Polymorphism of Long-Chain Alkane-r,ω-Diols with an EvenNumber 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

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 crystalstructure type has an advantage in the close packing for the methylene part with increasing hydrophobic interactions. Crystal structuresof a different polymorphic form in the even number of carbon atoms from 16 to 24 are also determined. The molecular and crystalstructures are similar to those of an odd number of carbon atoms analyzed previously. However, the structures are slightly differentfrom 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 observedjust below their melting points. From the viewpoint of the epitaxial growth at the surface and of the calculated density, it is foundthat the crystal structure of the polymorphic form has an advantage in exhibiting the rotator phase.

Introduction

Normal long-chain aliphatic hydrocarbons have been studiedas basic components in polymers, fats, lipids, soaps, and fuels.Normal alkanes are often used as simple representative modelsfor probing the physical properties of the more complex systems.Crystal structures of normal long-chain aliphatic compoundssuch as n-alkanes, R-monosubstituted n-alkanes, and R,ω-disubstituted n-alkanes have been investigated by many re-searchers, since Müller was the first to determine the crystalstructure of n-nonacosane.1 These researches are important indisclosing some principles of organic chemical crystallographyand basic polymer science, because the molecular skeletonconsists of a simple, all-trans, zigzag extended hydrocarbonchain.

Recently, Thalladi et al. analyzed the crystal structures ofrelatively short-chain alkane-R,ω-diols (called diols hereafter),HO-(CH2)n-OH, with n ) 2–10 to find the relevant require-ments for hydrogen bonding and hydrophobic interactions andto establish their mutual dependence.2 The results showed aconsistent distinction between the structures with even and oddnumbers of carbon atoms in the diols (n g 4); in the even-numbered diols, the molecular skeleton that included bothterminal hydroxy groups has an all-trans conformation. Themolecules formed a layer structure in which the molecular axeswere inclined with respect to the basal plane formed by thehydroxy groups. The layers were alternately stacked in a zigzagmanner on the molecular inclination angle. In contrast, themolecular structure of the odd-numbered diols adopted a gaucheconformation at one of the hydroxy groups, whereas the otherwas a trans. The molecules formed layers in which the molecularaxes were aligned almost perpendicular to the basal plane. Itwas also found that the packing patterns in the diols (n ) 2, 3)were different from those in the longer diols (n ) 4–10) becausethe hydrogen bonds overruled the hydrophobic interactions.

When the hydrophobic interactions have an advantage overthe hydrogen bonds, namely, the number of carbon atomsincreases; additionally, how does it affect the crystal-packingarrangement? To elucidate this question, we have systematicallyanalyzed the crystal structures of the long-chain diols (n )10–19, 21, 23).3 These results showed that the higher homolo-gous series of even and odd diols adopted isomorphic structureswith those of the lower ones (n ) 4–10) as described above.Hereafter, the even-numbered diols (n ) 10–18) and the odd-numbered diols (n ) 11–23) are abbreviated to n-diol-I andn-diol-II, respectively, where n is the number of carbon atoms.On the other hand, in our previous study on the phase-transitionbehavior of the long-chain diols (n ) 13–24) applying adifferential scanning calorimetry and an X-ray powder diffrac-tion method, it was confirmed that the even-numbered diols (ng 16) exhibited a polymorphism depending on the crystallizingcondition, slow or rapid cooling from a solution.4

Against these backgrounds, we have carried out the crystalstructure analyses of the long-chain diols (n ) 20, 22, 24 andn ) 16, 18, 20, 22, 24 with a different polymorphic form). Inthis paper, the structures newly analyzed are described andcompared with those of the homologous series. Furthermore,we discuss the polymorphism in the even-numbered diols (n g16).

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 acidswere obtained from Tokyo Kasei (n ) 16, 18, 20) and Aldrich (n )22) and that with n ) 24 was prepared by us using the Wolff–Kishnerreduction of 7,18-dioxotetracosanedioic acid prepared by reaction of1-morpholino-1-cyclohexene and dodecanedioyl chloride.5 These acidswere converted into the dimethyl esters by conventional procedures.The pure dimethyl esters which were obtained through fractionaldistillation and recrystallization were further reduced to the diols usingLiAlH4. The purities were more than 99.9%.

In the diols (n ) 20, 22, 24), rectangular, thin platelike, and colorlesssingle crystals which showed the cleavage parallel to the long axis weregrown by slow evaporation from a solution containing a mixture of

* E-mail: rc002961@se.ritsumei.ac.jp.† Ritsumeikan University.‡ Kumamoto University.

CRYSTALGROWTH& DESIGN

2008VOL. 8, NO. 2

592–599

10.1021/cg700729q CCC: $40.75 2008 American Chemical SocietyPublished on Web 12/21/2007

Tab

le1.

Cry

stal

logr

aphi

cD

ata

ofn-

Dio

l-I′

and

n-D

iol-

II′a

20-d

iol-

I′22

-dio

l-I′

24-d

iol-

I′16

-dio

l-II

′18

-dio

l-II

′20

-dio

l-II

′22

-dio

l-II

′24

-dio

l-II

′m

olec

ular

form

ula

HO-

(CH

2)20-

OH

HO-

(CH

2)22-

OH

HO-

(CH

2)24-

OH

HO-

(CH

2)16-

OH

HO-

(CH

2)18-

OH

HO-

(CH

2)20-

OH

HO-

(CH

2)22-

OH

HO-

(CH

2)24-

OH

mol

ecul

arw

eigh

t31

4.54

342.

5937

0.64

258.

4328

6.48

314.

5434

2.59

370.

64ce

llse

tting

,sp

ace

grou

pm

onoc

linic

,C

2/c

mon

oclin

ic,

C2/

cm

onoc

linic

,C

2/c

mon

oclin

ic,

P2 1

mon

oclin

ic,

P2 1

mon

oclin

ic,

P2 1

mon

oclin

ic,

P2 1

mon

oclin

ic,

P2 1

a(Å

)9.

697

(3)

9.69

1(4

)9.

682

(3)

5.08

6(2

)5.

0706

(17)

5.05

8(3

)5.

045

(2)

5.03

6(3

)b

(Å)

5.23

13(1

6)5.

235

(3)

5.24

34(1

8)7.

193

(2)

7.21

2(3

)7.

219

(4)

7.25

6(4

)7.

250

(4)

c(Å

)39

.674

(3)

43.6

16(6

)46

.952

(3)

22.6

641

(14)

25.2

067

(12)

27.7

46(2

)30

.290

0(1

5)32

.833

0(1

8)�

(deg

)91

.747

(15)

95.2

1(2

)93

.688

(18)

94.2

50(1

6)93

.831

(14)

93.4

7(2

)93

.226

(17)

92.9

4(2

)V

(Å3)

2011

.6(9

)21

93.5

(16)

2378

.7(1

1)82

6.9

(4)

919.

7(5

)10

11.3

(8)

1107

.1(8

)11

97.2

(10)

Z4

44

22

22

2ca

lcul

ated

dens

ity(M

gm

-3)

1.03

91.

037

1.03

51.

038

1.03

51.

033

1.02

81.

028

pack

ing

coef

ficie

nt8

0.66

90.

671

0.67

00.

663

0.66

40.

665

0.66

40.

667

radi

atio

nty

peC

uKR

Cu

KR

Cu

KR

Cu

KR

Cu

KR

Cu

KR

Cu

KR

Cu

KR

µ(m

m-

1)

0.48

0.48

0.47

0.50

0.49

0.48

0.47

0.47

tem

pera

ture

(K)

296

(2)

296

(2)

296

(2)

296

(2)

296

(2)

296

(2)

296

(2)

296

(2)

diff

ract

omet

erri

gaku

AFC

-5R

riga

kuA

FC-5

Rri

gaku

AFC

-5R

riga

kuA

FC-5

Rri

gaku

AFC

-5R

riga

kuA

FC-5

Rri

gaku

AFC

-5R

riga

kuA

FC-5

Rab

sorp

tion

corr

ectio

nψ

scan

6ψ

scan

6ψ

scan

6ψ

scan

6ψ

scan

6ψ

scan

6ψ

scan

6ψ

scan

6

Tm

in0.

897

0.91

30.

905

0.91

70.

855

0.90

70.

917

0.89

2T

max

0.98

90.

991

0.99

70.

994

0.97

70.

992

0.99

30.

996

no.

ofm

easu

red,

inde

pend

ent

and

obse

rved

(F2>

2σ(F

2))

refle

ctio

ns21

9323

9625

1422

2224

6927

8029

7537

3418

5420

1921

1216

5818

3720

8322

0523

9380

569

910

3095

110

2811

1312

5816

36R

int

0.06

40.

052

0.06

20.

017

0.02

00.

062

0.02

60.

032

θ max

(deg

)70

.170

.670

.170

.170

.170

.670

.670

.6in

tens

ityde

cay

(%)

5.5

3.4

0.1

1.2

0.3

0.8

1.2

3.6

R[F

2>

2σ(F

2)]

,w

R(F

2),

S0.

069,

0.23

5,1.

050.

050,

0.17

4,0.

910.

056,

0.20

0,1.

030.

036,

0.11

7,0.

980.

038,

0.12

2,1.

000.

045,

0.15

9,0.

960.

045,

0.15

1,0.

970.

039,

0.12

6,1.

00no

.of

refle

ctio

ns18

5420

1921

1216

5818

3720

8322

0523

93no

.of

para

met

ers

103

112

121

170

188

205

223

242

∆F m

ax,

∆F m

in(e

Å-

3)

0.17

,-

0.20

0.13

,-

0.16

0.16

,-

0.20

0.12

,-

0.10

0.12

,-

0.13

0.11

,-

0.15

0.11

,-

0.21

0.15

,-

0.17

extin

ctio

nm

etho

dno

neno

neno

neSH

EL

XL

8SH

EL

XL

8no

neno

neSH

EL

XL

8

extin

ctio

nco

effic

ient

0.00

34(8

)0.

0030

(6)

0.00

38(7

)

aD

ata

colle

ctio

n,M

SC/A

FCD

iffr

acto

met

erC

ontr

olSo

ftw

are;

9ce

llre

finem

ent,

MSC

/AFC

Dif

frac

tom

eter

Con

trol

Soft

war

e;9

data

redu

ctoi

n,C

ryst

alSt

ruct

ure;

10

prog

ram

used

toso

lve

stru

ctur

e,SH

EL

XS-

97;1

1

prog

ram

used

tore

fine

stru

ctur

e,SH

EL

XL

-97;

7m

olec

ular

grap

hics

,O

RT

EP-

3fo

rW

indo

ws;

12

soft

war

eus

edto

prep

are

mat

eria

lfo

rpu

blic

atio

n,W

inG

Xpu

blic

atio

nro

utin

es.1

3

Polymorphism of Alkane-R,ω-Diols Crystal Growth & Design, Vol. 8, No. 2, 2008 593

ethanol and n-heptane (1:1). In contrast, by slow evaporation from asolution in a mixed three-solvent system consisting of methanol, ethylacetate, and n-heptane (1:1:2), single crystals of a different polymorphicform in the diols (n ) 16, 18, 20, 22, 24) were obtained. The singlecrystals did not show the cleavage clearly, and the morphologicalfeatures were lozenged, thin platelike, and colorless. Hereafter, theformer single crystals are abbreviated to n-diol-I′, and the latter, ton-diol-II′, where n is the number of carbon atoms.

Data Collection, Structure Solution, and Refinement. All mea-surements were carried out at the same temperature, 296(2) K, using aRigaku AFC-5R diffractometer with graphite-monochromated Cu KRradiation. The intensity data were corrected for Lorentz and polarizationeffects. Empirical absorption corrections by Ψ scan6 and decaycorrections were applied. In the data of n-diol-II′ (n ) 16, 18, 24),secondary extinction corrections7 were also applied. Other details aresummarized in Table 1, together with details of the software used.

All nonhydrogen atoms were refined anisotropically. Isotropicdisplacement 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 wereallowed to ride on the parent carbon atoms (C-H ) 0.95 Å). Theterminal hydroxy-H atoms were located from a difference Fourier map,and the positional parameters were allowed to refine with the restraineddistance 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) resembleeach other in the molecular and crystal structures. The molecularstructure, the projection of the crystal structure along the a axis,

and that along the b axis in 20-diol-I′, as a representativeexample, are displayed in Figures 1, 2, and 3, respectively.Selected torsion angles are listed in Table 2.

The molecule is centrosymmetric, and all torsion anglesformed by nonhydrogen atoms are close to (180°, that is, themolecular skeleton that included both terminal hydroxy groupshas an all-trans conformation. The molecules are inclined withrespect to the basal plane formed by the hydroxy groups andform a layer which has a thickness of (c sin �)/2 and is built upin a zigzag manner along the layer stacking direction, the c axis.

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

Figure 2. Projection of the crystal structure of 20-diol-I′ along the aaxis. Dotted lines indicate the hydrogen bonding.

Figure 3. Projection of the crystal structure of 20-diol-I′ along the baxis.

Table 2. Selected Torsion Angles of n-Diol-I′ (degree)

20-diol-I′ 22-diol-I′ 24-diol-I′O1-C1-C2-C3 179.9 (3) -179.6 (2) -179.83 (16)C1-C2-C3-C4 177.0 (3) 176.8 (2) -176.44 (16)C2-C3-C4-C5 -179.3 (3) -180.0 (2) -179.95 (16)C3-C4-C5-C6 179.6 (3) 179.3 (2) -179.14 (16)C4-C5-C6-C7 179.6 (3) 179.4 (2) -179.39 (16)C5-C6-C7-C8 -179.9 (3) 179.6 (2) -179.82 (16)C6-C7-C8-C9 179.2 (3) 179.6 (2) -179.45 (16)C7-C8-C9-C10 179.1 (3) 179.9 (2) -179.64 (16)C8-C9-C10-C11 179.8 (2) -179.86 (16)C9-C10-C11-C12 179.97 (16)C8-C9-C10-C10a 179.4 (3)C9-C10-C10a-C9a 180.0 (3)C9-C10-C11-C11b 179.9 (2)C10-C11-C11b-C10b -180.0 (3)C10-C11-C12-C12c 180.00 (17)C11-C12-C12c-C11c 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.

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

At the interlayer, the molecules form hydrogen bonds. Theinterlayer hydrogen-bond parameters of n-diol-I′ (n ) 20, 22,24) are shown in Table 3. These features are similar to those ofthe 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 even-numbered n-diol-I (n ) 10–18).3 The former has a monoclinicsystem 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), andthe latter has a monoclinic system with the P21/c or P21/n spacegroups and with a ) 4.9395 (15)-4.998 (2) Å, which is abouta half of the former, and b ) 5.1592 (18)-5.220 (2) Å. In thecase of 18-diol-I as a representative example of n-diol-I (n )10–18), the molecules are aligned on the (106) plane (Figures4 and 5). On the other hand, the molecules of 20-diol-I′ arealigned on the (206) plane, and the molecular plane slides b/2with respect to the neighboring one, as can be seen in Figure 2.Therefore, the molecules of n-diol-I′ appearing in the projectionalong the a axis double in number compared with those ofn-diol-I (see Figures 2 and 4).

The difference between the crystal structure of n-diol-I andn-diol-I′ has an influence on calculated densities (Figure 6). TheX-ray data for all diols were collected at the same temperature,296 (2) K, to allow a comparison of calculated densities. Theslope 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), whereasthe calculated densities of diols tend to decrease monotonicallywith increasing the number of carbon atoms because of theincreased hydrophobic content. This means that the crystalstructure of n-diol-I′ is more efficient in the methylene chainpacking than that of n-diol-I. This fact may be caused bythe competition between hydrogen bond interactions formedby the terminal hydroxy groups and hydrophobic interactionsof the methylene chains. In consequence, the crystal structuretype of n-diol-I′ has an advantage in the close packing forthe methylene part, with an increasing number of carbonatoms. In Figure 6, two dotted lines indicate the extrapolationof the calculated densities in n-diol-I and n-diol-I′ , respec-tively. It is found out that there is a border at 20-diol on thecrystal structure type. Moreover, based on the extrapolated lineof n-diol-I (n ) 10–18) in Figure 6, we suggest that the n ) 20diol may adopt the crystal structure of the n-diol-I type asanother polymorphic form.

The difference of the crystal structure type between even-numbered n-diol-I and n-diol-I′ is also recognized as that ofthe methylene chain packing in a layer between basal planesformed by hydroxy groups, i.e., the subcell. In the subcellconcept, the chains are assumed to be infinitely long to form a

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

D-H. . .A D-H H. . .A D. . .A D-H. . .A

20-diol-I′ O1-H1. . .O1a 0.82 (2) 2.02 (2) 2.836 (4) 174 (5)22-diol-I′ O1-H1. . .O1b 0.825 (18) 2.015 (18) 2.8387 (18) 176 (4)24-diol-I′ O1-H1. . .O1c 0.836 (19) 2.004 (19) 2.8401 (14) 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 4. Projection of the crystal structure of 18-diol-I along the aaxis.3 Dotted lines indicate the hydrogen bonding.

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

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

Polymorphism of Alkane-R,ω-Diols Crystal Growth & Design, Vol. 8, No. 2, 2008 595

three-dimensional crystal with an asymmetric unit consistingof methylene groups with the subcell constant cs in the chaindirection and as and bs in the other direction, where Rs, �s, andγs are the interaxial angles. As the center of gravity in the subcellunit is matched against the average for the gravity points ofmolecules comprised in the subcell unit, the subcell constantsare 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 Figure7a which shows the subcell of 18-diol-I as a representativeexample, there are infinite rows of nearly coplanar zigzag chainswith a chain-chain distance of bs ) 4.570 (6)-4.622 (7) Å.The chains are related by a simple translation almost perpen-dicular to the chain axes, Rs ) 88.2 (3)-88.6 (4)°. Adjacentrows are also related by a simple translation, as ) 4.144(6)-4.167 (7) Å, almost perpendicular to the chain axes, �s )92.1 (4)-93.1 (4)°. The angle between as and bs, i.e. γs, is110.01 (12)-110.52 (14)°. According to the Segerman’s subcellclassification,14 this packing type is M1

|. On the other hand, inthe subcell of n-diol-I′, as illustrated in Figure 7b, which showsthe subcell of 20-diol-I′ as a representative example, the zigzagchains in a row are related by a translation almost perpendicularto the chain axes, Rs ) 87.9 (6)-87.9 (10)°. However, the zigzagplanes are not parallel to the translation bs ) 4.668 (17)-4.679(11) Å. Neighboring rows are related by another translationalmost perpendicular to the chain axes, �s ) 90.6 (8)-90.7(10)°, and a 180° rotation around the chain axes. The distancebetween 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 zigzag-chain plane and the row plane, are increasing slightly with theaddition of the number of the methylene carbon atoms (see Table4). The δs values of n-diol-I′ are remarkably large comparedwith those of n-diol-I, by introducing 2-fold axes between the

rows. The increase of the δs values means that the hydrocarbonchain swivels gradually from the fulcrum of the terminalhydroxy groups to the molecular center, in order to fill the gapsbetween hydrocarbon chains. Therefore, these results alsoendorse the conclusions of an advantage in the close packingfor 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 16-diol-II′ as a representative example of n-diol-II′. Selected torsionangles of n-diol-II′ (n ) 16, 18, 20, 22, 24) are listed in Table5. The terminal torsion angle O1-C1-C2-C3 is in the rangefrom -62.2 (5)° to -63.7 (3)°, while the other terminal torsionangle is close to 180°. This means that the former has a gaucheconformation with respect to the hydrocarbon skeleton whichis all-trans and the latter has a trans conformation. The moleculein which one of the hydroxy groups adopts a gauche conforma-tion is noncentrosymmetric, but we cannot determine theabsolute configuration because the diols are composed of lightatoms. Therefore, the absolute configurations are tentativelyassigned.

The crystal structures of n-diol-II′ for n ) 16, 18, 20, 22, 24all have the same packing pattern. As an example, Figure 9aand b shows the projection of the crystal structure of 16-diol-II′ along the a and b axes, respectively. The projection alongthe chain axes in a layer between the basal planes formed byhydroxy groups is shown in Figure 9c. The molecules are nearlynormal to the basal plane and form a layer with a thickness ofc sin �, in which the molecules are arranged in a typicalherringbone motif of aliphatic compounds.15 The molecules alsoform two different types of hydrogen bond, i.e. interlayer andintralayer. The hydrogen-bond parameters of n-diol-II′ (n ) 16,18, 20, 22, 24) are shown in Table 6. These features are similarto those of the long-chain odd-numbered diols with n ) 11–23,n-diol-II, analyzed previously.3 The relation between thecalculated 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 slightinclination angle of the molecules with respect to the basal planeformed by hydroxy groups. The crystal structure of n-diol-IIhas a double-layered structure of an orthorhombic system withthe P212121 space group (Figure 10). In the layer, the inclinationangles of the molecules are less than 0.5° with respect to theline normal to the basal plane. In contrast, the crystal structuresof n-diol-II′ have a single layered structure of a monoclinicsystem with the P21 space group. In the layer, the molecularaxes of n-diol-II′ (n ) 16, 18, 20, 22, 24) are inclined alongthe a axis at 1.87, 1.53, 1.30, 1.05, and 0.93°, respectively.Therefore, as the number of carbon atoms increases, the crystalstructure of n-diol-II′ becomes closer to that of n-diol-II.Intralayer hydrogen-bond patterns are also different betweenn-diol-II′ and n-diol-II. In the former, the hydrogen-bondsequence is formed toward the b axis, and in the latter, it is

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

10-diol-I 12-diol-I 14-diol-I 16-diol-I 18-diol-I 20-diol-I′ 22-diol-I′ 24-diol-I′as (Å) 4.144 (6) 4.153 (4) 4.163 (4) 4.159 (7) 4.167 (7) 8.021 (16) 8.017 (13) 8.011 (10)bs (Å) 4.570 (6) 4.583 (4) 4.601 (5) 4.607 (7) 4.622 (7) 4.668 (17) 4.670 (13) 4.679 (11)cs (Å) 2.544 (13) 2.543 (10) 2.545 (10) 2.543 (15) 2.545 (15) 2.55 (4) 2.55 (3) 2.54 (2)Rs (deg) 88.2 (3) 88.4 (3) 88.4 (3) 88.6 (4) 88.6 (4) 87.9 (10) 87.9 (8) 87.9 (6)�s (deg) 92.1 (4) 92.2 (3) 92.6 (3) 92.6 (4) 93.1 (4) 90.7 (10) 90.6 (8) 90.6 (6)γs (deg) 110.01 (12) 110.08 (9) 110.26 (10) 110.34 (15) 110.52 (14) 104.7 (3) 104.8 (2) 104.83 (16)Vs (Å3) 45.2 (2) 45.44 (19) 45.67 (19) 45.6 (3) 45.8 (3) 92.1 (14) 92.1 (11) 92.1 (9)V per -CH2- (Å3) 22.62 (12) 22.72 (10) 22.84 (10) 22.82 (15) 22.92 (15) 23.0 (4) 23.0 (3) 23.0 (2)δs (deg) 1.86 (13) 2.34 (10) 2.77 (11) 2.81 (15) 3.35 (16) 9.6 (4) 9.3 (3) 9.5 (2)

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

596 Crystal Growth & Design, Vol. 8, No. 2, 2008 Uno et al.

toward the a axis, which is related to the 2-fold screw axis (SeeFigures 9c and 10c).

By the way, Popovitz-Biro et al. applied a grazing incidencesynchrotron X-ray diffraction method (GID) for determiningthe packing arrangements of the self-assembled multilayerstructures of the diols n ) 16, 22, 23, and 30.16 Some of normallong-chain aliphatic compounds have attracted attention as icenucleators for cloud-seeding, because the monolayer or multi-layer films which are self-assembled on the water surfacepromote an epitaxial nucleation of hexagonal ice crystals fromsupercooled water.17 They derived the models of a rectangular

cell of a ≈ 5.0 Å and b ≈ 7.3 Å with a herringbone motif inwhich the molecules were aligned nearly perpendicular to thelayer plane. The models were introduced a relaxed a-glide planeand a 2-fold screw axis parallel to the a axis for the even- andodd-numbered diols, respectively, based on the requirementsof the unit cell dimensions, the hydrogen-bonds, and the spacegroups. The single crystals of n-diol-I′ and n-diol-II′ wereobtained from the bottom and the side of the sample bottle,respectively. That is, the former was grown in the solution andthe latter at the air–solution interface. Therefore, the crystalstructures of n-diol-II′ are probably grown epitaxially from thenuclei formed at the air–solution interface and are correspondingto 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, becauseof a minor molecular tilt with respect to the normal line to thelayer plane. Furthermore, the glide plane was allowed a minorrelaxation by inclining the glide plane by an angle with respectto the normal plane to the layer plane to obtain the best fit ofthe GID data. However, the proposed models of the even-numbered diols made up of one or three layers did not fit theobserved GID data satisfactorily. The proposed models may beimproved due to introducing not a relaxed a-glide plane butrather a 2-fold screw axis which were observed in the crystalstructure of n-diol-II′.

Relationship between the Polymorphism and the Phase-Transition Behavior in the Long-Chain Even-NumberedDiols. In our previous study on the phase-transition behaviorof the long-chain diols (n ) 13–24), it was found that the even-

Figure 8. Molecular structure of 16-diol-II′ showing the crystallographic numbering scheme. Displacement ellipsoids are drawn at the 50% probabilitylevel.

Table 5. Selected Torsion Angles of n-Diol-II′ (degree)

16-diol-II′ 18-diol-II′ 20-diol-II′ 22-diol-II′ 24-diol-II′O1-C1-C2-C3 -63.4 (5) -63.0 (5) -62.2 (5) -63.5 (5) -63.7 (3)C1-C2-C3-C4 175.2 (3) 175.3 (3) 175.1 (3) 175.2 (3) 175.5 (2)C2-C3-C4-C5 -178.9 (3) -178.9 (3) -179.1 (3) -178.7 (3) -179.15 (18)C3-C4-C5-C6 178.6 (3) 177.9 (3) 177.9 (3) 178.6 (3) 178.01 (18)C4-C5-C6-C7 -179.7 (3) 179.7 (3) 179.6 (3) 180.0 (3) -179.94 (17)C5-C6-C7-C8 179.8 (3) -179.9 (3) 179.6 (3) 179.5 (2) 179.73 (17)C6-C7-C8-C9 179.5 (3) -180.0 (3) 179.5 (3) 179.4 (2) 179.51 (16)C7-C8-C9-C10 179.8 (3) -179.9 (3) -179.7 (3) -179.5 (2) -179.76 (16)C8-C9-C10-C11 -179.9 (3) 179.5 (3) -180.0 (3) 179.8 (2) 179.85 (16)C9-C10-C11-C12 179.7 (3) 179.7 (3) -179.8 (3) 179.8 (2) -179.79 (16)C10-C11-C12-C13 -179.8 (3) -179.5 (3) 179.8 (3) 179.9 (2) 179.84 (16)C11-C12-C13-C14 179.9 (3) 179.9 (3) 179.8 (3) -179.9 (3) 179.99 (17)C12-C13-C14-C15 180.0 (3) -179.6 (3) -179.6 (3) -180.0 (2) -179.95 (17)C13-C14-C15-C16 179.9 (3) 179.7 (3) 179.4 (3) 179.9 (2) -179.97 (16)C14-C15-C16-C17 180.0 (3) -179.7 (3) -179.4 (2) -179.65 (16)C15-C16-C17-C18 179.6 (3) 179.7 (3) 179.2 (3) 179.78 (17)C16-C17-C18-C19 180.0 (3) -179.8 (2) -179.89 (16)C17-C18-C19-C20 179.7 (3) 179.8 (3) 179.40 (17)C18-C19-C20-C21 179.9 (2) -179.61 (16)C19-C20-C21-C22 179.9 (3) 179.24 (17)C20-C21-C22-C23 179.86 (17)C21-C22-C23-C24 179.99 (17)C14-C15-C16-O2 179.0 (3)C16-C17-C18-O2 179.0 (3)C18-C19-C20-O2 179.3 (3)C20-C21-C22-O2 179.3 (3)C22-C23-C24-O2 179.76 (17)

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

D-H. . .A D-H H. . .A D. . .A D-H. . .A

16-diol-II′ O1-H1. . .O2a 0.821 (19) 1.97 (2) 2.783 (4) 170 (4)O2-H2. . .O1b 0.838 (18) 1.877 (19) 2.710 (3) 173 (4)

18-diol-II′ O1-H1. . .O2c 0.831 (18) 1.97 (2) 2.786 (4) 168 (4)O2-H2. . .O1d 0.840 (18) 1.870 (19) 2.710 (3) 179 (4)

20-diol-II′ O1-H1. . .O2e 0.847 (19) 1.94 (2) 2.783 (5) 177 (4)O2-H2. . .O1f 0.846 (19) 1.87 (2) 2.711 (3) 176 (4)

22-diol-II′ O1-H1. . .O2g 0.860 (19) 1.94 (2) 2.792 (4) 170 (4)O2-H2. . .O1h 0.838 (19) 1.878 (19) 2.715 (3) 176 (4)

24-diol-II′ O1-H1. . .O2i 0.840 (18) 1.961 (18) 2.790 (3) 169 (3)O2-H2. . .O1j 0.837 (17) 1.873 (18) 2.7093 (18) 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.

Polymorphism of Alkane-R,ω-Diols Crystal Growth & Design, Vol. 8, No. 2, 2008 597

numbered diols (n ) 16–24) and the odd ones (n ) 13–23)had a rotator phase just below their melting points.4 In the even-numbered diols, the carbon number starting to exhibit apolymorphic form, n-diol-II′, coincides with that starting to showa rotator phase. Calculated density gives a measure of compact-ness, and in a homologous series, it may be correlated with aphase-transition point. However, the phase-transition tempera-tures from the crystal phase to the rotator phase in the even-numbered diols (n ) 16–24) increase monotonically when thenumber of carbon atoms are increased, whereas a border onthe crystal structure type exists at n ) 20. In differential scanningcalorimetry, the sample is rapidly cooled from the molten stateobtained from the first heating process, and then, the samplewas used on the second heating process to obtain the crystal-rotator phase transition point. In the cooling process, the

molecules probably undergo “surface freezing” which is con-sidered to be when a single crystalline monolayer is formed atthe surface of the isotropic liquid bulk above the bulk freezingtemperature.18 And then, crystals are grown epitaxially from asingle crystalline monolayer. This monolayer structure isprobably similar to n-diol-II′ because the crystal of n-diol-II′ isgrown at the surface of a solution. Moreover, the calculateddensities of n-diol-II′ are lower than those of n-diol-I and n-diol-I′ as can be seen in Figure 6, that is, the crystal of n-diol-II′ isconsidered to be a metastable crystalline phase. Therefore, thecrystal structure of n-diol-II′ has an advantage in exhibiting therotator phase.

Conclusion

The crystal structure analyses of the long-chain diols (n-diol-I′ of n ) 20, 22, 24 and n-diol-II′ of n ) 16, 18, 20, 22, 24

Figure 9. Projections of the crystal structure of 16-diol-II′ (a) along the aaxis and (b) along the b axis. (c) Projection along the chain axis in a layerwith a thickness of c sin �. Dotted lines indicate the hydrogen bonding.

Figure 10. Projections of the crystal structure of the 15-diol-II (a) alongthe a axis and (b) along the b axis. (c) Projection along the chain axisin a layer with a thickness of c/2. Dotted lines indicate the hydrogenbonding.3

598 Crystal Growth & Design, Vol. 8, No. 2, 2008 Uno et al.

with a different polymorphic form) have been carried out, andthe structures are compared with those of homologues.

The molecular structures of n-diol-I′ are similar to those ofthe lower diols with an even number of carbon atoms of n-diol-I(n ) 10–18) analyzed previously, while the crystal structuresare different from each other. The former has a monoclinicsystem with the C2/c space group, and the latter has amonoclinic system with the P21/c or P21/n space groups. Inn-diol-I (n ) 10–18), the slope of calculated densities on thedecline with an increasing number of carbon atoms is steeperthan 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 thezigzag-chain plane and the row plane, are increasing slightlywith the addition of the number of the methylene carbon atoms,and the δs values of n-diol-I′ are remarkably large compared withthose of n-diol-I. The increase of the δs value means that thehydrocarbon chain swivels gradually from the fulcrum of theterminal hydroxy groups to the molecular center, in order to fillthe gaps between hydrocarbon chains. Therefore, these resultsendorse the conclusions of an advantage in the close packing forthe methylene part on the crystal structure type of n-diol-I′.

The molecular and crystal structures of n-diol-II′ are similarto those of the odd-number diols, n-diol-II (n ) 11–23), analyzedpreviously but are slightly different from the self-assembledmultilayer structures whose models are derived from grazingincidence synchrotron X-ray diffraction data. Therefore, it isbetter that the proposed models are improved due to introducinga 2-fold screw axis which was observed in the crystal structureof n-diol-II′. On the other hand, the carbon number starting toexhibit a polymorphic form coincides with structures startingto show a rotator phase which is observed just below theirmelting points. From the viewpoint of the epitaxial crystalgrowth at the surface and of the calculated density, it is foundthat the crystal structure of n-diol-II′ has an advantage inexhibiting the rotator phase.

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

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