Glycinium trifluoroacetate

In the title compound, C₂H₆NO₂⁺·C₂F₃O₂^-, the main N-C-COOH skeleton of the glycinium cation is almost perfectly planar. The tri­fluoro­acetate anion has a staggered conformation with typical bond distances and angles. The CF₃ group is slightly disordered. The structure is stabilized by an extensive network of strong O-H...O hydrogen bonds and weaker N-H...O bonds.

S1. Comment

Glycine (aminoethanoic acid) is the simplest amino acid, and is the only one that is not optically active (it has no stereoisomers). This amino acid is essential for the biosynthesis of nucleic acids, as well as of bile acids, porphyrins, creatine phosphate and other amino acids. On a molar basis, glycine is the second most common amino acid found in proteins and enzymes, being incorporated at the rate of 7.5% compared with the other amino acids. Glycine is also similar to γ-aminobutyric acid and glutamic acid in its ability to inhibit neurotransmitter signals in the central nervous system, with important functions centrally and peripherally. Glycine systems may be important in controlling epilepsy and other central nervous system disorders.

In its pure form, glycine exists as a zwitterion, where the carboxyl H atom has been transferred to the amino group. In common with most amino acids, glycine has an amphoteric character, being able to accept an H atom at the carboxylate group from even moderately weak acids, and to donate the amino H atom in basic environments. Anionic glycine chelates 3 d and 4 d transition metals.

Our main interest in glycine compounds relates to their physical properties. The most well known glycine compounds are those that undergo ferroelectric transitions. Other closely related compounds, which also show phase transitions, are simple salts of sarcosine (N-methylglycine) and betaine (N,N,N-trimethylglycine). Therefore, it would be reasonable to expect that simple salts of glycine, sarcosine or betaine also show interesting properties at low temperature; however, many of them do not.

Trifluoroacetic acid is a very strong carboxylic acid, due to the charge-drawing effect of the F atoms on the C_α carbon. Its dissociation constant is K = 0.66 mol dm^-3 (Strehlow & Hildebrandt, 1990), as determined by Raman spectroscopy. Phase transitions in crystalline trifluoroacetic acid tetrahydrate at low temperature have recently been observed for both deuterated and nondeuterated samples (Mootz & Schilling, 1992).

The present study of glycinium trifluoroacetate, (I), performed at room temperature, completes a series of structural investigations. The crystal structures of sarcosine, dimethylglycine and betaine trifluoroacetates have already been determined (Rodrigues et al., 2000, 2001a,b). Complementary differential scanning calorimetry and low-temperature X-ray diffraction have not revealed any phase transitions of an electrical nature in these substances; nevertheless, the true significance of these results has not yet been discussed. \sch

The ionization states of the glycine and trifluoroacetic acid molecules in (I) were determined from the objective localization of the H atoms bonded to the carboxylic acid groups, but could easily be inferred from the bond distances within these groups. The glycine molecule exists in the cationic form, with a mono-positively charged amino group and a neutral carboxylic acid group, in agreement with the large asymmetry between the C—O bond lengths of this functional group. The trifluoroacetate molecules are in the ionized state, as expected from the strength of the acid and the required charge neutrality of the salt.

The glycine carboxyl skeleton, which includes atoms O1, O2, C1 and C2, is planar to within 0.0010 (4) Å. The N atom is slightly displaced out of this plane, by -0.072 (3) Å, corresponding to a small rotation around the C1—C2 single bond. The relevant torsion angles are O1—C1—C2—N 3.3 (2)° and O2—C1—C2—N -177.09 (13)°. These are to be compared with the corresponding values in pure γ-glycine, 167.1 (1) and -15.4 (1)°, respectively (Kvick et al., 1980), which is more distorted from planarity.

The trifluoroacetate anion in (I) has a staggered conformation, as indicated by the O—C4—C5—F torsion angles. The geometry of the CF₃ group is similar to that found in other structures (Nahringbauer et al., 1979), with an average C—F bond length and F—C—F angle of 1.327 (7) Å and 106.8 (2)°, respectively. The average F—C—C angle is 112 (2)°. The carboxylate group of the anion is planar to within 0.0089 (4) Å; the C4—C5 bond length of 1.541 (2) Å is longer than the average value for a Csp³—Csp² bond (Reference?), but is within the normal range of values found in trifluoroacetic acid and trifluoroacetate compounds (Lundgren, 1978). The CF₃ group is disordered over two very close positions. The most probable of these occurs with roughly 0.75 probability. Low-temperature measurements seem to support the possible dynamic nature of this disorder, as no refinement of two different positions for each F atom is then necessary.

The glycine cations in (I) interact directly, via hydrogen bonds, with two neighbouring trifluoroacetate anions, forming cation-anion-cation-anion rings lying in the bc plane. The glycine cation acts as a donor of four hydrogen bonds; the N atom is engaged in three hydrogen bonds as a donor, and the O atom of the carboxylic acid group is also the donor of the strongest hydrogen bond found in this structure. Both O atoms of the carboxylate group of the anion act as acceptors of hydrogen bonds. The above-mentioned ring is formed via two very strong O—H···O and two weaker N—H···O hydrogen bonds. The N—H···O distances and angles are in the range 2.859–3.000 Å and 161.48–174.72°, respectively, which allows a classification of these hydrogen bonds as relatively weak (Reference?). The strongest hydrogen bond in (I) involves the carboxylic acid O atom of the cation as a donor and a carboxylate O atom of the anion as an acceptor [O—H···O 2.597 (2) Å and O—H—O 175 (2)°].

S2. Experimental

Colourless prismatic crystals of (I) were obtained by evaporation of the solvent from a solution of glycine in trifluoroacetic acid. Good quality single crystals recrystallized after a few hours at room temperature. A small single-crystal was cast and checked by photographic methods prior to data collection.

S3. Refinement

All H atoms were located on a difference Fourier map and refined isotropically. Please clarify - CIF table shows H atoms on C were constrained. Examination of the crystal structure with PLATON (Spek, 1995) 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, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); 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 view of the cation and anion of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A projection of the structure of (I) along the bc plane, showing the four-molecule hydrogen-bonded rings.
	Fig. 3. A projection of the difference Fourier map for (I) on the plane of the main F atoms, showing the disordered charge (when disorder is not accounted for).

Glycinium trifluoroacetate

Crystal data

C2H6NO2^+·C2F3O2^−	F(000) = 384
Mr = 189.10	Dx = 1.798 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 25 reflections
a = 4.9598 (6) Å	θ = 9.6–15.4°
b = 12.239 (2) Å	µ = 0.20 mm^−1
c = 12.015 (8) Å	T = 293 K
β = 106.67 (7)°	Prismatic, colourless
V = 698.7 (5) Å^3	0.47 × 0.40 × 0.15 mm
Z = 4

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.021
Radiation source: fine-focus sealed tube	θmax = 27.5°, θmin = 3.3°
Graphite monochromator	h = −6→0
profile data from ω/2θ scans	k = −15→0
1772 measured reflections	l = −14→15
1599 independent reflections	25 standard reflections every 180 min
1285 reflections with I > 2σ(I)	intensity decay: 7%

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)] = 0.030	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.092	w = 1/[σ^2(Fo^2) + (0.0538P)^2 + 0.1642P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
1599 reflections	Δρmax = 0.32 e Å^−3
154 parameters	Δρmin = −0.19 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.197 (10)

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)
O1	0.5549 (2)	0.64019 (9)	0.14246 (10)	0.0417 (3)
H1	0.660 (5)	0.6601 (17)	0.100 (2)	0.065 (6)*
O2	0.6954 (2)	0.47019 (8)	0.11800 (9)	0.0389 (3)
N	0.3357 (3)	0.37703 (10)	0.22041 (10)	0.0335 (3)
H5	0.278 (4)	0.3452 (15)	0.1473 (19)	0.053 (5)*
H4	0.215 (4)	0.3557 (14)	0.2604 (16)	0.047 (5)*
H6	0.508 (5)	0.3435 (17)	0.2557 (19)	0.061 (6)*
C1	0.5528 (2)	0.53342 (11)	0.15255 (11)	0.0289 (3)
C2	0.3451 (3)	0.49723 (12)	0.21543 (13)	0.0365 (3)
H2	0.3997	0.5267	0.2937	0.044*
H3	0.1596	0.5251	0.1754	0.044*
F1	0.6043 (15)	0.1099 (4)	−0.0188 (5)	0.0464 (13)	0.75 (5)
F2	0.812 (2)	0.2257 (6)	0.1144 (9)	0.0560 (13)	0.75 (5)
F3	1.0226 (17)	0.0808 (6)	0.0920 (9)	0.0613 (14)	0.75 (5)
O4	1.1501 (2)	0.29041 (9)	−0.00972 (8)	0.0401 (3)
O3	0.9317 (2)	0.17262 (8)	−0.14903 (8)	0.0365 (3)
C3	0.9888 (2)	0.21341 (10)	−0.05132 (10)	0.0277 (3)
C4	0.8537 (3)	0.15886 (11)	0.03542 (11)	0.0329 (3)
F1A	0.658 (9)	0.101 (3)	−0.006 (3)	0.084 (6)	0.25 (5)
F2A	0.775 (5)	0.2334 (15)	0.102 (2)	0.050 (3)	0.25 (5)
F3A	1.031 (4)	0.094 (3)	0.111 (3)	0.065 (5)	0.25 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0466 (6)	0.0362 (5)	0.0504 (6)	−0.0024 (4)	0.0269 (5)	0.0014 (4)
O2	0.0349 (5)	0.0420 (6)	0.0454 (6)	0.0033 (4)	0.0206 (4)	0.0024 (4)
N	0.0325 (6)	0.0410 (7)	0.0286 (5)	−0.0096 (5)	0.0114 (5)	0.0002 (5)
C1	0.0224 (5)	0.0360 (6)	0.0270 (6)	−0.0017 (5)	0.0052 (4)	0.0007 (5)
C2	0.0293 (6)	0.0406 (7)	0.0440 (7)	−0.0015 (5)	0.0174 (6)	0.0004 (6)
F1	0.0397 (16)	0.059 (2)	0.0427 (14)	−0.0251 (15)	0.0159 (12)	−0.0086 (9)
F2	0.077 (3)	0.0540 (18)	0.0504 (18)	−0.023 (2)	0.040 (2)	−0.0189 (14)
F3	0.083 (3)	0.053 (2)	0.052 (3)	0.0187 (13)	0.025 (2)	0.0222 (16)
O4	0.0438 (6)	0.0449 (6)	0.0341 (5)	−0.0179 (4)	0.0153 (4)	−0.0057 (4)
O3	0.0380 (5)	0.0457 (6)	0.0274 (5)	−0.0041 (4)	0.0118 (4)	−0.0059 (4)
C3	0.0267 (6)	0.0302 (6)	0.0270 (6)	0.0004 (5)	0.0089 (5)	0.0012 (5)
C4	0.0377 (7)	0.0329 (7)	0.0300 (6)	−0.0057 (5)	0.0129 (5)	−0.0026 (5)
F1A	0.071 (11)	0.114 (12)	0.064 (6)	−0.064 (8)	0.013 (7)	−0.010 (6)
F2A	0.049 (4)	0.067 (6)	0.046 (5)	0.011 (5)	0.033 (4)	−0.016 (3)
F3A	0.052 (6)	0.102 (13)	0.037 (6)	0.011 (6)	0.005 (4)	0.038 (7)

Geometric parameters (Å, º)

O1—C1	1.3127 (17)	F1—C4	1.361 (7)
O1—H1	0.86 (2)	F2—C4	1.313 (6)
O2—C1	1.1997 (17)	F3—C4	1.323 (7)
N—C2	1.4736 (18)	O4—C3	1.2446 (17)
N—H5	0.93 (2)	O3—C3	1.2317 (17)
N—H4	0.91 (2)	C3—C4	1.5430 (19)
N—H6	0.93 (2)	C4—F1A	1.19 (3)
C1—C2	1.5084 (18)	C4—F3A	1.33 (2)
C2—H2	0.9700	C4—F2A	1.341 (15)
C2—H3	0.9700
C1—O1—H1	111.0 (14)	H2—C2—H3	108.1
C2—N—H5	112.7 (11)	O3—C3—O4	129.44 (12)
C2—N—H4	110.0 (11)	O3—C3—C4	116.28 (12)
H5—N—H4	107.9 (16)	O4—C3—C4	114.21 (11)
C2—N—H6	114.9 (13)	F2—C4—F3	106.6 (6)
H5—N—H6	103.9 (17)	F1A—C4—F3A	103.3 (18)
H4—N—H6	107.0 (17)	F1A—C4—F2A	108.2 (15)
O2—C1—O1	126.11 (12)	F2—C4—F1	108.1 (4)
O2—C1—C2	122.61 (12)	F3—C4—F1	106.4 (4)
O1—C1—C2	111.28 (11)	F2—C4—C3	114.0 (3)
N—C2—C1	110.34 (11)	F3—C4—C3	109.0 (4)
N—C2—H2	109.6	F3A—C4—C3	112.8 (12)
C1—C2—H2	109.6	F2A—C4—C3	111.3 (8)
N—C2—H3	109.6	F1—C4—C3	112.2 (3)
C1—C2—H3	109.6
O2—C1—C2—N	−3.27 (18)	O4—C3—C4—F3	−87.8 (5)
O1—C1—C2—N	177.08 (12)	O3—C3—C4—F3A	101.3 (19)
O3—C3—C4—F1A	−17 (2)	O4—C3—C4—F3A	−75.9 (19)
O4—C3—C4—F1A	165 (2)	O3—C3—C4—F2A	−141.7 (12)
O3—C3—C4—F2	−151.6 (6)	O4—C3—C4—F2A	41.1 (12)
O4—C3—C4—F2	31.2 (6)	O3—C3—C4—F1	−28.2 (3)
O3—C3—C4—F3	89.4 (5)	O4—C3—C4—F1	154.5 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O4^i	0.86 (2)	1.74 (3)	2.597 (2)	175 (2)
N—H5···O4^ii	0.93 (2)	1.93 (2)	2.856 (2)	175.5 (17)
N—H4···O3^iii	0.91 (2)	2.04 (2)	2.941 (2)	173.0 (16)
N—H6···O3^iv	0.93 (2)	2.09 (2)	2.984 (2)	159.2 (18)

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