Desynchronization of pedal motion: Crystallographic and theoretical study of (E)-4-[(4-ethylphenyl)diazenyl]-2-methylphenol

Article abstract of DOI:10.1007/s10870-008-9371-5

The single crystal X-ray diffraction analysis of the title compound, C ₁₅H₁₆N₂O, reveals that its molecules exhibit whole-molecule disorder at both crystal lattice sites due to pedal motion in solid state. The compound crystallizes in the monoclinic space group P 2 ₁/c with a = 20.5504(14) A, b = 10.8887(5) A, c = 12.0191(8) A and β = 96.927(5)°. While major pedal conformers of the compound in solid state are stabilized by intermolecular O-H...N type hydrogen bonds leading to the formation of C(7) chains at Site 1 and C(8) chains at Site 2 along [0 1 0] axis, C-H...π type intermolecular interactions between major and minor conformers also serve to stabilize minor pedal conformers. An interesting feature about the crystal structure is that pedal conformers at Site 1 have two different occupancy factors arising from desynchronization of pedal motion along [2 1 0] direction in crystal phase. Quantum chemical calculations at the B3LYP/6-31++G level suggest that the desynchronization of pedal motions make more unstable pedal conformers at Site 1 than those at Site 2.

Full text of DOI:10.1007/s10870-008-9371-5

J Chem Crystallogr (2008) 38:671–677
DOI 10.1007/s10870-008-9371-5
ORIGINAL PAPER
Desynchronization of Pedal Motion: Crystallographic
and Theoretical Study of (E)-4-[(4-ethylphenyl)diazenyl]-
2-methylphenol
_
Nazan Ocak-Iskeleli Æ Hasan Karabıyık Æ
˘
˘
C¸ igdem Albayrak Æ Erbil Agar
Received: 16 April 2007 / Accepted: 27 March 2008 / Published online: 9 April 2008
Ó Springer Science+Business Media, LLC 2008
Introduction
Abstract The single crystal X-ray diffraction analysis of
the title compound, C15H16N2O, reveals that its molecules
exhibit whole-molecule disorder at both crystal lattice sites
due to pedal motion in solid state. The compound crys-
tallizes in the monoclinic space group P 21/c with
Solid-state conformational changes have received consid-
erable attention for the past three decades in order to
explain the dynamic aspects associated with certain fea-
tures of the molecules having photoreactivity and
photochemical properties [1–4]. In this regard, molecular
motions in crystals are of great importance in determining
the effects on their physical properties.
˚
˚
˚
a = 20.5504(14) A, b = 10.8887(5) A, c = 12.0191(8) A
and b = 96.927(5)°. While major pedal conformers of the
compound in solid state are stabilized by intermolecular
O–HÁÁÁN type hydrogen bonds leading to the formation of
C(7) chains at Site 1 and C(8) chains at Site 2 along [0 1 0]
axis, C–HÁÁÁp type intermolecular interactions between
major and minor conformers also serve to stabilize minor
pedal conformers. An interesting feature about the crystal
structure is that pedal conformers at Site 1 have two dif-
ferent occupancy factors arising from desynchronization of
pedal motion along [2 1 0] direction in crystal phase.
Quantum chemical calculations at the B3LYP/6-31++G**
level suggest that the desynchronization of pedal motions
make more unstable pedal conformers at Site 1 than those
at Site 2.
Some molecules having molecular skeletons similar to
those of (E)-stilbenes and azobenzenes show orientational
disorder in the crystal phase [5–10]. The disorder observed
in some azobenzenes is dynamic and therefore ascribed to a
conformational interconversion between major and minor
conformers [11]. The conformational interconversion
observed in such compounds is due to the pedal (or crank-
shaft) motion: a pair of benzene rings moving like the
pedals of a bicycle. Pedal motion is not only peculiar to
stilbene- and azobenzene-type molecules, it is also one of
the key processes in the photochromism of salicylidene-
anilines [12] and the photodimerization of trans-
cinnamides [13, 14]. The absence of orientational disorder
in such compounds can be attributable to the destabiliza-
tion of the minor conformer and the non-disordered crystal
structures of them do not mean that the pedal motion is
inhibited. In fact, the low occupancy factor of the minor
pedal conformer makes the disorder invisible in most
crystals. Ogawa and Harada reported that the pedal motion
takes place even in non-disordered crystals [15]. As a result
of this, orientational disorder arising from pedal motion has
been overlooked in most of crystal structures of such
compounds [15]. On the other hand, it is very difficult to
determine the structure of the minor conformer whose
occupancy factor is very low (generally less than 0.10) as
being in most of disordered azobenzenes. The occupancy
Keywords Pedal motion Á Crystal structure Á
Whole-molecule disorder Á Azobenzene Á DFT/B3LYP
_
N. Ocak-Iskeleli
Department of Science Education, Ondokuz Mayıs University,
55200 Samsun, Turkey
H. Karabıyık (&)
Department of Physics, Dokuz Eylu¨l University, Fen Edebiyat
¨ ¨
Faku¨ltesi, Fizik Bolumu, Tinaztepe Kampusu,
35160 Izmir, Turkey
¨
e-mail: hasan.karabiyik@deu.edu.tr
˘
C¸ . Albayrak Á E. Agar
Department of Chemistry, Ondokuz Mayıs University, 55139
Samsun, Turkey
123

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J Chem Crystallogr (2008) 38:671–677
factor of the minor conformers of azobenzene-type mole-
cules decreases as the temperature is lowered [15].
According to the Boltzmann distribution law, the popula-
tion of an energetically unfavorable conformer in
equilibrating systems increases as the temperature is raised.
Therefore, the population of a minor conformer of such a
compound cannot be easily detectable at a low tempera-
ture. In agreement with experiment, molecular simulations
of solid stilbene show the onset of pedal motion at one of
the crystal lattice sites between 150 and 200 K, while such
a motion starts at both lattice sites at higher temperatures
[16].
This solution was cooled to 273–278 K and a solution
sodium nitrite (1.42 g, 29.2 mmol) in water was added
dropwise while the temperature was maintained below
278 K. The resulting mixture was stirred for 30 min in an ice
bath. o-Cresol (2.23 g, 20.6 mmol) solution (pH * 8–9)
was gradually added to a cooled solution of 4-ethyl-
benzenediazonium chloride, prepared as described above,
and the resulting mixture was stirred at 273–278 K for
60 min in ice bath. The product was recrystallized from ethyl
alcohol to obtain solid (E)-4-[(4-ethylphenyl)diazenyl]-2-
methylphenol. Crystals of (E)-4-[(4-ethylphenyl)diazenyl]-
2-methylphenol were obtained from ethyl alcohol at room
temperature via slow evaporation (yield 76%, m.p. 398–
400 K). Synthesis route of the title compound is shown in
Scheme 1.
The disordered pedal conformers of azobenzene-type
molecules are usually located at one of the crystal lattice sites
[15, 17]. The single crystal X-ray diffraction method gives us
valuable information regarding the populations of major and
minor conformers for the structures in which conformational
changes occur in solid-state. In the present paper, we report
that the wholly disordered molecules of the title azobenzene
derivative are located at both crystal lattice sites and de-
synchronization of pedal motion results in different-site
occupancy factors in the crystal phase. Stability levels of
crystallographically observed pedal conformers were quan-
titatively compared by using the results from quantum
chemical calculations at level of B3LYP/6-31++G**.
X-ray Crystallography
A suitable sample of size 0.60 9 0.33 9 0.11 mm was cho-
sen for the crystallographic study and then carefully mounted
on goniometer of a STOE IPDS 2 diffractometer. All dif-
fraction measurements were performed at 300 K using
˚
graphite monochromated MoKa radiation (k = 0.71073 A).
The systematic absences and intensity symmetries indicated
the monoclinic P 21/c space group. A total of 36,045 reflec-
tions (5,240 unique) within the h range of [2.00°
\ h \ 26.00°] for -24 B h B 25, -13 B k B 13, -14
B l B 14 were collected in --scan mode with Rint = 0.0802.
The intensities collected were corrected for Lorentz and
Experimental
Synthesis of the Title Compound
polarization factors, absorption correction (l = 0.076 mm-1
)
by integration method via X-RED software [18]. Extinction
correction was not applied. Cell parameters were determined
by using X-AREA software [18]. The structure was solved by
direct methods using SHELXS-97 [19]. The refinement was
A mixture of 4-ethylaniline (2.5 g, 20.6 mmol), water
(50 mL) and concentrated hydrochloric acid (5 mL,
61.8 mmol) was stirred until a clear solution was obtained.
Scheme 1 Synthesis route for
the title compound
H3C
H3C
NaNO2
HCl
NH2
-
N
N+Cl
OH
pH ~ 8-9
CH3
H3C
N
N
OH
CH3
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J Chem Crystallogr (2008) 38:671–677
673
hybrid exchange-correlation functional (B3LYP) [20]
incorporating B88 gradient-corrected exchange [21] and
Lee–Yang–Parr non-local correlation functional [22] by
means of 6-31++G** basis set [23–25] implemented in
HyperChem 7.52 program package [26]. Numerical inte-
grations involving the exchange-correlation functional
were carried out on 32-point Gauss–Chebyshev radial and
50-point Lebedev angular integration grid by using cut-off
Table 1 Crystallographic data and the details of refinement process
for the title compound
Chemical formula
Color, shape
C15H16N2O
Orange, prism
240.30
Formula weight
Crystal system
Space group
Monoclinic
P 21/c
˚
a = 20.5504(14) A
Unit cell parameters
for density of 10-6 eA-3. Geometry optimization of the
˚
˚
b = 10.8887(5) A
title compound was achieved by the application of a conju-
gate gradient method known as Polak-Ribiere optimization
algorithm [27] with convergence limit of 10-2 kcal/mol
˚
c = 12.0191 (8) A
b = 96.927(5)°
3
˚
Volume
2669.8(3) A
and quite precise RMS gradient of 10-3 kcal/Amol. In order
˚
Formula Z
Dx (Mg m-3
l (mm-1
8
to compare stabilization levels of the pedal conformers in
solid state, total energies of pedal conformers were also
calculated for their crystallographically observed atomic
configurations (single-point calculations).
)
1.196
)
0.076
F000
1024
T
min, Tmax
0.965, 0.990
36045, 5240, 2648
No. of measured, independent
and observed reflections
Results and Discussion
hmax (°)
26.00
F2
Refinement on
R[I [ 2r(I)], S
No. of parameters
Weighting scheme
X-ray diffraction analysis of the title compound,
C15H16N2O, revealed that its thermodynamically unstable
minor pedal conformers coexist with stable major com-
ponents at the same crystal lattice sites. In the asymmetric
unit, there are two crystallographically independent mol-
ecules (Residue 1 at lattice Site 1 and Residue 2 at lattice
Site 2) each of which is refined as a superposition of
major and minor pedal conformer of the compound as
shown in Fig. 1 [28]. Residue 1 represents the monomer
containing O1A and O1B atoms. Other O atoms in the
asymmetric unit are included in Residue 2. The major (A)
and minor (B) pedal conformers at Site 2 have 0.72 and
0.28 site occupancy factors, respectively, whereas site
occupancy factors of the disordered atoms at Site 1 have
two different ratios: 0.53:0.47 for atoms C9A/B to C15A/
B and 0.72:0.28 for the remaining disordered fragment at
Site 1. This interesting feature of the crystal structure is
related to a possible displacive phase transition accom-
panied with conformational interconversion between
major and minor conformers at Site 1 along [2 1 0]
direction due to desynchronization of the pedal motions
and indicates that the compound is probably open to
polymorphism.
0.059, 0.974
640
w = 1/[r2(F2o)+(0.1447P)2]
P = (F2o + 2F2c)/3
0.012
(D/r)max
-3
)
˚
Dqmax, Dqmin (e A
0.27, -0.45
carried out by full-matrix least-squares method on the
positional and anisotropic temperature parameters of the
non-hydrogen atoms. All H atoms were refined using appro-
˚
priate riding models with C–H distances of 0.96 A for methyl
˚
˚
groups, 0.97 A for methylene groups and 0.93 A for aromatic
CH groups. The displacement parameters of H atoms were
fixed at 1.2 Ueq of their parent carbon atoms for aromatic
groups and methylene groups, 1.5 Ueq of their parent atoms for
methyl and hydroxyl groups. The disordered N atoms were
refined using the SADI restraints. In addition, the disordered
aromatic rings were adapted to their regular hexagonal
geometries by DFIX restraints. The structure was refined to
R1 = 0.059 for observed 2648 reflections prescribed by the
condition of I [2r(I) and to wR1 = 0.148 for all 5,240 data
used in refinement process. The scattering factors were taken
from SHELXL-97 [19]. Other details of the data collection
conditions and parameters of refinement process are summa-
rized in Table 1.
Some selected geometrical parameters for both con-
formers obtained from crystallographic analysis are listed
in Table 2. It is worth noting some conformational differ-
ences between the orientations of the major and minor
components at both crystal sites. The most remarkable
difference in the disordered orientations is observed in the
dihedral angle between the mean planes through major and
minor part of the 4-ethylphenyl ring at Site 1 with the angle
of 11(1)°, while the dihedral angle between the mean
Computational Procedures
All quantum chemical calculations were performed by
DFT calculations with the use of Becke’s three-parameters
123

674
J Chem Crystallogr (2008) 38:671–677
Fig. 1 Superposition of thermal
ellipsoid views of the major
(full lines) and minor (broken
lines) pedal conformers of the
title compound with the atom
numbering scheme of non-H
atoms in the asymmetric unit.
Displacement ellipsoids are
depicted at the 30% probability
level
Table 2 Some geometrical
˚
Bond lengths
parameters (A, °) for the
crystallographically observed
conformers of the title
compound
O1A–C3A
O1B–C3B
N1A–N2A
N1A–C6A
N1B–C6B
N1B–N2B
N2A–C8A
N2B–C8B
1.36(2)
1.37(6)
O2A–C16A
O2B–C16B
C19A–N3A
C19B–N3B
N3A–N4A
N3B–N4B
N4A–C23A
N4B–C23B
1.38(1)
1.32(2)
1.283(4)
1.446(8)
1.43(1)
1.472(9)
1.41(3)
1.219(5)
1.240(8)
1.467(7)
1.402(9)
1.233(9)
1.428(8)
1.37(1)
Bond angles
N2A–N1A–C6A
N2B–N1B–C6B
N4A–N3A–C19A
N3A–N4A–C23A
110.6(4)
N1A–N2A–C8A
N1B–N2B–C8B
N4B–N3B–C19B
N3B–N4B–C23B
110.5(4)
123(2)
125(2)
109.7(6)
108.7(5)
122(1)
119.3(8)
Torsion angles
C6A–N1A–N2A–C8A
C6B–N1B–N2B–C8B
-179(1)
C19A–N3A–N4A–C23A
C19B–N3B–N4B–C23B
C10B–C11B–C14B–C15B
C25B–C26B–C29B–C30B
180.0(5)
176(2)
-167(4)
159(1)
C10A–C11A–C14A–15A
C25A–C26A–C29A–C30A
-97(1)
-158(2)
-97(1)
planes of major and minor components of 4-ethylphenyl
ring at Site 2 is 7.1(7)°. Moreover, the dihedral angles
between the mean planes of 2-methylphenol rings of minor
and major components at Site 1 and Site 2 are 8(1)° and
3(1)°, respectively. Consequently, the disordered fragment
having different occupancy ratio from the remaining part of
Site 1, atoms C9A/B to C15A/B, is of evident orientational
difference. On the other hand, positional separations of
some atom pairs at crystal lattice Site 1 are not sufficiently
distinguishable. C6A and C6B have almost coincident
atomic positions, as do atoms C8A and C8B. None of the
pedal conformers in the asymmetric unit is planar around
the azo bridges and each pedal conformer is adapted to
trans stereo-configuration. Dihedral angles between the
aromatic rings of major and minor conformers at Site 1 are
found as 11(2)° and 6(2)°, respectively, these angles at Site
2 are 16.3(5)° and 26(1)°. Although the mentioned values
for the dihedral angles between the aromatic rings at Site 1
are in agreement with the values of 5°–15° observed for
(E)-azo benzenes [10], it is inferred from these results that
non-planarities of the conformers become more visible at
Site 2 than those at Site 1.
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J Chem Crystallogr (2008) 38:671–677
675
The r.m.s. fit of the bond distances of major con-
˚
former to those of minor conformer is 1.0742 A at Site
Table 3 Geometrical details of H-bonds and C–HÁÁÁp interactions
˚
(A, °)
˚
1 and 0.9417 A at Site 2. These values indicate that
D–HÁÁÁA
D–H
HÁÁÁA
DÁÁÁA
\D–HÁÁÁA
there are remarkable discrepancies between in bond
distances of the major and minor conformer at both
sites. As far as geometrical parameters of the major and
minor components are concerned that the most impor-
tant differentiation is observed for N=N bonds in both
lattice sites and for C–O bonds at Site 2. The unex-
pected shortenings in N3A–N4A and N1B–N2B bonds
and elongations in N1A–N2A and N3B–N4B bonds can
be explained by low stabilization levels of the minor
conformers during solid-state conformational intercon-
version and the presence of intermolecular hydrogen
bonds in the crystal structure. Pairs of formally single
C–N bonds at opposite sides around azo bridges in both
pedal conformers of the compound are slightly different
from each other due to a possible through-resonance
effect between the electron-donating O atom and the
two-electron accepting N atom in azo bridge. This
tendency is observed for similar azo compounds
[29–34].
O1A–H3AÁÁÁN1Aa
0.82
0.82
2.15
2.01
2.97(2)
2.83(2)
174.3(3)
172.2(3)
O2A–H16AÁÁÁN4Ab
C–HÁÁÁCg
C–H
HÁÁÁCg
CÁÁÁCg
\C–HÁÁÁCg
C1B–H1EÁÁÁCg1c
0.96
0.96
2.51
2.69
3.37(2)
3.52(3)
150(2)
145(2)
C22B–H22FÁÁÁCg2d
Note: D: donor, A: acceptor. Cg1 and Cg2 denote the centroids of the
rings formed by atoms C2A–C7A and C16A–C21A. Symmetry
a
operations: 1 - x, y - 1/2, 3/2 - z; -x, 1/2 + y, 1/2 - z; 1 - x,
b
c
d
1 - y, 2 - z; -x, 1 - y, 1 - z
compound is major conformer at Site 2. As expected,
major pedal conformers are more stable than minor
conformers at both crystal sites. Furthermore, pedal
conformers located at Site 2 are more stable than their
counterparts at Site 1. These inferences are reasonable
in order that the desynchronization of pedal motions
resulting in two different occupancy ratios along [2 1 0]
direction makes somewhat unstable pedal conformers at
Site 1 than the conformers at Site 2.
Solid-state conformational changes are proceeded in a
different way from those in gas-phase and coupled with
changes in supramolecular environment of the com-
pounds. In this sense, changes in supramolecular
environment as well as the conformational changes
accompanied with intermolecular interactions are of spe-
cial importance in terms of conformational stabilities of
the conformers. In this sense, there are two remarkable
intermolecular hydrogen bonds between hydroxyl groups
and azobridges of major pedal conformers. Intermolecular
hydrogen bonding geometries and details of C–HÁÁÁp
interactions are given in Table 3. According to graph-set
notation [35], the major pedal conformers of the title
compound at Site 1 and Site 2 are linked into C(7) and
C(8) hydrogen-bonded polymeric chains generated by
translation along the [0 1 0] direction as shown in Figs. 2
and 3, respectively. Furthermore, there are two remark-
able C–HÁÁÁp interactions between methyl groups of minor
conformers and 2-methylphenol rings of major conform-
ers at both lattice sites as shown in Fig. 4. It can be
inferred from these results that O–HÁÁÁN type intermo-
lecular hydrogen bonds supply leading contribution to the
stabilization of major conformers, while minor pedal
conformers are stabilized by C–HÁÁÁp type edge-to-face
interactions in solid state.
It is worth noting some conformational discrepancies
between the optimized geometry and experimentally
observed structures. While N = N bond is optimized
˚
˚
geometry is 1.232 A, it ranges from 1.22 to 1.28 A in
pedal conformers at both sites. The optimized geometry
of the compound is exactly planar. The most remarkable
conformational discrepancy is observed in the orientations
of ethyl groups. Although ethyl group in the optimized
geometry lies in the same plane with its parent ring,
C15A and C15B atoms are out of plane with r.m.s.
˚
deviations of 1.32(5) and -0.56(3) A from the related
ring planes. Similarly, C30A and C30B are also out of
˚
plane with r.m.s. deviations of 1.31(2) and 0.73(5) A from
the related ring planes. This result is not surprising
because the most flexible part of the molecule in solid
state is 4-ethylphenyl fragment participating in neither
intra- nor intermolecular interaction according to crystal-
lographic analysis and indicates that orientations of
substituent ethyl groups have a primary influence on the
stabilization levels of the conformers. The stabilities of
pedal conformers at the same lattice site are proportional
to the out-of-plane separation. Keeping in mind that the
planar geometry of the molecule is the most stable con-
formation in gas-phase, it can be stated that O–HÁÁÁN and
C–HÁÁÁp intermolecular interactions besides crystal pack-
ing interactions prescribed by the space group symmetry
operations supply leading contribution to the stabilizations
of the conformers in solid state.
Total energy values for the crystallographically
observed geometries of pedal conformers in comparison
with total energy value of the optimized geometry of the
title compound are listed in Table 4. According to these
results, the most stable pedal conformer of the title
123

676
J Chem Crystallogr (2008) 38:671–677
Fig. 2 C(7) zigzag chain
formed by the major conformer
at Site 1. Minor conformers and
H atoms not involved in the
motif have been omitted for the
sake of clarity
Fig. 3 C(8) zigzag chain
formed by the major conformers
at Site 2. Minor conformers and
H atoms not involved in the
motif have been omitted for the
sake of clarity
Table 4 Total energy values of the crystallographically observed
pedal conformers of the title compound in comparison with the most
stable in vacuo optimized geometry
Conformer
Site
Energya
Conformer
Site
Energya
Minor
Minor
a
1
2
0.194
0.149
Major
Major
1
2
0.127
0.102
Energy values related to the pedal conformers are calculated in
atomic units (a.u.)
Supplementary Material
Crystallographic data (excluding structure factors) for the
structure reported in this article have been deposited with
the Cambridge Crystallographic Data Centre as supple-
mentary publication number CCDC 631822. Copies of the
data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax:
+44-1223-336033; e-mail: data_request@ccdc.cam.ac.uk).
Fig. 4 C–HÁÁÁp type interactions involving major and minor pedal
conformer in the unit cell. Only H atoms involved in the interactions
(H1E and H22F) have been plotted for the sake of clarity
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J Chem Crystallogr (2008) 38:671–677
677
¨
_
Acknowledgment Hasan Karabıyık would like to thank TUBITAK
(The Scientific and Technical Research Council of Turkey) for partial
financial support.
19. Sheldrick GM (2008) Acta Cryst A 64:112–122
20. Stephens PJ, Devlin FJ, Chabalowski CF, Frisch MJ (1994)
J Phys Chem 98:11623–11627
21. Becke AD (1988) Phys Rev A 38:3098–3100
22. Lee C, Yang W, Parr RG (1988) Phys Rev B 37:785–789
23. Hariharan PC, Pople JA (1973) Theor Chim Acta 28:213–222
24. Francl MM, Petro WJ, Hehre WJ, Binkley JS, Gordon MS, De-
Frees DJ, Pople JA (1982) J Chem Phys 77:3654–3665
25. Rassolov V, Pople JA, Ratner M, Windus TL (1998) J Chem Phys
109:1223–1229
26. Hypercube (2003). HyperChem, Version 7.52. Hypercube Inc.,
1115 NW 4th Street, Gainsville, FL 32601-4256, USA
27. Fletcher P (1990) Practical methods of optimization. Wiley, New
York. pp 84–86
References
1. Paul IC, Curtin DY (1973) Acc Chem Res 6:217–225
2. Gavezzotti A, Simonetta M (1982) Chem Rev 82:1–13
3. Harris KDM, Aliev AE (1995) Chem Br 31:132–136
4. Bu¨rgi HB (2002) Faraday Discuss 122:41–63
5. Finder CJ, Newton MG, Allinger NL (1974) Acta Cryst
B30:411–415
6. Bernstein J (1975) Acta Cryst B31:1268–1271
7. Hoekstra A, Meertens P, Vos A (1975) Acta Cryst B31:2813–2817
8. Adams H, Allen RWK., Chin J, O’Sullivan B, Styring P, Sutton
LR (2004) Acta Cryst E60:o289–o290
9. Bouwstra JA, Schouten A, Kroon J (1984) Acta Cryst C40:428–431
10. Brown CJ (1966) Acta Cryst 21:146–152
28. Spek AL (2003) J Appl Cryst 36:7–13
˘
¨ ¨
29. Albayrak C¸ , Odabas¸oglu M, Bu¨yu¨kgu¨ngor O, Lonnecke P (2004)
Acta Cryst C60:o318–o320
_
˘
30. Ocak-Iskeleli N, Karabıyık H, Albayrak C¸ , Petek H, Agar E
(2006) J Chem Cryst 36:709–714
_
˘
31. Ocak-Iskeleli N, Karabıyık H, Albayrak C¸ , Petek H, Agar E
(2006) Struct Chem 17:393–399
11. Harada J, Ogawa K, Tomoda S (1997) Acta Cryst B53:662–672
12. Harada J, Uekusa H, Ohashi Y (1999) J Am Chem Soc 121:5809–
5810
_
˘
32. Karabıyık H., Ocak-Iskeleli N., Albayrak C¸ , Agar E (2007) Struct
Chem 18:87–93
13. Ito Y, Hosomi H, Ohba S (2000) Tetrahedron 56:6833–6844
14. Ohba S, Hosomi H, Ito Y (2001) J Am Chem Soc 123:6349–6352
15. Harada J, Ogawa K (2001) J Am Chem Soc 123:10884–10888
16. Murugan NA, Yashonath S (2004) J Phy Chem B 108:17403
17. Harada J, Ogawa K (2004) J Am Chem Soc 126:3539–3544
18. Stoe & Cie, X-AREA (Version 1.18) and X-RED32 (Version
1.04), Darmstadt, Germany
_
˘
33. Ocak-Iskeleli N, Karabıyık H, Albayrak C¸ , Agar E, Gu¨mru¨k-
_
c¸u¨oglu IE (2008) Struct Chem (in press)
34. Jimenez-Cruz F, Perez-Caballero G, Hernandez-Ortega S, Rubio-
Arroyo M (2000) Acta Cryst C 56:1028–1029
35. Bernstein J, Davis RE, Shimoni L, Chang N-L (1995) Angew
Chem Int Ed Engl 34:1555–1573
˘
´
´
´
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