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Citation
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Infrared Spectra and Molecular Configuration of Benzoic Acid
(Special Issue on Physical Chemistry)
Hayashi, Soichi; Kimura, Noriyuki
Bulletin of the Institute for Chemical Research, Kyoto
University (1966), 44(4): 335-340
1966-10-31
http://hdl.handle.net/2433/76131
Right
Type
Textversion
Departmental Bulletin Paper
publisher
Kyoto University
Infrared
Spectra
and
of
Molecular
Configuration
Chenzoic Acid
Soichi HAYASIIIand Noriyuki KIMURA*
(Gotoh Laboratory)
ReceivedAugust 16, 1966
Infrared spectra of benzoic and deuterobenzoicacids were measured at low temperatures. The characteristic bands of the COOHgroup near 1700,1300and 950 cm.-1 were
observed as pairs. This appears to indicate that two kinds of configurationshaving different energies and different spectra exist in the crystal. The energy difference was estimated to be about 0.1 Kcal./mole from measurements of intensity ratios of pairs at
various temperatures. The nearly equal distances of the two C—Obonds in benzoicacid,
i.e. 1.29 and 1.24A as determined by X-ray measurements, were interpreted as the average
for the mixture of the two configurations.
INTRODUCTION
The crystal and molecular structures of benzoic acid were accurately determined by Sim, Robertson and Goodwin", who reported that the molecules form nearly planar, centrosymmetrical dimers, with hydrogen bonds (2.64A) between adjacent carboxyl groups. They assigned the hydrogen atom of the carboxyl group
to 02, as shown in Figure 1 (A), though the resolution of this hydrogen atom was
not good enough to confirm the assignment.
This lack of resolution may indeed
be connected with the nearly equal distances of the two C-0 bonds, i.e. 1.29 and
1.24 A, and indicate a ready transfer of hydrogen to the other oxygen across the
OzOz
OiOi
ofdi
OzO'z
Fig. 1.
*
(A)(B)
Two kinds of configurations
-ff
(335 )
of benzoic acid.
Soichi HAYASIII and Noriyuki
hydrogen
bond.
If
they
the
two
must
have
in the
KIMURA
configurations
the
crystalline
energies
and
frared
same
show
of
dimers
energy
potential
spectra
of
and
of benzoic
spectrum.
spectra.
benzoic
and
of Figure
However,
field of Ci symmetry,
different
acid
In order
they
when
may
acids
slightly
these
were
isolated,
the dimers
have
to investigate
deuterobenzoic
1 are
matters,
measured
are
different
the in-
at low tem-
peratures.
EXPERIMENTAL
The
benzoic
acid
supplied
The
deuterobenzoic
recrystalization.
crystalization
of these
from
by
the
aceton-deuterium
samples
were
Tokyo
acid,
oxide
obtained
by
Kasei
Co.
C6H5COOD,
solution.
cooling
the
was
further
was
Well oriented
melts
purified
prepared
between
by the
crystal
two
by
re-
layers
optically
flat
plates
of rock-salt.
Infrared
spectra
were
measured
by the Perkin
Elmer 521
spectrometer,
and the Koken DS-301 spectrometer
equipped
with two NaC1 prisms
and
an AgC1 polarizer.
RESULTS
The
infrared
peratures
infrared
1.
spectra
There
of
two
C=0
at room
OH bending,
at room
temperature,
of C—O stretching
respond
to
z
the
of the
at
1706
bonds
of
the dimeric
respectively,
broad
and
units.
are
OH bending,
band
at
about
in the
and
in the
The
region
at
935 cm.-1
region
polarized
or
of another
959
at
and
room
of the
stretching
at 1432 cm.-1
vibrations
1298 cm.-1,
those
at low tem-
one of the
band
coupled
at 1334 and
measured
and
COOD groups
1684 cm.-1
is due to one of the
the two bands
and
and
acids
3, respectively,
COOH
bands
temperature
ing and
and deuterobenzoic
Figures
2 and
in Figure
4.
characteristic
are
vibration
cm.-1
of benzoic
are shown
in
spectra
is shown
Frequencies
AND DISCUSSION
1324 and
1288 cm.-1
coupled
948 cm.-1,
temperature
vibration
which
are
0
rc
Ia
^
3200
2800
2400
Fig. 2.
2000
1800 1600
1400
1200 1000
800
WAVE NUMBER(cm-')
Infrared spectra of benzoic acid at —150°C.
(336)
^
or 1421
of C—O stretch-
600
corin the
a
Infrared
Spectra
and Molecular
Configuration
Ir~
ZI
a
of Benzoic
Acid
Ilill ^ .
ii
H
i
Z LA
V
rr
WI
2400
2200
2000
1800
1600
WAVE
Fig.
3.
Infrared
1400
1200
1000
800
600
NUMBER (cm-1)
spectra
of d-benzoic
acid
at —150°C.
....
_/,,
,,,,,, ,i,_.....,x_v,
111
'11I
a!N^^'i"I
,,,.,,,,,~
I1111
N
11---------------ij
1I1---------------iI~III
u)1r,iI
i----------------II
II
III ---------
cr
jilIIi1
zr--------CC
+
W
IIV
W
1
(I
1600
region
ration.
acids
2.
I
1600
14001200
1100
1000900
WAVE NUMBER(cm-')
Fig. 4. Polarized infrared spectra of benzoic acid at —150°C.
E vector parallel to b axis.
----E vector perpendicular ......
to b axis.
of O—H out-of-plane
The
is given
Causes
Polarized
bending
correspondence
in Table
of the
of the
1, together
vibration2'.
bands
with
between
bands
the
benzoic
assigments
disappear
and
on deute-
deuterobenzoic
of them.
splitting
infrared
spectra
of
Figure
( 337)
I
These
4 measured
at
—150°C show
that
bands
Soichi HAYASHI and Noriyuki
Table 1.
Characteristic
KIMURA
Frequencies of Benzoic and Deuterobenzoic
at —150°C
Acids.
(RCOOH)2(RCOOD)z
Frequency
Configurations and
cm.-1assignments**cm.-'assignments
1706
1684Bv
Frequency
Configurations
A1700
and
A
C=01684Bv
C=0
1674
1432
1334
1298
v C-0
-I-40H1375*v
C-0
1350
B1321*
AOH+vC-01293*
1064
1044
959
948
707due
808v
670
B 6OH930*
A917*
to benzene ring730*
C—C783Av
8 OD
4 OD
A
B
A
a' OH+benzene
725
a OH
o' OH+benzene
718*
710
690
A —C~Oscisseringa654
A
554B/Ca>546B/C
543AC\
B
A
C—C
ring
ring
—C‹0scissering'1
0scissering536
A —C\ 0 scissering5l
* Bands perhaps due to RCOOH•RCOOD
dimers.
** v=stretching ; S=inplane bending ; u=out-of-plane bending
at 1184 and 1174 cm.-1 have high dichroic ratios. This means that the orientation
of samples is good. Hence, the splitting of the bands of low dichroic ratios at
1706 and 1684 cm.-1 must not be due to the "factor group splitting", caused by
the interaction with neighbouring molecules of the crystal. A pair of bands at
1334 and 1298 cm.-1 as well as the doublet at 959 and 948 cm.-1 must not be due
to the "factor group splitting" either. Moreover, it is difficult to consider that
many pairs of the bands are caused by the "Fermi resonance". Another type of
explanation') of the splitting of bands of the carboxylic group is the tunnelling
motion of hydrogen atoms in the O—H groups of the acid dimers from one equiva-
E
R(=Rc21-,=Rc2'ri)
Fig. 5.
Asymmetric
double minimum
(338 )
potencial.
Infrared Spectra and Molecular Configuration of Benzoic Acid
lent position to another.
Although such a phenomenon
would give rise to the
splitting of the Raman-active vO—H energy levels into pairs',
infrared-active
bands
of vC=O, vC-0+(5OH
and oOH would be difficult to give rise to the splitting by
tunnelling effect.
Consequently, we concider that the two configurations of Figure 1(A) and (B)
with different energies and different spectra exist in the crystal.
Figure 5 shows
the potential energy curve of the dimer in the crystal.
The two potential minima
would be different from each other.
The lower and higher potential
respond to the configurations A and B of Figure 1, respectively.
3.
Energy
minima
cor-
difference
If the two configurations exist and the ratio of absorption coefficients of a pair
is constant, the energy difference
(4E0) between configurations A and B can be
obtained by measuring
intensity ratios of the pair at various temperatures
from
6080-
7070
-
GO60
(-
5050
f-
40z
40 -
aa
UJ
0 30a30
0
2020
1010
-
0 o------------------------F—
175017001650
WAVE
NUMBER
(cm-1)WAVE
Fig.
6(a). C=0
17501700
bands at —160°C.Fig.
6(b).
1650
NUMBER
(cm-')
C=0
bands at —80°C.
-0 .51.0
Q
J
I.OO.s
1100
—1
,5•0
O
510
/I xiO3
Fig. 7. Temperature
dependences of band areas of pairs.
—6-0—
A pair at ca . 1300 em''.
—O—Q— A pair at ca . 1700 cm-I.
( 339 )
Soichi HAYASIII and Noriyuki
the
following
equation
KIMURA
:
1n AA
A
) RT'
1nKBeAs/R_LIE(1)
K
A.
where A,, and An are the band areas for absorption assigned to the configurations
A and B respectively, KA and KBthe absorption coefficients of corresponding bands,
and AS and dE the differences of entropy and enthalpy between the configurations
A and B, respectively.
Figure 6 shows the pair of bands due to C=0 stretching
vibration at various temperatures.
From the linearity of ln(A,,/An) against 1/T,
shown in Figure 7, the energy difference is estimated to be about 0.1 Kcal./mole.
4.
Interpretation
for nearly equal C—O distances
The C—O distances of gaseous dimers of formic and acetic acids have been
determined by using the electron diffractionG). The longer and shorter values for
these acids are 1.36 and 1.25A, and those for nicotinic acid are 1.18 and 1.34A,
respectively.
The differences of these distances are larger than those for the
benzoic acid. The nearly equal C—O distances for the benzoic acid, i.e. 1.29 and
1.24A, may be interpreted as averages for the mixture of the two configurations
with a larger difference in C—O distances than that actually observed.
ACKNOWLEDGMENT
The authors wish to thank Professor R. Gotoh for his kind advices and encouragement throughout this work.
REFERENCES
(1)
(2)
(3)
(4)
(5)
(6)
G. A. Sim, J. Monteath Robertson and T. H. Goodwin,Acta Cryst., 8, 157 (1955).
D. Hadji and N. Sheppard, Proc. Roy. Soc., A216, 247 (1953).
S. Braid, D. Hadji and N. Sheppard, Spectrochem.Acta, 8, 249 (1956).
R. Blinc and D. Hadji, "Hydrogen Bonding" Pergamon Press, London, 1959,p. 148.
F. F. Bentley and M. T. Ryan, Spectrochim.Acta, 20, 685 (1964).
"Tables of Interatomic Distances and Configuration in Molecules and Ions," The
Chemical Society, Burlington House, W. I. London, 1958.
( 340)