Sarcosinium tri­fluoro­acetate

The title compound, C₃H₈NO₂⁺·C₂F₃O₂^-, crystallizes in space group C2/c. The main N-C-COOH skeleton of the protonated sarcosine mol­ecule is almost perfectly planar. The tri­fluoro­acetate anion has a staggered conformation and typical bond distances and angles. The CF₃ group is probably 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

Sarcosine (N-methylglycine, CH₃NH₂⁺CH₂COO⁻) is an α-amino acid found in many biological materials and also used in cosmetics (Meister, 1965).

In pure form sarcosine exists as a zwitterion, where the carboxylic proton has been transferred to the amino group. As most aminoacids, sarcosine has an amphoteric character, being able to accept a proton at the carboxylate group from even moderately weak acids, and to donate the amino proton in basic environments. The anionic form of sarcosine is a well known chelating agent of 3 d and 4 d transition metals (Guha, 1973; Krishnakumar et al., 1994; Darensbourg et al., 1994). On the other hand, structural data on compounds where sarcosine is in the cationic form are rather scarce.

Our main interest in sarcosine compounds relates to their physical properties. The most well known sarcosine compound, tris(sarcosine) calcium chloride (TSCC), undergoes a ferroelectric transition at T_c = 127 K (Pepinsky & Makita, 1962). Its crystal structure both in the paraelectric phase at room temperature (Ashida et al., 1972) and in the ferroelectric phase (Mishima et al., 1984) have been reported. Another closely related compound, which also shows a `devil stair' series of phase transitions is betaine (trimethylglycine) calcium chloride (BCCD) (Almeida et al., 1992). A number of simple salts of betaine show ferroelectric or antiferroelectric phase transitions at low temperature as well as ferroelastic behaviour inducing phase transitions to both commensurate and incommensurate superstructures (Shildkamp & Spilker, 1984; Haussül, 1984, 1988). Therefore it is reasonable to expect that simple salts of sarcosine also show interesting properties at low temperature. The trifluoroacetic acid is a very strong carboxylic acid due to the charge withdrawing effect of the fluorine atoms on the C_α atom. Its dissociation constant is K = 0.66 mol dm⁻³ (Strehlow & Hildebrandt, 1990), as determined by Raman spectroscopy. Phase transitions at low temperature on crystalline trifluoroacetic acid tetrahydrate have been recently discovered on undeuterated and deuterated samples (Mootz & Schilling, 1992). Thus, sarcosinium trifluoroacetate is expected to be a good candidate to exhibit phase transitions and superstructures at low temperature. This is the first report of a research project on a systematic study of the structural and physical properties of sarcosine compounds. The present study, performed at room temperature, will be completed by other low-temperature structural and spectroscopic studies.

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

The sarcosine carboxy skeleton which includes atoms O1, O2, C1 and C2 is planar within 0.0015 (13) Å. The N atom is slightly displaced out of this plane by −0.0557 (30) Å, corresponding to a small rotation around the single C1–C2 bond. The relevant torsion angles are O1–C1–C2–N [177.75 (14)°] and O2–C1–C2–N [−2.5 (2)°]. These values are to be compared with the corresponding ones in pure sarcosine, 173.7 (2) and −6.8 (2)°, respectively (Mostad & Natarajan, 1989), which is more distorted from planarity. The methyl group breaks the almost perfect planarity of the main chain. The torsional angle C1—C2—N—C3 is 172.39 (14)° and the C3 atom is 0.1256 (46) Å away from the plane, in the opposite direction of the N atom. This deviation of the methyl group from the plane of the main skeleton is, however, smaller than that observed in pure sarcosine for which the torsion angle is −166.3° (Mostad & Natarajan, 1989).

The trifluoroacetate anion has a staggered conformation as indicated by the torsional angle O4—C4—C5—F1 of −24.4 (3)°. 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 angles of 1.32 (12) and 106 (2)°, respectively. The average F–C–C angle is 112 (1)°. The carboxylate group of the anion is planar within 0.0065 (14) Å; the C4—C5 bond length [1.539 (2) Å] is longer than the average Csp³—Csp² bond but is within the normal range of values found in trifluoroacetic acid and trifluoroacetate compounds (Lundgren, 1978).

It should be pointed out that the atomic displacement tensors of the F atoms have a strong anisotropic character which indicates some disorder, probably of dynamic nature, of these atoms. As is often found in trifluoroacetate compounds, it is plausible that at room temperature the CF₃ groups rotate undergoing angular oscillations around the single C–C bond. Such a minor disorder may account for the relatively large final R and wR values.

The sarcosine cations interact directly, via hydrogen-bonds, with two other neighbouring sarcosine molecules and three trifluoroacetate anions. The N atom is only bonded to two H atoms but is engaged in three hydrogen bonds as a donor. One of the two H atoms is shared between two carboxylate O atoms of two distinct trifluoroacetate anions in a bifurcated hydrogen bond. The other H atom is shared with the unprotonated O atom of another sarcosine molecule. The N–H···O distances and angles are in the range 2.880 (2)–2.919 (2) Å and 121 (3)–146 (3)°, respectively, which allows a classification of these hydrogen bonds as relatively weak.

The strongest hydrogen bond in the title compound involves the carboxylic O atom of the cation as a donor and a carboxylate O atom of the anion as an acceptor [O–H···O 2.575 (2) Å; O–H–O 163 (4)°].

Like other carboxylic acid complexes viz. D-tryptophan hydrogen oxalate (Bakke & Mostad, 1980) and L-histidine oxalate and DL-histidine oxalate (Prabu et al., 1996), the present compound has no solvate molecules in which it differs from the more closely related complex sarcosine hydrogen oxalate (Krishnakumar et al., 1998).

S2. Experimental

Large colourless crystals of tabular form were obtained after one day evaporation of the solution obtained from adding an excess of trifluoroacetic acid (Aldrich, 99%) directly to 1 g of pure sarcosine as purchased from Aldrich (98%). Most of these crystals were too big to be used on the X-ray diffractometer and had to be cut. However, twinned crystals were systematically obtained under application of pressure. Therefore, a small single-crystal cast as grown from the solution was used and checked prior to data collection by photographic methods.

S3. Refinement

All H atoms were located on a difference Fourier map and refined isotropically.

Examination of the crystal structure with PLATON (Spek, 1995) showed that there are no solvent-accessible voids in the crystal lattice. All calculations were performed on a Pentium 350 MHz PC running LINUX.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA (Spek, 1997); 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. ORTEPII (Johnson, 1976) plot of the title compound. Displacement ellipsoids are drawn at the 50% level.
	Fig. 2. Projection of the structure along the b axis showing the hydrogen bonds as dashed lines.

N-methylglycinium trifluoroacetate

Crystal data

C3H8NO2^+·C2F3O2^−	F(000) = 832
Mr = 203.12	Dx = 1.666 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 14.5912 (19) Å	Cell parameters from 25 reflections
b = 7.0951 (11) Å	θ = 7.1–18.0°
c = 15.8414 (13) Å	µ = 0.18 mm^−1
β = 99.138 (8)°	T = 293 K
V = 1619.2 (4) Å^3	Block, clear, colourless
Z = 8	0.45 × 0.40 × 0.15 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.011
Radiation source: fine-focus sealed tube	θmax = 30.0°, θmin = 3.2°
Graphite monochromator	h = 0→20
profile data from ω–2θ scans	k = 0→9
2462 measured reflections	l = −22→22
2350 independent reflections	3 standard reflections every 180 min
1883 reflections with I > 2σ(I)	intensity decay: 10%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.056	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.179	Difmap
S = 1.04	w = 1/[σ^2(Fo^2) + (0.1088P)^2 + 1.4397P] where P = (Fo^2 + 2Fc^2)/3
2350 reflections	(Δ/σ)max < 0.001
150 parameters	Δρmax = 0.63 e Å^−3
0 restraints	Δρmin = −0.38 e Å^−3

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
C1	0.25143 (10)	0.0700 (2)	0.36853 (10)	0.0296 (3)
O1	0.30053 (11)	0.1504 (2)	0.43438 (9)	0.0456 (4)
O2	0.23728 (10)	0.1290 (2)	0.29639 (9)	0.0427 (3)
C2	0.21252 (11)	−0.1155 (2)	0.39371 (10)	0.0293 (3)
N	0.15374 (11)	−0.2048 (2)	0.32066 (10)	0.0327 (3)
C3	0.12451 (15)	−0.3983 (3)	0.33993 (16)	0.0427 (4)
H1	0.319 (3)	0.257 (5)	0.418 (2)	0.078 (10)*
H2	0.1784 (17)	−0.098 (3)	0.4359 (15)	0.039 (6)*
H3	0.2669 (18)	−0.199 (4)	0.4143 (16)	0.042 (6)*
H4	0.180 (2)	−0.208 (4)	0.276 (2)	0.057 (8)*
H5	0.105 (2)	−0.140 (4)	0.3074 (18)	0.054 (7)*
H6	0.092 (2)	−0.440 (5)	0.293 (2)	0.071 (9)*
H7	0.181 (3)	−0.466 (6)	0.355 (2)	0.087 (11)*
H8	0.091 (2)	−0.374 (5)	0.386 (2)	0.070 (9)*
C4	−0.07118 (11)	0.0083 (2)	0.38100 (10)	0.0305 (3)
O3	−0.14113 (10)	−0.0119 (2)	0.41607 (11)	0.0457 (4)
O4	−0.03378 (10)	−0.1039 (2)	0.33831 (10)	0.0448 (4)
C5	−0.02818 (12)	0.2064 (2)	0.39269 (13)	0.0384 (4)
F1	0.06028 (10)	0.2099 (2)	0.38432 (17)	0.0864 (7)
F2	−0.07389 (12)	0.3281 (2)	0.33934 (13)	0.0768 (5)
F3	−0.03152 (16)	0.2770 (3)	0.47001 (12)	0.0853 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0271 (6)	0.0275 (7)	0.0365 (7)	−0.0010 (5)	0.0116 (5)	−0.0013 (5)
O1	0.0550 (8)	0.0379 (7)	0.0435 (7)	−0.0171 (6)	0.0061 (6)	−0.0042 (6)
O2	0.0469 (7)	0.0416 (7)	0.0403 (7)	−0.0089 (6)	0.0091 (5)	0.0081 (5)
C2	0.0301 (7)	0.0277 (7)	0.0306 (7)	−0.0023 (5)	0.0065 (5)	0.0010 (5)
N	0.0328 (7)	0.0289 (6)	0.0365 (7)	−0.0037 (5)	0.0058 (5)	−0.0016 (5)
C3	0.0417 (9)	0.0279 (8)	0.0602 (12)	−0.0076 (7)	0.0131 (9)	−0.0052 (8)
C4	0.0287 (7)	0.0274 (7)	0.0356 (7)	−0.0011 (5)	0.0059 (6)	−0.0008 (5)
O3	0.0413 (7)	0.0339 (6)	0.0681 (9)	−0.0085 (5)	0.0274 (6)	−0.0072 (6)
O4	0.0413 (7)	0.0446 (7)	0.0508 (8)	0.0036 (6)	0.0143 (6)	−0.0127 (6)
C5	0.0310 (8)	0.0301 (8)	0.0547 (10)	−0.0023 (6)	0.0083 (7)	0.0024 (7)
F1	0.0375 (7)	0.0527 (9)	0.175 (2)	−0.0101 (6)	0.0360 (9)	0.0030 (10)
F2	0.0764 (11)	0.0464 (8)	0.1021 (13)	0.0012 (7)	−0.0026 (9)	0.0310 (8)
F3	0.1138 (15)	0.0669 (10)	0.0770 (11)	−0.0331 (10)	0.0204 (10)	−0.0331 (9)

Geometric parameters (Å, º)

C1—O2	1.204 (2)	C3—H6	0.87 (4)
C1—O1	1.300 (2)	C3—H7	0.95 (4)
C1—C2	1.512 (2)	C3—H8	0.96 (4)
O1—H1	0.86 (4)	C4—O4	1.227 (2)
C2—N	1.470 (2)	C4—O3	1.245 (2)
C2—H2	0.90 (3)	C4—C5	1.538 (2)
C2—H3	1.00 (3)	C5—F2	1.314 (2)
N—C3	1.484 (2)	C5—F1	1.319 (2)
N—H4	0.86 (3)	C5—F3	1.331 (3)
N—H5	0.84 (3)
O2—C1—O1	126.80 (16)	N—C3—H6	106 (2)
O2—C1—C2	122.55 (15)	N—C3—H7	105 (3)
O1—C1—C2	110.65 (14)	H6—C3—H7	112 (3)
C1—O1—H1	108 (2)	N—C3—H8	101 (2)
N—C2—C1	111.35 (13)	H6—C3—H8	116 (3)
N—C2—H2	108.7 (15)	H7—C3—H8	115 (3)
C1—C2—H2	110.4 (16)	O4—C4—O3	129.71 (16)
N—C2—H3	109.4 (15)	O4—C4—C5	116.77 (15)
C1—C2—H3	106.9 (15)	O3—C4—C5	113.51 (15)
H2—C2—H3	110 (2)	F2—C5—F1	109.15 (18)
C2—N—C3	112.81 (15)	F2—C5—F3	104.80 (19)
C2—N—H4	113 (2)	F1—C5—F3	105.36 (19)
C3—N—H4	109 (2)	F2—C5—C4	111.55 (15)
C2—N—H5	109 (2)	F1—C5—C4	113.09 (16)
C3—N—H5	108 (2)	F3—C5—C4	112.39 (16)
H4—N—H5	106 (3)
O2—C1—C2—N	−2.5 (2)	O4—C4—C5—F1	−24.4 (3)
O1—C1—C2—N	177.75 (14)	O3—C4—C5—F1	156.8 (2)
C1—C2—N—C3	172.39 (14)	O4—C4—C5—F3	−143.6 (2)
O4—C4—C5—F2	99.1 (2)	O3—C4—C5—F3	37.7 (2)
O3—C4—C5—F2	−79.7 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O3^i	0.84 (4)	1.76 (4)	2.575 (2)	163 (4)
N—H4···O2^ii	0.85 (3)	2.14 (3)	2.880 (2)	146 (3)
N—H5···O4^iii	0.84 (3)	2.40 (3)	2.918 (2)	121 (3)
N—H5···O4	0.84 (3)	2.18 (3)	2.885 (2)	141 (3)

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