The γ-form of n-eicosanol

In the crystal of the title compound, C₂₀H₄₂O, the mol­ecules are packed in layers parallel to the (100) plane. The alkyl chains are parallel to the [30] direction and these molecular chains are hydrogen-bonded into chains parallel to the c axis. All C—C bonds of the alkyl chain show an antiperiplanar (trans) conformation, with a slight deviation from the ideal value (180°) in the C—C bonds close to the hydrogen bonds. The length of the alkyl chain is 27.92 (2) Å and the tilt angle is 59.7 (2)°.

S1. Comment

The n-alkanols (normal fatty alcohols C_nH_2n+1OH) show a complicated polymorphic behaviour, only partially characterized to date. The principle of a common structural description is summed up by Small (1986). This description is based on the assumption that the presence of long alkanol chains packed in a regular and parallel crystallographic array would contribute a small periodic structure within the real unit cell that defines the repeating unit of the whole structure. For a given mode of alkanol chain packing, this small internal periodic structure describes the three-dimensional relationship between equivalent positions of translationally related positions within a single alkanol chain and between its lateral neighbours. This three-dimensional subcell is defined by three axes: c_s for the translation between equivalent positions within a chain and a_s and b_s for the lateral translations. In the title compound, (I), c_s = 2.553 Å and a_s and b_s are given by the rectangular planar subcell, 4.889 × 6.032 Å.

When the long axis of the molecule tilts over the short axis of the rectangular planar subcell (4.889 Å), the modification is called γ, and if the long axis of the molecule tilts over the long axis of the rectangular subcell (6.032 Å), the modification is called β. The γ-modifications are typically exhibited by even-numbered n-alkanols. They are subdivided into several forms γ_i, depending on the degree of tilting of the molecules with respect to the molecular layers (Precht, 1974). The structure of two such crystals, n-hexadecanol (Abrahamsson et al., 1960) and n-octadecanol (Seto, 1962; Fujimoto et al., 1985), have already been determined. They were proved to be of the γ₄ form and were reported in the space group A2/a. The cell transformation between the conventional and the reported cell is a_R = c, b_R = −b and c_R = a - nc, where n is 5 for n-octadecanol and 6 for n-hexadecanol. For the rest of the series, as well as for other γ_i forms, powder diffraction patterns are indexed by extrapolation, using a constant orthorhombic subcell connected with the –CH₂– repetition in the structure (Precht, 1974, 1976). In spite of the consistency of the subcell description, a complete and unambiguous indexing of the patterns becomes difficult as the length of the chain increases.

The present study is part of our work dedicated to n-alkanol mixed crystals. The investigation of the crystal structure of (I) was undertaken in order to support powder diffraction analyses.

The average C—C bond length is 1.515 (7) Å. All C—C bonds show an anti-periplanar (trans) conformation. The C2—C3—C4—C5 [177.86 (8)°], C1—C2—C3—C4 [−179.04 (9)°] and O—C1—C2—C3 [175.77 (9)°] torsion angles all deviate significantly from the ideal 180°. This is explained by the steric hindrance produced by the hydrogen bond. The chains are parallel to [3020] direction, while they are parallel to [106] in n-octadecanol and [3017] in n-hexadecanol if the conventional cell is used. Precht (1974) classified the three compounds in the [106] group.

The length of the alkyl chain is 27.92 (2) Å. This value is obtained by dividing the modulus of the [3020] vector by 12, the number of chains in this period; the value is 27.08 Å if Precht's aproximation is used. The corresponding chain lengths are 24.9 for n-octadecanol and 21.6 Å for n-hexadecanol. The chain length for (I) can also be obtained in Å from the equation l = 1.246 n + 3, where n is the number of C atoms. The long axis of the molecule in (I) tilts 59.7 (2)° over the short axis (4.889 Å) of the orthorhombic subcell defined by Precht (1974), so the structure corresponds to the γ₄ form according to the classification of Precht.

A rigid body treatment of the atomic displacement parameters for the whole chain has been carried out, using the EKRT program (Craven et al., 1992). The inertial axes are defined as follows: the first is normal to the plane of the chain, the second is in the chain plane and perpendicular to the long axis of the molecule and the third is the long axis of the molecule. The principal values of the translational tensor are 0.043, 0.051 and 0.019 Ų, where the first two principal axes of the translational tensor are the first two inertial axes rotated 60° around the third inertial axis and the third is parallel to the long axis of the molecule. The principal values of the librational tensor are 0.4, 1.0 and 97.2°², where the first two principal axes of the librational tensor are the first two inertial axes rotated 74° around the third inertial axis and the third is parallel to the long axis of the molecule. These results could explain the formation of gauche conformers in the phases at high temperature.

The alkyl chains are hydrogen-bonded, producing chains parallel to the c axis. The direction of the alkyl chains and the direction of the hydrogen bond produce a layered structure, with the layers parallel to the (100) plane. This fact explains the pseudo-rhombohedral morphology of the crystals with two more-developed opposite faces, which are (100). The short diagonal of these rhombic faces is the direction of the b axis. The angles between the edges of the main face are close to 120 and 60°.

S2. Experimental

Crystals of (I) were obtained by slowly evaporating saturated solutions of the compound in 4-chlorotoluene at room temperature.

S3. Refinement

Intensities were collected to θ = 30°. (hkl) reflections with θ > 26° were omitted due to peak overlapping problems (asymmetric background) and to a low intensity value. The positions of 41 H atoms linked to C atoms were geometrically calculated. The H atom linked to O was located from a difference map and found to be on a disordered site. An occupancy factor of 0.5 was assigned according to the peak heights in the difference map. Each O atom is close to two further O atoms (related by an inversion centre and a twofold axis) at a distance appropriate to a hydrogen bond, so an ordered hydrogen-bond scheme is incompatible with the space group symmetry. An overall isotropic displacement parameter for the H atoms was used in the refinement and all H atoms were refined using a riding model. The anisotropic displacement parameters for the O atom revealed a possible disorder in the localization of this atom. The maximum and minimum principal axes of the displacement ellipsoids are 0.131 (1) and 0.015 (1) Ų, respectively. The direction of that corresponding to the maximum eigenvalue is practically perpendicular to the C—O bond.

Computing details

Data collection: CAD-4-PC (Kretschmar, 1996); cell refinement: CAD-4-PC; data reduction: CFEO (Solans, 1978); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP (Johnson, 1965; Brueggermann & Schmid, 1990); software used to prepare material for publication: PLATON (Spek, 1990).

Figures

	Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids and the disorder of atom H1O.
	Fig. 2. The projection of the cell content parallel to the (010) plane, showing the hydrogen bonds.

γ-form of n-Eicosanol

Crystal data

C20H42O	F(000) = 1360
Mr = 298.54	Dx = 0.982 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71069 Å
a = 91.116 (17) Å	Cell parameters from 25 reflections
b = 4.932 (4) Å	θ = 8–15°
c = 8.997 (10) Å	µ = 0.06 mm^−1
β = 93.01 (4)°	T = 293 K
V = 4038 (6) Å^3	Lozenge, colourless
Z = 8	0.6 × 0.6 × 0.2 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.023
Radiation source: fine-focus sealed tube	θmax = 26.0°, θmin = 2.2°
Graphite monochromator	h = −110→112
ω–2θ scans	k = 0→6
4539 measured reflections	l = 0→10
3932 independent reflections	3 standard reflections every 120 min
2007 reflections with I > 2σ(I)	intensity decay: none

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)] = 0.041	Hydrogen site location: mixed
wR(F^2) = 0.159	H atoms treated by a mixture of independent and constrained refinement
S = 0.98	Calculated w = 1/[σ^2(Fo^2) + (0.0879P)^2] where P = (Fo^2 + 2Fc^2)/3
3932 reflections	(Δ/σ)max = 0.009
198 parameters	Δρmax = 0.15 e Å^−3
2 restraints	Δρmin = −0.12 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
O	0.005323 (9)	0.3433 (2)	0.11346 (8)	0.0658 (4)
C1	0.018551 (13)	0.2041 (3)	0.09344 (13)	0.0583 (4)
H1	0.0164	0.0138	0.0757	0.0989 (16)*
H1A	0.0248	0.2171	0.1840	0.0989 (16)*
C2	0.026676 (12)	0.3140 (2)	−0.03390 (12)	0.0499 (3)
H2	0.0280	0.5074	−0.0194	0.0989 (16)*
H2A	0.0206	0.2891	−0.1249	0.0989 (16)*
C3	0.041483 (12)	0.1857 (2)	−0.05364 (11)	0.0407 (3)
H3	0.0401	−0.0072	−0.0702	0.0989 (16)*
H3A	0.0475	0.2080	0.0377	0.0989 (16)*
C4	0.049606 (11)	0.3033 (2)	−0.18150 (11)	0.0376 (3)
H4	0.0437	0.2750	−0.2731	0.0989 (16)*
H4A	0.0506	0.4974	−0.1667	0.0989 (16)*
C5	0.064749 (11)	0.1846 (2)	−0.19995 (11)	0.0400 (3)
H5	0.0638	−0.0094	−0.2153	0.0989 (16)*
H5A	0.0707	0.2127	−0.1085	0.0989 (16)*
C6	0.072729 (10)	0.3052 (2)	−0.32812 (11)	0.0366 (3)
H6	0.0668	0.2768	−0.4195	0.0989 (16)*
H6A	0.0736	0.4993	−0.3128	0.0989 (16)*
C7	0.087904 (11)	0.1878 (2)	−0.34692 (11)	0.0398 (3)
H7	0.0870	−0.0063	−0.3619	0.0989 (16)*
H7A	0.0938	0.2166	−0.2556	0.0989 (16)*
C8	0.095919 (11)	0.3082 (2)	−0.47584 (11)	0.0386 (3)
H8	0.0900	0.2794	−0.5672	0.0989 (16)*
H8A	0.0968	0.5024	−0.4609	0.0989 (16)*
C9	0.111082 (11)	0.1903 (2)	−0.49424 (12)	0.0411 (3)
H9	0.1102	−0.0040	−0.5087	0.0989 (16)*
H9A	0.1170	0.2199	−0.4031	0.0989 (16)*
C10	0.119043 (12)	0.3101 (2)	−0.62378 (11)	0.0407 (3)
H10	0.1131	0.2814	−0.7149	0.0989 (16)*
H10A	0.1200	0.5043	−0.6090	0.0989 (16)*
C11	0.134116 (12)	0.1918 (2)	−0.64282 (12)	0.0428 (3)
H11	0.1331	−0.0025	−0.6572	0.0989 (16)*
H11A	0.1401	0.2209	−0.5518	0.0989 (16)*
C12	0.142101 (12)	0.3098 (2)	−0.77209 (11)	0.0435 (3)
H12	0.1361	0.2808	−0.8631	0.0989 (16)*
H12A	0.1431	0.5040	−0.7576	0.0989 (16)*
C13	0.157136 (12)	0.1919 (2)	−0.79133 (12)	0.0454 (3)
H13	0.1562	−0.0023	−0.8057	0.0989 (16)*
H13A	0.1631	0.2210	−0.7003	0.0989 (16)*
C14	0.165162 (13)	0.3092 (2)	−0.92044 (12)	0.0454 (3)
H14	0.1592	0.2790	−1.0117	0.0989 (16)*
H14A	0.1661	0.5036	−0.9065	0.0989 (16)*
C15	0.180218 (13)	0.1918 (2)	−0.93831 (12)	0.0499 (3)
H15	0.1793	−0.0025	−0.9524	0.0989 (16)*
H15A	0.1861	0.2215	−0.8468	0.0989 (16)*
C16	0.188386 (13)	0.3090 (2)	−1.06745 (12)	0.0495 (3)
H16	0.1826	0.2769	−1.1592	0.0989 (16)*
H16A	0.1892	0.5037	−1.0542	0.0989 (16)*
C17	0.203576 (13)	0.1924 (2)	−1.08302 (13)	0.0543 (4)
H17	0.2027	−0.0024	−1.0957	0.0989 (16)*
H17A	0.2094	0.2254	−0.9915	0.0989 (16)*
C18	0.211663 (14)	0.3085 (2)	−1.21240 (12)	0.0545 (4)
H18	0.2059	0.2733	−1.3042	0.0989 (16)*
H18A	0.2124	0.5036	−1.2004	0.0989 (16)*
C19	0.226915 (15)	0.1938 (3)	−1.22607 (14)	0.0720 (4)
H19	0.2262	−0.0016	−1.2363	0.0989 (16)*
H19A	0.2327	0.2314	−1.1347	0.0989 (16)*
C20	0.235048 (16)	0.3051 (3)	−1.35654 (14)	0.0863 (5)
H20	0.2446	0.2228	−1.3577	0.0989 (16)*
H20A	0.2361	0.4979	−1.3462	0.0989 (16)*
H20B	0.2296	0.2647	−1.4479	0.0989 (16)*
H1O	−0.0004 (3)	0.313 (4)	0.204 (2)	0.0989 (16)*	0.50
H1O'	−0.0010 (3)	0.376 (4)	0.021 (2)	0.0989 (16)*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O	0.0457 (5)	0.1042 (8)	0.0502 (5)	0.0385 (5)	0.0294 (4)	0.0157 (6)
C1	0.0350 (6)	0.0849 (9)	0.0573 (7)	0.0157 (6)	0.0256 (5)	0.0133 (6)
C2	0.0318 (6)	0.0754 (8)	0.0441 (6)	0.0077 (6)	0.0183 (5)	0.0016 (6)
C3	0.0341 (6)	0.0487 (7)	0.0408 (6)	0.0048 (5)	0.0146 (4)	−0.0033 (5)
C4	0.0213 (5)	0.0517 (7)	0.0414 (6)	0.0051 (5)	0.0167 (4)	0.0023 (5)
C5	0.0296 (6)	0.0530 (7)	0.0390 (6)	0.0006 (5)	0.0157 (4)	−0.0035 (5)
C6	0.0198 (5)	0.0514 (7)	0.0402 (6)	0.0065 (5)	0.0165 (4)	0.0015 (5)
C7	0.0282 (6)	0.0497 (7)	0.0430 (6)	−0.0003 (5)	0.0180 (5)	−0.0007 (5)
C8	0.0228 (5)	0.0526 (7)	0.0420 (6)	0.0040 (5)	0.0186 (4)	0.0048 (5)
C9	0.0298 (6)	0.0521 (7)	0.0432 (6)	0.0017 (5)	0.0192 (5)	0.0021 (5)
C10	0.0283 (6)	0.0523 (7)	0.0434 (6)	0.0030 (5)	0.0204 (4)	0.0034 (5)
C11	0.0366 (7)	0.0503 (7)	0.0434 (6)	−0.0003 (5)	0.0205 (5)	−0.0010 (5)
C12	0.0341 (6)	0.0547 (7)	0.0438 (6)	0.0028 (5)	0.0227 (5)	0.0040 (5)
C13	0.0398 (7)	0.0517 (7)	0.0468 (6)	−0.0030 (5)	0.0216 (5)	0.0023 (5)
C14	0.0382 (6)	0.0532 (7)	0.0470 (6)	0.0045 (5)	0.0224 (5)	0.0051 (5)
C15	0.0438 (7)	0.0580 (7)	0.0506 (6)	−0.0019 (6)	0.0267 (5)	0.0001 (6)
C16	0.0409 (7)	0.0619 (8)	0.0479 (6)	0.0020 (6)	0.0250 (5)	0.0076 (6)
C17	0.0489 (7)	0.0637 (8)	0.0534 (7)	0.0007 (6)	0.0312 (6)	0.0023 (6)
C18	0.0570 (8)	0.0607 (8)	0.0485 (6)	−0.0021 (6)	0.0291 (6)	0.0049 (6)
C19	0.0774 (10)	0.0815 (10)	0.0605 (7)	0.0073 (8)	0.0354 (7)	0.0069 (7)
C20	0.0819 (11)	0.1154 (12)	0.0653 (8)	−0.0090 (9)	0.0394 (8)	0.0000 (8)

Geometric parameters (Å, º)

O—C1	1.407 (1)	C10—H10A	0.9700
O—H1O	1.001 (10)	C11—C12	1.520 (2)
O—H1O'	1.000 (10)	C11—H11	0.9700
C1—C2	1.498 (2)	C11—H11A	0.9700
C1—H1	0.9700	C12—C13	1.507 (1)
C1—H1A	0.9700	C12—H12	0.9700
C2—C3	1.509 (1)	C12—H12A	0.9700
C2—H2	0.9700	C13—C14	1.519 (2)
C2—H2A	0.9700	C13—H13	0.9700
C3—C4	1.516 (2)	C13—H13A	0.9700
C3—H3	0.9700	C14—C15	1.506 (1)
C3—H3A	0.9700	C14—H14	0.9700
C4—C5	1.516 (1)	C14—H14A	0.9700
C4—H4	0.9700	C15—C16	1.526 (2)
C4—H4A	0.9700	C15—H15	0.9700
C5—C6	1.517 (2)	C15—H15A	0.9700
C5—H5	0.9700	C16—C17	1.512 (2)
C5—H5A	0.9700	C16—H16	0.9700
C6—C7	1.517 (1)	C16—H16A	0.9700
C6—H6	0.9700	C17—C18	1.522 (2)
C6—H6A	0.9700	C17—H17	0.9700
C7—C8	1.523 (2)	C17—H17A	0.9700
C7—H7	0.9700	C18—C19	1.511 (2)
C7—H7A	0.9700	C18—H18	0.9700
C8—C9	1.516 (1)	C18—H18A	0.9700
C8—H8	0.9700	C19—C20	1.523 (2)
C8—H8A	0.9700	C19—H19	0.9700
C9—C10	1.524 (2)	C19—H19A	0.9700
C9—H9	0.9700	C20—H20	0.9600
C9—H9A	0.9700	C20—H20A	0.9600
C10—C11	1.510 (1)	C20—H20B	0.9600
C10—H10	0.9700
C1—O—H1O	121.3 (15)	H10—C10—H10A	107.6
C1—O—H1O'	115.7 (17)	C10—C11—C12	114.4 (1)
H1O—O—H1O'	113 (2)	C10—C11—H11	108.7
O—C1—C2	112.5 (1)	C12—C11—H11	108.7
O—C1—H1	109.1	C10—C11—H11A	108.7
C2—C1—H1	109.1	C12—C11—H11A	108.7
O—C1—H1A	109.1	H11—C11—H11A	107.6
C2—C1—H1A	109.1	C13—C12—C11	114.5 (1)
H1—C1—H1A	107.8	C13—C12—H12	108.6
C1—C2—C3	114.8 (1)	C11—C12—H12	108.6
C1—C2—H2	108.6	C13—C12—H12A	108.6
C3—C2—H2	108.6	C11—C12—H12A	108.6
C1—C2—H2A	108.6	H12—C12—H12A	107.6
C3—C2—H2A	108.6	C12—C13—C14	114.7 (1)
H2—C2—H2A	107.6	C12—C13—H13	108.6
C2—C3—C4	113.8 (1)	C14—C13—H13	108.6
C2—C3—H3	108.8	C12—C13—H13A	108.6
C4—C3—H3	108.8	C14—C13—H13A	108.6
C2—C3—H3A	108.8	H13—C13—H13A	107.6
C4—C3—H3A	108.8	C15—C14—C13	114.4 (1)
H3—C3—H3A	107.7	C15—C14—H14	108.7
C5—C4—C3	114.8 (1)	C13—C14—H14	108.7
C5—C4—H4	108.6	C15—C14—H14A	108.7
C3—C4—H4	108.6	C13—C14—H14A	108.7
C5—C4—H4A	108.6	H14—C14—H14A	107.6
C3—C4—H4A	108.6	C14—C15—C16	114.8 (1)
H4—C4—H4A	107.6	C14—C15—H15	108.6
C4—C5—C6	114.0 (1)	C16—C15—H15	108.6
C4—C5—H5	108.7	C14—C15—H15A	108.6
C6—C5—H5	108.7	C16—C15—H15A	108.6
C4—C5—H5A	108.7	H15—C15—H15A	107.5
C6—C5—H5A	108.7	C17—C16—C15	114.3 (1)
H5—C5—H5A	107.6	C17—C16—H16	108.7
C7—C6—C5	114.3 (1)	C15—C16—H16	108.7
C7—C6—H6	108.7	C17—C16—H16A	108.7
C5—C6—H6	108.7	C15—C16—H16A	108.7
C7—C6—H6A	108.7	H16—C16—H16A	107.6
C5—C6—H6A	108.7	C16—C17—C18	114.2 (1)
H6—C6—H6A	107.6	C16—C17—H17	108.7
C6—C7—C8	114.4 (1)	C18—C17—H17	108.7
C6—C7—H7	108.7	C16—C17—H17A	108.7
C8—C7—H7	108.7	C18—C17—H17A	108.7
C6—C7—H7A	108.7	H17—C17—H17A	107.6
C8—C7—H7A	108.7	C19—C18—C17	113.9 (1)
H7—C7—H7A	107.6	C19—C18—H18	108.8
C9—C8—C7	114.2 (1)	C17—C18—H18	108.8
C9—C8—H8	108.7	C19—C18—H18A	108.8
C7—C8—H8	108.7	C17—C18—H18A	108.8
C9—C8—H8A	108.7	H18—C18—H18A	107.7
C7—C8—H8A	108.7	C18—C19—C20	114.5 (1)
H8—C8—H8A	107.6	C18—C19—H19	108.6
C8—C9—C10	114.1 (1)	C20—C19—H19	108.6
C8—C9—H9	108.7	C18—C19—H19A	108.6
C10—C9—H9	108.7	C20—C19—H19A	108.6
C8—C9—H9A	108.7	H19—C19—H19A	107.6
C10—C9—H9A	108.7	C19—C20—H20	109.5
H9—C9—H9A	107.6	C19—C20—H20A	109.5
C11—C10—C9	114.1 (1)	H20—C20—H20A	109.5
C11—C10—H10	108.7	C19—C20—H20B	109.5
C9—C10—H10	108.7	H20—C20—H20B	109.5
C11—C10—H10A	108.7	H20A—C20—H20B	109.5
C9—C10—H10A	108.7
O—C1—C2—C3	175.77 (9)	C9—C10—C11—C12	179.84 (8)
C1—C2—C3—C4	−179.04 (9)	C10—C11—C12—C13	−179.97 (8)
C2—C3—C4—C5	177.86 (8)	C11—C12—C13—C14	179.98 (8)
C3—C4—C5—C6	−179.85 (8)	C12—C13—C14—C15	179.60 (8)
C4—C5—C6—C7	179.85 (8)	C13—C14—C15—C16	−179.83 (8)
C5—C6—C7—C8	179.86 (8)	C14—C15—C16—C17	179.19 (9)
C6—C7—C8—C9	−179.99 (8)	C15—C16—C17—C18	179.68 (9)
C7—C8—C9—C10	179.71 (8)	C16—C17—C18—C19	179.23 (9)
C8—C9—C10—C11	−179.68 (8)	C17—C18—C19—C20	179.12 (10)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O—H1O···O^i	1.00 (2)	1.73 (2)	2.688 (3)	159 (2)
O—H1O′···O^ii	1.00 (2)	1.87 (2)	2.700 (3)	139 (2)

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