Trimethyl­urea

The title compound, C₄H₁₀N₂O, crystallizes with two independent mol­ecules in the space group Cmc2₁. The first mol­ecule lies in a crystallographic mirror plane with x = 0, whereas the second mol­ecule lies on a general position with x ≃ . Each mol­ecule forms classical N—H...O=C hydrogen bonds, thereby leading to chains of mol­ecules parallel or anti­parallel, respectively, to the polar c axis.

S1. Comment

In the course of our studies of adducts of di(organosulfonyl)amines with various N-methyl-substituted ureas [methylurea (Henschel et al., 2002; Wölper et al., 2010), 1,1-dimethylurea (Henschel et al., 2002; Wölper et al., 2010), 1,3-dimethylurea (Wijaya et al., 1997; Hamann, Henschel et al., 2002), tetramethylurea (Hamann, Wijaya et al., 2002; Zerbe et al., 2008) and trimethylurea (Wölper et al., 2011; Döring et al., 2012)], we recently noted to our surprise that the structure of trimethylurea itself has never been published. The structures of the other N-methylureas have been determined as follows: methylurea, space group P2₁2₁2₁, Huiszoon & Tiemessen (1976); 1,1-dimethylurea, P2₁/n, Pathirana et al. (1994), and as a footnote/supplementary material also by Fairlie et al. (1994); 1,3-dimethylurea, space group corrected from Cc to Fdd2 by Marsh (2004), Pérez-Folch et al. (1997); tetramethylurea, a liquid at room temperature, C2/c, Frampton & Parkes (1996). The latter two structures both display imposed twofold symmetry.

We report here the hitherto `missing' structure of trimethylurea, (I). The compound as purchased proved to contain single crystals and one of these was used to determine the structure. The suppliers Alfa Aesar were unfortunately unable to provide details of crystallization conditions; recrystallization experiments in our hands have so far failed to provide evidence of any other crystalline forms. The same crystal form was obtained from e.g. tetrahydrofuran/n-heptane by liquid diffusion.

The title compound crystallizes in the space group Cmc2₁ with Z = 12 (Z' = 1.5). The first independent molecule (unprimed atoms) lies in the mirror plane at x = 0, whereas the second molecule (primed atoms) occupies a general position in the plane at x ≈ 1/3 (Fig. 1). Molecule 2 has an r.m.s. deviation from planarity of only 0.03 Å; a least-squares fit of both molecules also gives an r.m.s. deviation of 0.03 Å. In both molecules, the NH hydrogen is trans to the C═O group, as is observed for all the methylated ureas for which this configuration is possible; one may speculate that this facilitates the formation of hydrogen-bonded chains of molecules.

Molecular dimensions, such as the C═O bond lengths of 1.244 (2) and 1.243 (3) Å and the N—C—N angles of 117.25 (15) and 117.4 (2)°, may be considered normal.

The packing involves classical N—H···O═C hydrogen-bond systems (Table 1), linking adjacent molecules via the c-glide operator and thus leading to chains of molecules with the simple and common graph-set C(4), parallel to the polar c axis (Fig. 2). The chains involve exclusively either molecule 1 or molecule 2, and, as defined by the general direction of the H···O vector, are, respectively, parallel or antiparallel to the c axis. The chain pattern as seen along the b axis (Fig. 3) is thus AABAAB at intervals of ca a/6; in other words, there are twice as many chains antiparallel to the c axis as parallel to the c axis, which is an unusual packing feature. The pattern of chains then extends itself by b-axis translation to form the layers perpendicular to the a axis as noted above for the individual molecules.

For 1,3-dimethylurea, which also forms hydrogen-bonded chains in a polar space group but has only one independent molecule, all chains are necessarily parallel to the polar c axis (Pérez-Folch et al., 1997). We have recently reinvestigated the 1,3-dimethylurea system and have established the presence of a new polymorph, upon which we shall report.

Related literature

For related literature, see: Döring et al. (2012); Fairlie et al. (1994); Frampton & Parkes (1996); Hamann, Henschel, Lange, Moers, Blaschette & Jones (2002); Hamann, Wijaya, Moers, Blaschette & Jones (2002); Henschel et al. (2002); Huiszoon & Tiemessen (1976); Marsh (2004); Pérez-Folch, Subirana & Aymami (1997); Pathirana et al. (1994); Wölper et al. (2010, 2011); Wijaya et al. (1997); Zerbe et al. (2008).

S2. Experimental

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S3. Refinement

The treatment of hydrogen atoms for molecule 1 (in the mirror plane): the NH hydrogen atom was refined freely. Methyl H atoms were identified in difference syntheses; the geometry was idealized (C—H 0.98 Å, H—C—H 109.5 °) and the methyl groups refined as rigid groups allowed to rotate but not tip. For all methyl H, U(H) was set to 1.5U_eq(C). For the carbons C2 and C4 one hydrogen lay to a good approximation in the mirror plane; this hydrogen was retained, together with one of the other methyl H atoms, with half and full occupation, respectively, and the group was refined using a riding model. For carbon C3 the H atoms were not in the mirror plane and a disordered model with three half-occupied H atoms was thus used.

The treatment of hydrogen atoms for molecule 2 (in the general position): the NH hydrogen was refined freely. The methyl H atoms were identified and incorporated into rigid groups as above. For carbons C2' and C4', the hydrogen positions were acceptable, but the methyl group at C3' showed indistinct maxima and refined slowly. For this reason a model of six half H atoms was used, corresponding to two equally spaced and half-occupied positions of the methyl H atoms. Even this model converged slowly, and the true disorder may be more serious. In both molecules there is a short intramolecular contact from the NH hydrogen to one of the disordered H atoms at C3 (1.96 Å for molecule 1 and 1.84 Å for molecule 2; the latter leads to an `Alert B' message from CHECKCIF).

In the absence of significant anomalous dispersion, Friedel opposite reflections were merged and the Flack parameter is thus meaningless.

Computing details

Data collection: CrysAlis PRO (Oxford Diffraction, 2011); cell refinement: CrysAlis PRO (Oxford Diffraction, 2011); data reduction: CrysAlis PRO (Oxford Diffraction, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. Both molecules of the title compound, projected parallel to the a axis. Only one position of each disordered methyl group (at C3, C3') is shown. Ellipsoids represent 50% probability levels.
	Fig. 2. (a) Hydrogen-bonded chain of molecule 1 in the region x ≈ 0. (b) Hydrogen-bonded chain of molecule 2 in the region x ≈ 1/3. In both cases hydrogen bonds are indicated by dashed lines, and hydrogen atoms not involved in hydrogen bonding are omitted.
	Fig. 3. Packing diagram of the title compound viewed parallel to the b axis, showing mutually antiparallel chains of molecules 1 and 2 (correspondingly labelled).

Trimethylurea

Crystal data

C4H10N2O	Dx = 1.221 Mg m^−3
Mr = 102.14	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmc21	Cell parameters from 11117 reflections
a = 20.5281 (15) Å	θ = 2.7–30.7°
b = 8.1474 (3) Å	µ = 0.09 mm^−1
c = 9.9637 (4) Å	T = 100 K
V = 1666.43 (15) Å^3	Block, colourless
Z = 12	0.4 × 0.3 × 0.2 mm
F(000) = 672

Data collection

Oxford Diffraction Xcalibur Eos diffractometer	1172 reflections with I > 2σ(I)
Radiation source: Enhance (Mo) X-ray Source	Rint = 0.027
Graphite monochromator	θmax = 29.1°, θmin = 2.7°
Detector resolution: 16.1419 pixels mm^-1	h = −27→28
ω scans	k = −11→11
36795 measured reflections	l = −13→13
1214 independent reflections

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)] = 0.031	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.087	H atoms treated by a mixture of independent and constrained refinement
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0501P)^2 + 0.5442P] where P = (Fo^2 + 2Fc^2)/3
1214 reflections	(Δ/σ)max = 0.040
117 parameters	Δρmax = 0.28 e Å^−3
1 restraint	Δρmin = −0.19 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.

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

20.5281 (0.0015) x + 0.0000 (0.0000) y + 0.0000 (0.0001) z = 0.0000 (0.0000)

* 0.0000 (0.0000) C1 * 0.0000 (0.0000) C2 * 0.0000 (0.0000) C3 * 0.0000 (0.0000) C4 * 0.0000 (0.0000) N1 * 0.0000 (0.0000) N3 * 0.0000 (0.0000) O1

Rms deviation of fitted atoms = 0.0000

20.4297 (0.0018) x + 0.6938 (0.0032) y - 0.4785 (0.0073) z = 7.1982 (0.0022)

Angle to previous plane (with approximate e.s.d.) = 5.61 (0.01)

* -0.0095 (0.0013) C1' * 0.0496 (0.0011) C2' * -0.0109 (0.0010) C3' * 0.0106 (0.0010) C4' * -0.0418 (0.0014) N1' * 0.0284 (0.0013) N3' * -0.0265 (0.0011) O1'

Rms deviation of fitted atoms = 0.0294

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)
C1	0.0000	0.4081 (2)	0.8582 (2)	0.0180 (4)
C2	0.0000	0.1132 (3)	0.8124 (2)	0.0241 (4)
H2A	0.0000	0.1531	0.7218	0.036*
H2B	0.0382	0.0476	0.8273	0.036*
C3	0.0000	0.2106 (3)	1.0478 (3)	0.0289 (5)
H3A	0.0188	0.1013	1.0611	0.043*	0.50
H3B	−0.0448	0.2119	1.0817	0.043*	0.50
H3C	0.0260	0.2920	1.0966	0.043*	0.50
C4	0.0000	0.6993 (2)	0.9065 (3)	0.0246 (5)
H4A	0.0000	0.7698	0.9837	0.037*
H4B	0.0382	0.7202	0.8536	0.037*
N1	0.0000	0.2505 (2)	0.9046 (2)	0.0253 (4)
H01	0.0000	0.508 (5)	1.030 (4)	0.024 (7)*
N3	0.0000	0.5296 (2)	0.95021 (19)	0.0239 (4)
O1	0.0000	0.4384 (2)	0.73592 (17)	0.0259 (4)
C1'	0.33529 (6)	0.5919 (2)	0.15028 (16)	0.0199 (3)
C2'	0.32948 (8)	0.8835 (2)	0.20144 (19)	0.0272 (3)
H2'1	0.3288	0.8395	0.2929	0.041*
H2'2	0.2916	0.9548	0.1880	0.041*
H2'3	0.3695	0.9471	0.1879	0.041*
C3'	0.32403 (9)	0.7936 (3)	−0.03531 (18)	0.0312 (4)
H3'1	0.3267	0.9132	−0.0440	0.047*	0.50
H3'2	0.2828	0.7549	−0.0734	0.047*	0.50
H3'3	0.3604	0.7429	−0.0837	0.047*	0.50
H3'4	0.3199	0.6942	−0.0900	0.047*	0.50
H3'5	0.3638	0.8524	−0.0606	0.047*	0.50
H3'6	0.2862	0.8644	−0.0503	0.047*	0.50
C4'	0.34493 (8)	0.30117 (18)	0.09814 (18)	0.0261 (3)
H4'1	0.3843	0.2878	0.1527	0.039*
H4'2	0.3478	0.2306	0.0187	0.039*
H4'3	0.3067	0.2702	0.1512	0.039*
N1'	0.32734 (7)	0.74860 (17)	0.10550 (15)	0.0249 (3)
H01'	0.3371 (9)	0.484 (4)	−0.031 (3)	0.037 (6)*
N3'	0.33904 (7)	0.47151 (18)	0.05633 (14)	0.0239 (3)
O1'	0.33841 (7)	0.55955 (15)	0.27212 (11)	0.0262 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0254 (9)	0.0143 (9)	0.0145 (9)	0.000	0.000	0.0011 (8)
C2	0.0336 (10)	0.0168 (9)	0.0219 (11)	0.000	0.000	−0.0021 (9)
C3	0.0509 (13)	0.0180 (10)	0.0177 (11)	0.000	0.000	0.0042 (9)
C4	0.0399 (11)	0.0159 (9)	0.0181 (10)	0.000	0.000	0.0003 (9)
N1	0.0488 (11)	0.0143 (8)	0.0128 (8)	0.000	0.000	0.0014 (8)
N3	0.0451 (10)	0.0160 (9)	0.0107 (9)	0.000	0.000	0.0016 (8)
O1	0.0454 (9)	0.0208 (9)	0.0116 (8)	0.000	0.000	0.0003 (7)
C1'	0.0232 (6)	0.0194 (7)	0.0170 (7)	−0.0007 (5)	−0.0007 (5)	0.0005 (6)
C2'	0.0405 (8)	0.0157 (7)	0.0254 (8)	0.0010 (5)	−0.0027 (7)	−0.0024 (7)
C3'	0.0493 (9)	0.0254 (9)	0.0190 (8)	0.0056 (6)	0.0003 (7)	0.0057 (7)
C4'	0.0396 (8)	0.0169 (7)	0.0219 (8)	0.0004 (6)	−0.0003 (6)	−0.0015 (6)
N1'	0.0405 (7)	0.0171 (6)	0.0171 (6)	0.0030 (4)	−0.0010 (5)	0.0003 (6)
N3'	0.0390 (7)	0.0176 (6)	0.0152 (7)	−0.0014 (5)	0.0006 (5)	0.0000 (6)
O1'	0.0434 (7)	0.0202 (5)	0.0150 (6)	0.0004 (5)	−0.0012 (5)	0.0014 (5)

Geometric parameters (Å, º)

C1—O1	1.243 (3)	C2'—N1'	1.458 (2)
C1—N3	1.349 (3)	C2'—H2'1	0.98
C1—N1	1.364 (3)	C2'—H2'2	0.98
C2—N1	1.447 (3)	C2'—H2'3	0.98
C2—H2A	0.96	C3'—N1'	1.452 (2)
C2—H2B	0.96	C3'—H3'1	0.98
C3—N1	1.464 (3)	C3'—H3'2	0.98
C3—H3A	0.98	C3'—H3'3	0.98
C3—H3B	0.98	C3'—H3'4	0.98
C3—H3C	0.98	C3'—H3'5	0.98
C4—N3	1.450 (3)	C3'—H3'6	0.98
C4—H4A	0.96	C4'—N3'	1.4540 (19)
C4—H4B	0.96	C4'—H4'1	0.98
N3—H01	0.81 (4)	C4'—H4'2	0.98
C1'—O1'	1.2438 (19)	C4'—H4'3	0.98
C1'—N3'	1.358 (2)	N3'—H01'	0.88 (3)
C1'—N1'	1.362 (2)
O1—C1—N3	121.3 (2)	N1'—C2'—H2'3	109.5
O1—C1—N1	121.3 (2)	H2'1—C2'—H2'3	109.5
N3—C1—N1	117.4 (2)	H2'2—C2'—H2'3	109.5
N1—C2—H2A	109.6	N1'—C3'—H3'1	109.5
N1—C2—H2B	109.4	N1'—C3'—H3'2	109.5
H2A—C2—H2B	109.5	H3'1—C3'—H3'2	109.5
N1—C3—H3A	109.5	N1'—C3'—H3'3	109.5
N1—C3—H3B	109.5	H3'1—C3'—H3'3	109.5
H3A—C3—H3B	109.5	H3'2—C3'—H3'3	109.5
N1—C3—H3C	109.5	N1'—C3'—H3'4	109.5
H3A—C3—H3C	109.5	N1'—C3'—H3'5	109.5
H3B—C3—H3C	109.5	H3'4—C3'—H3'5	109.5
N3—C4—H4A	109.3	N1'—C3'—H3'6	109.5
N3—C4—H4B	109.6	H3'4—C3'—H3'6	109.5
H4A—C4—H4B	109.5	H3'5—C3'—H3'6	109.5
C1—N1—C2	120.8 (2)	N3'—C4'—H4'1	109.5
C1—N1—C3	122.61 (19)	N3'—C4'—H4'2	109.5
C2—N1—C3	116.55 (19)	H4'1—C4'—H4'2	109.5
C1—N3—C4	119.72 (19)	N3'—C4'—H4'3	109.5
C1—N3—H01	120 (3)	H4'1—C4'—H4'3	109.5
C4—N3—H01	120 (3)	H4'2—C4'—H4'3	109.5
O1'—C1'—N3'	121.13 (17)	C1'—N1'—C3'	123.96 (15)
O1'—C1'—N1'	121.61 (15)	C1'—N1'—C2'	119.25 (15)
N3'—C1'—N1'	117.25 (15)	C3'—N1'—C2'	116.42 (15)
N1'—C2'—H2'1	109.5	C1'—N3'—C4'	119.77 (15)
N1'—C2'—H2'2	109.5	C1'—N3'—H01'	126 (2)
H2'1—C2'—H2'2	109.5	C4'—N3'—H01'	114 (2)
O1—C1—N1—C2	0.000	O1'—C1'—N1'—C3'	179.36 (17)
N3—C1—N1—C2	180.0	N3'—C1'—N1'—C3'	−1.3 (2)
O1—C1—N1—C3	180.0	O1'—C1'—N1'—C2'	6.6 (2)
N3—C1—N1—C3	0.000	N3'—C1'—N1'—C2'	−174.01 (14)
O1—C1—N3—C4	0.000	O1'—C1'—N3'—C4'	1.8 (2)
N1—C1—N3—C4	180.0	N1'—C1'—N3'—C4'	−177.57 (13)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N3—H01···O1^i	0.81 (4)	2.10 (4)	2.859 (3)	155 (4)
N3′—H01′···O1′^ii	0.88 (3)	2.00 (3)	2.8431 (19)	162 (3)

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