Structure and infrared spectrum of 2-hydroxyphenyl radical

Article abstract of DOI:10.1016/j.cplett.2004.04.120

Infrared spectrum of 2-hydroxyphenyl, ^·C ₆H₄-OH, produced from 2-iodophenol in a low-temperature argon matrix upon UV irradiation (λ>280 nm) was measured with an FT-IR spectrophotometer. This radical was found to be less stable by 120 kJ mol ^-1 than phenoxyl, C₆H₅-O^·, by a density functional theory calculation. Two final photoproducts were identified as cyclopentadienylidenemethanone and 4-iodo-2,5-cyclohexadienone in analogy with the photoproducts of 2-chloro and 2-bromophenols. A kinetic analysis shows that the latter is produced via 2-hydroxyphenyl by hydrogen migration and iodine recombination while the former is produced by Wolff rearrangement after elimination of hydrogen iodide.

Full text of DOI:10.1016/j.cplett.2004.04.120

Chemical Physics Letters 392 (2004) 259–264
www.elsevier.com/locate/cplett
Structure and infrared spectrum of 2-hydroxyphenyl radical
Mitsuhiro Nagata, Yoshisuke Futami, Nobuyuki Akai, Satoshi Kudoh,
Munetaka Nakata *
Graduate School of BASE (Bio-Applications and Systems Engineering), Tokyo University of Agriculture and Technology,
2
-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
Received 31 March 2004; in final form 7 April 2004
Available online 11 June 2004
Abstract
Å
Infrared spectrum of 2-hydroxyphenyl, C6H4-OH, produced from 2-iodophenol in a low-temperature argon matrix upon UV
ꢀ
1
irradiation (k > 280 nm) was measured with an FT-IR spectrophotometer. This radical was found to be less stable by 120 kJ mol
Å
than phenoxyl, C6H5-O , by a density functional theory calculation. Two final photoproducts were identified as cyclopentadieny-
lidenemethanone and 4-iodo-2,5-cyclohexadienone in analogy with the photoproducts of 2-chloro and 2-bromophenols. A kinetic
analysis shows that the latter is produced via 2-hydroxyphenyl by hydrogen migration and iodine recombination while the former is
produced by Wolff rearrangement after elimination of hydrogen iodide.
ꢀ
2004 Elsevier B.V. All rights reserved.
1
. Introduction
Phenoxyl radical, C6H5-O , is well known as a tran-
In the present Letter, we report an infrared spectrum
of 2-hydroxyphenyl produced from 2-iodophenol in a
low-temperature argon matrix upon UV irradiation,
from which the structure of this radical has been deter-
mined with an aid of a density functional theory (DFT)
calculation. Furthermore, a kinetic analysis for the
growth behavior of 2-hydroxyphenyl and final photo-
products is performed to elucidate the photoreaction
mechanism of 2-iodophenol. The obtained reaction
mechanism is compared with the results of our previous
studies for 2-chloro [7] and 2-bromophenols [8].
Å
sient species in chemical and biological processes such as
combustion of aromatic compounds [1] and protein re-
dox reactions [2]. The molecular structure and chemical
properties of this radical have been studied extensively by
electron-spin resonance, electronic absorption, resonance
Raman, and infrared spectroscopies [3]. For example, the
infrared spectra of this radical including isotopic species
were measured in low-temperature matrices [4].
In contrast, many fewer studies have been made on
Å
2
-hydroxyphenyl radical, C H -OH, which is an iso-
6
4
meric structure of phenoxyl, because 2-hydroxyphenyl is
much less stable than phenoxyl. Kasai and McLeod Jr. [5]
observed the ESR spectrum of an intermediate produced
from 2-iodophenol in an argon matrix and identified it
from the splitting pattern as 2-hydroxyphenyl. Schuler
et al. [6] reported the conversion of hydroxyphenyl to
phenoxyl in aqueous solutions by ESR spectroscopy. No
other spectroscopic studies have been reported to our
knowledge.
2. Experimental and calculation methods
The sample of 2-iodophenol (Aldrich, 98% purity) was
premixed with argon (Nippon Sanso, 99.9999% purity)
after vacuum distillation. The mixing ratio of sample/
rare gas was 1/1000, and the deposition time was about
1 h. The premixed gas was deposited on a CsI plate,
cooled by a closed-cycle helium refrigerator (Iwatani,
Model CW303), at about 10 K. Spectra of the matrix
sample were measured with an FT-IR spectrophotome-
ter (JEOL, Model JIR-WINSPEC50). The spectral res-
olution was 0.5 cm 1, and the number of accumulation
was 64. UV radiation from a superhigh-pressure mercury
ꢀ
*
Corresponding author. Fax: +81-42-388-7349.
E-mail address: necom@cc.tuat.ac.jp (M. Nakata).
0
009-2614/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.04.120

260
M. Nagata et al. / Chemical Physics Letters 392 (2004) 259–264
lamp was used to induce photodissociation, where a UV-
8 cutoff optical filter (HOYA) was used to choose ra-
O
C
2
diation energy (k > 280 nm). Other experimental details
were reported elsewhere [9,10].
The DFT calculations were performed by using the
O
G
6
AUSSIAN 03 program [11] with the basis set of
-31++G**, where Becke’s three-parameter hybrid
H
I
H
O
density functional [12], in combination with the Lee–
Yang–Parr correlation functional (B3LYP) [13], was
used to optimize geometrical structures and to estimate
vibrational wavenumbers of 2-hydroxyphenyl and
phenoxyl radicals.
Irradiation Time / min
Fig. 2. Time dependence of infrared band intensity. Symbols s, h, and
n represent the bands appearing at 1233 (2-hydroxyphenyl), 896
3
. Results and discussion
ꢀ
1
(
cyclopentadienylidenemethanone) and 1257 cm (4-iodo-2,5-cyclo-
hexadienone). Solid lines represent calculated values obtained by least-
squares fitting, where Scheme 3 was assumed for photoreaction
mechanism.
3.1. Infrared spectra of 2-hydroxyphenyl and final prod-
ucts
Infrared spectral changes caused by photodissocia-
amount is found at the irradiation time of 10 min. The
wavenumbers of the bands, which showed the same in-
tensity change, are summarized in Table 1. A reasonable
interpretation is that 2-hydroxyphenyl is produced from
2-iodophenol by elimination of iodine atom upon UV
irradiation in the matrix, as shown in Scheme 1, where
the conformation of the reactant, 2-iodophenol, is as-
sumed to be syn in analogy with 2-chloro [7] and 2-
bromophenols [8] in consideration of the intramolecular
hydrogen bonding. This radical also has two confor-
mations around the C–O bond as drawn in Fig. 3; we
call them syn and anti, where the hydrogen atom posi-
tions in the hydroxyl group of the former and the latter
are near and far from the unpaired electron, respec-
tively. The optimized geometries of syn- and anti-2-hy-
droxyphenyl were obtained by a DFT calculation, which
showed that both conformers are planer. The anti con-
former is estimated by this calculation to be less stable
tion were observed when 2-iodophenol in an argon
matrix was irradiated by UV light through a UV-28
cutoff optical filter. A difference spectrum between the
spectra measured after 10-min and before irradiation is
shown in Fig. 1a, where the decreasing and increasing
bands represent the reactant, 2-iodophenol, and photo-
products, respectively. The bands marked with an open
circle are assignable to an intermediate because their
intensity increased at the beginning of irradiation time
and decreased afterwards. A typical intensity change of
ꢀ
1
the 1233 cm band is shown in Fig. 2. The maximum
than syn by 7.7 kJ mol 1. Their bond lengths and angles
of the optimized geometries are compared in Table 2.
The corresponding bond lengths of anti and syn are
ꢀ
ꢀ
agree within 0.003 A, but the C3–H9 length in anti is
slightly longer. This finding suggests repulsion between
H8 and H9 atoms, which is supported by the fact that
the C2–C3–H9 and C2–O7–H8 angles in anti are larger
than the corresponding values in syn.
The calculated infrared spectral patterns of syn and
anti are compared with the observed spectrum in Fig. 1.
The spectral pattern for syn is found to reproduce the
observed spectrum more satisfactorily than anti. Even if
the less stable anti is produced upon UV irradiation, it is
probably converted immediately to the more stable syn
in a low-temperature matrix by hydrogen tunneling like
hydroquinone derivatives [14–16]. The calculated
wavenumbers and the relative intensities of the two
conformers for 2-hydroxyphenyl are compared with the
observed values in Table 1.
Fig. 1. Infrared spectra of 2-hydroxyphenyl: (a) observed difference
spectrum between spectra measured after 10-min minus before UV
irradiation (k > 280 nm), (b) and (c) calculated spectral patterns of syn
and anti obtained at the DFT/B3LYP/6-31++G** level. A scaling
factor of 0.98 was used. Symbols s, h, and n represent an interme-
diate (2-hydroxyphenyl) and final products (cyclopentadienylidene-
methanone and 4-iodo-2,5-cyclohexadienone), respectively.

M. Nagata et al. / Chemical Physics Letters 392 (2004) 259–264
261
Table 1
Wavenumbers (cm 1) and relative intensities of 2-hydroxyphenyl
8
8
ꢀ
H
H
7O
7O
Observed
Calculated
Intensity Syn
9
9
m
Anti
H
H
2
2
5
a
a
m
Intensity
m
Intensity
3
4
1
6
3
4
1
6
3
739
144
133
125
118
613
557
57
8
3749
3143
3129
3118
3088
1625
1550
1467
1441
1325
1285
1258
1157
1149
1088
1023
958
39
8
5
3
3
3
3
1
1
H
10
H
H
10
H
1
2
12
9
7
3
<1
10
27
27
16
26
20
100
12
18
20
28
5
H
H
1
1
11
<1
50
11
59
33
2
b
b
b
b
syn
anti
1
1
1
1
1
1
448
440
325
284
233
163
80
71
4
1452
1445
1328
1295
1237
1153
Fig. 3. Two possible conformations of 2-hydroxyphenyl with atom
numbering.
5
5
Table 2
69
34
87
90
1
ꢀ
Bond lengths (A) and bond angles (ꢁ) for optimized geometry of
2
-hydroxyphenyl
1
148
a
Parameter
Syn
1.3825
Anti
1.3842
1
1
094
021
8
12
1085
1017
13
12
<1
<1
2
r(C1–C2)
r(C2–C3)
r(C3–C4)
r(C4–C5)
r(C5–C6)
r(C6–C1)
r(C2–O7)
r(O7–H8)
r(C3–H9)
r(C4–H10)
r(C5–H11)
r(C6–H12)
9
9
60
47
<1
<1
1
1.4043
1.3970
1.3994
1.4068
1.3753
1.3687
0.9671
1.0865
1.0859
1.0859
1.0863
1.4058
1.3995
1.3968
1.4081
1.3737
1.3680
0.9665
1.0895
1.0859
1.0859
1.0866
940
899
c
8
96
20
906
811
8
30
21
5
810
798
18
2
809
725
728
100
60
<1
<1
<1
1
726
615
54
<1
<1
<1
3
6
6
5
5
4
3
3
2
20
13
11
08
03
80
59
20
612
512
509
404
4
5
10
100
1
389
322
6
h(C1–C2–C3)
h(C2–C3–C4)
h(C3–C4–C5)
h(C4–C5–C6)
h(C5–C6–C1)
h(C6–C1–C2)
h(C1–C2–O7)
h(C3–C2–O7)
h(C2–O7–H8)
h(C2–C3–H9)
h(C3–C4–H10)
h(C4–C5–H11)
h(C5–C6–H12)
a
116.59
116.38
70
2
119.88
121.33
119.46
116.99
125.75
124.46
118.95
109.12
118.65
119.03
120.65
121.13
120.23
121.12
119.33
117.45
125.49
119.99
123.63
109.71
119.59
119.03
120.64
120.97
216
a
Calculated by DFT/B3LYP/6-31++G**. A scaling factor of 0.98
was used.
b
Disturbed by vibrational rotational lines of H2O in atmosphere.
Overlapped with bands of final products.
c
H
O
H
O
hν
I
I
Atom numbering is given in Fig. 3.
ꢀ
1
Scheme 1.
strong band appearing at 1680 cm in the C@O stretch-
ing region. These bands are similar to those of 4-bromo-
2,5-cyclohexadienone identified as a final product in the
photolysis of 2-bromophenol [8]. Hence, we tentatively
assign these bands to 4-iodo-2,5-cyclohexadienone
(CHD). To confirm this assignment, we tried to perform a
DFT calculation to obtain the optimized geometry and
infrared spectral pattern for CHD but in vain, because no
suitable basis set for iodine atom was available.
There are some photoproducts bands unassigned to
-hydroxyphenyl in the infrared spectrum measured after
UV irradiation. A spectrum measured after the 2-h irra-
diation is compared with those of 2-chloro [7] and
2
2
-bromophenols [8] in Fig. 4. The bands marked with an
open square show the same growth behavior as the strong
band appearing at 2134 cm 1 in the C@C@O stretching
region. These bands correspond with those of cyclo-
pentadienylidenemethanone (CPM) identified as a final
product in the photoreactions of 2-chloro and 2-brom-
ophenols. On the other hand, the bands marked with an
open triangle show the same growth behavior as the
ꢀ
3.2. Mechanism of photoreaction
ꢀ
1
The intensity changes of the 896 cm (CPM) and
1
ꢀ
1257 cm (CHD) bands are compared with that of the

262
M. Nagata et al. / Chemical Physics Letters 392 (2004) 259–264
ꢀ
1
1
233 cm band (2-hydroxyphenyl) in Fig. 2. The 896
ꢀ
1
cm band increases rapidly in the beginning of photo-
reaction, implying that CPM is produced from 2-iodo-
phenol by one-step reaction. On the other hand, the
ꢀ
1
1
initial irradiation time, implying that CHD is produced
257 cm band shows a clear induction period in the
ꢀ
1
by two-step reaction. The intensity of the 1257 cm
1
ꢀ
band increases as that of the 1233 cm band decreases
during prolonged irradiation, while that of the 896 cm 1
band remains constant after 90-min irradiation. There-
fore, we conclude that CHD is produced from 2-iodo-
phenol via 2-hydroxyphenyl, as shown in Scheme 2.
When hydrogen iodide is eliminated by UV irra-
diation, ketocarbene is produced form 2-iodophenol.
Since ketocarbene is reactive, the final product CPM
is produced immediately by Wolff rearrangement.
This reaction pathway is found in the photoreaction
of 2-chloro [7] and 2-bromophenols [8]. On the other
hand, 2-hydroxyphenyl is produced by elimination of
iodine atom from 2-iodophenol. The hydrogen atom
ꢀ
Fig. 4. Comparison of infrared spectra of final products: (a) 2-iodo-,
b) 2-bromo-, (c) 2-chlorophenols. Irradiation time was 2 h. Bands
marked with h and n are assigned to cyclopentadienylidenemetha-
(
none and 4-halogeno-2,5-cyclohexadienone, respectively.
marked with · is due to water.
A
band
O
O
H
H
C
I
I
I
hν
H
O
k1
I
[CPM]
H
hν
O
O
O
k2
hν
I
k3
[HPR]
H I
H
[
CHD]
Scheme 2.
O
C
O
H
H
I
I
hν
H
O
k1
I
H
O
hν
O
O
I
k2
hν
I
k3
H I
H
k4 k '
4
H
O
I
Scheme 3.

M. Nagata et al. / Chemical Physics Letters 392 (2004) 259–264
263
in the hydroxyl group migrates to the unpaired
electron by absorption of the second photon with
electron rearrangement, which may be phenoxyl rad-
ical in an electronically excited state. Then another
final product CHD is produced by recombination
with iodine atom. A similar final product, 4-bromo-
droxyl group and causes elimination of hydrogen chlo-
ride. As a result, 2-hydroxyphenyl and CHD are not
produced upon UV irradiation (see Fig. 4c). On the
other hand, bromine atom migrates to produce CHD
more easily than iodine because of the smaller size of
bromine than iodine; this implies that the rate constant
k3 in the photoreaction of 2-bromophenol is too large
for the radical to survive in the matrix. If this is the case,
then the rate constant k3 for 2-iodophenol in a xenon
matrix should be larger than that in an argon matrix
because the matrix cage made of xenon atoms is larger
than that of argon and favors migration of iodine atom
for recombination. We performed a similar experiment
on 2-iodophenol using xenon gas. No infrared spectrum
of 2-hydroxyphenyl was observable, as expected.
2
,5-cyclohexadienone, was identified in the photore-
action of 2-bromophenol with an aid of a DFT
calculation [8].
To confirm this photoreaction mechanism, we tried to
determine the rate constants of k1, k2, and k3 in Scheme
2. The following equations can be derived by solving the
rate formulae:
ACPM A0k1
eCPM k1 þ k2
À
Á
ꢀðk þk Þt
1
2
¼
1 ꢀ e
;
ð1Þ
ð2Þ
A
HPR
A
0
k
2
À
ꢀ eꢀk3tÁ
ꢀ
ðk1þk2Þt
¼ k
e
;
4. Concluding remarks
e
HPR
3
ꢀ k
2
ꢀ k
1
ꢀ
À
ðk1þk2ÞtÁ
The infrared spectrum of 2-hydroxyphenyl has been
observed in UV photolysis of 2-iodophenol in an argon
matrix. The conformation around the C–O bond of the
radical is determined to be syn by a comparison of the
observed spectrum with the calculated spectral patterns
obtained by DFT calculations. Namely, the hydrogen
atom of the hydroxyl group lies closer to the unpaired
electron. Final photoproducts are assigned to cyclo-
pentadienylidenemethanone and 4-iodo-2,5-cyclohexa-
dienone in analogy with the photoreaction of 2-chloro
and 2-bromophenols. A kinetic analysis has shown that
A
CHD
1
¼
A0k2
1 ꢀ eꢀ
e
CHD
k
1
þ k
2
ꢁ
1
À
Á
ꢀ
ðk1þk2Þt
ꢀk3t
ꢀ k
e
ꢀ e
;
ð3Þ
3
ꢀ k
2
ꢀ k
1
where A and e represent absorbance and absorption
coefficients, respectively, for the 896 (CPM), 1233 (HPR:
-hydroxyphenyl) and 1257 cm (CHD) bands. In the
least-squares fitting, the absorbance of 2-iodophenol at
t ¼ 0, A , was fixed to the experimental value measured
ꢀ
1
2
0
before UV irradiation, while the ratio of eHPR=eCPM was
fixed to the value of 3.0 obtained by the DFT calcula-
tion. Then the five parameters among seven were vari-
able in the least-squares analysis. However, a significant
discrepancy between the observed and calculated values
was found in the decreasing behavior of 2-hydroxy-
phenyl and the increasing behavior of CHD in the
prolonged irradiation time. This finding suggests that a
portion of 2-hydroxyphenyl cannot be converted to
CHD by recombination with iodine atom. Then we
considered diffusion of the iodine atom eliminated from
4-iodo-2,5-cyclohexadienone is produced via 2-hy-
droxyphenyl, where the recombination with iodine atom
is partially disturbed by argon atoms.
Acknowledgements
The authors thank Professor Kozo Kuchitsu (BASE,
Tokyo University A&T) for his helpful discussions. S.K.
thanks the Steel Industry Foundation for the Ad-
vancement of Environmental Protection Technology.
2
3
-iodophenol in the photoreaction mechanism (Scheme
0
), where the rate constants k and k denote the leaving
4
4
and returning rates of iodine atom, respectively. A nu-
merical integral method was used in the least-squares
fitting. The obtained rate constants are 0.011 ± 0.001,
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