Intramolecular hydrogen-atom tunneling and photoreaction mechanism of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices

Article abstract of DOI:10.1016/j.molstruc.2012.02.018

Infrared spectra of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices were measured with an FTIR spectrometer. Despite that an F-type geometrical isomer, which has an intramolecular hydrogen bond of OH...F, is as stable in the CCl₄ solution at room temperature as a Cl-type geometrical isomer, which has that of OH...Cl, no infrared bands of the F-type were observed in an argon matrix at 20 K. This implied that the F-type changed to the more stable Cl-type by hydrogen-atom tunneling in the argon matrix. To confirm the tunneling isomerization, similar experiments on the deuterated species were performed, resulting in that an OD stretching band of the F-type appeared as well as that of the Cl-type. This finding suggested that a deuterium is inhibited from the tunneling migration. The UV-induced photoreaction pathways of 4-bromo-2-chloro-6-fluorophenol were additionally examined, resulting in that 2-fluoro-4-bromocyclopentadienylidenemethanone was produced from the Cl-type by Wolff rearrangement after dissociation of the H atom in the OH group and the Cl atom. However no cyclohexadienone derivative, which is analogous to a photoproduct of a parent molecule, 2-chloro-6- fluorophenol, was produced upon UV irradiation. It is concluded that the bromine atom located at the 4th position disturbs the intramolecular migration of the Cl atom.

Full text of DOI:10.1016/j.molstruc.2012.02.018

Journal of Molecular Structure 1025 (2012) 69–73
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Intramolecular hydrogen-atom tunneling and photoreaction mechanism
of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices
⇑
Shota Nanbu, Masahiko Sekine, Munetaka Nakata
Graduate School of BASE (Bio-Applications and Systems Engineering), Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 16 February 2012
Infrared spectra of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices were measured
with an FTIR spectrometer. Despite that an F-type geometrical isomer, which has an intramolecular
hydrogen bond of OHÁ Á ÁF, is as stable in the CCl4 solution at room temperature as a Cl-type geometrical
isomer, which has that of OHÁ Á ÁCl, no infrared bands of the F-type were observed in an argon matrix at
20 K. This implied that the F-type changed to the more stable Cl-type by hydrogen-atom tunneling in the
argon matrix. To confirm the tunneling isomerization, similar experiments on the deuterated species
were performed, resulting in that an OAD stretching band of the F-type appeared as well as that of
the Cl-type. This finding suggested that a deuterium is inhibited from the tunneling migration. The
UV-induced photoreaction pathways of 4-bromo-2-chloro-6-fluorophenol were additionally examined,
resulting in that 2-fluoro-4-bromocyclopentadienylidenemethanone was produced from the Cl-type by
Wolff rearrangement after dissociation of the H atom in the OH group and the Cl atom. However no
cyclohexadienone derivative, which is analogous to a photoproduct of a parent molecule, 2-chloro-
6-fluorophenol, was produced upon UV irradiation. It is concluded that the bromine atom located at
the 4th position disturbs the intramolecular migration of the Cl atom.
Keywords:
4-Bromo-2-chloro-6-fluorophenol
Hydrogen-atom tunneling
UV photoreaction
Matrix isolation
DFT calculation
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
The branching ratio of the two photoreaction pathways depends
on the kind of halogen atoms, i.e., 2-bromophenol prefers
Recently we have investigated the stable conformational iso-
mers, intramolecular hydrogen-atom tunneling, and UV-induced
photoreaction mechanisms of a series of 2-halogenophenols in
low-temperature rare-gas matrices [1–8]. It was concluded that
only the syn isomer, which has an intramolecular hydrogen bond
of OHÁ Á ÁX, exists in the low-temperature rare-gas matrices. In
addition, the intermediates and final products yielded from
2-halogenophenols upon UV irradiation were identified with an
aid of density-functional-theory (DFT) calculations. For example,
an infrared (IR) spectrum of 2-hydroxyphenyl radical (HPR) pro-
duced by detachment of the iodine atom from 2-iodophenol was
first observed and assigned [1]. As for 2-chlorophenol [4] and
2-bromophenol [2], two photoreaction pathways were confirmed;
one product is a five-membered ring ketene, 2,4-cyclopentadiene-
1-ylidenemethanone (CPYM), via a ketocarbene by Wolff rear-
rangement immediately after capture of the H atom in the OH
group by a halogen atom X, as shown in Scheme 1, where X = Cl
or Br. The other product is 4-halogeno-2,5-cyclohexadienone
(4X-CHD) yielded by migration of a halogen atom X to the 4th
position and the H atom in the OH group to the 2nd position.
4X-CHD, whereas 2-chlorophenol prefers CPYM [2,4].
On the other hand, 2,6-heterodihalogenophenols have two sta-
ble isomers having an intramolecular hydrogen bond of OHÁ Á ÁX or
OHÁ Á ÁY. The energy difference between the two isomers is so small
that they co-exist in the gas phase and in the CCl4 solution [9].
However, we observed an IR spectrum of only one isomer of
2-chloro-6-fluorophenol in low-temperature matrices [7]. In addi-
tion, the UV-induced photoproducts of only the Cl-type were
obtained by detachment or intramolecular migration of H and Cl
atoms, but no photoproducts of the F-type were observed. To
explain this notable finding, we assumed the tunneling isomeriza-
tion of the H atom in the OH group from the F type, which has an
intramolecular hydrogen bond of OHÁ Á ÁF, to the more stable
Cl-type geometrical isomer, which has an intramolecular hydrogen
bond of OHÁ Á ÁCl. This assumption was confirmed by a similar
experiment on the deuterated species, where two OAD stretching
IR bands for the both isomers were observed even in a low-temper-
ature matrix by inhibition of deuterium tunneling migration.
In the present study, 4-bromo-2-chloro-6-fluorophenol (4Br-
2Cl-6F-Phe) has been chosen as a target molecule. This molecule
is expected to have two stable isomers of the Cl-type and the
F-type like 2-chloro-6-fluorophenol. We examine the influence of
the bromine atom located at the 4th position on tunneling
⇑
Corresponding author. Tel./fax: +81 42 388 7349.
E-mail address: necom816@cc.tuat.ac.jp (M. Nakata).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.02.018

70
S. Nanbu et al. / Journal of Molecular Structure 1025 (2012) 69–73
O
C
Pure Chemical Industries, and the concentration of the solution
HX
O
was 48 mg/mL. The spectral resolution was 4 cmÀ1
.
HX
DFT calculations were carried out with the 6-31++GÃÃ basis set
using the Gaussian03 program [12]. Beck’s three-parameter hybrid
density functional [13], in combination with the Lee–Yang–Parr
correlation functional (B3LYP) [14], was used to optimize the geo-
metrical structures and to obtain relative energies and IR spectral
H
O
OH
X
(CPYM)
(ketocarbene)
X
patterns. A linear scaling formula of 0.9894–1.04 Â 10À5
mcalcd pro-
O
H
posed in our previous study [15] was used to reproduce the ob-
served wavenumbers in the wide range of 600 and 4000 cmÀ1
.
(HPR)
H
X
3. Results and discussion
(4X-CHD)
3.1. IR spectra and conformation of the reactant
Scheme 1.
As shown in Fig. 1, 4Br-2Cl-6F-Phe has two stable isomers. Since
there are neither experimental nor theoretical reports on the en-
ergy difference between the two isomers, to our knowledge, we
measured an IR spectrum of the sample in CCl4 solution at room
temperature before the matrix-isolation experiments.
isomerization by using normal and deuterated isotope species. We
also examine the influence of the bromine atom on photoreaction
pathways and conclude that the intramolecular migration of the Cl
atom to the 4th position to produce a CHD derivative is disturbed
by the bromine atom.
The OAH stretching region of the solution is shown in Fig. 2,
where two strong bands appear around 3574 and 3542 cmÀ1. The
former is assignable to the F-type, while the latter to the Cl-type,
judging from the calculation results obtained by the DFT method,
as described later, and by comparison with the previous studies
on other heterodihalogenophenols in CCl4 solution by IR spectros-
copy [9]. The absorption ratio for the observed two IR bands is
0.85, from which the energy difference between the two isomers
can be approximately estimated to be 0.99 kJ molÀ1, according to
the Boltzmann’s distribution law at 300 K, if the ratio of the absorp-
tion coefficients for the IR bands obtained by the DFT method, 43:34,
is true. We also calculated the energy difference between the iso-
mers by the DFT method, resulting in that the Cl-type is more stable
than the F-type by 0.05 kJ molÀ1 after zero-point-energy (ZPE) cor-
rection. The calculated value is smaller than that of our experimen-
tal result. Though IR spectral patterns calculated by the DFT method
can reproduce the corresponding observed matrix IR spectra satis-
factorily, it is known that the quantum-chemical calculations are
difficult to reproduce the small energy difference between isomers,
depending on the calculation methods and the size of basis sets [16].
Though the population ratio of isomers in the gas phase at room
temperature before deposition may be changed to some extent in
the low-temperature matrix by conformational cooling [17], IR
bands of the two isomers of 4Br-2Cl-6F-Phe could be observed in
2. Experimental and calculation methods
A sample of 4Br-2Cl-6F-Phe was purchased from Wako Pure
Chemical Industries and used after vacuum distillation. The solid
sample was put in a way of a vacuum line, on the surface of which
argon gas (Taiyo Toyo Sanso, 99.9999% purity) was flowed to
vaporize the solid sample. After mixing the sample/argon gas with
pure argon gas, introduced through another vacuum line so as to
get the satisfaction for the isolation, the mixed gas was deposited
on a CsI plate in a vacuum chamber, which was cooled to about
20 K by a closed-cycle helium-refrigeration unit (CTI Cryogenics,
Model SC). A deuterated sample, in which the H atom in the OH
group was replaced with a D atom, was prepared as follows: The
solid sample of 4Br-2Cl-6F-Phe (0.5 g) was solved in heavy water,
D2O (purity > 99.8%, 20 ml); the solution was stirred for 1 week;
the solid sample obtained by vacuum filtration was dried in a des-
iccator for 5 days. The deuteration ratio (D/H) was estimated from
the IR band intensities in an argon matrix to be about 0.19/1. IR
spectra of the matrix samples were measured with an FT-IR spec-
trometer (JEOL, Model JIR-7000). The band resolution was 0.5 cmÀ1
and the number of accumulation was 64. UV radiation from a
superhigh-pressure mercury lamp (Ushio, SX-UI-500HQ) was used
to induce photoreaction, where a water filter was used to remove
thermal reactions and a cut-off optical filter, UV25 (HOYA), to
choose irradiation wavelength (k > 240 nm). Other experimental
details were reported previously [10,11].
3574
3542
0.3
An IR spectrum of 4Br-2Cl-6F-Phe in CCl4 solution was mea-
sured with the same spectrometer. A liquid cell composed of KBr
crystal plates with a space of a thickness of 100
lm was used.
The solvent of CCl4 (purity > 99.8%) was purchased from Wako
H
H
O
O
Cl
F
Cl
F
0.0
Br
Br
Wavenumber / cm-1
Cl-type
F-type
Fig. 2. An observed IR spectrum in the OAH stretching region of 4Br-2Cl-6F-Phe in
Fig. 1. Two stable isomers of 4Br-2Cl-6F-Phe.
CCl4 solution measured at room temperature: The concentration is 48 mg/mL.

S. Nanbu et al. / Journal of Molecular Structure 1025 (2012) 69–73
71
Table 1
0.3
0.0
Observed and calculated wavenumbers and relative intensities for 4Br-2Cl-6F-Phe.
(a) Obsd.
(4Br-2Cl-6F-Phe)
(b) Calcd.
(Cl-type)
(c) Calcd.
(F-type)
Wavenumber / cm-1
Fig. 3. Observed and calculated IR spectra of 4Br-2Cl-6F-Phe: (a) Observed
spectrum measured in an argon matrix at 20 K and spectral patterns of (b) Cl-
type and (c) F-type, calculated at the DFT/B3LYP/6-31++GÃÃ level. A linear scaling
formula of 0.9894–1.04 Â 10À5 mcalc is used [15].
the matrix as well as in the CCl4 solution. However, only one band
was observed at 3556 cmÀ1 in the OAH stretching region between
3400 and 3800 cmÀ1, as shown in Fig. 3. This implies that the less
stable isomer changed to the more stable isomer through the
hydrogen-atom tunneling isomerization around the CAOH bond.
As a result, the population ratio of the two isomers changed to
be less than 0.01, which was calculated at the matrix temperature
of 20 K using the energy difference of 0.99 kJ molÀ1 obtained from
the relative intensity of the IR bands in the CCl4 solution at room
temperature. The barrier to interconversion is estimated to be at
least 21 kJ molÀ1, which is the sum of the rotational barrier of phe-
nol [18], 14 kJ molÀ1, and the stabilizing energy due to hydrogen
bonding of 2-chlorophenol [7], 7 kJ molÀ1. Then, our assumption
is reasonable because the barrier to interconversion is too high
for thermal equilibrium to be approached at 20 K without tunnel-
ing isomerization [17].
aCalculated at DFT/B3LYP/6-31++GÃÃ level. A linear scaling formula of 0.9894–
1.04 Â 10À5mcalc is used.
bLetters represent relative intensities; vs, s, m, and w denote very strong, strong,
medium, and weak, respectively.
cRelative intensities.
dOAD stretching mode of deuterated species in an argon matrix.
To confirm which isomer exists in the matrix, we calculated the
spectral patterns of the Cl-type and the F-type by the DFT method
and compared them with the observed IR spectrum. As shown in
Fig. 3, the spectral patterns of the two isomers in the spectral re-
gion between 800 and 1800 cmÀ1 are similar to each other. How-
ever, the wavenumber of the OAH stretching band for the F-type,
3608 cmÀ1, is different from that of the Cl-type, 3576 cmÀ1. Since
the latter is close to the observed wavenumber, 3556 cmÀ1, we as-
sumed that the species existing in the matrix is only the Cl-type,
which is more stable than the F-type in the CCl4 solution at room
temperature by 0.99 kJ molÀ1. This is consistent with the fact that
only the Cl-type for 2Cl-6F-Phe exists in an argon matrix [7]. The
observed and calculated wavenumbers and relative intensities
are compared in Table 1. Since most of the bands show splitting,
one may claim that both the Cl-type and the F-type co-exist in
the matrix. However, our assignment is credible because only
one OAH stretching band is observed. Then we assume that the
band splitting is due to the matrix effect.
0.04
(a) Obsd.
(4Br-2Cl-6F-Phe)
0.00
(b) Calcd.
(Cl-type)
(c) Calcd.
(F-type)
Wavenumber / cm-1
Fig. 4. Observed and calculated IR spectra of deuterated species in the OAD
stretching region. See the caption of Fig. 3.
To confirm the tunneling isomerization, we carried out a similar
experiment using the deuterated species. In contrast to the result
for the normal species, two IR bands were observed at 2627 and
2659 cmÀ1 in the OAD stretching region, as shown in Fig. 4, imply-
ing that the deuterium tunneling isomerization is inhibited. Since
the calculated wavenumbers for the Cl-type and the F-type are
2632 and 2656 cmÀ1, respectively, the observed bands of 2627
and 2659 cmÀ1 are assignable to the Cl-type and the F-type,
respectively. The intensity of the Cl-type band is much stronger
than that of the F-type band. The ratio of the intensities is esti-
mated to be 0.58, which is inconsistent to some extent with the re-
sult observed in the CCl4 solution shown in Fig. 2, probably because
the gas-phase molecules were frozen gradually with thermal isom-
erization on the cold plate during sample deposition by conforma-
tional cooling. If this is true, the thermal equilibrium temperature
in deposition can be estimated to be 140 K from the population ra-
tio of the IR bands, according to Boltzmann’s distribution law using
the energy difference of 0.99 kJ molÀ1
.
3.2. Photoproducts by UV irradiation
When the matrix sample was exposed to UV light coming from
superhigh-pressure mercury lamp, spectral changes were
a

72
S. Nanbu et al. / Journal of Molecular Structure 1025 (2012) 69–73
l
0.03
C
-
H
0.00
0.07
(a) Obsd. (2Cl-6F-Phe)
O
C
(b) Obsd. (4Br-2Cl-6F-Phe)
O
C
O
0.0
C
F
F
(c) Calcd. (O-Complex)
(d) Calcd. (F-Complex)
(e) Calcd. (Br-Complex)
F
Br
Cl-H
Br
F-Complex
Br
Wavenumber / cm-1
Br-Complex
O-Complex
Fig. 5. Observed spectral changes and calculated IR spectral patterns: (a) Difference
spectrum of 2Cl-6F-Phe between spectra measured after minus before 60-min UV
irradiation (k > 240 nm), (b) difference spectrum of 4Br-2Cl-6F-Phe between spectra
measured after minus before 60-min UV irradiation (k > 240 nm), and calculated
spectral patterns of (c) O-Complex, (d) F-Complex, and (e) Br-Complex between
Fig. 6. Structures of three possible complexes between 4Br-2Cl-6F-Phe and HCl.
istic band due to the C@O stretching mode was observed around
1700 cmÀ1, as shown in Fig. 5b, Therefore we conclude that 4Br-
4Cl-2F-CHD was not produced because the bromine atom located
at the 4th position disturbs recombination of the Cl atom detached
by UV irradiation. A photoreaction mechanism of 4Br-2Cl-6F-Phe is
summarized in Scheme 2.
4Br-2Cl-6F-Phe and HCl. A linear scaling formula of 0.9894–1.04 Â 10À5
mcalc is used
[15]. See Fig. 6 for the structures of the complexes. A band with a symbol of à is due
to CO2 in atmosphere.
observed. Fig. 5b shows a difference spectrum between the spectra
measured before and after 2-min UV irradiation (k > 240 nm). The
decreasing and increasing bands are due to a reactant and a photo-
product, respectively. A weak band appearing at 2858 cmÀ1 is due
to HCl, suggesting that the H atom in the OH group and the Cl atom
were dissociated upon UV irradiation. The strongest band around
2150 cmÀ1 is characteristic for the C@C@O stretching mode of ke-
tene derivatives. Since similar strong bands were observed in the
photolysis of other 2-halogenophenols [1–8], the 2150 cmÀ1 band
is assignable to a five-member-ring ketene, i.e., 4-bromo-2-flu-
oro-CPYM (4Br-2F-CPYM). Neither bands of the F-type before UV
irradiation nor a band of HF after the irradiation were observed,
leading to the conclusion that 4-bromo-2-chloro-CPYM (4Br-2Cl-
CPYM) was not produced by detachment of HF from the F-type
upon UV irradiation.
In the case of 2Cl-6F-Phe, another product, 4Cl-2F-CHD, was ob-
tained upon UV irradiation by migration of the Cl atom to the 4th
position and the H atom in the OH group to the 2nd position
besides 2F-CPYM [7]. The difference spectrum of 2Cl-6F-Phe is re-
drawn in Fig. 5a, where the C@O stretching mode of 4Cl-2F-CHD
appears at 1694 cmÀ1. If a similar photoreaction occurs in the
present sample of 4Br-2Cl-6F-Phe, 4Br-4Cl-2F-CHD could be pro-
duced by migration of the Cl and H atoms. However, no character-
In the production of 4Br-2F-CPYM, the detached HCl band that
appeared at 2858 cmÀ1 was shifted from the argon matrix value
of the isolated monomer at 2888 cmÀ1 [19], implying that HCl
interacts with 4Br-2F-CPYM. There are three possible complexes
between 4Br-2F-CPYM and HCl, i.e., O-complex, where HCl inter-
acts at the part of C@C@O, and F-complex and Br-complex, as
shown in Fig. 6. To determine which complex was produced in
the matrix upon UV irradiation, we calculated the spectral patterns
of the three complexes and compared them with the observed
spectrum in Fig. 5. The spectral patterns of 4Br-2F-CPYM in the
three complexes are nearly equal to one another, implying that
there is little influence of the position on the intermolecular inter-
action. On the other hand, the wavelengths of HCl are totally differ-
ent from one another, being calculated to be 2812, 2786, and
2689 cmÀ1 for the O-complex, the F-complex, and the Br-complex,
respectively, reflecting the strength of intermolecular hydrogen
bonds. Since the wavenumber of the O-complex corresponds to
that of the observed band, we conclude that HCl interacts with
4Br-2F-CPYM at the part of C@C@O. This result is consistent with
those of 2Cl-6F-Phe [7] and 2-chlorophenols [4].
Our assignment can be supported by the analysis of the C@C@O
stretching mode. The calculated wavenumbers of the O-complex,
O
O
C
HCl
HCl
F
F
H
H
O
O
Cl
Br
Br
F
Cl
F
UV
4Br-2F-CPYM
O
O
Br
Cl-type
Br
F
F
4Br-6F-HPR
Tunneling
Isomerization
Br
Cl
Br
Cl
4Br-4Cl-2F-CHD
H
O
F
Cl
Br
F-type
Scheme 2.

S. Nanbu et al. / Journal of Molecular Structure 1025 (2012) 69–73
73
Table 2
band of the Cl-type was observed in the argon matrix at 20 K, sug-
gesting that the less stable F-type changed to the more stable Cl-
type by intramolecular hydrogen-atom tunneling. This was sup-
ported by a similar experiment of the deuterated species, where
two OAD stretching IR bands for the both isomers were observed
by inhibition of the hydrogen-atom tunneling. When the matrix
sample was exposed to UV light (k > 240 nm), 2F-4Br-CPYM was
produced from the Cl-type by Wolff rearrangement after dissocia-
tion of the H atom in the OH group and the Cl atom. The HCl mol-
ecule was found to interact with 2F-4Br-CPYM at the C@C@O part.
The absence of another product, 4Br-4Cl-2F-CHD, which could be
produced by migration of the Cl atom to the 4th position and the
H atom to the 2nd position, is interpreted as that the Br atom lo-
cated at the 4th position disturbs the migration of the Cl atom.
Observed and calculated wavenumbers and relative intensities for 4Br-2F-CPYM-HCl
complexes.
Obsd.
Calcd.a
b
b
b
O-Complex
F-Complex
/cmÀ1
Br-Complex
/cmÀ1
m
/cmÀ1
Int.c
m
/cmÀ1
Int.d
10
m
Int.d
12
m
Int.d
31
2858
2177
2157
2145
1591
1488
1436
1320
1289
1284
1186
1120
1106
889
m
w
w
vs
m
w
w
w
w
2812
2786
2689
2151
100
2153
100
2142
1582
1494
1426
1301
1273
100
13
2
4
3
1576
1490
1432
1303
1273
14
3
5
5
5
1576
1491
1431
1304
1280
14
4
5
4
4
4
m
w
w
w
w
w
w
1174
1095
1054
915
867
770
734
687
660
0
2
2
1
1
0
3
2
0
1174
1090
1058
951
869
776
731
692
660
0
4
2
2
1
0
4
2
0
1175
1100
1057
905
865
770
734
678
659
0
2
1
2
2
0
3
2
0
Acknowledgements
The authors thank Professor Kozo Kuchitsu (BASE, Tokyo Uni-
versity of A & T) for his helpful discussions.
877
788
739
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a
Calculated at DFT/B3LYP/6-31++GÃÃ level. A linear scaling formula of 0.9894–
1.04 Â 10À5 mcalc is used.
b
Structures are defined in Fig. 6.
Letters represent relative intensities; vs, m, and w denote very strong, medium,
c
and weak, respectively.
d
Relative intensities.
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side of the C@C@O stretching band, a few very weak bands ap-
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least stable one is the O-complex, though the relatively small en-
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This finding implies one possibility that the HCl molecule gener-
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the C@C@O part and immediately forms the O-complex before
migration to the distant Br and F atoms in an argon cage.
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