Crystal Structure Theory and Applications, 2012, 1, 121-127 Published Online December 2012 (
Crystal Structure and Solution Structural Dynamic
Feature of 1,8-Dibenzoyl-2,7-Dimethoxynaphthalene
Akiko Okamoto, Shoji Watanabe, Kosuke Nakaema, Noriyuki Yonezawa
Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and Technology, Tokyo, Japan
Received October 26, 2012; revised November 30, 2012; accepted December 8, 2012
The crystal structure and the dynamic feature of molecular structure in solution for 1,8-dibenzoyl-2,7-dimethoxynaph-
thalene are revealed by X-ray crystallographic analysis and VT-NMR measurements. In crystal, the molecule of the title
compound is located on a twofold rotation axis. The two benzoyl groups are situated in an opposite direction. The dihe-
dral angle between the mean planes of the phenyl ring and the naphthalene ring system is 80.25(6)˚. The benzene ring
and carbonyl moiety in each benzoyl group are almost coplanar. The molecular packing is stabilized by weak C–H…O
hydrogen bonds and a π-π stacking interaction between the benzene rings [centroid-centroid and interplanar distances of
3.6383(10) and 3.294 Å, respectively]. In solution, the temperature-dependent rotation behavior of the C–C bond be-
tween the benzene ring and the ketonic carbonyl group has been observed by 1H VT-NMR measurements. Furthermore,
comparison of the C–C bond rotation behavior between the benzene ring and the carbonyl group with 1-ben-
zoyl-2,7-dimethoxynaphthalene has clarified that the C–C bond between the ketonic carbonyl group and the naphtha-
lene ring rotates slower than the 1,8-dibenzoylated homologue.
Keywords: Non-Coplanarly Accumulated Aromatic Rings Molecules; Crystal Structural Features; Bond Rotation
Behavior in Solution; X-Ray Crystallography; VT-NMR Spectroscopy
1. Introduction
Non-coplanarly accumulated aromatic rings molecules,
such as binaphthyl and biphenyl compounds, have at-
tracted significant attention because of their characteris-
tic properties, unique shapes, and various applications
[1-7]. Peri-substituted naphthalenes have also received
much attention as unique structured aromatic core com-
pounds for variety of the functional materials [8-12].
Therefore, the structural analyses have been actively per-
formed [13-15]. Recently, the authors’ group has re-
vealed that diaroylation at 1,8-positions of 2,7-dimeth-
oxynaphthalene smoothly proceeds [16,17]. According to
X-ray crystal structural study, the obtained 1,8-diaroyl-
naphthalene has unique non-coplanar alignment of aro-
matic rings [18]. The curious reversible aroylation be-
havior of the naphthalene derivative [16] and chemo-
specific and regioselective ethereal alkyl-oxygen bond
cleavage reaction of aroylated naphthalenes [19] can be
explained on the basis of the structural features of the
aroylated naphthalenes. Under these circumstances, the
authors have undertaken the structural studies of the
aroylnaphthalne compounds [20-26] with investigation of
the formation and the related reaction behaviors.
In this article, the crystallographical structural charac-
teristics and the dynamic feature in solution structure of a
1,8-diaroylated naphthalene derivative having two me-
thoxy groups at the 2,7-positions are described as the most
simple homologue of monoaroylated derivative.
2. Experimental
All reagents were of commercial quality and were used
as received. Solvents were dried and purified using stan-
dard techniques. Pentoxide-methanesulfonic acid (P2O5-
MsOH) was prepared according to literature [27].
2.1. Measurements
1H NMR spectra were recorded on a JEOL JNM-AL300
spectrometer (300 MHz) and a JEOL ECX400 spec-
trometer (400 MHz). Chemical shifts are expressed in
ppm relative to internal standard of Me4Si (δ 0.00). 13C
NMR spectra were recorded on a JEOL JNM-AL300
spectrometer (75 MHz). Chemical shifts are expressed in
ppm relative to internal standard of CDCl3 (δ 77.0). IR
spectra were recorded on a JASCO FT/IR-4100 spec-
trometer. High-resolution FAB mass spectra were re-
corded on a JEOL MStation (MS700) ion trap mass
spectrometer in positive ion mode.
opyright © 2012 SciRes. CSTA
2.2. Synthetic Procedure of the Title Compound
The title compound was synthesized via direct condensa-
tion mediated by P2O5-MsOH of 2,7-dimethoxynaphtha-
lene with benzoic acid (Scheme 1). To a mixture of
2,7-dimethoxynaphthalene (0.200 mmol, 37.6 mg) and
benzoic acid (0.440 mmol, 174 mg), P2O5-MsOH (0.88
mL) was added by portions at rt. After the reaction mix-
ture was stirred at 60 ̊C for 3 h, it was poured into iced
water (20 mL) and the mixture was extracted with CHCl3
(15 mL × 3). The combined extracts were washed with 2
M NaOH aq and followed by sat NaCl aq. The organic
layers thus obtained were dried over anhydrous sodium
sulfate. The solvent was removed under reduced pressure
to give powdery product. Isolation of the title compound
was carried out by column chromatography [hexane:
AcOEt = 2:1] (1,8-diaroylnaphthalene 63%; 3-mono-
aroylnaphthalene 19%; 1-monoaroylnaphthalene 3%).
Colorless single crystals suitable for X-ray diffraction
were obtained by recrystallization from ethanol.
1,8-benzoyl-2,7-dimethoxynaphthalene: Colorless nee-
dle (EtOH); m.p. = 530 K; IR (KBr): 1665, 1626 cm–1;
1H NMR (400 MHz, CDCl3): 3.68 (6H, s), 7.21 (2H, d,
J = 9.2 Hz), 7.34 (4H, dd, J = 7.6, 7.6 Hz), 7.49 (2H, t, J
= 7.4 Hz), 7.70 (4H, d, J = 7.4 Hz), 7.95 (2H, d, J = 9.2
Hz) ppm; 13C NMR (75 MHz, CDCl3):56.40, 111.24,
121.47, 125.55, 127.95, 129.09, 129.84, 132.03, 132.64,
138.61, 156.28, 196.875 ppm. The above melting point
and spectral data are compatible with the literature [28].
1-benzoy l-2,7-dimetho xynapht halene:Colorless plate
(Hexane + methylenechloride); m.p. = 358.5 - 362 K; IR
(KBr): 1663, 1627 cm–1; 1H NMR δ (400 MHz, CDCl3):
3.71 (3H, s), 3.79 (3H, s), 6.80 (1H, d, J = 2.8 Hz), 7.01
(1H, dd, J = 9.2, 2.8 Hz), 7.17(1H, d, J = 9.2 Hz), 7.43
(2H, t, J = 8.0 Hz), 7.57 (1H, t, J = 8.2 Hz), 7.72 (1H, d,
J = 8.0 Hz), 7.84 - 7.89 (3H, m) ppm; 13C NMR (75
MHz, CDCl3): 55.09, 56.24, 102.02, 110.17, 117.01,
121.67, 124.29, 128.48, 129.44, 129.62, 130.96, 132.98,
133.31, 137.98, 154.93, 158.77, 198.07 ppm; HRMS
(FAB; m-nitrobenzyl alcohol [m-NBA]) m/z: [M + H]+;
Calcd for C19H17O3, 293.3365; found, 293.1185.
2.3. X-Ray Crystallography
For the crystal structure determination, the single-crystal
Scheme 1. Synthetic reaction of the title compound: P2O5-
MsOH mediated direct condensation of 2,7-dimethoxyna-
phthalene with benzoic acid.
of the compound C26H20O4 was used for data collection
on a four-circle Rigaku R-AXIS RAPID diffractometer
(equipped with a two-dimensional area IP detector). The
graphite-mono-chromated Cu Kα radiation (λ = 1.54187
Å) was used for data collection. The lattice parameters
were determined by the least-squares methods on the
basis of all reflections with F2 > 2σ(F2). The data collec-
tion and cell refinement were performed using PROC-
ESS-AUTO [29] software. The data reduction was per-
formed using CrystalStructure [30]. The structures were
solved by direct methods using SIR2004 [31] and refined
by a full-matrix least-squares procedure using the pro-
gram SHELXL97 [32]. All H atoms were found in a dif-
ference map and were subsequently refined as riding a-
toms, with the aromatic C–H = 0.95 Å and methyl C–H =
0.98 Å, and with Uiso(H) = 1.2Ueq(C). Molecular struc-
ture of the compound showing the atomic numbering
scheme is shown in Figure 1. The crystallography details
for the structures determination of the compound are
displayed presented in Table 1.
2.4. Variable Temperature NMR
Variable temperature 1H NMR spectra were recorded on
an FT-NMR operating at 400 MHz. Chemical shift val-
ues were reported in parts per million (ppm) relative to
(CH3)4Si (TMS). The solvent used in all cases was
CDCl3 and CS2 (1:4 v/v) solution. Low temperature spec-
tra were obtained with the use of a JEOL cooling system.
3. Results and Discussion
The title compound was synthesized via direct condensation
Figure 1. Molecular structure of the title compound, with
the atom-labeling scheme and displacement ellipsoids drawn at
the 50% probability level. The symbol “_2” refers to sym-
metry code: x, y, z + 1/2.
Copyright © 2012 SciRes. CSTA
mediated by P2O5—MsOH of 2,7-dimethoxynaphthalene
with benzoic acid (see Experimental section). Table 1
shows the crystallographic data of the title compound.
Table 2 shows selected bond lengths and angles. Table 3
gives selected torsion angles. Figure 1 gives ORTEP re-
presentation of the molecular structure of the title com-
pound, as determined by the structured X-ray analysis
[33]. The molecule of the title compound lies across a
crystallographic 2-fold axis so that the asymmetric unit
contains one half of the molecules. Thus, the two benzoyl
groups are situated in an opposite direction (anti-orien-
tation). The benzoyl groups are twisted away from the
naphthalene moiety, and the dihedral angle is 80.25(6)˚.
The torsion angle between the carbonyl group and the
naphthalene ring is –76.73(16)˚ [C6–C1–C7–O1] and
that between the carbonyl group and the phenyl group is
179.76(13)˚ [O1–C7–C8–C13].
Table 1. Crystallographic data and structure refinement pa-
Empirical formula C26H20O4
Formula weight 396.43 g·mol–1
Crystal shape, colour Needle, colorless
Temperature 93(2) K
Wavelength 1.54187 Å
Crystal sytem Monoclinic
Space group C 2/c
Unit cell dimensions
a = 13.9677 (4) Å
b = 10.2145 (3) Å
c = 14.6966 (4) Å
b = 109.711(2)˚
Volume 1973.95 (10) Å3
Z, calculated density 4, 1.334 Mg·m3
Absorption coefficient 0.72 mm1
F(000) 832
Crystal size 0.50 × 0.10 × 0.10 mm
Theta range for data collection 3.2˚ to 68.1˚
Limiting indices
16 h 16
12 k 12
17 l 17
Reflections collected/unique 17362/1807 [Rint = 0.027]
Completeness to theta = 68.21˚ 99.6%
Max. and min transmission 0.930 and 0.838
Refinement method Full-matrix least-squares on F2
Data/restraints/parameters 1807/0/139
Goodness-of-fit on F2 1.08
Final R indices [I > 2 sigma (I)] R1 = 0.039, wR2 = 0.107
R indices (all data) R1 = 0.047, wR2 = 0.115
Largest diff. peak and hole 0.19 e·Å3 and 0.21 e·Å3
Table 2. Selected bond lengths (Å) and angles (˚).
O1—C7 1.2197 (16)C2—C1—C7 115.70 (13)
C1—C2 1.382 (2) C6—C1—C7 123.36 (13)
C1—C6 1.4264 (17)O1—C7—C8 121.63 (13)
C1—C7 1.5158 (19)O1—C7—C1 118.49 (12)
C7—C8 1.4814 (19)C8—C7—C1 119.88 (12)
C8—C131.3908 (19)C13—C8—C7 122.02 (13)
C8—C9 1.396 (2) C9—C8—C7 118.88 (13)
Table 3. Selected torsion angles (˚).
O1—C7—C8—C13 179.76 (13)
O1—C7—C8—C9 0.4 (2)
C2—C1—C7—O1 97.99 (16)
C6—C1—C7—O1 76.73 (16)
In the crystal structure, the molecular packing of the
title compound is mainly stabilized by van der Waals
interaction. In addition, the packing of the molecule is
stabilized by relatively weak C–H…O hydrogen bonding,
namely, C12–H12…O1i [symmetry code: i) x, –y + 1, z
+ 1/2], C14–H14B…O1ii [symmetry code: ii) –x + 1/2, y
– 1/2, –z + 1/2], and a π-π stacking interaction [centroid-
centroid and interplanar distances of 3.6383 (10) and
3.294 Å, respectively]. In the packing, the molecules are
arranged by C–H…O hydrogen bonding along the c axis
of the unit cell, and by a π-π stacking interaction perpen-
dicular the bc plane of the unit cell (Figures 2 and 3).
The nonbonding distances are listed in Table 4.
The dynamic behavior of the title compound in solu-
tion was estimated by variable temperature NMR (VT-
NMR) measurement performed in chloroform-d and CS2
(1:4, v/v) solution from 293 to 173 K. Figure 4(a) shows
the 1H VT-NMR spectra of the title compound in the
aromatic region. At 293 K, two signals of δ 7.21 and 7.95
ppm are assigned the protons at 3(6)- and 4(5)-positions
of the naphthalene ring, respectively. The signals of δ
7.34, 7.49, and 7.70 ppm are assigned to the protons at
m-positions, p-position, and o-positions of benzoyl
groups. The signals of δ 7.21 and 7.95 ppm (at 293 K)
are scarcely changed from 293 to 173 K. On the other
hand, the figures of the three signals of δ 7.34, 7.49, and
7.70 ppm (at 293 K) are drastically changed. The signal
of δ 7.49 ppm (at 293 K) is broadened from 293 to 233 K
then sharpened from 213 to 173 K. The signals of δ 7.34
and 7.70 ppm (at 293 K) are broadened from 293 to 213
K and each of them splits into two broad signals at 193 K.
The both of the pairs of broad signals are sharpened
again at 173 K.
Copyright © 2012 SciRes. CSTA
O1 H14 C14
i, ii
Figure 2. C–H…O interactions of methoxy group and ben-
zene ring with carbonyl group [Symmetry codes: (i) x, y +
1, z + 1/2; (ii) x + 1/2, y 1/2, z + 1/2].
Figure 3. A partial packing diagram of the title compound,
viewed down the b axis. The dashed lines indicate hydro-
gen bonds (blue dashed lines) and π-π stacking interactions
(green lines).
Table 4. Nonbonding distances and related geometrical pa-
rameters in 1,8-dibenzoyl-2,7-dimethoxynaphthalene (˚).
D—H···A D—H
C12—H12···O1(i) 0.95 2.60 3.4987 (19) 159
C14—H14B···O1(ii) 0.98 2.39 3.344 (2) 164
Symmetry codes: (i) x, y + 1, z + 1/2; (ii) x + 1/2, y 1/2, z + 1/2.
Figure 4. Variable-temperature 1H NMR study in CDCl3/
CS2 (1:4 v/v): (a) the title compound (6.5 - 8.0 ppm); (b)
1-benzoyl-2,7-dimethoxynaphthalene (6.0 - 8.0 ppm).
The sequent signal changes for the protons at o- and
m-positions of the benzoyl group as shown in the VT-
NMR spectra of the title compound suggest that the C–C
Copyright © 2012 SciRes. CSTA
bond rotation between benzene ring and ketonic carbonyl
group is gradually slowed down with decreasing tem-
perature then the magnetically non-equivalent environ-
ment is made on the benzene ring. The rotation barrier of
the C(benzene)—C(carbonyl) bond was calculated as G =
9.63 kcal/mol on the basis of the detailed VT-NMR
measurements (Figure 5) [34].
In a similar manner, VT-NMR measurement of the
homologous compound of 1-benzoyl-2,7-dimethoxyna-
phthalene was carried out (Figure 4(b)). For spectrum at
293 K, the signals of δ 6.80, 7.01, 7.17, and 7.72 ppm are
assigned to the protons at 8-, 6-, 5-, and 3-positions of
the naphthalene ring, respectively. The two signals of δ
7.43 and 7.57 ppm are assigned to the protons at m-posi-
tions and p-position of the benzoyl groups. The signals
from δ 7.84 to 7.89 ppm are overlapped by the two kinds
of signals, the proton at 4-position of the naphthalene
ring and the protons at o-positions of the benzoyl group.
The signals assigned to the protons of the benzoyl group
are broadened in the temperature range from 293 to 173
K. However, no split behavior is observed at 173 K.
The title compound and the homologue have two kinds
of the C–C bonds allowed to rotate, i.e., C(benzene)–
C(carbonyl) and C(naphthalene)–C(carbonyl) bonds. The
steric environment around the neighboring carbonyl groups
in the title compound is highly congested as shown in
Figure 1. Naturally, the rotation behavior of C(naphtha-
lene)–C(carbonyl) bond should be slower than 1-benzoyl-
naphthalene homologue. In the consequence, the tempe-
rature-dependent C(benzene)–C(carbonyl) bond rotation
behavior might be observed. In other words, C(naphtha-
lene)–C(carbonyl) bond in the title compound rotates
slowly in the temperature range that the C(benzene)–
C(carbonyl) bond is allowed to rotate freely.
4. Conclusion
Conclusively, the crystal structural shapes and the dy-
9.08.0 7.06.0
203 K
201 K
200 K
198 K
Figure 5. The temperature-dependence of 1H NMR signals
of the title compound in CDCl3/CS2 (1:4 v/v) from 203 to
198 K (6.0 - 9.0 ppm).
namic feature of solution structure of 1,8-dibenzoyl-2,7-
dimethoxynaphthalene are clarified. In crystal, the aroyl
groups of the compound are perpendicularly attached to
the naphthalene ring core and situated in an opposite di-
rection. The two types of C–H…O interactions of ketonic
carbonyl group with benzene ring and methoxy group
and π-π interactions between benzene rings mainly stabi-
lize the molecular packing. According to VT-NMR study,
the split signals assigned to the o- and m-protons of the
benzene ring are observed from 193 to 173 K. Compari-
son of the dynamic behavior with 1-benzoylnaphthalene
homologue shows that the C–C bond rotation between
ketonic carbonyl group and naphthalene ring in the title
compound is enough slow to detect the rotation of the
C–C bond rotation between benzene ring and ketonic
carbonyl group. The structural information of the title
compound in solid state and solution affords some hith-
erto-unknown aspects in molecular formula and intramo-
lecular rotation properties relationship of these non-cop-
lanarly accumulated aromatic rings molecules.
5. Acknowledgements
This work was partially supported by the Shorai Founda-
tion for Science and Technology (Tokyo, Japan) and the
Ogasawara Foundation for the Promotion of Science and
Engineering (Tokyo, Japan).
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