organic compounds
4′-Octyloxybiphenyl-4-carbonitrile polymorph III
aDepartment of Chemical Engineering, UMIST, PO Box 88, Manchester M60 1QD, England
*Correspondence e-mail: mike.j.quayle@bnfl.com
The title compound, C21H25NO, is a member of a well known family of liquid crystals (4-oxy-4′-cyanobiphenyls, OCBs) and packs in lamellar-type bilayers in the solid state, through CN⋯H hydrogen bonds. This packing type is analogous to that found of other members of the n-OCB homologous series, viz. 7-OCB and 9-OCB.
Comment
The OCB family of compounds (4-n-alkyloxy-4′-cyanobiphenyls) are well known and well utilized for their thermotropic (melt) behaviour, with the members in the range n = 5–8 forming both smectic A and nematic phases. Since these molecules readily order themselves to some degree in the phases, we were interested in probing structural mechanisms during the crystallization
event. On cooling the systems from the higher-temperature melt through the different stages, through to crystallization, the long-range d-spacing was tracked by in situ powder diffraction. The aim was to understand the molecular–molecular associations, through the various stages of assembly, before the system crystallizes. During the course of this work, a number of new crystal structures were encountered, as identified by X-ray powder diffraction patterns. To exploit as much structural information as possible, attempts at growing single crystals with diffraction patterns matching those of the unknown crystalline phases were made.The title compound, (I), is the third identified polymorph of 8-OCB (4-octyloxy-4′-cyanobiphenyl polymorph), and packs in the same arrangement as polymorphs of 7-OCB (Hori, Koma et al., 1996) and 9-OCB (Hori & Wu, 1999). The molecular structure of (I) is shown in Fig. 1 and selected geometric parameters are given in Table 1. The two rings of the biphenyl group are rotated by ∼42°, and the C15—C14—O1—C11 torsion angle is 168.7 (2)°. The aliphatic chain is in a rigid conformation, apparently to aid efficient packing. The molecular packing, which is rich in CN⋯H hydrogen bonds, is shown in Fig. 2. A number of notable features are apparent from the packing arrangement. Molecules pack head-to-head, forming bilayers (ca 36 Å in length) extending along the a axis. Each molecule forms four hydrogen bonds (two symmetry unique) with two head-on molecules. Pairs of hydrogen bonds (centrosymmetric dimers) in turn form catemeric chains along the b axis. The head-to-head packing of molecules in the a direction has a lateral displacement of 1.9 Å relative to the mean planes of the pairs of terminal arene groups. A second, identical, set of catemeric chains (not shown in Fig. 2) extends in the b direction but at ca 45° to the first set.
The two unique CN⋯H hydrogen bonds are N1⋯H7 and N1⋯H3 (Table 2), as shown in Fig. 2, with C1—N1⋯H7 and C1—N1⋯H3 angles of 135 and 131°. The C1—C2 bond is unusually long for a Csp2—Csp bond [1.455 (4) Å], possibly as a result of the π-electron withdrawing cyano group and its associated hydrogen bonds.
Polymorphs of 7-OCB (Hori, Koma et al., 1996) and 9-OCB (Hori & Wu, 1999) also display this packing configuration, with CN⋯H hydrogen-bond lengths over the narrow range 2.68–2.76 Å. Another polymorph of 8-OCB (Hori, Kurosaki et al., 1996; Rajnikant et al., 2000) also has layered-type packing, but in this configuration dimers are again formed through CN⋯H interactions, which extend only to tetramers. In this form, the direction of adjacent pairs of molecules at the interface is different from that in (I), i.e. chains in the a direction are antiparallel. In a final type of polymorph displayed by 7-OCB (Hori et al., 1995) and 8-OCB (Hori, Kurosaki et al., 1996), another mixed bilayer structure is formed, but notably in this case the CN⋯H dimer is absent; a CN group is close not to another CN group but to the biphenyl moiety. The O atom is utilized forming O⋯H hydrogen bonds along with CN⋯H hydrogen bonds.
The number and structural variety of polymorphs (at least five known) exhibited by the n-OCB series of compounds (the structural identity of some is still unknown) is typical of compounds that exhibit liquid crystalline behaviour. The apparent ease at which one form might be crystallized over another is caused by weak and competing intermolecular interactions.
Experimental
Crystals of 8-OCB form III were grown from propan-2-ol (0.1 g ml−1) at 278 K, yielding transparent crystals with a plate morphology.
Crystal data
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Refinement
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H atoms were constrained to idealized geometries and assigned Uiso(H) values of 1.2 times Ueq of their attached C atom for aromatic H atoms and 1.5Ueq for all others.
Data collection: COLLECT (Nonius, 2000); cell SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXL97; software used to prepare material for publication: SHELXL97.
Supporting information
10.1107/S0108270105000326/sq1183sup1.cif
contains datablocks global, I. DOI:Structure factors: contains datablock I. DOI: 10.1107/S0108270105000326/sq1183Isup2.hkl
Crystals of 8-OCB form III were grown from propan-2-ol (0.1 g ml−1) at 278 K, yielding transparent crystals with a plate morphology.
All non-H atoms were refined with anisotropic displacement parameters. H atoms were constrained to idealized geometries and assigned Uiso(H) values of 1.2 times the Ueq value of their attached C atom for aromatic H atoms and 1.5Ueq for all others.
Data collection: Collect (Nonius, 2000); cell
HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO (Otwinowski & Minor 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: program (reference)?; software used to prepare material for publication: program (reference)?.Fig. 1. A view of (I), showing 50% probability displacement ellipsoids. | |
Fig. 2. A packing diagram of (I), shown along the a axis. [Symmetry codes: (i) −x, 2 − y, 1 − z, (ii) −x, 1 − y, 1 − z.] |
C21H25NO | F(000) = 1328 |
Mr = 307.42 | Dx = 1.153 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 35865 reflections |
a = 73.814 (15) Å | θ = 5–50° |
b = 7.0080 (14) Å | µ = 0.07 mm−1 |
c = 6.8710 (14) Å | T = 293 K |
β = 94.98 (3)° | Plate, transparent |
V = 3540.9 (12) Å3 | 0.20 × 0.20 × 0.05 mm |
Z = 8 |
Nonius KappaCCD diffractometer | 1551 reflections with I > 2σ(I) |
ϕ or ω scans | Rint = 0.032 |
Absorption correction: multi-scan (SORTAV; Blessing, 1995) | θmax = 25.0°, θmin = 2.9° |
Tmin = 0.976, Tmax = 0.996 | h = −87→88 |
6626 measured reflections | k = −8→8 |
3073 independent reflections | l = −8→8 |
Refinement on F2 | H-atom parameters constrained |
Least-squares matrix: full | w = 1/[σ2(Fo2) + (0.0687P)2] where P = (Fo2 + 2Fc2)/3 |
R[F2 > 2σ(F2)] = 0.059 | (Δ/σ)max < 0.001 |
wR(F2) = 0.159 | Δρmax = 0.22 e Å−3 |
S = 0.99 | Δρmin = −0.23 e Å−3 |
3073 reflections | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
210 parameters | Extinction coefficient: 0.0017 (4) |
0 restraints |
C21H25NO | V = 3540.9 (12) Å3 |
Mr = 307.42 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 73.814 (15) Å | µ = 0.07 mm−1 |
b = 7.0080 (14) Å | T = 293 K |
c = 6.8710 (14) Å | 0.20 × 0.20 × 0.05 mm |
β = 94.98 (3)° |
Nonius KappaCCD diffractometer | 3073 independent reflections |
Absorption correction: multi-scan (SORTAV; Blessing, 1995) | 1551 reflections with I > 2σ(I) |
Tmin = 0.976, Tmax = 0.996 | Rint = 0.032 |
6626 measured reflections |
R[F2 > 2σ(F2)] = 0.059 | 0 restraints |
wR(F2) = 0.159 | H-atom parameters constrained |
S = 0.99 | Δρmax = 0.22 e Å−3 |
3073 reflections | Δρmin = −0.23 e Å−3 |
210 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
C1 | 0.00861 (4) | 0.7581 (4) | 0.4897 (4) | 0.0311 (7) | |
C2 | 0.02773 (4) | 0.7545 (4) | 0.4531 (4) | 0.0299 (7) | |
C3 | 0.03421 (3) | 0.5998 (4) | 0.3505 (4) | 0.0311 (7) | |
H3 | 0.0265 | 0.4996 | 0.3112 | 0.037* | |
C4 | 0.05217 (3) | 0.5972 (4) | 0.3080 (4) | 0.0299 (7) | |
H4 | 0.0564 | 0.4947 | 0.239 | 0.036* | |
C5 | 0.06405 (3) | 0.7453 (4) | 0.3664 (4) | 0.0265 (7) | |
C6 | 0.05735 (4) | 0.8966 (4) | 0.4713 (4) | 0.0331 (7) | |
H6 | 0.0651 | 0.9958 | 0.5132 | 0.04* | |
C7 | 0.03931 (4) | 0.9019 (4) | 0.5144 (4) | 0.0348 (8) | |
H7 | 0.035 | 1.0039 | 0.5839 | 0.042* | |
C8 | 0.08311 (4) | 0.7460 (4) | 0.3120 (4) | 0.0281 (7) | |
C9 | 0.08753 (4) | 0.6594 (4) | 0.1392 (4) | 0.0319 (7) | |
H9 | 0.0784 | 0.5975 | 0.0608 | 0.038* | |
C10 | 0.10516 (4) | 0.6629 (4) | 0.0805 (4) | 0.0325 (7) | |
H10 | 0.1078 | 0.6031 | −0.0346 | 0.039* | |
C11 | 0.11866 (4) | 0.7559 (4) | 0.1949 (4) | 0.0285 (7) | |
C12 | 0.11484 (4) | 0.8367 (4) | 0.3713 (4) | 0.0312 (7) | |
H12 | 0.1241 | 0.8938 | 0.4517 | 0.037* | |
C13 | 0.09727 (3) | 0.8324 (4) | 0.4276 (4) | 0.0299 (7) | |
H13 | 0.0949 | 0.8885 | 0.5453 | 0.036* | |
C14 | 0.14020 (3) | 0.7111 (4) | −0.0449 (4) | 0.0337 (8) | |
H14A | 0.1312 | 0.7604 | −0.1437 | 0.04* | |
H14B | 0.1399 | 0.5728 | −0.0504 | 0.04* | |
C15 | 0.15880 (4) | 0.7827 (4) | −0.0802 (4) | 0.0339 (8) | |
H15A | 0.1675 | 0.7388 | 0.0244 | 0.041* | |
H15B | 0.1588 | 0.9211 | −0.0776 | 0.041* | |
C16 | 0.16482 (4) | 0.7153 (4) | −0.2747 (4) | 0.0342 (8) | |
H16A | 0.1561 | 0.76 | −0.379 | 0.041* | |
H16B | 0.1647 | 0.5769 | −0.2774 | 0.041* | |
C17 | 0.18371 (4) | 0.7850 (4) | −0.3129 (4) | 0.0334 (8) | |
H17A | 0.1923 | 0.7417 | −0.2076 | 0.04* | |
H17B | 0.1838 | 0.9234 | −0.3118 | 0.04* | |
C18 | 0.18993 (4) | 0.7156 (4) | −0.5066 (4) | 0.0345 (8) | |
H18A | 0.1895 | 0.5773 | −0.5088 | 0.041* | |
H18B | 0.1814 | 0.7616 | −0.6117 | 0.041* | |
C19 | 0.20894 (4) | 0.7787 (4) | −0.5461 (4) | 0.0366 (8) | |
H19A | 0.2093 | 0.917 | −0.5503 | 0.044* | |
H19B | 0.2174 | 0.7374 | −0.4387 | 0.044* | |
C20 | 0.21507 (4) | 0.7014 (4) | −0.7342 (4) | 0.0508 (9) | |
H20A | 0.2065 | 0.7425 | −0.8411 | 0.061* | |
H20B | 0.2147 | 0.5631 | −0.7295 | 0.061* | |
C21 | 0.23403 (4) | 0.7624 (5) | −0.7774 (5) | 0.0660 (11) | |
H21A | 0.2346 | 0.8991 | −0.7825 | 0.099* | |
H21B | 0.2367 | 0.7105 | −0.9009 | 0.099* | |
H21C | 0.2427 | 0.7162 | −0.6764 | 0.099* | |
N1 | −0.00632 (4) | 0.7582 (3) | 0.5162 (3) | 0.0445 (7) | |
O1 | 0.13627 (2) | 0.7761 (2) | 0.1462 (3) | 0.0354 (5) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C1 | 0.0336 (19) | 0.035 (2) | 0.0236 (16) | −0.0013 (14) | −0.0014 (14) | 0.0000 (14) |
C2 | 0.0251 (17) | 0.0366 (18) | 0.0278 (16) | 0.0015 (15) | 0.0006 (13) | 0.0020 (15) |
C3 | 0.0284 (17) | 0.0324 (19) | 0.0318 (17) | −0.0048 (14) | −0.0009 (14) | 0.0004 (14) |
C4 | 0.0300 (18) | 0.0305 (18) | 0.0292 (17) | 0.0014 (14) | 0.0033 (13) | 0.0000 (14) |
C5 | 0.0251 (17) | 0.0277 (17) | 0.0261 (15) | 0.0001 (14) | −0.0012 (13) | 0.0031 (14) |
C6 | 0.0314 (18) | 0.0304 (19) | 0.0375 (18) | −0.0020 (14) | 0.0023 (14) | −0.0028 (15) |
C7 | 0.0341 (18) | 0.0387 (19) | 0.0319 (17) | 0.0035 (15) | 0.0046 (14) | −0.0057 (15) |
C8 | 0.0287 (17) | 0.0285 (17) | 0.0270 (15) | 0.0023 (14) | 0.0017 (13) | 0.0020 (14) |
C9 | 0.0321 (18) | 0.0280 (18) | 0.0352 (17) | 0.0003 (13) | 0.0008 (14) | −0.0023 (14) |
C10 | 0.0316 (18) | 0.0319 (18) | 0.0342 (17) | 0.0021 (14) | 0.0050 (14) | −0.0048 (14) |
C11 | 0.0229 (17) | 0.0278 (18) | 0.0348 (17) | 0.0014 (13) | 0.0023 (13) | 0.0026 (14) |
C12 | 0.0302 (18) | 0.0325 (18) | 0.0300 (16) | −0.0014 (14) | −0.0022 (13) | −0.0010 (14) |
C13 | 0.0292 (18) | 0.0319 (18) | 0.0278 (16) | 0.0023 (14) | −0.0024 (13) | 0.0001 (14) |
C14 | 0.0315 (18) | 0.036 (2) | 0.0336 (17) | 0.0015 (14) | 0.0047 (14) | −0.0049 (15) |
C15 | 0.0299 (18) | 0.0338 (19) | 0.0384 (17) | 0.0002 (14) | 0.0057 (14) | −0.0023 (15) |
C16 | 0.0314 (18) | 0.0366 (19) | 0.0345 (17) | −0.0020 (14) | 0.0023 (14) | 0.0007 (15) |
C17 | 0.0306 (18) | 0.0366 (19) | 0.0328 (16) | 0.0006 (14) | 0.0025 (13) | 0.0005 (14) |
C18 | 0.0343 (18) | 0.039 (2) | 0.0301 (16) | 0.0013 (14) | 0.0021 (14) | −0.0004 (14) |
C19 | 0.0328 (18) | 0.039 (2) | 0.0385 (18) | 0.0007 (14) | 0.0029 (15) | −0.0002 (15) |
C20 | 0.049 (2) | 0.057 (2) | 0.049 (2) | −0.0041 (17) | 0.0161 (17) | −0.0034 (18) |
C21 | 0.045 (2) | 0.088 (3) | 0.068 (3) | −0.001 (2) | 0.021 (2) | 0.001 (2) |
N1 | 0.0435 (18) | 0.0455 (18) | 0.0450 (17) | −0.0034 (14) | 0.0063 (14) | −0.0015 (13) |
O1 | 0.0271 (12) | 0.0448 (13) | 0.0348 (11) | −0.0015 (9) | 0.0056 (9) | −0.0046 (10) |
C1—N1 | 1.132 (3) | C14—C15 | 1.502 (3) |
C1—C2 | 1.455 (4) | C14—H14A | 0.97 |
C2—C7 | 1.383 (3) | C14—H14B | 0.97 |
C2—C3 | 1.400 (3) | C15—C16 | 1.520 (3) |
C3—C4 | 1.382 (3) | C15—H15A | 0.97 |
C3—H3 | 0.93 | C15—H15B | 0.97 |
C4—C5 | 1.395 (3) | C16—C17 | 1.522 (3) |
C4—H4 | 0.93 | C16—H16A | 0.97 |
C5—C6 | 1.397 (3) | C16—H16B | 0.97 |
C5—C8 | 1.487 (3) | C17—C18 | 1.525 (3) |
C6—C7 | 1.389 (3) | C17—H17A | 0.97 |
C6—H6 | 0.93 | C17—H17B | 0.97 |
C7—H7 | 0.93 | C18—C19 | 1.518 (3) |
C8—C13 | 1.395 (4) | C18—H18A | 0.97 |
C8—C9 | 1.397 (3) | C18—H18B | 0.97 |
C9—C10 | 1.395 (3) | C19—C20 | 1.506 (4) |
C9—H9 | 0.93 | C19—H19A | 0.97 |
C10—C11 | 1.378 (4) | C19—H19B | 0.97 |
C10—H10 | 0.93 | C20—C21 | 1.518 (4) |
C11—O1 | 1.377 (3) | C20—H20A | 0.97 |
C11—C12 | 1.389 (3) | C20—H20B | 0.97 |
C12—C13 | 1.385 (3) | C21—H21A | 0.96 |
C12—H12 | 0.93 | C21—H21B | 0.96 |
C13—H13 | 0.93 | C21—H21C | 0.96 |
C14—O1 | 1.443 (3) | ||
N1—C1—C2 | 178.8 (3) | C16—C15—H15B | 109.2 |
C7—C2—C3 | 120.1 (2) | H15A—C15—H15B | 107.9 |
C7—C2—C1 | 121.2 (3) | C15—C16—C17 | 113.2 (2) |
C3—C2—C1 | 118.7 (3) | C15—C16—H16A | 108.9 |
C4—C3—C2 | 119.5 (3) | C17—C16—H16A | 108.9 |
C4—C3—H3 | 120.2 | C15—C16—H16B | 108.9 |
C2—C3—H3 | 120.2 | C17—C16—H16B | 108.9 |
C3—C4—C5 | 121.3 (3) | H16A—C16—H16B | 107.8 |
C3—C4—H4 | 119.3 | C16—C17—C18 | 113.5 (2) |
C5—C4—H4 | 119.3 | C16—C17—H17A | 108.9 |
C4—C5—C6 | 118.2 (2) | C18—C17—H17A | 108.9 |
C4—C5—C8 | 121.0 (2) | C16—C17—H17B | 108.9 |
C6—C5—C8 | 120.8 (2) | C18—C17—H17B | 108.9 |
C7—C6—C5 | 121.2 (3) | H17A—C17—H17B | 107.7 |
C7—C6—H6 | 119.4 | C19—C18—C17 | 114.6 (2) |
C5—C6—H6 | 119.4 | C19—C18—H18A | 108.6 |
C2—C7—C6 | 119.6 (3) | C17—C18—H18A | 108.6 |
C2—C7—H7 | 120.2 | C19—C18—H18B | 108.6 |
C6—C7—H7 | 120.2 | C17—C18—H18B | 108.6 |
C13—C8—C9 | 117.0 (2) | H18A—C18—H18B | 107.6 |
C13—C8—C5 | 122.5 (2) | C20—C19—C18 | 113.5 (2) |
C9—C8—C5 | 120.5 (3) | C20—C19—H19A | 108.9 |
C10—C9—C8 | 122.0 (3) | C18—C19—H19A | 108.9 |
C10—C9—H9 | 119 | C20—C19—H19B | 108.9 |
C8—C9—H9 | 119 | C18—C19—H19B | 108.9 |
C11—C10—C9 | 119.3 (3) | H19A—C19—H19B | 107.7 |
C11—C10—H10 | 120.3 | C19—C20—C21 | 114.7 (3) |
C9—C10—H10 | 120.3 | C19—C20—H20A | 108.6 |
O1—C11—C10 | 124.1 (2) | C21—C20—H20A | 108.6 |
O1—C11—C12 | 116.0 (2) | C19—C20—H20B | 108.6 |
C10—C11—C12 | 119.9 (2) | C21—C20—H20B | 108.6 |
C13—C12—C11 | 120.1 (3) | H20A—C20—H20B | 107.6 |
C13—C12—H12 | 120 | C20—C21—H21A | 109.5 |
C11—C12—H12 | 120 | C20—C21—H21B | 109.5 |
C12—C13—C8 | 121.6 (3) | H21A—C21—H21B | 109.5 |
C12—C13—H13 | 119.2 | C20—C21—H21C | 109.5 |
C8—C13—H13 | 119.2 | H21A—C21—H21C | 109.5 |
O1—C14—C15 | 107.5 (2) | H21B—C21—H21C | 109.5 |
O1—C14—H14A | 110.2 | C1—N1—H3i | 131 |
C15—C14—H14A | 110.2 | C1—N1—H7ii | 134.7 |
O1—C14—H14B | 110.2 | H3i—N1—H7ii | 94.2 |
C15—C14—H14B | 110.2 | C1—N1—C7ii | 133.8 (2) |
H14A—C14—H14B | 108.5 | H3i—N1—C7ii | 94.4 |
C14—C15—C16 | 112.3 (2) | C1—N1—C3i | 131.3 (2) |
C14—C15—H15A | 109.2 | H7ii—N1—C3i | 92.5 |
C16—C15—H15A | 109.2 | C7ii—N1—C3i | 94.92 (9) |
C14—C15—H15B | 109.2 | C11—O1—C14 | 117.2 (2) |
Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x, −y+2, −z+1. |
D—H···A | D—H | H···A | D···A | D—H···A |
C7ii—H7ii···N1 | 0.93 | 2.74 | 3.400 (4) | 129 |
C3i—H3i···N1 | 0.93 | 2.68 | 3.420 (3) | 137 |
Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x, −y+2, −z+1. |
Experimental details
Crystal data | |
Chemical formula | C21H25NO |
Mr | 307.42 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 293 |
a, b, c (Å) | 73.814 (15), 7.0080 (14), 6.8710 (14) |
β (°) | 94.98 (3) |
V (Å3) | 3540.9 (12) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 0.07 |
Crystal size (mm) | 0.20 × 0.20 × 0.05 |
Data collection | |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | Multi-scan (SORTAV; Blessing, 1995) |
Tmin, Tmax | 0.976, 0.996 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 6626, 3073, 1551 |
Rint | 0.032 |
(sin θ/λ)max (Å−1) | 0.595 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.059, 0.159, 0.99 |
No. of reflections | 3073 |
No. of parameters | 210 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.22, −0.23 |
Computer programs: Collect (Nonius, 2000), HKL SCALEPACK (Otwinowski & Minor 1997), HKL DENZO (Otwinowski & Minor 1997) and SCALEPACK, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), program (reference)?.
C1—N1 | 1.132 (3) | C11—O1 | 1.377 (3) |
C1—C2 | 1.455 (4) | C14—O1 | 1.443 (3) |
C5—C8 | 1.487 (3) | ||
N1—C1—C2 | 178.8 (3) | O1—C14—C15 | 107.5 (2) |
O1—C11—C10 | 124.1 (2) | C11—O1—C14 | 117.2 (2) |
O1—C11—C12 | 116.0 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
C7i—H7i···N1 | 0.93 | 2.74 | 3.400 (4) | 129 |
C3ii—H3ii···N1 | 0.93 | 2.68 | 3.420 (3) | 137 |
Symmetry codes: (i) −x, −y+2, −z+1; (ii) −x, −y+1, −z+1. |
Acknowledgements
The EPSRC is thanked for financial support to MJQ.
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The OCB family of compounds are well known and well utilized for their theromtropic (melt) liquid crystal behaviour, with the members in the range n = 5–8 (4 − n-oxy-4'cyanobiphenyl) forming both smectic A and nematic phases. Since these molecules readily order themselves to some degree in the liquid crystal phases, we were interested in probing structural mechanisms during the crystallization event. On cooling the systems from the higher-temperature melt through the different liquid crystal stages, through to crystallization, the long-range d-spacing was tracked by in situ powder diffraction. The aim was to understand the molecular–molecular associations, through the various stages of assembly, before the system crystallizes. During the course of this work, a number of new crystal structures were encountered, as identified by X-ray powder diffraction patterns. To exploit as much structural information as possible, attempts at growing single crystals with diffraction patterns matching those of the unknown crystalline phases were made.
The title compound, (I), is the third identified polymorph of 8-OCB (4-octoxy-4'-cyanobiphenyl polymorph), and packs in the same arrangement as polymorphs of 7-OCB (Hori, Koma et al., 1996) ?? and 9-OCB (Hori & Wu, 1999). The molecular structure and labelling is shown in Fig. 1. The two rings of the biphenyl group are rotated by ~42°, and the C15—C14—O1—C11 torsion angle is 168.7 (2)°. The aliphatic chain is in a rigid conformation, apparently to aid efficient packing. The molecular packing, which is rich in C—N···H hydrogen bonds, is shown in Fig. 2. A number of notable features are apparent from the packing arrangement. Molecules pack head-to-head, forming bilayers (ca 36 Å in length) extended along the a axis. Each molecule forms four hydrogen bonds (two symmetry unique) with two head-on molecules; pairs of hydrogen bonds (centrosymmetric dimers) in turn form catemeric chains along the b axis. The head-to-head packing of molecules in the a direction has a lateral displacement of 1.9 Å relative to the mean planes of the pairs of terminal arene groups. A second, identical, set of catemeric chains (not shown in Fig. 2) extends in the b direction but at ca 45° to the first set.
The two unique C—N···H hydrogen bonds are N1···H7 (2.74 Å) and N1···H3 (2.68 Å), as shown in Fig. 2, with C1—N1···H7 and C1—N1···H3 bond angles of 134.7 and 131°. The C1—C2 bond is unusually long for a Csp2—Csp1 bond [1.455 (4) Å], possibly as a result of the π electron withdrawing cyano group and its associated hydrogen bonds.
Polymorphs of 7-OCB (Hori, Koma et al., 1996) ?? and 9-OCB (Hori & Wu, 1999) also display this packing configuration, with C—N···H hydrogen-bond lengths over the narrow range 2.68–2.76 Å. Another polymorph of 8-OCB (Hori, Kurosaki et al., 1996; Rajnikant et al., 2000) also has layered-type packing, but in this configuration dimers are again formed through C—N···H interactions, which extend only to tetramers. In this form, the direction of adjacent pairs of molecules at the interface is different from 1, i.e. chains in the a direction are antiparallel. In a final type of polymorph displayed by 7-OCB (Hori et al., 1995) and 8-OCB (Hori, Kurosaki et al., 1996), another mixed bilayer structure is formed, but notably in this case the C—N···H dimer is absent; a CN group is close not to another CN group but to the biphenyl moiety. The O atom is utilized forming O···H hydrogen bonds along with C—N···H hydrogen bonds.
The number and structural variety of polymorphs (at least five known) exhibited by the n-OCB series of compounds (the structural identity of some is still unknown) is typical of compounds that exhibit liquid crystalline behaviour. The apparent ease at which one form might be crystallized over another is caused by weak and competing intermolecular interactions.