Polymorphism of Long-Chain Alkane-,-Diols with ... - ACS Publications

CRYSTAL GROWTH & DESIGN

Polymorphism of Long-Chain Alkane-r,ω-Diols with an Even Number of Carbon Atoms Kenjiro Uno,*,† Yoshihiro Ogawa,‡ and Naotake Nakamura† Department of Applied Chemistry, College of Science and Engineering, Ritsumeikan UniVersity 1-1-1, Nojihigashi, Kusatsu, Shiga 525-8577, Japan, and Department of Chemistry, Faculty of Science, Kumamoto UniVersity 2-39-1, Kurokami, Kumamoto 860-8555, Japan

2008 VOL. 8, NO. 2 592–599

ReceiVed August 3, 2007; ReVised Manuscript ReceiVed October 9, 2007

ABSTRACT: Crystal structures of long-chain alkane-R,ω-diols with an even number of carbon atoms from 20 to 24 are determined. The molecular structures are similar to those of the lower homologues with an even number of carbon atoms analyzed previously; however, the methylene chain packings are different from each other. In the newly analyzed structure, it is found that the crystal structure type has an advantage in the close packing for the methylene part with increasing hydrophobic interactions. Crystal structures of a different polymorphic form in the even number of carbon atoms from 16 to 24 are also determined. The molecular and crystal structures are similar to those of an odd number of carbon atoms analyzed previously. However, the structures are slightly different from self-assembled multilayer structures whose models are derived from a grazing incidence synchrotron X-ray diffraction data. The carbon number starting to exhibit a polymorphic form coincides with that starting to show a rotator phase which are observed just below their melting points. From the viewpoint of the epitaxial growth at the surface and of the calculated density, it is found that the crystal structure of the polymorphic form has an advantage in exhibiting the rotator phase. Introduction Normal long-chain aliphatic hydrocarbons have been studied as basic components in polymers, fats, lipids, soaps, and fuels. Normal alkanes are often used as simple representative models for probing the physical properties of the more complex systems. Crystal structures of normal long-chain aliphatic compounds such as n-alkanes, R-monosubstituted n-alkanes, and R,ωdisubstituted n-alkanes have been investigated by many researchers, since Müller was the first to determine the crystal structure of n-nonacosane.1 These researches are important in disclosing some principles of organic chemical crystallography and basic polymer science, because the molecular skeleton consists of a simple, all-trans, zigzag extended hydrocarbon chain. Recently, Thalladi et al. analyzed the crystal structures of relatively short-chain alkane-R,ω-diols (called diols hereafter), HO-(CH2)n-OH, with n ) 2–10 to find the relevant requirements for hydrogen bonding and hydrophobic interactions and to establish their mutual dependence.2 The results showed a consistent distinction between the structures with even and odd numbers of carbon atoms in the diols (n g 4); in the evennumbered diols, the molecular skeleton that included both terminal hydroxy groups has an all-trans conformation. The molecules formed a layer structure in which the molecular axes were inclined with respect to the basal plane formed by the hydroxy groups. The layers were alternately stacked in a zigzag manner on the molecular inclination angle. In contrast, the molecular structure of the odd-numbered diols adopted a gauche conformation at one of the hydroxy groups, whereas the other was a trans. The molecules formed layers in which the molecular axes were aligned almost perpendicular to the basal plane. It was also found that the packing patterns in the diols (n ) 2, 3) were different from those in the longer diols (n ) 4–10) because the hydrogen bonds overruled the hydrophobic interactions. * E-mail: [email protected]. † Ritsumeikan University. ‡ Kumamoto University.

When the hydrophobic interactions have an advantage over the hydrogen bonds, namely, the number of carbon atoms increases; additionally, how does it affect the crystal-packing arrangement? To elucidate this question, we have systematically analyzed the crystal structures of the long-chain diols (n ) 10–19, 21, 23).3 These results showed that the higher homologous series of even and odd diols adopted isomorphic structures with those of the lower ones (n ) 4–10) as described above. Hereafter, the even-numbered diols (n ) 10–18) and the oddnumbered diols (n ) 11–23) are abbreviated to n-diol-I and n-diol-II, respectively, where n is the number of carbon atoms. On the other hand, in our previous study on the phase-transition behavior of the long-chain diols (n ) 13–24) applying a differential scanning calorimetry and an X-ray powder diffraction method, it was confirmed that the even-numbered diols (n g 16) exhibited a polymorphism depending on the crystallizing condition, slow or rapid cooling from a solution.4 Against these backgrounds, we have carried out the crystal structure analyses of the long-chain diols (n ) 20, 22, 24 and n ) 16, 18, 20, 22, 24 with a different polymorphic form). In this paper, the structures newly analyzed are described and compared with those of the homologous series. Furthermore, we discuss the polymorphism in the even-numbered diols (n g 16). Experimental Section Synthesis and Preparation of Single Crystals. The long-chain diols (n ) 16, 18, 20, 22, 24) were synthesized from R,ω-alkanedioic acids, HOOC-(CH2)n-2-COOH, as described previously.4 The dioic acids were obtained from Tokyo Kasei (n ) 16, 18, 20) and Aldrich (n ) 22) and that with n ) 24 was prepared by us using the Wolff–Kishner reduction of 7,18-dioxotetracosanedioic acid prepared by reaction of 1-morpholino-1-cyclohexene and dodecanedioyl chloride.5 These acids were converted into the dimethyl esters by conventional procedures. The pure dimethyl esters which were obtained through fractional distillation and recrystallization were further reduced to the diols using LiAlH4. The purities were more than 99.9%. In the diols (n ) 20, 22, 24), rectangular, thin platelike, and colorless single crystals which showed the cleavage parallel to the long axis were grown by slow evaporation from a solution containing a mixture of

10.1021/cg700729q CCC: $40.75  2008 American Chemical Society Published on Web 12/21/2007

22-diol-I′ HO-(CH2)22-OH 342.59 monoclinic, C2/c 9.691 (4) 5.235 (3) 43.616 (6) 95.21 (2) 2193.5 (16) 4 1.037 0.671 Cu KR 0.48 296 (2) rigaku AFC-5R ψ scan6 0.913 0.991 2396 2019 699 0.052 70.6 3.4 0.050, 0.174, 0.91 2019 112 0.13, -0.16 none

20-diol-I′

HO-(CH2)20-OH 314.54 monoclinic, C2/c 9.697 (3) 5.2313 (16) 39.674 (3) 91.747 (15) 2011.6 (9) 4 1.039 0.669 Cu KR 0.48 296 (2) rigaku AFC-5R ψ scan6 0.897 0.989 2193 1854 805 0.064 70.1 5.5 0.069, 0.235, 1.05 1854 103 0.17, -0.20 none

24-diol-I′ HO-(CH2)24-OH 370.64 monoclinic, C2/c 9.682 (3) 5.2434 (18) 46.952 (3) 93.688 (18) 2378.7 (11) 4 1.035 0.670 Cu KR 0.47 296 (2) rigaku AFC-5R ψ scan6 0.905 0.997 2514 2112 1030 0.062 70.1 0.1 0.056, 0.200, 1.03 2112 121 0.16, -0.20 none

16-diol-II′ HO-(CH2)16-OH 258.43 monoclinic, P21 5.086 (2) 7.193 (2) 22.6641 (14) 94.250 (16) 826.9 (4) 2 1.038 0.663 Cu KR 0.50 296 (2) rigaku AFC-5R ψ scan6 0.917 0.994 2222 1658 951 0.017 70.1 1.2 0.036, 0.117, 0.98 1658 170 0.12, -0.10 SHELXL8 0.0034 (8)


20-diol-II′ HO-(CH2)20-OH 314.54 monoclinic, P21 5.058 (3) 7.219 (4) 27.746 (2) 93.47 (2) 1011.3 (8) 2 1.033 0.665 Cu KR 0.48 296 (2) rigaku AFC-5R ψ scan6 0.907 0.992 2780 2083 1113 0.062 70.6 0.8 0.045, 0.159, 0.96 2083 205 0.11, -0.15 none

22-diol-II′ HO-(CH2)22-OH 342.59 monoclinic, P21 5.045 (2) 7.256 (4) 30.2900 (15) 93.226 (17) 1107.1 (8) 2 1.028 0.664 Cu KR 0.47 296 (2) rigaku AFC-5R ψ scan6 0.917 0.993 2975 2205 1258 0.026 70.6 1.2 0.045, 0.151, 0.97 2205 223 0.11, -0.21 none


a Data collection, MSC/AFC Diffractometer Control Software;9 cell refinement, MSC/AFC Diffractometer Control Software;9 data reductoin, CrystalStructure;10 program used to solve structure, SHELXS-97;11 program used to refine structure, SHELXL-97;7 molecular graphics, ORTEP-3 for Windows;12 software used to prepare material for publication, WinGX publication routines.13

Rint θmax (deg) intensity decay (%) R [F2 > 2σ(F2)], wR(F2), S no. of reflections no. of parameters ∆Fmax, ∆Fmin (e Å-3) extinction method extinction coefficient

molecular formula molecular weight cell setting, space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z calculated density (Mg m-3) packing coefficient8 radiation type µ (mm-1) temperature (K) diffractometer absorption correction Tmin Tmax no. of measured, independent and observed (F2 > 2σ (F2)) reflections

Table 1. Crystallographic Data of n-Diol-I′ and n-Diol-II′a

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594 Crystal Growth & Design, Vol. 8, No. 2, 2008

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Figure 1. Molecular structure of 20-diol-I′ showing the crystallographic numbering scheme [symmetry code: (*) 1 - x, 2 - y, -z]. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2. Projection of the crystal structure of 20-diol-I′ along the a axis. Dotted lines indicate the hydrogen bonding. ethanol and n-heptane (1:1). In contrast, by slow evaporation from a solution in a mixed three-solvent system consisting of methanol, ethyl acetate, and n-heptane (1:1:2), single crystals of a different polymorphic form in the diols (n ) 16, 18, 20, 22, 24) were obtained. The single crystals did not show the cleavage clearly, and the morphological features were lozenged, thin platelike, and colorless. Hereafter, the former single crystals are abbreviated to n-diol-I′, and the latter, to n-diol-II′, where n is the number of carbon atoms. Data Collection, Structure Solution, and Refinement. All measurements were carried out at the same temperature, 296(2) K, using a Rigaku AFC-5R diffractometer with graphite-monochromated Cu KR radiation. The intensity data were corrected for Lorentz and polarization effects. Empirical absorption corrections by Ψ scan6 and decay corrections were applied. In the data of n-diol-II′ (n ) 16, 18, 24), secondary extinction corrections7 were also applied. Other details are summarized in Table 1, together with details of the software used. All nonhydrogen atoms were refined anisotropically. Isotropic displacement parameters of methylene-H atoms were set to be 1.2Ueq of the parent atom, and for hydroxy-H atoms, they were set to be 1.5Ueq. The methylene-H atoms were located at idealized positions and were allowed to ride on the parent carbon atoms (C-H ) 0.95 Å). The terminal hydroxy-H atoms were located from a difference Fourier map, and the positional parameters were allowed to refine with the restrained distance of O-H ) 0.82 Å (esd ) 0.02).

Results and Discussion Molecular and Crystal Structures of n-Diol-I′ (n ) 20, 22, 24). The n-diol-I′ compounds (n ) 20, 22, 24) resemble each other in the molecular and crystal structures. The molecular structure, the projection of the crystal structure along the a axis,

Figure 3. Projection of the crystal structure of 20-diol-I′ along the b axis. Table 2. Selected Torsion Angles of n-Diol-I′ (degree)

O1-C1-C2-C3 C1-C2-C3-C4 C2-C3-C4-C5 C3-C4-C5-C6 C4-C5-C6-C7 C5-C6-C7-C8 C6-C7-C8-C9 C7-C8-C9-C10 C8-C9-C10-C11 C9-C10-C11-C12 C8-C9-C10-C10a C9-C10-C10a-C9a C9-C10-C11-C11b C10-C11-C11b-C10b C10-C11-C12-C12c C11-C12-C12c-C11c

20-diol-I′

22-diol-I′

24-diol-I′

179.9 (3) 177.0 (3) -179.3 (3) 179.6 (3) 179.6 (3) -179.9 (3) 179.2 (3) 179.1 (3)

-179.6 (2) 176.8 (2) -180.0 (2) 179.3 (2) 179.4 (2) 179.6 (2) 179.6 (2) 179.9 (2) 179.8 (2)

-179.83 (16) -176.44 (16) -179.95 (16) -179.14 (16) -179.39 (16) -179.82 (16) -179.45 (16) -179.64 (16) -179.86 (16) 179.97 (16)

179.4 (3) 180.0 (3) 179.9 (2) -180.0 (3) 180.00 (17) 180.00 (15)

a Symmetry code: 1 – x, 2 - y, -z. b Symmetry code: 1 – x, 2 - y, -z. c Symmetry code: 1/2 - x, 3/2 - y, -z.

and that along the b axis in 20-diol-I′, as a representative example, are displayed in Figures 1, 2, and 3, respectively. Selected torsion angles are listed in Table 2. The molecule is centrosymmetric, and all torsion angles formed by nonhydrogen atoms are close to (180°, that is, the molecular skeleton that included both terminal hydroxy groups has an all-trans conformation. The molecules are inclined with respect to the basal plane formed by the hydroxy groups and form a layer which has a thickness of (c sin β)/2 and is built up in a zigzag manner along the layer stacking direction, the c axis.

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Crystal Growth & Design, Vol. 8, No. 2, 2008 595

Table 3. Hydrogen-Bond Parameters of n-Diol-I′ (angstrom, degree) D-H. . .A

D-H

H. . .A

D. . .A

20-diol-I′ O1-H1. . .O1a 0.82 (2) 2.02 (2) 2.836 (4) 22-diol-I′ O1-H1. . .O1b 0.825 (18) 2.015 (18) 2.8387 (18) 24-diol-I′ O1-H1. . .O1c 0.836 (19) 2.004 (19) 2.8401 (14)

D-H. . .A 174 (5) 176 (4) 178 (3)

a Symmetry codes: -1/2 - x, -1/2 + y, 1/2 - z. b Symmetry codes: -1/2 - x, -1/2 + y, 1/2 - z. c Symmetry codes: 5/2 - x, -1/2 + y, 1/2 - z.

Figure 6. Calculated densities in the even-numbered diols of n-diol-I with n ) 10–18 (b)3 and of n-diol-I′ with n ) 20–24 (9), the oddnumbered diols of n-diol-II with n ) 11–23 (O),3 and even-numbered diols of n-diol-II′ with n ) 16–24 (0) at the same temperature, 296 (2) K. Dotted lines indicate the extrapolated lines of the calculated densities in even-numbered diols of n-diol-I and n-diol-I′.

Figure 4. Projection of the crystal structure of 18-diol-I along the a axis.3 Dotted lines indicate the hydrogen bonding.

Figure 5. Projection of the crystal structure of 18-diol-I along the b axis.3

At the interlayer, the molecules form hydrogen bonds. The interlayer hydrogen-bond parameters of n-diol-I′ (n ) 20, 22, 24) are shown in Table 3. These features are similar to those of the long-chain even-numbered diols with n ) 10–18, n-diol-I, analyzed previously.3

However, the unit-cell parameters a and b in the n-diol-I′ (n ) 20, 22, 24) are different from those in the lower evennumbered n-diol-I (n ) 10–18).3 The former has a monoclinic system with the C2/c space group and with a ) 9.682 (3)-9.697 (3) Å and b ) 5.2313 (16)-5.2434 (18) Å (see Table 1), and the latter has a monoclinic system with the P21/c or P21/n space groups and with a ) 4.9395 (15)-4.998 (2) Å, which is about a half of the former, and b ) 5.1592 (18)-5.220 (2) Å. In the case of 18-diol-I as a representative example of n-diol-I (n ) 10–18), the molecules are aligned on the (106) plane (Figures 4 and 5). On the other hand, the molecules of 20-diol-I′ are aligned on the (206) plane, and the molecular plane slides b/2 with respect to the neighboring one, as can be seen in Figure 2. Therefore, the molecules of n-diol-I′ appearing in the projection along the a axis double in number compared with those of n-diol-I (see Figures 2 and 4). The difference between the crystal structure of n-diol-I and n-diol-I′ has an influence on calculated densities (Figure 6). The X-ray data for all diols were collected at the same temperature, 296 (2) K, to allow a comparison of calculated densities. The slope of a line formed by calculated densities of n-diol-I (n ) 10–18)3 is steeper than that of n-diol-I′ (n ) 20, 22, 24), whereas the calculated densities of diols tend to decrease monotonically with increasing the number of carbon atoms because of the increased hydrophobic content. This means that the crystal structure of n-diol-I′ is more efficient in the methylene chain packing than that of n-diol-I. This fact may be caused by the competition between hydrogen bond interactions formed by the terminal hydroxy groups and hydrophobic interactions of the methylene chains. In consequence, the crystal structure type of n-diol-I′ has an advantage in the close packing for the methylene part, with an increasing number of carbon atoms. In Figure 6, two dotted lines indicate the extrapolation of the calculated densities in n-diol-I and n-diol-I′, respectively. It is found out that there is a border at 20-diol on the crystal structure type. Moreover, based on the extrapolated line of n-diol-I (n ) 10–18) in Figure 6, we suggest that the n ) 20 diol may adopt the crystal structure of the n-diol-I type as another polymorphic form. The difference of the crystal structure type between evennumbered n-diol-I and n-diol-I′ is also recognized as that of the methylene chain packing in a layer between basal planes formed by hydroxy groups, i.e., the subcell. In the subcell concept, the chains are assumed to be infinitely long to form a


Uno et al.

Table 4. Subcell Parameters of n-Diol-I and n-Diol-I′

as (Å) bs (Å) cs (Å) Rs (deg) βs (deg) γs (deg) Vs (Å3) V per -CH2- (Å3) δs (deg)

10-diol-I

12-diol-I

14-diol-I

16-diol-I

18-diol-I

20-diol-I′

22-diol-I′

24-diol-I′

4.144 (6) 4.570 (6) 2.544 (13) 88.2 (3) 92.1 (4) 110.01 (12) 45.2 (2) 22.62 (12) 1.86 (13)

4.153 (4) 4.583 (4) 2.543 (10) 88.4 (3) 92.2 (3) 110.08 (9) 45.44 (19) 22.72 (10) 2.34 (10)

4.163 (4) 4.601 (5) 2.545 (10) 88.4 (3) 92.6 (3) 110.26 (10) 45.67 (19) 22.84 (10) 2.77 (11)

4.159 (7) 4.607 (7) 2.543 (15) 88.6 (4) 92.6 (4) 110.34 (15) 45.6 (3) 22.82 (15) 2.81 (15)

4.167 (7) 4.622 (7) 2.545 (15) 88.6 (4) 93.1 (4) 110.52 (14) 45.8 (3) 22.92 (15) 3.35 (16)

8.021 (16) 4.668 (17) 2.55 (4) 87.9 (10) 90.7 (10) 104.7 (3) 92.1 (14) 23.0 (4) 9.6 (4)

8.017 (13) 4.670 (13) 2.55 (3) 87.9 (8) 90.6 (8) 104.8 (2) 92.1 (11) 23.0 (3) 9.3 (3)

8.011 (10) 4.679 (11) 2.54 (2) 87.9 (6) 90.6 (6) 104.83 (16) 92.1 (9) 23.0 (2) 9.5 (2)

three-dimensional crystal with an asymmetric unit consisting of methylene groups with the subcell constant cs in the chain direction and as and bs in the other direction, where Rs, βs, and γs are the interaxial angles. As the center of gravity in the subcell unit is matched against the average for the gravity points of molecules comprised in the subcell unit, the subcell constants are calculated in this study. The subcell constants of n-diol-I (n ) 10–18) and n-diol-I′ (n ) 20, 22, 24) are listed in Table 4. In the subcell of n-diol-I (n ) 10–18), as can be seen in Figure 7a which shows the subcell of 18-diol-I as a representative example, there are infinite rows of nearly coplanar zigzag chains with a chain-chain distance of bs ) 4.570 (6)-4.622 (7) Å. The chains are related by a simple translation almost perpendicular to the chain axes, Rs ) 88.2 (3)-88.6 (4)°. Adjacent rows are also related by a simple translation, as ) 4.144 (6)-4.167 (7) Å, almost perpendicular to the chain axes, βs ) 92.1 (4)-93.1 (4)°. The angle between as and bs, i.e. γs, is 110.01 (12)-110.52 (14)°. According to the Segerman’s subcell classification,14 this packing type is M1|. On the other hand, in the subcell of n-diol-I′, as illustrated in Figure 7b, which shows the subcell of 20-diol-I′ as a representative example, the zigzag chains in a row are related by a translation almost perpendicular to the chain axes, Rs ) 87.9 (6)-87.9 (10)°. However, the zigzag planes are not parallel to the translation bs ) 4.668 (17)-4.679 (11) Å. Neighboring rows are related by another translation almost perpendicular to the chain axes, βs ) 90.6 (8)-90.7 (10)°, and a 180° rotation around the chain axes. The distance between adjacent chains in this direction is as/2 ) 4.006 (5)-4.011 (8) Å. The angle between both translations is γs ) 104.7 (3)-104.83 (16)°. The chain packing is described as M2 by Segerman’s notation.14 In the subcells of n-diol-I (n ) 10–18), the δs values, which are the angles between the zigzagchain plane and the row plane, are increasing slightly with the addition of the number of the methylene carbon atoms (see Table 4). The δs values of n-diol-I′ are remarkably large compared with those of n-diol-I, by introducing 2-fold axes between the

Figure 7. Subcell packings of (a) 18-diol-I and (b) 20-diol-I′, viewed along the chain axes. Gray and green spheres are carbon and hydrogen atoms, respectively.

rows. The increase of the δs values means that the hydrocarbon chain swivels gradually from the fulcrum of the terminal hydroxy groups to the molecular center, in order to fill the gaps between hydrocarbon chains. Therefore, these results also endorse the conclusions of an advantage in the close packing for the methylene part on the crystal structure type of n-diol-I′. Molecular and Crystal Structures of n-Diol-II′ (n ) 16, 18, 20, 22, 24). Figure 8 shows the molecular structure of 16diol-II′ as a representative example of n-diol-II′. Selected torsion angles of n-diol-II′ (n ) 16, 18, 20, 22, 24) are listed in Table 5. The terminal torsion angle O1-C1-C2-C3 is in the range from -62.2 (5)° to -63.7 (3)°, while the other terminal torsion angle is close to 180°. This means that the former has a gauche conformation with respect to the hydrocarbon skeleton which is all-trans and the latter has a trans conformation. The molecule in which one of the hydroxy groups adopts a gauche conformation is noncentrosymmetric, but we cannot determine the absolute configuration because the diols are composed of light atoms. Therefore, the absolute configurations are tentatively assigned. The crystal structures of n-diol-II′ for n ) 16, 18, 20, 22, 24 all have the same packing pattern. As an example, Figure 9a and b shows the projection of the crystal structure of 16-diolII′ along the a and b axes, respectively. The projection along the chain axes in a layer between the basal planes formed by hydroxy groups is shown in Figure 9c. The molecules are nearly normal to the basal plane and form a layer with a thickness of c sin β, in which the molecules are arranged in a typical herringbone motif of aliphatic compounds.15 The molecules also form two different types of hydrogen bond, i.e. interlayer and intralayer. The hydrogen-bond parameters of n-diol-II′ (n ) 16, 18, 20, 22, 24) are shown in Table 6. These features are similar to those of the long-chain odd-numbered diols with n ) 11–23, n-diol-II, analyzed previously.3 The relation between the calculated density and the number of carbon atoms in n-diol-II′ is also similar to that in n-diol-II as can be seen in Figure 6. However, the differences appear in the unit cell and the slight inclination angle of the molecules with respect to the basal plane formed by hydroxy groups. The crystal structure of n-diol-II has a double-layered structure of an orthorhombic system with the P212121 space group (Figure 10). In the layer, the inclination angles of the molecules are less than 0.5° with respect to the line normal to the basal plane. In contrast, the crystal structures of n-diol-II′ have a single layered structure of a monoclinic system with the P21 space group. In the layer, the molecular axes of n-diol-II′ (n ) 16, 18, 20, 22, 24) are inclined along the a axis at 1.87, 1.53, 1.30, 1.05, and 0.93°, respectively. Therefore, as the number of carbon atoms increases, the crystal structure of n-diol-II′ becomes closer to that of n-diol-II. Intralayer hydrogen-bond patterns are also different between n-diol-II′ and n-diol-II. In the former, the hydrogen-bond sequence is formed toward the b axis, and in the latter, it is



Figure 8. Molecular structure of 16-diol-II′ showing the crystallographic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Table 5. Selected Torsion Angles of n-Diol-II′ (degree)

O1-C1-C2-C3 C1-C2-C3-C4 C2-C3-C4-C5 C3-C4-C5-C6 C4-C5-C6-C7 C5-C6-C7-C8 C6-C7-C8-C9 C7-C8-C9-C10 C8-C9-C10-C11 C9-C10-C11-C12 C10-C11-C12-C13 C11-C12-C13-C14 C12-C13-C14-C15 C13-C14-C15-C16 C14-C15-C16-C17 C15-C16-C17-C18 C16-C17-C18-C19 C17-C18-C19-C20 C18-C19-C20-C21 C19-C20-C21-C22 C20-C21-C22-C23 C21-C22-C23-C24 C14-C15-C16-O2 C16-C17-C18-O2 C18-C19-C20-O2 C20-C21-C22-O2 C22-C23-C24-O2

16-diol-II′

18-diol-II′

20-diol-II′

22-diol-II′

24-diol-II′

-63.4 (5) 175.2 (3) -178.9 (3) 178.6 (3) -179.7 (3) 179.8 (3) 179.5 (3) 179.8 (3) -179.9 (3) 179.7 (3) -179.8 (3) 179.9 (3) 180.0 (3) 179.9 (3)

-63.0 (5) 175.3 (3) -178.9 (3) 177.9 (3) 179.7 (3) -179.9 (3) -180.0 (3) -179.9 (3) 179.5 (3) 179.7 (3) -179.5 (3) 179.9 (3) -179.6 (3) 179.7 (3) 180.0 (3) 179.6 (3)

-62.2 (5) 175.1 (3) -179.1 (3) 177.9 (3) 179.6 (3) 179.6 (3) 179.5 (3) -179.7 (3) -180.0 (3) -179.8 (3) 179.8 (3) 179.8 (3) -179.6 (3) 179.4 (3) -179.7 (3) 179.7 (3) 180.0 (3) 179.7 (3)

-63.5 (5) 175.2 (3) -178.7 (3) 178.6 (3) 180.0 (3) 179.5 (2) 179.4 (2) -179.5 (2) 179.8 (2) 179.8 (2) 179.9 (2) -179.9 (3) -180.0 (2) 179.9 (2) -179.4 (2) 179.2 (3) -179.8 (2) 179.8 (3) 179.9 (2) 179.9 (3)

-63.7 (3) 175.5 (2) -179.15 (18) 178.01 (18) -179.94 (17) 179.73 (17) 179.51 (16) -179.76 (16) 179.85 (16) -179.79 (16) 179.84 (16) 179.99 (17) -179.95 (17) -179.97 (16) -179.65 (16) 179.78 (17) -179.89 (16) 179.40 (17) -179.61 (16) 179.24 (17) 179.86 (17) 179.99 (17)

179.0 (3) 179.0 (3) 179.3 (3) 179.3 (3) 179.76 (17)

Table 6. Hydrogen-Bond Parameters of n-Diol-II′ (angstrom, degree) D-H. . .A a

16-diol-II′ O1-H1. . .O2 O2-H2. . .O1b 18-diol-II′ O1-H1. . .O2c O2-H2. . .O1d 20-diol-II′ O1-H1. . .O2e O2-H2. . .O1f 22-diol-II′ O1-H1. . .O2g O2-H2. . .O1h 24-diol-II′ O1-H1. . .O2i O2-H2. . .O1j

D-H

H. . .A

D. . .A

D-H. . .A

0.821 (19) 0.838 (18) 0.831 (18) 0.840 (18) 0.847 (19) 0.846 (19) 0.860 (19) 0.838 (19) 0.840 (18) 0.837 (17)

1.97 (2) 1.877 (19) 1.97 (2) 1.870 (19) 1.94 (2) 1.87 (2) 1.94 (2) 1.878 (19) 1.961 (18) 1.873 (18)

2.783 (4) 2.710 (3) 2.786 (4) 2.710 (3) 2.783 (5) 2.711 (3) 2.792 (4) 2.715 (3) 2.790 (3) 2.7093 (18)

170 (4) 173 (4) 168 (4) 179 (4) 177 (4) 176 (4) 170 (4) 176 (4) 169 (3) 178 (3)

a Symmetry codes: 1 - x, 1/2 + y, -z. b Symmetry codes: x, y, -1 + z. c Symmetry codes: 1 - x, 1/2 + y, 1 - z. d Symmetry codes: x, y, 1 + z. e Symmetry codes: 1 - x, 1/2 + y, 1 - z. f Symmetry codes: x, y, 1 + z. g Symmetry codes: 1 - x, 1/2 + y, 1 - z. h Symmetry codes: x, y, 1 + z. i Symmetry codes: 2 - x, 1/2 + y, 1 - z. j Symmetry codes: x, y, 1 + z.

toward the a axis, which is related to the 2-fold screw axis (See Figures 9c and 10c). By the way, Popovitz-Biro et al. applied a grazing incidence synchrotron X-ray diffraction method (GID) for determining the packing arrangements of the self-assembled multilayer structures of the diols n ) 16, 22, 23, and 30.16 Some of normal long-chain aliphatic compounds have attracted attention as ice nucleators for cloud-seeding, because the monolayer or multilayer films which are self-assembled on the water surface promote an epitaxial nucleation of hexagonal ice crystals from supercooled water.17 They derived the models of a rectangular

cell of a ≈ 5.0 Å and b ≈ 7.3 Å with a herringbone motif in which the molecules were aligned nearly perpendicular to the layer plane. The models were introduced a relaxed a-glide plane and a 2-fold screw axis parallel to the a axis for the even- and odd-numbered diols, respectively, based on the requirements of the unit cell dimensions, the hydrogen-bonds, and the space groups. The single crystals of n-diol-I′ and n-diol-II′ were obtained from the bottom and the side of the sample bottle, respectively. That is, the former was grown in the solution and the latter at the air–solution interface. Therefore, the crystal structures of n-diol-II′ are probably grown epitaxially from the nuclei formed at the air–solution interface and are corresponding to that of the multilayer film described by Popovitz-Biro et al. In the case of the proposed model of the even-numbered diols, the authors introduced a glide plane within the layer, because of a minor molecular tilt with respect to the normal line to the layer plane. Furthermore, the glide plane was allowed a minor relaxation by inclining the glide plane by an angle with respect to the normal plane to the layer plane to obtain the best fit of the GID data. However, the proposed models of the evennumbered diols made up of one or three layers did not fit the observed GID data satisfactorily. The proposed models may be improved due to introducing not a relaxed a-glide plane but rather a 2-fold screw axis which were observed in the crystal structure of n-diol-II′. Relationship between the Polymorphism and the PhaseTransition Behavior in the Long-Chain Even-Numbered Diols. In our previous study on the phase-transition behavior of the long-chain diols (n ) 13–24), it was found that the even-


Uno et al.

Figure 10. Projections of the crystal structure of the 15-diol-II (a) along the a axis and (b) along the b axis. (c) Projection along the chain axis in a layer with a thickness of c/2. Dotted lines indicate the hydrogen bonding.3 Figure 9. Projections of the crystal structure of 16-diol-II′ (a) along the a axis and (b) along the b axis. (c) Projection along the chain axis in a layer with a thickness of c sin β. Dotted lines indicate the hydrogen bonding.

numbered diols (n ) 16–24) and the odd ones (n ) 13–23) had a rotator phase just below their melting points.4 In the evennumbered diols, the carbon number starting to exhibit a polymorphic form, n-diol-II′, coincides with that starting to show a rotator phase. Calculated density gives a measure of compactness, and in a homologous series, it may be correlated with a phase-transition point. However, the phase-transition temperatures from the crystal phase to the rotator phase in the evennumbered diols (n ) 16–24) increase monotonically when the number of carbon atoms are increased, whereas a border on the crystal structure type exists at n ) 20. In differential scanning calorimetry, the sample is rapidly cooled from the molten state obtained from the first heating process, and then, the sample was used on the second heating process to obtain the crystalrotator phase transition point. In the cooling process, the

molecules probably undergo “surface freezing” which is considered to be when a single crystalline monolayer is formed at the surface of the isotropic liquid bulk above the bulk freezing temperature.18 And then, crystals are grown epitaxially from a single crystalline monolayer. This monolayer structure is probably similar to n-diol-II′ because the crystal of n-diol-II′ is grown at the surface of a solution. Moreover, the calculated densities of n-diol-II′ are lower than those of n-diol-I and n-diolI′ as can be seen in Figure 6, that is, the crystal of n-diol-II′ is considered to be a metastable crystalline phase. Therefore, the crystal structure of n-diol-II′ has an advantage in exhibiting the rotator phase. Conclusion The crystal structure analyses of the long-chain diols (n-diolI′ of n ) 20, 22, 24 and n-diol-II′ of n ) 16, 18, 20, 22, 24


with a different polymorphic form) have been carried out, and the structures are compared with those of homologues. The molecular structures of n-diol-I′ are similar to those of the lower diols with an even number of carbon atoms of n-diol-I (n ) 10–18) analyzed previously, while the crystal structures are different from each other. The former has a monoclinic system with the C2/c space group, and the latter has a monoclinic system with the P21/c or P21/n space groups. In n-diol-I (n ) 10–18), the slope of calculated densities on the decline with an increasing number of carbon atoms is steeper than that in n-diol-I′ (n ) 20, 22, 24). In the subcells of n-diol-I (n ) 10–18), the δs values, which are the angles between the zigzag-chain plane and the row plane, are increasing slightly with the addition of the number of the methylene carbon atoms, and the δs values of n-diol-I′ are remarkably large compared with those of n-diol-I. The increase of the δs value means that the hydrocarbon chain swivels gradually from the fulcrum of the terminal hydroxy groups to the molecular center, in order to fill the gaps between hydrocarbon chains. Therefore, these results endorse the conclusions of an advantage in the close packing for the methylene part on the crystal structure type of n-diol-I′. The molecular and crystal structures of n-diol-II′ are similar to those of the odd-number diols, n-diol-II (n ) 11–23), analyzed previously but are slightly different from the self-assembled multilayer structures whose models are derived from grazing incidence synchrotron X-ray diffraction data. Therefore, it is better that the proposed models are improved due to introducing a 2-fold screw axis which was observed in the crystal structure of n-diol-II′. On the other hand, the carbon number starting to exhibit a polymorphic form coincides with structures starting to show a rotator phase which is observed just below their melting points. From the viewpoint of the epitaxial crystal growth at the surface and of the calculated density, it is found that the crystal structure of n-diol-II′ has an advantage in exhibiting the rotator phase.


(4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)

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Supporting Information Available: X-ray crystallographic information file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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