Microwave spectrum of benzoic acid

Article abstract of DOI:10.1016/S0022-2860(98)00675-9

The microwave spectrum of benzoic acid was observed in the frequency range 30 ~ 40 GHz by using a Stark modulation spectrometer and 6-10 GHz by a molecular beam Fourier-transform microwave spectrometer (MB-FTMW). The rotational and centrifugal distortion

Full text of DOI:10.1016/S0022-2860(98)00675-9

Journal of Molecular Structure 482–483 (1999) 301–303
Microwave spectrum of benzoic acid
*
Masao Onda , Motoo Asai, Katsuyuki Takise, Koichi Kuwae, Koji Hayami,
Akihiko Kuroe, Masayoshi Mori, Hideya Miyazaki, Naoki Suzuki, Ichiro Yamaguchi
Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioicho 7-1, Chiyoda-ku, Tokyo 102, Japan
Abstract
The microwave spectrum of benzoic acid was observed in the frequency range 30 ϳ 40 GHz by using a Stark modulation
spectrometer and 6–10 GHz by a molecular beam Fourier-transform microwave spectrometer (MB-FTMW). The rotational and
centrifugal distortion constants of the normal and carboxylic deuterated species were obtained for the ground and C –COOH
1
torsional state. For the normal species in the ground state the rotational constants were determined by least-squares fitting using
low J transitions observed with FTMW: A ˆ 3872.2741(5) MHz, B ˆ 1227.3128(2) MHz, C ˆ 932.5657(2) MHz. The
2
˚
obtained inertial defect (D ˆ Ϫ 0.3656(2) uA ) indicated that the equilibrium conformation was of the planar structure with
˚
˚
large amplitude torsional motion. r . Coordinates of the hydroxyl hydrogen were |a| ˆ 2.91(1) A, |b| ˆ 1.14(1) A, indicating the
s
hydrogen was located close to the benzene ring. ᭧ 1999 Elsevier Science B.V. All rights reserved.
Keywords: Benzoic acid; rotational spectrum; MB-FTWM
1
. Introduction
This time we observed benzoic acid by microwave
spectroscopy and obtained the molecular constants
and the structural information on this molecule.
The benzoic acid is one of the most important acids
in chemistry. Its acid strength is the standard in the
Hammet rule. The structure of the carboxylic group is
interesting in view of the chemical behavior of the
acid.
The structure of benzoic acid had been investigated
with X-ray diffraction method by several investigators
2
. Experimental
The sample of normal benzoic acid was obtained
commercially and was used without further purifica-
tion. The sample of deuterated benzoic acid,
C H COOD, was prepared by the reaction of benzoic
anhydride with D O [5]. After refluxing the mixture of
the anhydride and D O for 72 h at 100ЊC t 110ЊC
without any catalysis, like SOCl , the deuterated
[1]. Most recent study clearly indicated that the dimer
6
5
unit existed in crystal [2]. The infrared studies on this
molecule in solutions showed the dimer structure [3].
The molecular beam spectroscopy revealed that the
dimer structure was dominant in He beam [4]. There
have been no structure study so far on the isolated
monomeric benzoic acid in the gas phase.
2
2
2
species was obtained with the yield of 80%.
The boiling point of benzoic acid is fairly high
(bp:249.2ЊC at 760 Torr) and it begins to sublime at
around 100ЊC [6]. Therefore it has very low vapor
pressure at room temperature. We started the
measurement of the spectrum with the sample gas
introduced under low sublimation pressure at 100ЊC.
*
Corresponding author. Tel: ϩ 81-3-3988-0223; fax: ϩ 81-3-
3
0
988-4796.
E-mail address: m_onda@hoffman.cc.sophia.ac.jp (M. Onda)
022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00675-9

3
02
M. Onda et al. / Journal of Molecular Structure 482–483 (1999) 301–303
Table 1
Molecular constants of benzoic acid
MB-FTMW
Stark modulation spectrometer
COOH (v ˆ 0)
C
H
6 5
COOH (v ˆ 0)
C
6
H
5
C
6
H
5
COOH (v ˆ 1)
C
6
H
5
COOD (v ˆ 0)
A/MHz
B/MHz
C/MHz
3872.2741(5)
1227.3128(2)
932.5657(2)
3877.94 (3)
1227.390(8)
932.466(8)
Ϫ 0.02 (2)
0.15 (2)
3910 (10)
1227.46(2)
932.74(2)
3841.0(11)
1202.67(8)
915.88(8)
D
D
D
J
/kHz
JK /Khz
/kHz
/kHz
K
17.9 (3)
d
d
J
J
Ϫ 0.051(2)
Ϫ 0.28 (5)
Ϫ 0.091(9)
/kHz
2
˚
DI/uA
Ϫ 0.3659(2)
0.01(9)
The X-band wave guide cell was heated with an usual
electrical ribbon heater surrounding the cell. Any
microwave absorption, however, was not observed
at the cell temperature of 50ЊC ϳ 120ЊC.
microwave propagation. We designed several model
of heated nozzles, modified versions of NIST group
[7]. The sample was heated 90ЊC in the nozzle pocket
and the stagnation pressure was 500 Torr with argon.
After several attempts were made, we could at last
observe the spectrum by adopting the following
heating method. The solid sample was mounted at
one internal end of the wave guide and the wave
guide and the sample handling vacuum system were
set together in a glass jacket and both were uniformly
heated up by heated air flow of above 100ЊC. Then the
temperature of the wave guide was decreased slowly
to 50ЊC so that the sample was deposited on the whole
inner surface of the wave guide. Then the whole
system was held at 50 ^; 1ЊC and sample pressure
was maintained at 0.01 t 0.02 Torr under flow condi-
tion.
The microwave spectrum was observed with a
conventional 100 kHz Stark modulated spectrometer
in the frequency region 8 t 40 GHz. The accuracy of
the measured frequency was 0.1 MHz for intense line
of the normal species and 0.5 MHz or less for the
torsionally excited state and deuterated species. This
poor accuracy came from the broadening of the line
widths of 1 t 3 MHz, presumably because of the
inhomogeneity of the Stark field by deformation of
Stark septum on heating and by the condensation of
solid sample on the septum. The other reason was that
the measured lines overlapped with many vibration-
ally excited lines because benzoic acid had many low
lying vibrational modes.
3
. Results and discussion
3.1. Wave guide spectrum
The spectrum was observed as a bush of lines
throughout the observed frequency region. Many
intense transitions accompanied by several satellites
which could be assigned to the low frequency vibra-
tionally excited state lines. We assigned 60 a-type R-
branch and 30 b-type Q-branch lines of the normal
species in the ground state. The lists of the observed
frequencies have been deposited with the B.L.L.D. as
Supplementary Publication No. SUP26611. The rota-
tional and centrifugal distortion constants were
obtained as shown in Table 1.
A series of clear progression with decreasing inten-
sity was assigned to the torsional state of C -carboxyl
1
group which had the lowest vibrational frequency in
this molecule. The 29 a-type R-branch lines of the first
excited state were assigned and the rotational
constants were determined as shown in Table 1. For
the deuterated species, 32 transitions were assigned
and the rotational constants were determined also as
shown in Table 1.
The small inertial defect indicates the planar
Low-J transitions were observed with MB-FTMW
spectrometer in the 6–10 GHz region. The direction
of the molecular beam was perpendicular to the
structure of this molecule. The r -coordinates of
s
the hydroxyl hydrogen were calculated as follows:
˚
˚
|a| ˆ 2.91(1) A, |b| ˆ 1.14(1) A. The absolute values

M. Onda et al. / Journal of Molecular Structure 482–483 (1999) 301–303
303
Table 2
C –COOH torsional mode, we can also calculate the
1
a
Observed transitions of benzoic acid by MB-FTMW
frequency from their inertial defects. According to the
method by Hanyu et al. [8] the approximate torsional
Transition
OBS (MHz)
O–C (kHz)
Ϫ1
frequency was estimated as 184 cm . The agreement
3
3
3
3
3
4
4
4
4
4
4
4
2
3
3
4
5
(2,1)–2 (2,0)
(0,3)–2 (0,2)
(1,2)–2 (1,1)
(2,2)–2 (2,1)
(1,3)–2 (1,2)
(3,2)–3 (3,1)
(1,4)–3 (1,3)
(2,3)–3 (2,2)
(3,1)–3 (3,0)
(1,3)–3 (1,2)
(2,2)–3 (2,1)
(0,4)–3 (0,3)
(1,2)–1 (0,1)
(1,3)–2 (0,2)
(2,1)–3 (1,2)
(2,2)–4 (1,3)
(2,3)–5 (1,4)
6571.8266
6387.4461
6906.5851
6479.6363
6023.4953
8682.8180
8006.7151
8621.3734
8689.7113
9178.8098
8845.9184
8414.9191
6669.9745
8396.9973
7623.4144
7290.5260
7043.4289
Ϫ 0.3
2.0
3.2
0.8
0.4
between calculated and experimental values is well
and this supports that the assigned vibrationally
excited state is of the C –COOH torsional mode.
1
0.7
3.2. MB-FTMW spectrum
Ϫ 1.6
Ϫ 1.7
Ϫ 2.3
1.3
Ϫ 2.3
Ϫ 1.9
3.1
The rotational constants were determined by least-
squares fitting of 17 low-J transitions observed with
MB-FTMW (Table 2). Any splitting or broadening of
the transition were not observed. The b-type transi-
tions were expected to be splitting as a result of the
COOH torsional motion or inversion of hydroxyl
hydrogen if their barrier height were low. No splitting
means the height may be fairly high. A theoretical
calculation about the torsional angle showed the
height was above 5 kcal/mol by a single point low
level treatment (HF/ 4-31G).
2.4
Ϫ 0.4
Ϫ 0.9
Ϫ 0.7
a
One standard deviation of the fit was 1.9 kHz.
of the coordinate of the hydrogen favor the syn posi-
tion to the carboxylic group.
The average relative intensity of 17 transitions of
the vibrationally excited state to the ground state was
References
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.47 ^ 0.05. The value was calculated simply from
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[
[
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