Glycinium trichloro­acetate

The title compound, C₂H₆NO₂⁺·C₂Cl₃O₂^-, crystallizes in the P4₁ space group with two glycinium cations and two trichloro­acetate anions in the asymmetric unit. The glycinium cations have nearly C_s point-group symmetry which is only broken by the H atoms of the amine group. The trichloro­acetate anions show typical bond lengths and angles, one of the trichloro­methyl groups being disordered. Chains of alternating anions and cations run along the c axis. Within these chains, consecutive anion-cation pairs are bound via strong hydrogen bonds involving the carboxyl­ate anions and the carboxyl or amine groups of the cations. Weaker hydrogen bonds bind neighbouring chains together.

S1. Comment

Glycine is a non-essential genetically coded amino acid and the second most abundant amino acid found in proteins and enzymes. It is similar to γ-aminobutyric and glutamic acids in inhibiting neurotransmitter signals in the nervous system. Hence, glycine systems are potential drugs to control some disorders of the nervous system. Glycine is also the only protein-forming amino acid without a centre of chirality (Meister, 1965). However, 103 chiral glycine compounds have been reported so far to the Cambridge Structural Database (CSD, Version?; Allen, 2002). This large and rapidly increasing number [by February 2004, only about 65 entries of this kind existed in the CSD (Fleck & Bohaty, 2004)] is indicative of active ongoing investigation regarding chiral glycine systems. Some examples of chiral structures involving glycine and other light non-chiral molecules are triglycine sulfate (TGS; Matthias et al., 1956), selenate (TGSe; Fugiel & Mierzwa, 1998), tetrafluoroberylate (TGFBe; Hoshino et al., 1957), diglycine nitrate (DGN; Pepinsky et al., 1958) and glycine phosphite (GPI; Tchukvinsky et al., 1998), all of which exhibit ferroelectricity within a particular temperature range. It is believed that the conformational variability of the glycine molecule in a crystalline environment is crucial for the mechanisms that lead to the observed ferroelectricity in these compounds.

In addition to glycine, for which 394 entries in the CSD have been reported so far, closely related N-methylated glycine derivatives, such as sarcosine (N-methylglycine), dimethylglycine and betaine (N,N,N-trimethylglycine), have also aroused much interest (Rodrigues et al., 2001).

Because of its amphoteric character, the glycine molecule can assume four possible forms: a neutral form stable in the gas phase, NH₂–CH₂–COOH, a neutral zwitterionic phase unstable in the gas phase but found in solids and in solutions, ⁺NH₃–CH₂–COO⁻, an anionic form, NH₂–CH₂–COO⁻, and finally, a cationic form, ⁺NH₃–CH₂–COOH. The up-to-date occurrence of each of these forms in the CSD is: NH₂–CH₂–COOH 33, ⁺NH₃–CH₂–COO⁻ 141, NH₂–CH₂–COO⁻ 137 and ⁺NH₃–CH₂–COOH 83.

In the title compound, (I), both crystallographically independent glycine molecules, A and B, are found in the cationic form with a neutral carboxylic acid group. The bond lengths C2—N1 [1.466 (8) Å in cation A and 1.475 (8) Å in cation B] and C1—C2 [1.522 (8) Å in cation A and 1.503 (8) Å in cation B], and angles N—C2—C1 [110.7 (5)° in cation A and 110.4 (5)° in cation B] and N—C2—C1—O2 [−2.6 (9)° in cation A and −2.2 (10)° in cation B] are within typical ranges for the glycinium cation. A cis conformation is observed for the carboxylic acid groups of both A and B cations.

The trichloroacetate anions, C and D, have typical geometries with average C—O and C—Cl distances [C3—O = 1.236 (6) Å in anion C and 1.233 (5) Å in anion D, and C4—Cl = 1.76 (2) Å in anion C and 1.75 (6) Å in anion D]. The C3—C4 distances [1.570 (9) Å in anion C and 1.559 (9) Å in anion D] and O1—C1—O2 angles [131.0 (6)° in anion C and 127.3 (6)° in anion D] are also within typical ranges. One of the trichloroacetate anions is disordered. This disorder was assumed and modelled as static according to a prior difference Fourier map, which showed unassigned peaks of charge close to the terminal Cl atoms. Disordered terminal halogens are rather common and the disorder is frequently found to be of a dynamic nature, corresponding to a rotation of the halogenated methyl group. In (I), this type of disorder is probably also present, this assumption being reinforced by the somewhat large and highly anisotropic vibration tensors of both anions.

The intermolecular bonds present in (I) are of two types, hydrogen bonding and van der Waals contacts. There are chains of alternating cations and anions running along the c axis. Within these chains, a disordered trichloroacetate ion is bonded via two hydrogen bonds to the carboxylic acid groups of two neighbouring cations, whereas the next trichloroacetate anion in the chain bonds to its cation neighbours via hydrogen bonds with the amine N atoms of the cation (Fig. 2). This pattern is then infinitely repeated along the chain. The chains are also interconnected through weaker hydrogen bonds and van der Waals interactions between the Cl atoms of the anions.

S2. Experimental

Colourless block-shaped crystals of (I) were obtained by recrystallization of an equimolar solution of glycine and trichloroacetic acid (from Aldrich, 98%) in water.

S3. Refinement

All H atoms positions were generated geometrically and subsequently refined as riding, with C—H = 0.97, N—H = 0.89 and O—H = 0.89 Å, with U_iso(H) = 1.2U_eq(C) or 1.5U_eq(N,O). Please check added text. Examination of the crystal structure with PLATON (Spek, 2003) showed that there are no solvent-accessible voids in the crystal lattice.

Computing details

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software; data reduction: PLATON (Spek, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A plot of (I), showing the four ions of the asymmetric unit and the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. The packing of the molecules of (I), showing the building pattern in the chains running along the c direction. A trichloroacetate anion (disordered) is bonded via two hydrogen bonds to the carboxylic acid groups of two neighbouring cations. The next trichloroacetate anion in the chain bonds to its cation neighbours via hydrogen bonds with the amine N atoms of the cations.

Glycinium trichloroacetate

Crystal data

C2H6NO2^+·C2Cl3O2^−	Dx = 1.758 Mg m^−3
Mr = 238.45	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P41	Cell parameters from 25 reflections
Hall symbol: P 4w	θ = 9.6–14.2°
a = 9.4416 (9) Å	µ = 0.99 mm^−1
c = 20.213 (4) Å	T = 293 K
V = 1801.9 (4) Å^3	Block, colourless
Z = 8	0.34 × 0.30 × 0.20 mm
F(000) = 960

Data collection

Enraf–Nonius CAD-4 diffractometer	1493 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.024
Graphite monochromator	θmax = 27.6°, θmin = 3.0°
Profile data from ω/2θ scans	h = 0→11
Absorption correction: ψ scan (North et al., 1968)	k = 0→11
Tmin = 0.722, Tmax = 0.817	l = −24→24
2596 measured reflections	3 standard reflections every 120 min
2106 independent reflections	intensity decay: 59%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.045	H-atom parameters constrained
wR(F^2) = 0.145	w = 1/[σ^2(Fo^2) + (0.0777P)^2 + 0.9874P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
2106 reflections	Δρmax = 0.33 e Å^−3
249 parameters	Δρmin = −0.32 e Å^−3
1 restraint	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881, with 24 Friedel pairs.
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.03 (13)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
O3	0.0569 (5)	0.2286 (6)	0.7501 (2)	0.0468 (12)
O4	0.1275 (5)	0.1834 (5)	0.6459 (2)	0.0472 (11)
C3	0.1277 (6)	0.2469 (6)	0.6997 (3)	0.0350 (12)
C4	0.2438 (7)	0.3659 (6)	0.7037 (3)	0.0377 (13)
Cl1	0.2524 (2)	0.4636 (2)	0.62889 (9)	0.0572 (5)
Cl2	0.2175 (3)	0.4845 (3)	0.76836 (10)	0.0855 (9)
Cl3	0.4063 (2)	0.2754 (3)	0.71384 (17)	0.0891 (9)
O3'	0.0524 (5)	0.3364 (5)	0.1480 (2)	0.0495 (12)
O4'	0.0901 (6)	0.2940 (5)	0.2541 (2)	0.0507 (12)
C3'	0.1123 (7)	0.3561 (6)	0.2012 (3)	0.0351 (13)
C4'	0.2366 (7)	0.4651 (7)	0.2016 (4)	0.0455 (15)
Cl1'	0.1949 (16)	0.6061 (10)	0.1467 (7)	0.065 (3)	0.53 (5)
Cl2'	0.272 (3)	0.531 (3)	0.2780 (8)	0.114 (6)	0.53 (5)
Cl3'	0.3812 (15)	0.3735 (16)	0.1654 (10)	0.097 (4)	0.53 (5)
Cl4'	0.237 (4)	0.590 (2)	0.1396 (9)	0.104 (6)	0.47 (5)
Cl5'	0.242 (2)	0.5598 (15)	0.2774 (9)	0.073 (4)	0.47 (5)
Cl6'	0.3921 (15)	0.3668 (18)	0.202 (2)	0.123 (8)	0.47 (5)
O1	0.1163 (7)	0.3896 (5)	0.0231 (2)	0.0554 (14)
H1	0.0988	0.3763	0.0624	0.083*
O2	0.0589 (6)	0.1641 (5)	0.0137 (2)	0.0526 (13)
C1	0.1015 (7)	0.2714 (6)	−0.0093 (3)	0.0327 (13)
C2	0.1370 (8)	0.2875 (7)	−0.0823 (3)	0.0402 (14)
H2A	0.0764	0.3592	−0.1018	0.048*
H2B	0.2345	0.3184	−0.0869	0.048*
N1	0.1174 (5)	0.1530 (5)	−0.1173 (2)	0.0346 (11)
H1A	0.1916	0.0971	−0.1100	0.052*
H1B	0.1094	0.1695	−0.1605	0.052*
H1C	0.0391	0.1107	−0.1027	0.052*
O1'	0.0408 (7)	0.3709 (5)	0.3801 (3)	0.0554 (13)
H1'	0.0609	0.3525	0.3416	0.083*
O2'	0.0151 (6)	0.1379 (5)	0.3896 (2)	0.0502 (12)
C1'	0.0228 (6)	0.2534 (6)	0.4129 (3)	0.0310 (12)
C2'	0.0215 (8)	0.2771 (7)	0.4864 (3)	0.0386 (14)
H2A'	0.1096	0.3214	0.4999	0.046*
H2B'	−0.0556	0.3404	0.4979	0.046*
N1'	0.0040 (6)	0.1413 (5)	0.5217 (2)	0.0355 (11)
H1A'	−0.0848	0.1112	0.5172	0.053*
H1B'	0.0233	0.1535	0.5644	0.053*
H1C'	0.0630	0.0773	0.5047	0.053*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O3	0.053 (3)	0.061 (3)	0.027 (2)	−0.010 (2)	0.004 (2)	0.000 (2)
O4	0.067 (3)	0.049 (3)	0.026 (2)	−0.013 (2)	−0.005 (2)	−0.008 (2)
C3	0.041 (3)	0.043 (3)	0.021 (3)	−0.003 (3)	−0.001 (2)	0.002 (3)
C4	0.048 (3)	0.041 (3)	0.024 (2)	−0.006 (3)	0.003 (3)	0.001 (3)
Cl1	0.0851 (13)	0.0479 (10)	0.0387 (9)	−0.0091 (9)	0.0095 (9)	0.0062 (8)
Cl2	0.148 (2)	0.0673 (13)	0.0414 (11)	−0.0408 (14)	0.0230 (13)	−0.0259 (10)
Cl3	0.0468 (11)	0.0981 (17)	0.122 (2)	0.0019 (11)	−0.0232 (14)	0.0200 (17)
O3'	0.060 (3)	0.063 (3)	0.025 (2)	−0.019 (2)	−0.005 (2)	−0.004 (2)
O4'	0.076 (3)	0.048 (3)	0.028 (2)	−0.022 (3)	0.004 (2)	0.006 (2)
C3'	0.049 (3)	0.033 (3)	0.024 (3)	0.003 (2)	0.000 (3)	0.005 (2)
C4'	0.051 (4)	0.050 (4)	0.036 (3)	−0.005 (3)	0.001 (3)	−0.003 (3)
Cl1'	0.107 (6)	0.030 (4)	0.058 (4)	−0.008 (4)	−0.001 (4)	0.007 (2)
Cl2'	0.145 (12)	0.161 (12)	0.034 (5)	−0.098 (10)	−0.030 (5)	0.014 (6)
Cl3'	0.052 (4)	0.100 (5)	0.138 (9)	0.019 (3)	0.026 (5)	−0.004 (6)
Cl4'	0.166 (16)	0.103 (10)	0.043 (5)	−0.085 (8)	−0.022 (7)	0.026 (6)
Cl5'	0.118 (8)	0.057 (5)	0.043 (6)	−0.030 (4)	−0.003 (5)	−0.021 (3)
Cl6'	0.046 (4)	0.100 (6)	0.22 (2)	0.001 (3)	0.018 (9)	−0.006 (11)
O1	0.098 (4)	0.042 (3)	0.026 (2)	−0.018 (3)	0.005 (3)	−0.007 (2)
O2	0.089 (4)	0.039 (3)	0.029 (2)	−0.015 (2)	0.002 (2)	0.001 (2)
C1	0.043 (3)	0.033 (3)	0.021 (3)	0.003 (3)	−0.005 (2)	0.002 (2)
C2	0.052 (4)	0.040 (3)	0.028 (3)	−0.003 (3)	0.006 (3)	0.002 (3)
N1	0.041 (3)	0.042 (3)	0.021 (2)	−0.001 (2)	0.001 (2)	−0.003 (2)
O1'	0.097 (4)	0.038 (2)	0.031 (3)	−0.005 (2)	0.010 (3)	0.005 (2)
O2'	0.086 (4)	0.041 (3)	0.024 (2)	−0.004 (2)	−0.006 (2)	−0.0031 (19)
C1'	0.036 (3)	0.028 (3)	0.029 (3)	0.002 (2)	−0.004 (2)	0.003 (2)
C2'	0.060 (4)	0.035 (3)	0.021 (3)	0.005 (3)	−0.001 (3)	−0.001 (2)
N1'	0.053 (3)	0.037 (3)	0.016 (2)	0.005 (2)	−0.003 (2)	0.0018 (19)

Geometric parameters (Å, º)

O3—C3	1.230 (8)	O2—C1	1.184 (7)
O4—C3	1.242 (7)	C1—C2	1.521 (8)
C3—C4	1.572 (9)	C2—N1	1.466 (8)
C4—Cl2	1.739 (7)	C2—H2A	0.9700
C4—Cl3	1.769 (7)	C2—H2B	0.9700
C4—Cl1	1.773 (7)	N1—H1A	0.8900
O3'—C3'	1.228 (8)	N1—H1B	0.8900
O4'—C3'	1.238 (7)	N1—H1C	0.8900
C3'—C4'	1.560 (9)	O1'—C1'	1.304 (7)
C4'—Cl2'	1.697 (17)	O1'—H1'	0.8200
C4'—Cl4'	1.721 (18)	O2'—C1'	1.190 (7)
C4'—Cl6'	1.737 (17)	C1'—C2'	1.502 (8)
C4'—Cl3'	1.773 (14)	C2'—N1'	1.477 (8)
C4'—Cl5'	1.776 (15)	C2'—H2A'	0.9700
C4'—Cl1'	1.776 (14)	C2'—H2B'	0.9700
Cl2'—Cl6'	2.46 (2)	N1'—H1A'	0.8900
Cl3'—Cl6'	0.74 (3)	N1'—H1B'	0.8900
O1—C1	1.302 (7)	N1'—H1C'	0.8900
O1—H1	0.8200
O3—C3—C4	115.9 (5)	C1—O1—H1	109.5
O4—C3—C4	113.0 (5)	O2—C1—O1	125.0 (6)
C3—C4—Cl2	113.5 (4)	O2—C1—C2	122.7 (6)
C3—C4—Cl3	105.4 (4)	O1—C1—C2	112.2 (5)
Cl2—C4—Cl3	110.3 (4)	N1—C2—C1	110.8 (5)
C3—C4—Cl1	111.1 (4)	N1—C2—H2A	109.5
Cl2—C4—Cl1	108.2 (3)	C1—C2—H2A	109.5
Cl3—C4—Cl1	108.1 (4)	N1—C2—H2B	109.5
O3'—C3'—O4'	127.3 (6)	C1—C2—H2B	109.5
O3'—C3'—C4'	116.8 (6)	H2A—C2—H2B	108.1
O4'—C3'—C4'	115.8 (6)	C2—N1—H1A	109.5
C3'—C4'—Cl2'	113.1 (8)	C2—N1—H1B	109.5
C3'—C4'—Cl4'	116.8 (10)	H1A—N1—H1B	109.5
Cl2'—C4'—Cl4'	114.4 (9)	C2—N1—H1C	109.5
C3'—C4'—Cl6'	106.5 (7)	H1A—N1—H1C	109.5
Cl2'—C4'—Cl6'	91.7 (10)	H1B—N1—H1C	109.5
Cl4'—C4'—Cl6'	111.3 (9)	C1'—O1'—H1'	109.5
C3'—C4'—Cl3'	104.8 (7)	O2'—C1'—O1'	125.8 (6)
Cl2'—C4'—Cl3'	113.8 (10)	O2'—C1'—C2'	121.9 (5)
Cl4'—C4'—Cl3'	91.8 (11)	O1'—C1'—C2'	112.2 (5)
Cl6'—C4'—Cl3'	24.3 (11)	N1'—C2'—C1'	110.4 (5)
C3'—C4'—Cl5'	111.1 (8)	N1'—C2'—H2A'	109.6
Cl2'—C4'—Cl5'	12.6 (15)	C1'—C2'—H2A'	109.6
Cl4'—C4'—Cl5'	106.5 (10)	N1'—C2'—H2B'	109.6
Cl6'—C4'—Cl5'	104.1 (15)	C1'—C2'—H2B'	109.6
Cl3'—C4'—Cl5'	125.2 (10)	H2A'—C2'—H2B'	108.1
C3'—C4'—Cl1'	108.9 (7)	C2'—N1'—H1A'	109.5
Cl2'—C4'—Cl1'	109.7 (11)	C2'—N1'—H1B'	109.5
Cl4'—C4'—Cl1'	14.7 (14)	H1A'—N1'—H1B'	109.5
Cl6'—C4'—Cl1'	126.0 (13)	C2'—N1'—H1C'	109.5
Cl3'—C4'—Cl1'	106.2 (7)	H1A'—N1'—H1C'	109.5
Cl5'—C4'—Cl1'	99.7 (8)	H1B'—N1'—H1C'	109.5
O3—C3—C4—Cl2	−18.2 (8)	O4'—C3'—C4'—Cl6'	74.4 (18)
O4—C3—C4—Cl2	164.4 (5)	O3'—C3'—C4'—Cl3'	−76.9 (10)
O3—C3—C4—Cl3	102.7 (6)	O4'—C3'—C4'—Cl3'	99.5 (10)
O4—C3—C4—Cl3	−74.7 (6)	O3'—C3'—C4'—Cl5'	145.2 (9)
O3—C3—C4—Cl1	−140.4 (5)	O4'—C3'—C4'—Cl5'	−38.3 (10)
O4—C3—C4—Cl1	42.2 (7)	O3'—C3'—C4'—Cl1'	36.3 (9)
O3'—C3'—C4'—Cl2'	158.6 (14)	O4'—C3'—C4'—Cl1'	−147.2 (7)
O4'—C3'—C4'—Cl2'	−24.9 (15)	O2—C1—C2—N1	−2.6 (9)
O3'—C3'—C4'—Cl4'	22.8 (16)	O1—C1—C2—N1	−179.2 (5)
O4'—C3'—C4'—Cl4'	−160.7 (15)	O2'—C1'—C2'—N1'	−2.3 (10)
O3'—C3'—C4'—Cl6'	−102.1 (18)	O1'—C1'—C2'—N1'	−178.8 (5)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O3′	0.82	1.83	2.644 (7)	176
N1—H1A···O3′^i	0.89	1.97	2.852 (7)	170
N1—H1B···O3^ii	0.89	1.96	2.832 (7)	168
N1—H1C···O4^iii	0.89	2.11	2.862 (7)	142
N1—H1C···O2′^iv	0.89	2.41	3.021 (7)	126
O1′—H1′···O4′	0.82	1.87	2.689 (7)	173
N1′—H1A′···O4′^v	0.89	2.00	2.877 (8)	167
N1′—H1B′···O4	0.89	1.94	2.797 (7)	161
N1′—H1B′···O2′^v	0.89	2.52	2.982 (7)	113
N1′—H1C′···O3^i	0.89	2.02	2.862 (7)	158

Symmetry codes: (i) y, −x, z−1/4; (ii) x, y, z−1; (iii) −y, x, z−3/4; (iv) −x, −y, z−1/2; (v) −y, x, z+1/4.