 e Å−3 |
968 reflections | Δρmin = −0.26 e Å−3 |
110 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 F against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F, 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 | |
NT | 0.8581 (3) | 0.9755 (2) | 0.3160 (3) | 0.0113 (4) | |
H1 | 1.0148 (3) | 0.9172 (2) | 0.3825 (3) | 0.0134 (2)* | |
H2 | 0.8127 (2) | 1.0890 (2) | 0.3827 (3) | 0.0134 (2)* | |
H3 | 0.8940 (3) | 1.0202 (2) | 0.2003 (1) | 0.0134 (2)* | |
CA | 0.6349 (3) | 0.8331 (1) | 0.2953 (1) | 0.0100 (5) | |
CT | 0.5604 (3) | 0.7934 (2) | 0.4781 (1) | 0.0112 (5) | |
CB | 0.7158 (2) | 0.6424 (4) | 0.2197 (2) | 0.0135 (5) | |
CG | 0.7937 (2) | 0.6576 (1) | 0.0378 (2) | 0.0125 (5) | |
OT1 | 0.7497 (2) | 0.7816 (2) | 0.6093 (1) | 0.0140 (3) | |
OT2 | 0.3184 (2) | 0.7691 (3) | 0.4818 (2) | 0.0148 (3) | |
OD1 | 1.0014 (2) | 0.5914 (3) | 0.0085 (1) | 0.0228 (4) | |
OD2 | 0.6126 (3) | 0.7418 (1) | −0.0835 (1) | 0.0148 (4) | |
HA | 0.4745 (3) | 0.8958 (2) | 0.2130 (3) | 0.0120 (2)* | |
HB1 | 0.8685 (1) | 0.5811 (2) | 0.3121 (3) | 0.0162 (3)* | |
HB2 | 0.5408 (3) | 0.5628 (2) | 0.2115 (1) | 0.0162 (3)* | |
HD | 0.6645 (1) | 0.7550 (1) | −0.1938 (2) | 0.0176 (3)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
NT | 0.0124 (7) | 0.0117 (8) | 0.0095 (7) | −0.0025 (6) | 0.0028 (6) | −0.0001 (6) |
CA | 0.0098 (9) | 0.0110 (8) | 0.0090 (8) | −0.0018 (7) | 0.0027 (6) | 0.0005 (6) |
CT | 0.0154 (8) | 0.0071 (8) | 0.0110 (8) | 0.0006 (7) | 0.0048 (7) | 0.0005 (7) |
CB | 0.0189 (9) | 0.0111 (8) | 0.0105 (8) | −0.0002 (7) | 0.0053 (7) | −0.0011 (7) |
CG | 0.0180 (9) | 0.0100 (8) | 0.0089 (8) | −0.0027 (7) | 0.0027 (7) | −0.0018 (7) |
OT1 | 0.0124 (6) | 0.0201 (7) | 0.0091 (6) | 0.0005 (6) | 0.0016 (4) | 0.0013 (6) |
OT2 | 0.0116 (6) | 0.0176 (7) | 0.0151 (6) | −0.0010 (6) | 0.0042 (5) | 0.0024 (6) |
OD1 | 0.0241 (7) | 0.0302 (8) | 0.0142 (6) | 0.0121 (7) | 0.0072 (5) | 0.0026 (7) |
OD2 | 0.0150 (7) | 0.0189 (7) | 0.0104 (6) | 0.0017 (5) | 0.0031 (5) | 0.0020 (5) |
Geometric parameters (Å, º) top
NT—H1 | 0.95 | CT—OT2 | 1.254 (3) |
NT—H2 | 0.98 | CB—CG | 1.510 (3) |
NT—H3 | 0.98 | CB—HB1 | 1.04 (3) |
NT—CA | 1.493 (2) | CB—HB2 | 1.04 (2) |
CA—CT | 1.530 (3) | CG—OD1 | 1.214 (3) |
CA—CB | 1.524 (4) | CG—OD2 | 1.315 (4) |
CA—HA | 1.03 | OD2—HD | 0.93 (1) |
CT—OT1 | 1.257 (2) | | |
| | | |
NT—CA—CT | 109.7 (5) | CA—CB—HB2 | 100 (9) |
NT—CA—CB | 110.7 (5) | CT—CA—CB | 108.5 (5) |
NT—CA—HA | 107.1 (3) | CT—CA—HA | 108 (8) |
H1—NT—H2 | 108 (6) | CB—CA—HA | 112 (1) |
H1—NT—H3 | 109 (2) | CB—CG—OD1 | 122.0 (7) |
H1—NT—CA | 109 (2) | CB—CG—OD2 | 113.6 (5) |
H2—NT—H3 | 108 (2) | CG—CB—HB1 | 110 (7) |
H2—NT—CA | 109 (3) | CG—CB—HB2 | 111 (5) |
H3—NT—CA | 112 (3) | CG—OD2—HD | 113 (2) |
CA—CT—OT1 | 116.5 (4) | OT1—CT—OT2 | 126.3 (5) |
CA—CT—OT2 | 117.2 (5) | OD1—CG—OD2 | 124.4 (7) |
CA—CB—CG | 114.9 (6) | HB1—CB—HB2 | 111 (8) |
CA—CB—HB1 | 108 (7) | | |
| | | |
NT—CA—CT—OT1 | −39.3 (5) | CA—CB—CG—OD2 | −53.7 (9) |
NT—CA—CT—OT2 | 143.1 (5) | CT—CA—CB—CG | 179.6 (3) |
NT—CA—CB—CG | −60.0 (8) | CB—CA—CT—OT1 | 81.8 (6) |
CA—CB—CG—OD1 | 128.5 (8) | CB—CA—CT—OT2 | −95.8 (6) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
OD2—HD···OT1i | 0.93 | 1.64 | 2.571 (3) | 179 |
NT—H2···OT2ii | 0.98 | 1.82 | 2.792 (3) | 169 |
NT—H3···OD1iii | 0.98 | 1.83 | 2.808 (3) | 173 |
NT—H1···OT2iv | 0.95 | 1.90 | 2.839 (4) | 170 |
Symmetry codes: (i) x, y, z−1; (ii) −x+1, y+1/2, −z+1; (iii) −x+2, y+1/2, −z; (iv) x+1, y, z. |
Experimental details
| (I_ELMAM) | (I_IAM) |
Crystal data |
Chemical formula | C4H7NO4 | C4H7NO4 |
Mr | 133.11 | 133.11 |
Crystal system, space group | Monoclinic, P21 | Monoclinic, P21 |
Temperature (K) | 100 | 100 |
a, b, c (Å) | 5.1135 (2), 6.9059 (3), 7.5925 (3) | 5.1135 (2), 6.9059 (3), 7.5925 (3) |
β (°) | 100.662 (4) | 100.662 (4) |
V (Å3) | 263.48 (3) | 263.48 (3) |
Z | 2 | 2 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.15 | 0.15 |
Crystal size (mm) | 0.2 × 0.15 × 0.1 | 0.2 × 0.15 × 0.1 |
|
Data collection |
Diffractometer | Oxford Diffraction Xcalibur-Sapphire2 diffractometer | Oxford Diffraction Xcalibur-Sapphire2 diffractometer |
Absorption correction | Integration (ABSORB; DeTitta, 1985) | Integration (ABSORB; DeTitta, 1985) |
Tmin, Tmax | 0.971, 0.982 | 0.971, 0.982 |
No. of measured, independent and observed [I > 0σ(I)] reflections | 7129, 995, 968 | 7129, 995, 968 |
Rint | 0.045 | 0.045 |
(sin θ/λ)max (Å−1) | 0.763 | 0.763 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.043, 0.019, 1.47 | 0.055, 0.026, 1.88 |
No. of reflections | 968 | 968 |
No. of parameters | 110 | 110 |
No. of restraints | 7 | 7 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.30, −0.20 | 0.43, −0.26 |
Selected geometric parameters (Å, º) for (I_ELMAM) topNT—H1 | 1.04 (4) | CT—OT2 | 1.254 (1) |
NT—H2 | 1.07 (3) | CB—CG | 1.510 (2) |
NT—H3 | 1.08 (3) | CB—HB1 | 1.09 (3) |
NT—CA | 1.492 (3) | CB—HB2 | 1.07 (2) |
CA—CT | 1.535 (2) | CG—OD1 | 1.220 (2) |
CA—CB | 1.532 (3) | CG—OD2 | 1.317 (3) |
CA—HA | 1.07 (3) | OD2—HD | 1.03 (2) |
CT—OT1 | 1.255 (2) | | |
| | | |
NT—CA—CT | 109.7 (3) | CA—CB—HB2 | 104 (2) |
NT—CA—CB | 110.7 (4) | CT—CA—CB | 108.0 (4) |
NT—CA—HA | 108 (2) | CT—CA—HA | 110 (3) |
H1—NT—H2 | 109 (5) | CB—CA—HA | 111 (3) |
H1—NT—H3 | 108 (5) | CB—CG—OD1 | 121.8 (5) |
H1—NT—CA | 109 (2) | CB—CG—OD2 | 113.9 (4) |
H2—NT—H3 | 111 (5) | CG—CB—HB1 | 109 (3) |
H2—NT—CA | 109 (2) | CG—CB—HB2 | 108 (2) |
H3—NT—CA | 111 (2) | CG—OD2—HD | 110 (3) |
CA—CT—OT1 | 116.5 (3) | OT1—CT—OT2 | 126.5 (4) |
CA—CT—OT2 | 116.9 (3) | OD1—CG—OD2 | 124.2 (5) |
CA—CB—CG | 114.6 (4) | HB1—CB—HB2 | 111 (4) |
CA—CB—HB1 | 110 (3) | | |
| | | |
CT—CA—CB—CG | 179.6 (2) | | |
Hydrogen-bond geometry (Å, º) for (I_ELMAM) top
D—H···A | D—H | H···A | D···A | D—H···A |
OD2—HD···OT1i | 1.027 | 1.546 | 2.567 (1) | 171 |
NT—H2···OT2ii | 1.066 | 1.738 | 2.792 (2) | 169 |
NT—H3···OD1iii | 1.077 | 1.731 | 2.806 (2) | 175 |
NT—H1···OT2iv | 1.037 | 1.811 | 2.838 (3) | 170 |
Symmetry codes: (i) x, y, z−1; (ii) −x+1, y+1/2, −z+1; (iii) −x+2, y+1/2, −z; (iv) x+1, y, z. |
Selected geometric parameters (Å, º) for (I_IAM) topNT—H1 | 0.95 | CT—OT2 | 1.254 (3) |
NT—H2 | 0.98 | CB—CG | 1.510 (3) |
NT—H3 | 0.98 | CB—HB1 | 1.04 (3) |
NT—CA | 1.493 (2) | CB—HB2 | 1.04 (2) |
CA—CT | 1.530 (3) | CG—OD1 | 1.214 (3) |
CA—CB | 1.524 (4) | CG—OD2 | 1.315 (4) |
CA—HA | 1.03 | OD2—HD | 0.93 (1) |
CT—OT1 | 1.257 (2) | | |
| | | |
NT—CA—CT | 109.7 (5) | CA—CB—HB2 | 100 (9) |
NT—CA—CB | 110.7 (5) | CT—CA—CB | 108.5 (5) |
NT—CA—HA | 107.1 (3) | CT—CA—HA | 108 (8) |
H1—NT—H2 | 108 (6) | CB—CA—HA | 112 (1) |
H1—NT—H3 | 109 (2) | CB—CG—OD1 | 122.0 (7) |
H1—NT—CA | 109 (2) | CB—CG—OD2 | 113.6 (5) |
H2—NT—H3 | 108 (2) | CG—CB—HB1 | 110 (7) |
H2—NT—CA | 109 (3) | CG—CB—HB2 | 111 (5) |
H3—NT—CA | 112 (3) | CG—OD2—HD | 113 (2) |
CA—CT—OT1 | 116.5 (4) | OT1—CT—OT2 | 126.3 (5) |
CA—CT—OT2 | 117.2 (5) | OD1—CG—OD2 | 124.4 (7) |
CA—CB—CG | 114.9 (6) | HB1—CB—HB2 | 111 (8) |
CA—CB—HB1 | 108 (7) | | |
| | | |
CT—CA—CB—CG | 179.6 (3) | | |
Hydrogen-bond geometry (Å, º) for (I_IAM) top
D—H···A | D—H | H···A | D···A | D—H···A |
OD2—HD···OT1i | 0.929 | 1.642 | 2.571 (3) | 179 |
NT—H2···OT2ii | 0.984 | 1.819 | 2.792 (3) | 169 |
NT—H3···OD1iii | 0.980 | 1.832 | 2.808 (3) | 173 |
NT—H1···OT2iv | 0.954 | 1.895 | 2.839 (4) | 170 |
Symmetry codes: (i) x, y, z−1; (ii) −x+1, y+1/2, −z+1; (iii) −x+2, y+1/2, −z; (iv) x+1, y, z. |
Despite the importance of L-aspartic acid (C4H7NO4), (I), only a few studies have been performed on the determination of its complexes. The X-ray crystal structure analysis of DL-aspartic acid was originally determined by Rao (1973) and a neutron structure analysis at room temperature has been reported by Sequeira et al. (1989). A topological analysis of the experimental charge density has been reported for DL-aspartic acid (Flaig et al., 1998). The crystal structure of DL-aspartic acid nitrate monohydrate has also been characterized (Asath Bahadur & Rajaram, 1995), and more recently bis(DL-aspartic acid) sulfate (Srinivasan et al., 2001) and L-aspartic acid nitrate L-aspartic acid (1/1) (Sridhar et al., 2002) have also been reported. It is worth noting that the atomic coordinates reported for L-aspartic acid monohydrate by Umadevi et al. (2003) correspond to the D configuration.
The purpose of this study is the accurate low-temperature redetermination of the crystal structure of L-aspartic acid and the comparison of the refinements using the same intensity data set using the experimental library multipolar atom model refinement (ELMAM) (Zarychta et al., 2007) and the standard independent atom model (IAM).
L-Aspartic acid, (I) (Fig. 1), was first prepared and its crystal and molecular structure characterized by Derissen et al. (1968) from single-crystal diffraction data at room temperature. However, a redetermination was considered worthwhile as, on the one hand, the atomic coordinates reported by Derissen et al. (1968) correspond to the D configuration, and on the other hand, fewer than 600 reflections were used to refine all structural parameters. The results reported here are based on data which are, to the best of our knowledge, the most extensive (sinθ/λ= 0.764 Å-1), collected at 100 K with a conventional X-ray source (Mo Kα) and a two-dimensional CCD detector.
Compound (I) crystallizes as a zwitterion, like many other α-amino acids. The deprotonated and protonated carboxyl groups can be clearly distinguished. In the deprotonated carboxyl group COO-, the two CT—O bonds to the terminal O atoms (OT1 and OT2) are identical within one s.u. [1.255 (2) and 1.254 (1) Å, respectively]. The protonated COOH group is, on the other hand, characterized by a double bond [CG—OD1 = 1.220 (2) Å] which is significantly shorter than the single bond involving the protonated O atom [CG—OD2 = 1.317 (3) Å] (Table 1). The C-atom skeleton is nearly fully extended, with a CT—CA—CB—CG torsion angle of 179.6 (2)°. The α-amino group is in a near perfect staggered conformation around the CA—NT bond.
The crystal structure of (I) is made up of chains of aspartic acid molecules, linked by O—H···O hydrogen bonds, approximately parallel to the c axis (Fig. 2). The interchain interactions are provided mainly by three NT—H···O hydrogen bonds. Atom OT2 is involved twice in hydrogen bonding, while OT1 and OD2 are only involved in the O—H···O bond in the zigzag chain and OD1 in another O—H···NT hydrogen bond, a total of four hydrogen bonds per molecule (Table 2). The O—H···O hydrogen bond between the two carboxyl groups is quite short and straight [OD2···OT1 = 2.567 (1) Å]. Such a short hydrogen bond is also seen in the crystal structures of other amino acids (Bendeif et al., 2005). There is no intramolecular hydrogen bond.
We now compare the two types of refinement models: the usual independent atom model (IAM) and the experimental library multipolar atom model (ELMAM) (Zarychta et al., 2007).
In the IAM refinement, a conventional spherical neutral atom model was applied. Scale factor, atomic positions and thermal displacement parameters for all atoms were refined using the MOPRO program (Guillot et al., 2001; Jelsch et al., 2005) until convergence. In the ELMAM refinements, the same parameters were varied, but a multipolar charged atom model was applied. The electron-density parameters were transferred from the library (Pichon-Pesme et al., 1995, 2004) and subsequently kept fixed. The thermal riding restraints on H-atom B factors were applied similarly to the IAM refinement. Both refinements were carried out using the same intensity data and cut-off criterion [I/σ(I) > 0] and with no X—H distance restraint.
The ELMAM refinement shows a good improvement in statistical indexes compared with the IAM refinement: the R(F) factor is reduced from 0.0553 to 0.0428, wR(F) from 0.0255 to 0.0190 and the goodness of fit from 1.88 to 1.46. The improvement of X—H distances towards the values obtained from neutron diffraction data on DL-aspartic acid (Sequeira et al., 1989) is clearly visible (Table 1, Fig. 3). Indeed, the O—H and the tertiary CA—HA bond lengths from the ELMAM refinement agree very well with the neutron diffraction values. For the CH2 group, the largest discrepancies are 0.011 and 0.041 Å with the ELMAM and IAM structure refinements, respectively. The N—H bond lengths in the ammonium (–NH3+) group obtained from the ELMAM refinement are slightly longer than those from the neutron diffraction experiment on DL-aspartic acid. The differences between the ELMAM and neutron diffraction values can be explained by the fact that the three H atoms are involved in different hydrogen-bond patterns in the L-aspartic and DL-aspartic crystal structures. The effect on other X—Y bonds (involving C, O and N atoms) is small.
We have also performed the rigid-bond test (Hirshfeld, 1976) for both refinements. In the ELMAM refinement, the rigid bond test fails only for the CT—OT2, CB—CG and CA—CB bonds, and the average for all bonds is Δ = 1.1 × 10 -3 Å2, whereas in the IAM refinement, all bonds fail except the CB—CG bond, and the average is Δ = 2.39 × 10 -3 Å2. This analysis demonstrates that the atomic displacement parameters are more properly determined with the ELMAM refinement.