2,4,6-Trifluoro­benzonitrile

The title compound, C₇H₂F₃N, contains three crystallographically independent mol­ecules in the crystal structure; two of these mol­ecules have symmetry m and the third has symmetry mm. Each independent mol­ecule forms a planar or approximately planar layer with its own kind. There are three different types of inter­layer contacts, two of which are similar to each other, while the third is distinctly different. The packing within the layers is similar to that found in 2,5- and 3,6-difluoro­benzonitrile, with weak C—H...N inter­actions holding the mol­ecules in the layers. The remarkable feature of this structure is the presence of more than one type of inter­layer inter­action.

S1. Comment

3,5-Difluorobenzonitrile (hereafter 35D; Britton, 2002) and 2,6-difluorobenzonitrile (hereafter 26D; Britton, 2004) both pack in slightly puckered molecular sheets held together by weak C—H···N and very weak C—H···F hydrogen bonds. The structure determination of the title compound (hereafter 246T) was undertaken to see the effect of replacing one more H atom with F, with the expectation that the C—H···N interactions would be slightly stronger (shorter).

There are two half-molecules and one quarter-molecule in the asymmetric unit (Z' = 5/4). Molecules 1 and 2 are normal to a mirror plane, and molecule 3 is on the intersection of two mirror planes. The Z value can be described, in the notation of Zorky (1996), as Z = 20(m²mm¹). The atom labelling and anisotropic displacement parameters for all three moleclules are shown in Fig. 1. The bond lengths and angles are all normal. The ring angles at atoms C11 and C13 and analogous atoms are less than 120°, and those at atoms C12 and C14 and analogous atoms are greater, in reasonable agreement with the substituent effects described by Domenicano (1992).

The molecules are arranged in layers normal to c. Each of the three types of molecules form layers with their own kind. The stacking of these layers is shown in Fig. 2; the unique layers are indicated by the labelling of the N atoms. Layer 3 is exactly planar. Molecules in layer 2 are tilted 2.9 (1)° away from parallel to layer 3. The N28 atoms are 2.862 (3) Å away from the plane of layer 3, and the F24 atoms 3.181 (2) Å away. A molecule in layer 1 is tilted 8.7 (1)° away from the plane of layer 3, and 11.6° away from the mean plane of a molecule in layer 2.

The molecules in the layers form C—H···N hydrogen bonds. The metric data for these bonds are given in Table 1, where they are compared with similar bonds in 35D and 26D. The H···N distances in molecules 2 and 3 are shorter than those in 35D and 26D, but the H···N distance in molecule 1 is not. This would appear to be a consequence of the deviation from planarity in the layer.

The stacking of the layers is shown in Fig. 3. The overlaps between layers 1 and 2 and between 2 and 3 are about the same, with no overlap between the rings and F atoms in one ring above the ring in the next layer. When layer 1 overlaps with layer 1', the rings do overlap, and the CN group in one molecule lies above the ring in the next layer. The two intralayer packing modes must have about the same energy; otherwise a structure with one of these modes exclusively ought to be favored.

Given the two distinctly different packing modes in the same structure, it seems likely that polymorphs might exist. Crystals were grown from acetone, benzene, CH₂Cl₂, CHCl₃, CCl₄ and acetonitrile. However, the crystals were all poor; in the two cases where a unit cell could be determined, it was the known cell.

Related literature

For related literature, see: Britton (2002, 2004); Domenicano (1992); Zorky (1996).

S2. Experimental

The compound was obtained from Lancaster Synthesis Inc. Attempts were made to recrystallize the compound from a number of solvents but none gave acceptable crystals. Crystals from the original sample were used for the data collection.

S3. Refinement

The crystals had a strong tendency to twin. When a single crystal was eventually found, the solution and refinement were straightforward. The H-atom positions and isotropic displacement parameters were refined [C—H = 0.970 (18) and 0.91 (3) Å].

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of the title compound. All of the molecules contain a mirror plane perpendicular to the plane of the molecule; only the crystallographically independent atoms are labelled. Displacement ellipsoids are drawn at the 50% probability level; H atoms are shown with arbitrary radius.
	Fig. 2. The crystal packing viewed along the a axis. One atom in each of the three independent layers is labeled.
	Fig. 3. The layers and their interactions with adjacent layers. In every case the C—H···N interactions are shown with dashed lines. Top: Layer 1, shown with solid bands, and layer 1', shown with dashed bonds, related to layer 1 by an inversion center at (1/2, 1/2, 0). Center: Layer 2, shown with solid bonds, and layer 1, shown with dashed bonds. Bottom: Layer 3, shown with solid bonds, and layer 2, shown with dashed bonds. Note that the overlap 1–1' is distinctly different from that of 2–1 and 3–2, which are similar to each other.

2,4,6-trifluorobenzonitrile

Crystal data

C7H2F3N	F(000) = 1560
Mr = 157.10	Dx = 1.631 Mg m^−3
Orthorhombic, Cmcm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2	Cell parameters from 2977 reflections
a = 7.5228 (10) Å	θ = 2.9–27.4°
b = 13.8222 (18) Å	µ = 0.16 mm^−1
c = 30.755 (4) Å	T = 173 K
V = 3198.0 (7) Å^3	Prism, colorless
Z = 20	0.5 × 0.5 × 0.4 mm

Data collection

Bruker SMART 1K CCD area-detector diffractometer	2004 independent reflections
Radiation source: fine-focus sealed tube	1721 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
ω scans	θmax = 27.5°, θmin = 1.3°
Absorption correction: multi-scan (SADABS; Sheldrick, 2003; Blessing, 1995)	h = −9→9
Tmin = 0.93, Tmax = 0.94	k = −17→17
18050 measured reflections	l = −39→39

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)] = 0.040	All H-atom parameters refined
wR(F^2) = 0.127	w = 1/[σ^2(Fo^2) + (0.064P)^2 + 1.81P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max = 0.001
2004 reflections	Δρmax = 0.35 e Å^−3
165 parameters	Δρmin = −0.27 e Å^−3
0 restraints	Extinction correction: SHELXTL (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0016 (3)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C11	0.5000	−0.01950 (12)	0.05738 (5)	0.0287 (4)
F12	0.19046 (11)	−0.01670 (7)	0.05722 (3)	0.0514 (3)
C12	0.34258 (17)	0.03226 (10)	0.05363 (4)	0.0341 (3)
C13	0.3385 (2)	0.13039 (11)	0.04635 (5)	0.0433 (4)
H13	0.233 (3)	0.1608 (13)	0.0436 (6)	0.053 (5)*
F14	0.5000	0.27250 (9)	0.03568 (5)	0.0714 (5)
C14	0.5000	0.17609 (15)	0.04288 (6)	0.0449 (5)
C17	0.5000	−0.12236 (14)	0.06366 (6)	0.0326 (4)
N18	0.5000	−0.20423 (13)	0.06765 (6)	0.0437 (4)
C21	0.5000	0.42065 (12)	0.15322 (5)	0.0256 (4)
F22	0.18998 (10)	0.41825 (7)	0.15336 (3)	0.0402 (3)
C22	0.34242 (16)	0.36836 (10)	0.15196 (4)	0.0279 (3)
C23	0.33765 (19)	0.26903 (10)	0.14946 (4)	0.0323 (3)
H23	0.225 (2)	0.2352 (12)	0.1486 (5)	0.038 (4)*
F24	0.5000	0.12533 (8)	0.14657 (4)	0.0462 (4)
C24	0.5000	0.22265 (13)	0.14846 (5)	0.0316 (4)
C27	0.5000	0.52423 (13)	0.15530 (6)	0.0317 (4)
N28	0.5000	0.60661 (13)	0.15692 (7)	0.0489 (5)
C31	0.5000	−0.01780 (17)	0.2500	0.0259 (5)
F32	0.18994 (14)	−0.01524 (9)	0.2500	0.0409 (3)
C32	0.3421 (2)	0.03461 (13)	0.2500	0.0280 (4)
C33	0.3377 (3)	0.13438 (13)	0.2500	0.0323 (4)
H33	0.232 (4)	0.1666 (19)	0.2500	0.052 (7)*
F34	0.5000	0.27807 (11)	0.2500	0.0453 (5)
C34	0.5000	0.18077 (18)	0.2500	0.0327 (6)
C37	0.5000	−0.12155 (19)	0.2500	0.0324 (5)
N38	0.5000	−0.20380 (18)	0.2500	0.0496 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C11	0.0299 (9)	0.0305 (9)	0.0257 (8)	0.000	0.000	0.0000 (6)
F12	0.0272 (4)	0.0619 (6)	0.0650 (6)	0.0003 (4)	0.0024 (4)	0.0032 (5)
C12	0.0302 (7)	0.0405 (7)	0.0316 (6)	0.0029 (5)	0.0024 (5)	−0.0014 (5)
C13	0.0511 (9)	0.0408 (8)	0.0381 (7)	0.0166 (7)	−0.0008 (6)	−0.0011 (6)
F14	0.1211 (15)	0.0285 (7)	0.0645 (9)	0.000	0.000	0.0033 (6)
C14	0.0726 (15)	0.0286 (10)	0.0337 (10)	0.000	0.000	−0.0001 (7)
C17	0.0311 (9)	0.0359 (10)	0.0306 (9)	0.000	0.000	−0.0006 (7)
N18	0.0498 (10)	0.0328 (9)	0.0487 (10)	0.000	0.000	0.0006 (7)
C21	0.0253 (8)	0.0236 (8)	0.0280 (8)	0.000	0.000	0.0018 (6)
F22	0.0210 (4)	0.0431 (5)	0.0566 (5)	0.0032 (3)	−0.0018 (3)	−0.0015 (4)
C22	0.0234 (6)	0.0310 (7)	0.0294 (6)	0.0005 (5)	−0.0012 (4)	0.0011 (4)
C23	0.0351 (7)	0.0312 (7)	0.0306 (6)	−0.0086 (5)	−0.0018 (5)	0.0002 (5)
F24	0.0747 (10)	0.0225 (6)	0.0413 (6)	0.000	0.000	−0.0016 (4)
C24	0.0491 (11)	0.0218 (8)	0.0238 (8)	0.000	0.000	0.0004 (6)
C27	0.0248 (8)	0.0270 (9)	0.0434 (10)	0.000	0.000	0.0023 (7)
N28	0.0398 (10)	0.0283 (9)	0.0787 (14)	0.000	0.000	0.0016 (8)
C31	0.0240 (11)	0.0251 (12)	0.0285 (11)	0.000	0.000	0.000
F32	0.0217 (5)	0.0427 (7)	0.0584 (7)	−0.0028 (4)	0.000	0.000
C32	0.0224 (8)	0.0315 (9)	0.0302 (8)	0.0010 (6)	0.000	0.000
C33	0.0340 (10)	0.0316 (9)	0.0313 (9)	0.0107 (7)	0.000	0.000
F34	0.0754 (13)	0.0212 (7)	0.0392 (9)	0.000	0.000	0.000
C34	0.0495 (16)	0.0220 (11)	0.0264 (11)	0.000	0.000	0.000
C37	0.0246 (12)	0.0276 (13)	0.0448 (14)	0.000	0.000	0.000
N38	0.0385 (13)	0.0281 (12)	0.082 (2)	0.000	0.000	0.000

Geometric parameters (Å, º)

C11—C17	1.435 (2)	C23—C24	1.3797 (16)
C11—C12	1.3884 (16)	C23—H23	0.970 (18)
F12—C12	1.3341 (16)	F24—C24	1.346 (2)
C12—C13	1.375 (2)	C27—N28	1.140 (3)
C13—C14	1.3733 (19)	C31—C32	1.391 (2)
C13—H13	0.905 (19)	C31—C37	1.434 (3)
F14—C14	1.351 (2)	F32—C32	1.336 (2)
C17—N18	1.138 (3)	C32—C33	1.379 (3)
C21—C22	1.3890 (15)	C33—C34	1.379 (2)
C21—C27	1.433 (2)	C33—H33	0.91 (3)
F22—C22	1.3387 (14)	F34—C34	1.345 (3)
C22—C23	1.3756 (19)	C37—N38	1.137 (4)
C17—C11—C12	121.45 (8)	C22—C23—H23	120.3 (10)
C12—C11—C12^i	117.07 (17)	C24—C23—H23	123.5 (10)
F12—C12—C13	119.66 (13)	F24—C24—C23	117.72 (8)
F12—C12—C11	117.60 (12)	C23—C24—C23^i	124.56 (17)
C13—C12—C11	122.74 (14)	N28—C27—C21	179.9 (2)
C12—C13—C14	116.54 (15)	C32—C31—C32^ii	117.3 (2)
C12—C13—H13	119.6 (12)	C32—C31—C37	121.37 (11)
C14—C13—H13	123.9 (12)	F32—C32—C33	119.69 (16)
F14—C14—C13	117.81 (9)	F32—C32—C31	117.58 (15)
C13^i—C14—C13	124.39 (18)	C33—C32—C31	122.73 (17)
N18—C17—C11	178.5 (2)	C32—C33—C34	116.35 (17)
C22—C21—C22^i	117.18 (15)	C32—C33—H33	120.7 (17)
C22—C21—C27	121.41 (8)	C34—C33—H33	123.0 (17)
F22—C22—C23	119.57 (12)	F34—C34—C33	117.71 (11)
F22—C22—C21	117.53 (11)	C33—C34—C33^ii	124.6 (2)
C23—C22—C21	122.90 (12)	N38—C37—C31	180.0
C22—C23—C24	116.23 (13)
C17—C11—C12—F12	1.7 (2)	F22—C22—C23—C24	−179.54 (12)
C12^i—C11—C12—F12	−179.95 (9)	C21—C22—C23—C24	0.33 (19)
C17—C11—C12—C13	−178.07 (15)	C22—C23—C24—F24	179.19 (12)
C12^i—C11—C12—C13	0.3 (3)	C22—C23—C24—C23^i	−0.5 (2)
F12—C12—C13—C14	−179.82 (14)	C32^ii—C31—C32—F32	180.0
C11—C12—C13—C14	0.0 (2)	C37—C31—C32—F32	0.0
C12—C13—C14—F14	179.94 (15)	C32^ii—C31—C32—C33	0.0
C12—C13—C14—C13^i	−0.2 (3)	C37—C31—C32—C33	180.0
C22^i—C21—C22—F22	179.71 (9)	F32—C32—C33—C34	180.0
C27—C21—C22—F22	−0.9 (2)	C31—C32—C33—C34	0.0
C22^i—C21—C22—C23	−0.2 (2)	C32—C33—C34—F34	180.0
C27—C21—C22—C23	179.24 (14)	C32—C33—C34—C33^ii	0.0

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