The liquid crystal state: An intermediate state to obtain crystal packing

Article abstract of DOI:10.1016/j.molliq.2014.10.032

Two 1-alkyl-3-methylimidazolium trifluoromethanesulfonate, [C_xMIM] [TfO] (x = 12 or 20), form in appropriate solvents liquid crystal phases from which crystals can be obtained. The cohesion of this crystal is ensured by a combination of dispersion forces, H-bonds and electrostatic interactions developed firstly in the liquid crystal phase.

Full text of DOI:10.1016/j.molliq.2014.10.032

Journal of Molecular Liquids 200 (2014) 283–288
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
The liquid crystal state: An intermediate state to obtain crystal packing
Loïc Leclercq a, Andreea R. Schmitzer b,
⁎
a
Université Lille 1 and ENSCL, EA 4478 Chimie Moléculaire et Formulation, F-59655 Villeneuve d'Ascq Cedex, France
Université de Montréal, Département de Chimie, CP 6128 Succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2014
Received in revised form 10 September 2014
Accepted 24 October 2014
Available online 30 October 2014
Two 1-alkyl-3-methylimidazolium trifluoromethanesulfonate, [CxMIM] [TfO] (x = 12 or 20), form in appropriate
solvents liquid crystal phases from which crystals can be obtained. The cohesion of this crystal is ensured by a
combination of dispersion forces, H-bonds and electrostatic interactions developed firstly in the liquid crystal
phase.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Imidazolium salts
Crystal growth
Liquid crystal
Hydrogen bonds
1. Introduction
coordination shells is generally similar in both solid and liquid state
[25]. To select or design imidazolium salts for a specific application, it
Ionic liquids (ILs) constitute an attractive field in chemistry due to
their solvent properties and the concept of “designer solvents” [1–3]. In-
deed, the solvent properties of ILs should be able to be tailored to meet
the requirements of specific applications, by varying the length and
branching of the alkyls chains on the cationic and the anionic species.
Imidazolium-based ILs were previously used as solvents in organic syn-
thesis, organometallic catalysis, nanochemistry, electrochemistry, sepa-
ration processes and as new materials [4–22], due to their unique
physicochemical properties. Imidazolium unit can also be considered
as a tecton because it possesses H-bonds donor through the three acidic
protons localized on the cationic heterocyclic ring [23,24]. These H-
bonds induce structural directionality contrary to classical salts in
which the cohesion is mainly due to ionic bonds (Fig. 1a) [25–37]. More-
over, complementary electrostatic, π-stacking and van der Waals inter-
actions can also occur in these salts (Fig. 1b) [38,39]. Therefore, in the
solid state, the formation of the supramolecular networks is governed
by molecular recognition processes between complementary tectons
(i.e. cations and anions) leading to assembled units, and the long
range translation of these units is leading to the formation of the 3D
molecular networks. It is noteworthy that the strongest H-bonds always
involves the most acidic proton H2 followed by H4 and H5 of the
imidazolium ring [40–42]. In the solid state, these H-bonds are weak
to moderate but they act as complements of other interactions (electro-
static, van der Waals and π-stacking) [43]. There is a relationship
between the liquid organization of ILs and the crystal structure. In-
deed, the molecular packing and interactions in the first two or three
is essential to understand the structure they will adopt in the liquid or
solid phase. Several attempts have been made to predict the molecular
packing in the solid phase [44,45]. However, this methodology is only
valid for simple rigid structures where only small C\C rotation can
occur [46]. Moreover, the crystallization of ILs with long alkyl tails (i.e.
the surfactants based on ILs) is still an important challenge. Indeed, as
pointed out by Drew Myers: “the packing of long hydrocarbon chains
into a crystalline alignment is difficult because of the many possible
variations in configuration for the units of the chain due to rotation
about the four bonds to each carbon atom (rotational isomers). That
difficulty is reflected in the relatively low melting points and poorly de-
fined crystal structure of most hydrocarbons under normal conditions”
[47]. On the other part, when water or other solvents are added to an
imidazolium salt, the crystal structure can be partially disrupted to
give a liquid crystal phase. This last possesses physical properties of
both crystalline and fluid phases. Moreover, this phase has at least one
highly ordered dimension [47]. We show herein that an intermediate
organized liquid crystal phase can provide a good platform for the crys-
tallization of these imidazolium salts. This new methodology can be ap-
plied for the crystallization of long alkyl chain length carried by an
imidazolium ring and maybe to other surfactants.
2. Experimental section
2.1. Materials
All reactions were carried in oven-dried glassware under nitrogen,
using standard Schlenk and vacuum line techniques. The 1H and 13
C
⁎
Corresponding author.
E-mail address: ar.schmitzer@umontreal.ca (A.R. Schmitzer).
NMR spectra were recorded on Advance 300 Brucker spectrometer at
http://dx.doi.org/10.1016/j.molliq.2014.10.032
0167-7322/© 2014 Elsevier B.V. All rights reserved.

284
L. Leclercq, A.R. Schmitzer / Journal of Molecular Liquids 200 (2014) 283–288
crystals. The imidazolium salt (1 g) was added to a water/ethanol solu-
-
-
a)
b)
A
A
tion (2.5 mL of each). The flask was sealed and placed in a thermoregu-
lated bath. To obtain a crystallization of the imidazolium salt, the
following temperature program was used: initial temperature 20 °C,
ramp to 80 °C at 20 °C/min (until homogenization of the solution
under stirring) and finally ramp to 20 °C at 0.5 °C/h (hold for 48 h).
After a week (i.e. at the end of the temperature program), needles are
obtained. A single crystal was mounted on a loop fiber. Data were
collected using a Bruker microstar diffractometer equipped with a Plat-
inum 135 CCD Detector, a Helios optics and a Kappa goniometer (Cu/Kα
radiation, 1.54178 Å, at 150 K). The crystal-to-detector distance was
4.0 cm, and the data collection was carried out in 512 × 512 pixel
mode. The initial unit cell parameters were determined by a least-
squares fit of the angular setting of strong reflections, collected by a
10.0 degree scan in 33 frames over three different parts of the reciprocal
space (99 frames total). Due to geometrical constraints of the instru-
ment and the use of copper radiation, we obtained consistently a data
completeness lower than 100% in dependence of the crystal system
and the orientation of the mounted crystal, even with appropriate
data collection routines. Typical values for data completeness ranged
from 83 to 92% for triclinic, 85–97% for monoclinic and 85–98% for all
other crystal systems. The weighted R-factor wR and goodness of fit S
were based on F2, conventional R-factors R were based on F, with F set
to zero for negative F2. The threshold expression of F2 N 2σ(F2) was
used only for calculating R-factors (gt) etc. and was not relevant to the
choice of reflections for refinement. R-factors based on F2 were statisti-
cally about twice as large as those based on F, and R-factors based on all
data will be even larger.
H
H
+
N
N
Cx
Me
H
-
A
Fig. 1. Schematic representation of: a) the three H-bond donors (purple dotted line), and
b) π-stacking interactions between imidazolium cations (blue pentagons) and triflate an-
ions (red spheres).
300.13 and 75.49 MHz, respectively, with the samples nonspinning.
Chemical shifts are given in ppm (δ) and measured relative to residual
solvent. HR-MS analyses were obtained at the Université de Montréal fa-
cility. Trifluoromethanesulfonic anhydride was freshly prepared prior to
use by distillation of a 1:1 w/w mixture of trifluoromethanesulfonic acid
and phosphorus pentoxide. All other chemicals were purchased from
Aldrich and used without further purification. “Distilled” solvents were
obtained using a GlassContour system (Irvine, CA).
2.2. Synthesis of 1-alkyl-3-methylimidazolium triflate [CxMIM] [TfO] with
x = 12 or 20
3. Results and discussion
A dry flask (250 mL), equipped with a magnetic stir bar and a septum-
inlet for nitrogen, was charged with n-alkyl alcohol (5.4 mmol, 1 eq.) and
poly(vinylpyridine) (1.15 eq.) in dichloromethane (50 mL). At 0 °C,
trifluoromethanesulfonic anhydride (1.1 eq.) was dropwise added. The
reaction mixture was stirred for 3 h to 0 °C at room temperature and
then filtered under gravity. The poly(vinylpyridinium triflate) precipitate
was washed with 5 mL of dichloromethane. The organic layer was
washed with saturated NaHCO3 solution and when the organic layer be-
came transparency, we dried (MgSO4) and concentrated under vacuum
to give the n-alkyl triflate (yellowish oil). This oil was dissolved in anhy-
drous toluene under a nitrogen atmosphere and the solution was cooled
to 0 °C. The 1-methylimidazole (1 eq.) was added and the resulting solu-
tion was stirred during 16 h at 57 °C. The ionic liquid layer appeared be-
neath the toluene phase or the ionic salt precipitated. The resulting salt
was dried overnight at 120 °C under vacuum. The product was stored
under dry nitrogen.
3.1. Synthesis and crystallization
3.1.1. Synthesis of imidazolium triflate salts
Our strategy of synthesis is based on the reaction of an intermediate
(trifluoromethanesulfonic)acid ester with the 1-methylimidazolium.
Indeed, n-alcohols can be easily converted into n-alkyl triflates in the
presence of triflic anhydride and a non-nucleophilic base providing
the expected intermediate, which is further reacted with the 1-
methylimidazolium in dry toluene. Poly(4-vinylpyridin) was used
in order to avoid the base alkylation and to facilitate the purification
of 1-alkyl-3-methylimidazolium triflate salts ([CxMIM] [TfO], with
x = 12 or 20) [48]. Highly pure imidazolium triflates were isolated
in excellent yields (N80%) and were characterized by 1H and 13C
NMR, and High Resolution Mass Spectrometry (HR-MS). All salts
were kept in a glove box under argon atmosphere.
[C12MIM] [TfO]: Yield: 85%. m.p.: 40 °C. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 0.87 (broad signal, 3H); 1.30 (broad signal, 18H); 1.90
(broad signal, 2H); 3.98 (s, 3H); 4.18 (t, 2H, 3J = 7.1 Hz); 7.31 (s,1H);
7.39 (s,1H); 9.13 (s,1H). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 137.7;
124.5; 122.8; 119.6 (q, JCF = 319.0 Hz); 51.0; 37.2; 32.7; 30.9; 30.4;
30.3; 30.1; 30.1; 29.7; 26.9; 23.5; 14.9. HR-MS: m/z found: 251.2482
([C12MIM]+), calc: 251.2487.
3.1.2. Crystallization in liquid crystal phase
The crystallization of pure imidazolium salts generally results in
lamellar structures with alternating head-to-head and tail-to-tail
arrangements. The energy of the crystal, as reflected by their fusion
point (40 and 76 °C for [C12MIM] and [C20MIM], respectively), is
governed by the nature of the salt. The salts with highly polar and
small hydrophilic groups generally provide enhanced crystal stability.
In contrast, the salts with long hydrocarbon chains provide a poor crystal
packing due to a difficult crystalline alignment of the alkyl tails. Indeed,
the flexibility of the alkyl chain provides a very important number of pos-
sible rotational isomers.
On the other hand, when water or other solvent was added to these
long alkyl chain imidazolium salts, organized liquid crystalline phase
were formed. A liquid crystalline phase can be described as a phase
containing both specific molecular organization in a region surrounded
by more liquid or amorphous regions. Therefore, the liquid crystalline
phase can be seen as a prolific intermediate state to obtain pre-
organization and initiate the crystallization process. In other words,
the crystallization process goes through an initial pre-organization
[C20MIM] [TfO]: Yield: 91%. m.p.: 76 °C. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 0.88 (t, 3H); 1.28 (s, 34H); 1.83 (m, 2H); 3.98 (s, 3H);
4.18 (t, 2H); 7.27 (s,1H); 7.29 (s,1H); 9.14 (s,1H). 13C NMR (75 MHz,
CDCl3, 25 °C): δ = 137.8; 124.4; 122.8; 121.0 (q, JCF = 320.2 Hz);
51.0; 39.6; 37.2; 35.6; 32.7; 30.9; 30.7; 30.6; 30.6; 30.6; 30.4; 30.3;
30.2; 30.2; 29.8; 29.7; 27.0; 23.5; 23.5; 14.9. HR-MS: m/z found:
363.3743 ([C20MIM]+), calc: 363.3739.
2.3. Crystallization and X-ray diffraction
Both imidazolium salts were submitted to different crystallization
conditions, but any of these classical conditions did not allow obtaining

L. Leclercq, A.R. Schmitzer / Journal of Molecular Liquids 200 (2014) 283–288
285
Table 1
state in the solvent, before forming crystals from which the solvent is
excluded.
After several attempts, crystallizations were carried out efficiently
Crystal, collection and refinement information for [C20MIM] [TfO] crystal.a
Crystal data
from water/ethanol solutions. Thus, water/ethanol (1/1) solutions of
each imidazolium triflate salt were heated until homogenization (80 °C)
in order to accelerate the dissolution process. Then, the temperature
was gradually decreased from 80 to 20 °C (0.5 °C/h) then left at 20 °C
for 48 h. The solutions have been observed during a week using cross po-
larized optical microscopy (Fig. 2).
After the first heating (i.e. at 80 °C), we observed the absence of any
birefringence. After 1 day at 68 °C, Maltese crosses were observed, sug-
gesting the formation of a liquid crystalline phase. With the progressive
decrease of the temperature, the number of Maltese crosses increased,
as well as their sizes. At 32 °C (after 4 days), needles appeared in the liq-
uid crystalline phase. Indeed, crystallization from the liquid crystal
phase resulted in a highly oriented structure in which the molecules
may possess extended chain morphology. After a week, the crystals
were mounted on a loop fiber to perform X-rays diffraction.
Cation Formula
Anion Formula
Molecular weight 512.71
Crystal system
Space group
a/Å
b/Å
c/Å
α/deg
C
24H47N2
β/deg
96.953 (2)
96.814 (3)
2814.98 (19)
4
Colorless
0.20 × 0.16 × 0.04
1.421
150
1.210
CF3SO3
γ/deg
V/Å3
Triclinic
P − 1
9.4917 (4)
9.9553 (4)
30.5188 (11) Temperature/K
97.017 (2)
Z
Color
Crystal dim./mm
μ/mm−1
Dcalc/g·cm−3
Data collection and refinement
Tmin
Tmax
0.718
0.945
θ
θ
min/deg.
max/deg.
1.47
67.95
−9 ≤ h ≤ 10
Measured
reflections
Independent
reflections
Completeness
to θmax/%
Data
Parameters
Restraints
Goodness-of-fit
on F2
45,559
8850
86.3
Limiting indices
−11 ≤ k ≤ 11
−36 ≤ l ≤ 36
8850
843
4042
Δρmin and Δρmax/eA−3
Final R indices
[I N 2σ (I)]
R indices (all data) R1
−0.29 and 0.37
0.0480
wR2 0.1348
3.2. Solid state analysis
R1
1.035
0.034
0.0605
3.2.1. Crystal information and structure of the cation
For [C12MIM] [TfO], the displacement parameters are high for fluo-
rine and oxygen atoms of the trifluoromethanesulfonate ions and the
residual electronic densities at the end of the refinement are all located
in the vicinity of O and F positions: this observation can point to some
rotational disorder in the anion. The final R-factor (19.32%) indicates a
very poor agreement between the observed and calculated intensities.
In contrast, the [C20MIM] [TfO] salt crystallizes in a triclinic system
with P − 1 as space group and a disorder on the lateral eicosyl chain
was observed due to little rotations of some C–C bonds (Tables 1 and 2).
No water or residual solvent molecules were present in the lattice,
showing that the cohesion of the crystal is ensured only by the ionic
components. Therefore, the asymmetric unit of the crystal structure is
composed of two imidazolium cations and obviously two triflate anions.
We used here the atom numbering of crystal by using the first digit of
the atom's label as indication of the corresponding moiety (Fig. 3).
Rint
wR2 0.1427
a
Diffractometer: Bruker microstar equipped with a Platinum 135 CCD Detector, a Helios
optics and a Kappa goniometer (Cu/Kα radiation, 1.54178 Å).
Table 2 reports the geometry of the cations obtained from the X-ray
analysis.
3.2.2. Supramolecular assembly
The crystal structure reveals a typical organization compared to
those previously observed for other imidazolium crystals, i.e. an extend-
ed 3-D network of imidazolium cations and triflate anions connected to-
gether by H-bonds [37]. In the present system, the H-bonds network is
relatively weak according to published criteria. Indeed, as depicted in
Fig. 2. Cross polarized optical microphotograph of water/ethanol (1/1) solutions of [C20MIM] [TfO] salt as a function of time and temperature. The red circle shows the image of the
polarization box.

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L. Leclercq, A.R. Schmitzer / Journal of Molecular Liquids 200 (2014) 283–288
Table 2
Table 3
Experimental bond distances (Å), bond angles (∠, °), and dihedral angles (D, °) of
Distances (Å) and angles (°) of C\H⋯Y hydrogen bonds in [C20MIM] [TfO] crystal.
[C20MIM] cations.
C⋯H (Å)
H⋯Y (Å)
C⋯Y (Å)
C\H⋯Y (°)
X-rays diffraction
a
b
c
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
12\H12⋯O22
14\H14⋯O21
15\H15⋯O13
0.95
0.95
0.95
0.98
0.98
0.99
0.99
0.99
0.99
0.99
0.99
0.95
0.95
0.95
0.98
0.98
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
2.4274
2.3543
2.5178
2.5760
2.5963
2.5825
2.4494
2.5431
2.5887
2.6326
2.7448
2.2796
2.6629
2.5089
2.5417
2.4900
2.4341
2.4125
2.5433
2.6888
2.4935
2.6334
2.7728
2.4790
2.6614
3.245 (3)
3.255 (2)
3.343 (3)
3.466 (3)
3.249 (3)
3.552 (3)
3.275 (3)
3.275 (3)
3.275 (3)
3.552 (3)
3.552 (3)
3.147 (2)
3.286 (3)
3.088 (2)
3.242 (3)
3.465 (2)
3.372 (2)
3.372 (2)
3.363 (3)
3.363 (3)
3.363 (3)
3.151 (12)
3.293 (20)
3.175 (10)
3.417 (17)
144.12
158.08
145.37
151.03
124.10
166.35
140.60
130.64
126.46
154.56
139.04
151.39
123.71
119.41
128.33
173.68
157.96
163.16
140.09
125.63
146.42
112.70
113.36
127.02
133.33
x = 1
x = 2
N
C
N
x1\Cx2
x2\Nx3
x3\Cx4
1.318 (3)
1.335 (2)
1.373 (3)
1.348 (3)
1.364 (3)
1.467 (2)
1.463 (3)
109.12 (18)
107.30 (17)
1.0 (2)
−0.8 (2)
−101.1 (5)
−109.5 (7)
−118.4 (6)
–
1.333 (2)
1.323 (2)
1.368 (2)
1.348 (3)
1.374 (2)
1.469 (3)
1.480 (2)
108.62 (15)
107.07 (17)
−0.3 (2)
0.4 (2)
a
16\H16A⋯O22
16\H16C⋯O11
b
b
17\H17A⋯O23
17\H17B⋯O12
17\H17D⋯O12
17\H17F⋯O12
C
C
x4\Cx5
x5\Nx1
N
N
x1\CHx6
x3\CHx7
b
17\H17C⋯O23
∠Nx1\Cx2\Nx3
∠Nx3\Cx4\Cx5
b
17\H17E⋯O23
d
22\H22⋯O11
DNx1\Cx2\Nx3\Cx4
DCx2\Nx3\Cx4\Cx5
DCx2\Nx3\Cx7\Cx8
DC12\N13\C17\C38
DC12\N13\C17\C58
DC22\N23\C27\C48
DC22\N23\C27\C68
DCx7\Nx8\Cx9\Cx10
DC17\N38\C39\C310
DC17\N58\C59\C510
DC27\N48\C49\C410
DC27\N68\C69\C610
e
24\H24⋯O23
d
25\H25⋯O13
66.6 (5)
–
d
26\H26A⋯O21
f
26\H26B⋯O12
–
g
27\H27A⋯O12
73.5 (9)
124.2 (5)
167.5 (12)
–
g
27\H27E⋯O12
27\H27B⋯O23
27\H27C⋯O23
–
e
e
−78.3 (1)
−75 (13)
73 (2)
e
27\H27F⋯O23
–
d
d
28\H28A⋯F13
48\H48A⋯F13
–
170 (2)
−177 (2)
–
h
210\H21B⋯F22
410\H41B⋯F22
h
Symmetry operators for Y:
a
1 + x, −1 + y, z.
b
1 + x, y, z.
c
3 − x, 1 − y, 1 − z.
Table 3, the C\H⋯Y (Y_O or F) distances are comprised between 2.2
and 2.8 Å and C\H⋯Y angles between 112° and 174° [49–51]. The
weakness of these H-bonds may be due to the steric hindrance of the
triflate anions. However, the number of H-bonds compensates their
weaknesses.
d
1 − x, −y, 1 − z.
e
−x, 1 − y, 1 − z.
f
−1 + x, y, z.
g
1 − x, −y, 1 − z.
h
1 − x, 1 − y, 1 − z.
Each imidazolium cation is surrounded by three anions and vice
versa. Overall, H-bonds, electrostatic interactions and the dispersion
forces which operate between the alkyl tails are play the key role in
the assembly of this ordered structure (Fig. 4). Finally, no π-stacking in-
teractions can be observed in this structure due to the presence of the
large triflate anions.
To get better insights in the molecular organization of the [C20MIM]
[TfO], we compared the structural organization of the salt with the
[C2MIM] [TfO] and [C4MIM] [TfO] previously reported by Choudhury
et al. (Fig. 5) [52,53]. It is important to note that the authors also report-
ed at that time a phase transition for these short-chain imidazolium
salts at very low temperatures.
In these salts, cations and anions are typically connected by H-bonds
between the acidic protons of the cation (H2, H4, H5, N\CH2\ and
N\CH3) and the oxygen atom of the triflate anions. For [C4MIM] [TfO],
the H-bonds allow the formation of layers and each layer are head to
tail in order to give weak hydrophobic interactions between two alkyl
tails. Although the N-alkyl substituents are very different, the organiza-
tion of [C4MIM] [TfO] and [C20MIM] [TfO] are very similar, suggesting
that the dispersion forces are the driving forces of the crystallization
process. In contrast, in the [C2MIM] [TfO] crystal no dispersion forces be-
tween the alkyl tails of the two cations can be observed: the cohesion in
Fig. 3. ORTEP view of cations in the [C20MIM] [TfO] crystal (for more details see ESI). Ellipsoids drawn at 30% probability level.

L. Leclercq, A.R. Schmitzer / Journal of Molecular Liquids 200 (2014) 283–288
287
γ
β
Fig. 4. Crystal packing of [C20MIM] [TfO] salt obtained by X-ray diffraction: a) view down crystallographic a axis and b) view down crystallographic b axis.
this structure is essentially maintained by the H-bonds and electrostatic
interactions.
that allows the formation of crystals (Fig. 6). The crystallizations of
1-dodecyl- and 1-eicosyl-3-methylimidazolium triflate were per-
formed from their intermediate liquid crystal phase. The crystalliza-
tion process is probably favored for compounds possessing higher
fusion temperatures, allowing to achieve a correct crystal packing.
The structure of the cation, as well as the H-bonds, the electrostatic
and the dispersion interactions govern the general packing and the
lattice energy of these salts in the crystal. Fruitful developments
could be probably made for the crystallization of other imidazolium
salts and other ionic surfactants from a liquid crystal phase.
4. Conclusion
Imidazolium ILs form in the solid state extended supramolecular
networks of interconnected cations and anions. The dilution of these
solid molecular networks in hot water/ethanol results in the disruption
of the supramolecular network. However, when the temperature de-
creases, the separated ions form an intermediate liquid crystal phase
a)
b)
c)
Fig. 5. Crystal packing of: a) [C2MIM] [TfO], b) [C4MIM] [TfO] and c) [C20MIM] [TfO]. The dispersion forces are shown as red double arrow.

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Appendix A. Supplementary data
Supplementary data associated with this article can be found in the
online version, at http://dx.doi.org/10.1016/j.molliq.2014.10.032. CCDC-
688096 contains the Supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge Crystallo-
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