Infrared spectra of monosubstituted toluene derivatives in cyclodextrin: Orientation of guest molecules in included complexes

Article abstract of DOI:10.1016/j.molstruc.2009.04.002

Infrared spectroscopy was applied to determine the orientations of guest molecules (m- and p-chlorotoluene, m- and p-tolunitrile, and m- and p-toluidine) in cyclodextrin (α-, β-, or γ-cyclodextrin (CD)). Some bands of the guest molecules were found to shi

Full text of DOI:10.1016/j.molstruc.2009.04.002

Journal of Molecular Structure 929 (2009) 43–47
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Infrared spectra of monosubstituted toluene derivatives in cyclodextrin:
Orientation of guest molecules in included complexes
Akemi Nagao a, Akira Kan-no b, Masao Takayanagi c,
*
a United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo 183-8509, Japan
b Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo 183-8509, Japan
c Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo 183-8509, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Infrared spectroscopy was applied to determine the orientations of guest molecules (m- and p-chlorotol-
Received 20 November 2008
Received in revised form 2 April 2009
Accepted 2 April 2009
uene, m- and p-tolunitrile, and m- and p-toluidine) in cyclodextrin (a-, b-, or c-cyclodextrin (CD)). Some
bands of the guest molecules were found to shift by several wavenumbers upon formation of inclusion
complexes or upon variation of the diameters of CD. The shifts could be explained by the steric hindrance
imparted on the guest molecules in the CD cavities. Moreover, the bands were thought to be due to the
vibrations of the moieties of guest molecules in the cavity of CD. The bands were assigned with the aid of
quantum chemical calculations, which revealed that chlorotoluene, tolunitrile, and p-toluidine were able
to enter the CD cavity, respectively, from the Cl, CN, and CH3 sides. On the other hand, both the CH3 and
NH2 groups of m-toluidine direct to the outside of the CD ring. The orientation of a guest molecule in a CD
cavity was found to be determined by the charge distribution in the guest molecule; the negatively
charged moiety of a guest molecule is attracted to the inside of CD.
Available online 10 April 2009
Keywords:
Cyclodextrin
Toluene derivatives
Inclusion compounds
Infrared spectra
Intermolecular interaction
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
imental results including vibrational spectra of a number of other
aromatic molecules in the CD. A discussion of the intermolecular
Cyclodextrin (CD) is a cyclic oligosaccharide consisting of six
), seven (b), and eight ( -glucose units connected through
glycosidic -1,4 bonds. CD, as a host, is able to include a number
interactions and the orientation of the guest molecule in the host
cavity are also presented.
Herein, infrared-absorption spectra of monosubstituted tolu-
enes, such as m- and p-chlorotoluene, m- and p-tolunitrile, and
(a
c) a-D
a
of guest compounds that fit into a hydrophobic cavity of 4.5–
8.5 Å in diameter. The inclusion ability can be widely applied in
numerous fields including the food and cosmetic industries, as well
as the pharmaceutical industry. CD also is used in the field of ana-
lytical chemistry, where it is used for molecular recognition and
enantioseparation in chromatographic methods. Understanding
of the intermolecular interactions between the host and guest mol-
ecules is of great help in the development of further applications.
Small shifts can be observed for some bands in Raman and infra-
red-absorption spectra of the guest molecules of various types
upon formation of inclusion complexes. These shifts have been dis-
cussed from the standpoint of intermolecular interactions in the
complexes [1–8]. Although changes in the FT-Raman spectra of
some fundamental aromatic guest molecules such as o-, m-, and
p-chlorostyrene [9] and o-, m-, and p-nitrophenol [10] have been
reported upon complex formation with CD and upon variation of
the host molecule, neither the interactions between the host and
guest molecules nor the orientation of guest molecule in the host
cavity has been discussed. The present study provides new exper-
m- and p-toluidine, included in
a-, b-, or c-CD were measured.
Some of the IR bands were observed to shift upon variation of
the ring size of CD. The moieties of the guest molecules that suffer
from intermolecular interactions were identified by assigning the
shifted bands using quantum chemical calculations. Experimental
results are shown in detail together with a discussion of the orien-
tation of the guest molecules in the inclusion complexes.
2. Experimental
Host compounds,
a-, b-, and c-CD (Wacker), were generously
provided by CycloChem Co., Ltd., Japan, and the guest compounds
(reagent grade) were purchased from Wako Chemicals Inc., Japan.
All reagents were used without further purification. Water distilled
after deionization was used as the solvent.
A typical host–guest experiment was carried out as follows. A
guest compound was added to a saturated aqueous solution of
CD so that the molar concentration of the guest compound in the
solution was half that of CD. The mixture was treated with super-
sonic waves to promote the inclusion. The inclusion complex was
obtained as a precipitate, which was separated by centrifugation
* Corresponding author. Tel./fax: +81 42 367 5614.
E-mail address: masaot@cc.tuat.ac.jp (M. Takayanagi).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.04.002

44
A. Nagao et al. / Journal of Molecular Structure 929 (2009) 43–47
and subsequent filtration. The complex was then dried in the air at
the room temperature prior to spectroscopic measurements. The
amount of obtained precipitate differed greatly among complexes
of different host–guest combinations, presumably due to the dif-
ference in the binding constant. Band shifts in the IR spectra upon
precipitate formation both for the host and guest compounds sup-
port the formation of inclusion complexes.
The band around 1580 cmÀ1 was assigned to the aromatic C@C
stretching mode, and was observed to shift upon variation of the
host. A reduction of the ring size of CD caused a shift of several
wavenumbers towards a higher wavenumber;
a-CD > b-CD > c-
CD. The bands around 1480 and 680 cmÀ1 were assigned to the
CH3-bending and the aromatic CH-bending modes, respectively,
and were found to shift in the same way as the band around
IR absorption spectra of the precipitates in KBr pellets were
measured using an FT-IR spectrophotometer (Shimadzu, FT-
IR8400) equipped with a DTGS detector. Preparation of KBr pellets
was not thought affect the quality of the inclusion complexes; re-
peated preparation of disks and their spectral measurements for a
host–guest combination gave the same spectra. Further, we as-
sumed that the interaction of KBr with sample compounds was
so small that it did not affect the results.
1580 cmÀ1
.
Fig. 2 shows IR absorption spectra of p-chlorotoluene included
in
a
-, b-, and
c
-CD in the 1650–1450 and 900–700 cmÀ1 spectral
regions. The band observed around 805 cmÀ1 assigned to the aro-
matic CH-bending mode was found to shift to a higher wavenum-
ber with a decrease in the cavity size of CD. On the other hand, the
wavenumber of the band observed at 1490 cmÀ1 assigned to the
CH3-bending mode was found to be independent of the cavity size
of CD.
Spectra were recorded in the 4000–400 cmÀ1 spectral region
with the accumulation number of 40. A spectral resolution of
2 cmÀ1 was applied. The interval of spectral data under these con-
ditions is about 0.96 cmÀ1, therefore, we can examine qualitative
The observed wavenumbers of bands attributed to m- and p-
chlorotoluene in CD are summarized in Table 1, where the results
of the DFT calculations for isolated chlorotoluenes and the ob-
served wavenumbers for the liquid film samples are also shown.
Calculated wavenumbers were higher than those observed because
they are not scaled.
band shifts greater than about 1 cmÀ1
.
DFT calculations were performed with the GAUSSIAN03 pack-
age [11] using the 6-31G* basis set. Becke’s three-parameter hybrid
density-functional in combination with the Lee–Yang–Parr correla-
tion functional (B3LYP) was used to optimize the geometrical
structures and estimate the vibrational wavenumbers.
3.2. FT-IR spectra of m- and p-tolunitrile included in
a-, b-, or c-CD
Fig. 3 shows the IR absorption spectra of m-tolunitrile included
in
a-, b-, and c
-CD in the 2300–2100 and 710–670 cmÀ1 spectral
3. Results
regions. The bands observed around 2230 and 685 cmÀ1 were as-
signed to the CN-stretching and the aromatic CH-bending modes,
respectively, and were found to shift to higher wavenumbers upon
In the observed spectra of the inclusion complexes, bands due
to CD were prominent. However, some bands attributed to the
guest molecules could also be observed. Since intense broad bands
due to CD often interfere with the observation of weak bands due
to guest molecules, two spectral regions (1700–1450 and 850–
600 cmÀ1) were selected for a more detailed examination. In this
region, bands due to CD were not as dominant and bands attrib-
uted to the guest molecules were more obvious. Characteristic aro-
matic C@C stretching modes and the CH3-bending modes were
observed in the 1700–1450 cmÀ1 region, and the band due to the
aromatic CH-bending mode was observed in the 850–600 cmÀ1 re-
gion. A band around 2250 cmÀ1 could be observed, which was as-
signed to the CN-stretching of tolunitrile.
a decrease in the ring size of CD;
a-CD > b-CD P
c-CD.
0.6
*
0.4
*
*
* *
0.3
*
(a)
0.4
0.2
0
* *
*
*
(a)
(b)
*
*
*
*
0.2
*
*
(b)
*
0.1
*
(c)
(c)
0
900
850
800
750
700
1650 1600 1550 1500 1450
wavenumber / cm-1
-1
wavenumber / cm
3.1. FT-IR spectra of m- and p-chlorotoluene included in a-, b-, or c-CD
Fig. 2. IR absorption spectra of p-chlorotoluene included in (a) a-, (b) b-, and (c) c-
CD. Bands marked with asterisks are those attributed to CD. Dashed lines help
visualize the bands shift.
Fig. 1 shows IR absorption spectra of m-chlorotoluene included
-, b-, and
-CD in the 1650–1450 and 800–600 cmÀ1 spectral
in
a
c
regions. The spectra shown in Fig. 1 (and in all figures appearing
later) are depicted with shifted baselines, so that they do not over-
lap. Bands marked with asterisks are attributed to CD. The dashed
lines in the figure help visualize the band shifts more clearly. See
tables for quantitative shifts which are hardly seen in figures.
Table 1
Wavenumbers of IR bands of m- and p-chlorotoluene included in
a-, b-, and c-CD.
Aromatic C@C
CH3-bending
Aromatic CH-bending
stretching (cmÀ1
)
(cmÀ1
)
(cmÀ1
)
m-Chlorotoluene
Monomer (calc.)a,b
Liquid filmb
1632
1577
1579
1576
1575
1529
1477
1478
1476
1475
696
682
685
682
676
0.4
*
*
*
*
(a)
0.6
0.4
0.2
0
Included in
Included in b-CD
Included in -CD
a-CD
(a)
(b)
*
*
*
0.3
0.2
0.1
0
*
*
*
c
*
*
*
(b)
(c)
*
*
*
*
p-Chlorotoluene
Monomer (calc.)a,b
Liquid filmb
*
1522
1492
1490
1490
1490
822
806
806
805
804
*
(c)
c
Included in
a-CD
–
c
Included in b-CD
–
1650
1600
1550
1500
1450
800
750
700
650
600
c
Included in
c
-CD
–
wavenumber / cm-1
wavenumber / cm-1
a
Calculated by the DFT method using B3LYP/6-31G*. Not scaled.
Those wavenumbers are shown to confirm the assignments of bands of inclu-
b
Fig. 1. IR absorption spectra of m-chlorotoluene included in (a) a-, (b) b-, and (c) c-
ded complexes.
CD. Bands marked with asterisks are those attributed to CD. Dashed lines help
visualize the band shifts.
c
Not observed.

A. Nagao et al. / Journal of Molecular Structure 929 (2009) 43–47
45
0.15
0.10
0.05
0.00
0.4
0.5
0.4
0.3
0.2
0.1
0
*
*
*
*
*
*
(a)
(b)
(c)
0.43
0.33
0.23
0.13
(a)
(a)
*
0.3
0.2
0.1
0
(a)
(b)
*
*
*
*
*
*
*
(b)
*
(c)
* *
* *
(b)
(c)
*
*
(c)
2300
2250
2200
2150
2100
710
690
670
wavenumber / cm-1
wavenumber / cm
-1
1650 1625 1600
wavenumber / cm
1650
1600
1550
1500
1450
-1
-1
wavenumber / cm
Fig. 3. IR absorption spectra of m-tolunitrile included in (a)
Bands marked with asterisks are those attributed to CD. Dashed lines help visualize
the bands shift.
a
-, (b) b-, and c-CD.
Fig. 5. IR absorption spectra of m-toluidine included in (a)
a
-, (b) b-, and (c)
c
-CD.
Bands marked with asterisks are those attributed to CD. Dashed lines help visualize
the bands shift. The 1650–1600 cmÀ1 spectral region is expanded on the right-hand
side to show a weak band at 1627 cmÀ1, which can be observed on the intense
broad bands of CD.
Fig. 4 shows the IR absorption spectra of p-tolunitrile included
in
a-, b-, and c
-CD in the 2300–2100 and 840–800 cmÀ1 spectral
regions. The bands observed around 2230 and 817 cmÀ1 were as-
signed to the CN-stretching and the aromatic CH-bending modes,
respectively, and were found to shift higher wavenumbers in the
in CD are observed at 1627 cmÀ1 on the intense broad bands of CD.
Neither the band at 1627 cmÀ1 assignable to the NH-bending mode
nor the band at 1497 cmÀ1 assignable to the CH3-bending mode
was found to shift upon variation of the cavity size of CD.
same manner as m-tolunitrile in CD;
a
-CD > b-CD ꢀ
c-CD.
The wavenumbers of bands observed for m- and p-tolunitrile in
CD are summarized in Table 2, where the results of DFT calcula-
tions and liquid-film observations are also shown.
Fig. 6 shows IR absorption spectra of p-toluidine included in
a-,
b-, and
c
-CD in the 1700–1500 cmÀ1 spectral region. In the spectra
on the right-hand side which show the 1650–1600 cmÀ1 spectral
region in expanded scales, weak bands due to p-toluidine included
in CD are observed at 1627 cmÀ1 on the intense broad bands of CD.
No shift upon variation of the ring size of CD was observed for the
band at 1627 cmÀ1 which was assigned to the NH-bending mode,
while the band around 1520 cmÀ1 assigned to the CH3-bending
mode was found to shift to the higher wavenumber with the de-
3.3. FT-IR spectra of m- and p-toluidine included in
a-, b-, or c-CD
Fig. 5 shows IR absorption spectra of m-toluidine included in
a-,
b-, and c
-CD in the 1650–1450 cmÀ1 spectral region. In the spectra
on the right-hand side which show the 1650–1600 cmÀ1 spectral
region in expanded scales, weak bands due to m-toluidine included
crease of the ring size of CD;
a
-CD > b-CD ꢀ
c-CD.
The wavenumbers of bands observed for m- and p-toluidine in
CD are summarized in Table 3, where the results of DFT calcula-
tions and the wavenumbers observed under the liquid-film or in
the KBr-pellet are also shown.
0.1
0.08
0.06
0.04
0.02
0
0.06
0.04
0.02
0
*
(a)
*
*
(a)
*
*
*
4. Discussion
(b)
(c)
(b)
(c)
*
*
*
Three functional groups, chloro, cyano, and amino, are well
known to affect the stereoelectronic properties of a benzene ring
in different ways [12]. In tolunitrile, the cyano group functions to
pull electron density out of the benzene ring due to the mesomeric
effect. This causes the electronic polarization with the negatively
charged cyano group and the positively charged benzene ring.
The electron-withdrawing inductive effect of the chloro group ex-
ceeds the electron-donating mesomeric effect, thus resulting in
electronic polarization with the negatively charged chloro group
and the positively charged aromatic ring. The amino group in tolu-
idine, which is an electron donor to benzene ring through the res-
*
2300 2250 2200 2150 2100
840
820
800
wavenumber /cm-1
wavenumber / cm-1
Fig. 4. IR absorption spectra of p-tolunitrile included in (a) a-, (b) b-, and (c) c-CD.
Bands marked with asterisks are those attributed to CD. Dashed lines help visualize
the bands shift.
Table 2
Wavenumbers of IR bands of m- and p-tolunitrile included in a-, b-, and c-CD.
CN-stretching (cmÀ1
)
Aromatic CH-bending (cmÀ1
)
0.4
0.3
0.2
0.1
0.5
0.4
0.3
0.2
0.1
0
m-Tolunitrile
(a)
(b)
Monomer (calc.)a,b
2348
2228
2234
2230
2227
705
686
686
684
684
(a)
(b)
Liquid filmb
Included in
a-CD
Included in b-CD
Included in c-CD
(c)
p-Tolunitrile
(c)
Monomer (calc.)a,b
2347
2228
2233
2227
2227
836
817
817
815
815
Liquid filmb
Included in
a-CD
1700
1650
1600
1550
1500
1650
1625
1600
-1
wavenumber / cm-1
Included in b-CD
Included in c-CD
wavenumber / cm
Fig. 6. IR absorption spectra of p-toluidine included in (a)
a
-, (b) b-, and (c) c-CD.
a
Calculated by the DFT method using B3LYP/6-31G*. Not scaled.
Bands marked with asterisks are those attributed to CD. Dashed lines help visualize
the band shifts. The 1650–1600 cmÀ1 spectral region is expanded on the right-hand
side to show a weak band at 1627 cmÀ1, which can be observed on the intense
broad bands of CD.
b
Those wavenumbers are shown to confirm the assignments of bands of inclu-
ded complexes.

46
A. Nagao et al. / Journal of Molecular Structure 929 (2009) 43–47
Table 3
chloro and methyl groups in m-chlorotoluene is smaller than that
in p-chlorotoluene, and is expected to be in the CD cavity. The
wavenumber for the band due to the CH3-bending mode, therefore,
depends on the CD cavity size; the larger steric hindrance in a
Wavenumbers of IR bands of m- and p-toluidine included in a-, b-, and c-CD.
NH-bending (cmÀ1
)
CH3-bending (cmÀ1
)
m-Toluidine
Monomer (calc.)a,b
1649
1622
1627
1627
1627
1532
1495
1497
1497
1497
smaller cavity causes the shift of the band from 1475 in
c-CD to
Liquid filmb
1478 cmÀ1 in
a-CD (Table 1). The benzene ring is included in the
Included in
Included in b-CD
Included in c-CD
a-CD
cavity of CD, therefore, the bands due to the aromatic C@C stretch-
ing and aromatic CH-bending modes also shift upon variation of
the cavity size of CD (Table 1).
p-Toluidine
Tolunitrile enters CD from the cyano-group side bearing a
highly negative charge. The cyano group of included tolunitrile is
always in the CD cavity, therefore, its wavenumber depends on
the cavity size. The bands attributed to the CN-stretching mode
of the m- and p-tolunitrile shift, by 7 or 6 cmÀ1, respectively, to-
wards the higher wavenumber upon a decrease in the cavity size
of CD (Table 2). The band due to aromatic CH-bending modes of
m- and p-tolunitrile included in CD also shifts upon variation of
the cavity size of CD (Table 2); the benzene rings of these mole-
cules are included in the CD cavity. No bands due to the methyl
group of m- and p-tolunitrile are observed in the IR spectra, there-
fore, no information on the interaction between the methyl group
of tolunitrile and CD was obtained.
Monomer (calc.)a,b
1696
1627
1627
1627
1627
1571
1517
1521
1517
1517
KBr pelletb
Included in
Included in b-CD
Included in c-CD
a-CD
a
Calculated by the DFT method using B3LYP/6-31G*. Not scaled.
Those wavenumbers are shown to confirm the assignments of bands of inclu-
b
ded complexes.
onance effect, causes the electronic polarization with the positively
charged amino group.
All IR absorption spectra observed in this work could be ex-
plained with these charge distributions in the guest molecules by
assuming that negatively charged moieties of the guest molecules
are preferentially drawn into the cavity of the CD, because the elec-
tron withdrawing OH groups at the exterior of the cyclic CD mole-
cule are expected to cause a cavity with high electric potential
where is preferable for a negatively charged moiety of guest mol-
ecule. Orientations of inclusions are shown schematically in
Fig. 7, where the structures of the host and guest molecules are cal-
culated using the B3LYP/6-31G* method and are shown in a same
scale together with the arrows representing the orientations of
the inclusions.
Chlorotoluene presumably enters from the chloro-group side
into a CD ring, because the chloro group is the most negatively
charged part of the molecule. Therefore, the methyl group of p-
chlorotoluene leaves the outside of the cavity. No shift of the band
due to the methyl group is expected upon variation of the ring size
of CD. On the other hand, the methyl group of m-chlorotoluene in
CD inevitably interacts with CD because the distance between the
The wavenumbers of the observed bands for the NH-bending
of m- and p-toluidine were found to be independent of the cav-
ity size of CD (Figs. 5 and 6). The positively charged amino
group hardly enters the cavity of CD, whereas toluidine enters
the CD from the opposite side of the amino group. In the case
of m-toluidine, the wavenumber of the aromatic CH-bending
mode also was found to be independent of the cavity size of
CD, suggesting that m-toluidine enters the CD only shallowly
and interacts weakly with CD. On the other hand, the wavenum-
ber of the aromatic CH-bending mode of p-toluidine was found
to shift upon variation of the cavity size of CD. The dipole mo-
ment of p-toluidine was calculated to be 1.535 D and to direct
from the methyl group to the amino group; the methyl group
is negatively charged compared with the amino group. There-
fore, p-toluidine is included in the CD from the methyl-group
side. The straight structure of p-toluidine enables deep inclusion
into CD only with the small steric hindrance, which results in
the shift of the aromatic CH-bending band upon variation in
the size of the CD ring.
Fig. 7. Molecular structures calculated using the B3LYP/6-31GÃ method shown together with the arrows representing the orientations of inclusions. ‘‘M” denotes a methyl
group.

A. Nagao et al. / Journal of Molecular Structure 929 (2009) 43–47
47
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Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-
Laham, C.Y. Peng, A. Nanayakkara, M. Challacomb, P.M. W. Gill, B. Johnson,
W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision C.02,
Gaussian, Inc., Wallingford, CT, 2004.
5. Conclusions
A detailed investigation of the band shifts upon formation of the
inclusion complex and upon a change in the cavity size of CD re-
vealed that the charge distribution of monosubstituted toluene
derivatives determines the orientations and depths of inclusion in
the CD cavity. The orientation of a guest molecule in CD is deter-
mined not by the steric effect of the functional group, but mainly
by the electronic character of the functional group. This finding is ex-
pected to be useful in discussing the guest orientation in CD. Future
work will involve measuring Raman spectra of the inclusion com-
plexes to confirm the present conclusions. Only the band shifts of
the guest molecules were examined in the present study, because
they were expected to contain information on the orientation of
the guest in CD. Examination of the observed shifts in the bands
due to the host molecules will also be examined in the future.
Acknowledgements
This work was supported in part by Dr. Keiji Terao of Cyclo-
Chem Co., Ltd., Japan. The authors also appreciate Prof. Yoshiaki
Hamada and Mr. Ei-ichi Masuko of University of the Air, Japan
for their technical assistance and helpful discussions.
References
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