Matrix-isolation infrared spectra of HOOBr and HOBrO produced upon VUV light irradiation of HBr/O2/Ne system

Article abstract of DOI:10.1016/j.cplett.2010.09.039

Vacuum ultraviolet (VUV) light photolysis of an HBr/O₂ mixture in a Ne matrix has produced HO₂Br isomers (HOOBr and HOBrO), which are important reaction intermediates in atmospheric chemistry. The observed bands have been assigned with an aid of a quantum chemical calculation at CCSD/aug-cc-pVDZ. These assignments have been confirmed by the experimental results using isotopic species of ¹⁸O₂ or DBr. Their characteristic bands are discussed in comparison with those of HOOCl and HOClO from an HCl/O₂ mixture [7]. Both HOOBr and HOBrO are found to be photolyzed with the UV light below 385 nm.

Full text of DOI:10.1016/j.cplett.2010.09.039

Chemical Physics Letters 499 (2010) 117–120
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Matrix-isolation infrared spectra of HOOBr and HOBrO produced upon VUV
light irradiation of HBr/O2/Ne system
⇑
Nobuyuki Akai, Daisuke Wakamatsu, Takeo Yoshinobu, Akio Kawai, Kazuhiko Shibuya
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-H57 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Vacuum ultraviolet (VUV) light photolysis of an HBr/O2 mixture in a Ne matrix has produced HO2Br iso-
mers (HOOBr and HOBrO), which are important reaction intermediates in atmospheric chemistry. The
observed bands have been assigned with an aid of a quantum chemical calculation at CCSD/aug-cc-pVDZ.
These assignments have been confirmed by the experimental results using isotopic species of 18O2 or DBr.
Their characteristic bands are discussed in comparison with those of HOOCl and HOClO from an HCl/O2
mixture [7]. Both HOOBr and HOBrO are found to be photolyzed with the UV light below 385 nm.
Ó 2010 Elsevier B.V. All rights reserved.
Received 12 August 2010
In final form 16 September 2010
Available online 18 September 2010
1. Introduction
bromine cycle to decompose ozone, on which the following reac-
tions have been proposed [18]:
Accurate knowledge of halogen-atom cycles in the atmosphere
is imperative for understanding ozone depletion in the strato-
sphere. Since Molina and Rowland reported that the chlorine atom
produced from chlorofluorocarbons (CFCs) upon UV irradiation
catalytically consumed numerous ozone molecules [1], the chlo-
rine cycle has been investigated by numerous experimental and
theoretical researchers [2,3]. In order to understand the accurate
reaction mechanisms, it is required to know molecular properties
of the related species as well as individual elementary reactions
in great detail. However, it is hard to characterize short-lived spe-
cies produced in the collisions or reactions using experimental data
based on spectroscopy and thermodynamics. A powerful tool to
study such unstable species is the technique of low-temperature
matrix isolation, which has provided infrared (IR) spectra of many
related species such as ClO [4], ClOO radical [5], HOClO [6], HClO2
[6], HOOCl [7], HOOClO2 [8], ClOOCl [9] and ClOx [10].
In contrast, atmospheric bromine chemistry has not been ex-
plored in such detail, though Br atom is suggested to decompose
ozone more effectively than Cl atom [11–13]. Nonetheless, the
spectra of Br-containing free radicals related to atmospheric chem-
istry have been measured by matrix-isolation IR spectra for BrO,
HOBr, BrBrO, BrOO and OBrO [14–17]. Since the BrO radical is sup-
posed to effectively contribute to ozone depletion, the reactions re-
lated to the production and depletion of this radical deserve careful
studies. The reaction between the BrO and OH radicals is consid-
ered to be especially important as an elementary reaction of the
BrO þ OH ! ðHOOBrÞ ! Br þ HO2
BrO þ OH ! ðHOOBrÞ ! HBr þ O2
ð1Þ
ð2Þ
The expected short-lived intermediate, HOOBr (bromous acid),
has never been identified [19–23], and accurate spectroscopic
information is required for systematic detection of the various
intermediates in the atmosphere. Three isomeric structures for
HOOBr have been proposed theoretically [24], as shown in
Figure 1.
In the present study, we have measured the IR spectrum of
HOOBr produced upon the VUV irradiation of HBr/O2 in a low-tem-
perature Ne matrix. Unlike all reported studies based on gaseous
discharge flows [2,3,15], where unstable species like intermediates
of atmospheric reactions are frequently produced, the present
method of VUV photolysis under isolated matrix conditions at
low-temperature provides precise information on such chemi-
cally-unstable species. We have recently reported the IR spectrum
of HOOCl for the first time using the same photolysis/analysis
method [7]. The present letter follows this line to report the IR
spectroscopic detection and characterization of HOOBr and HOBrO
isomers, being supported by quantum chemical calculations.
2. Experimental and calculation methods
Hydrogen bromide (HBr) and deuterium bromide (DBr) were
prepared from hydrobromic acid (Wako) and deuterium hydrobro-
mic acid (Tokyo Kasei), respectively, by passing through a silica-gel
trap to remove water vapor. Oxygen gas (Taiyo Nissan, 99.999%)
and 18O2 (Shoko, 99%) were used without further purification. The
mixed samples of HBr/O2/excess Ne (Spectra Gases, 99.9999%) were
⇑
Corresponding author. Fax: +81 3 5734 2224.
E-mail addresses: shibuya.k.ac@m.titech.ac.jp, kshibuya@chem.titech.ac.jp (K.
Shibuya).
0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.09.039

118
N. Akai et al. / Chemical Physics Letters 499 (2010) 117–120
Figure 1. Three isomers of bromous acid. Optimized geometrical parameters and
relative energies (in kJ molꢁ1) calculated at the CCSD/aug-cc-pVDZ level are shown
in parentheses.
introducedinto a vacuumchamberthrougha stainlesssteelpipe and
deposited on a CsI plate cooled at ꢀ6 K by a closed-cycle helium
refrigerator (Iwatani, Cryomini).
Figure 2. IR spectra of VUV photoproducts from HBr/O2. (a)
A difference IR
spectrum associated with the 240-min VUV irradiation of an HBr/O2/Ne system (the
mixing ratio of 1/1/1000), and (b) a 1:1 composite spectrum of HOOBr and HOBrO
simulated at the CCSD/aug-cc-pVDZ level.
Infrared spectra of the matrix isolated samples were measured
witha Fouriertransform(FT)IR spectrophotometer(Jeol, SPX200ST).
The spectral resolution was 0.5 cmꢁ1, and the accumulation number
was 100. Two xenon lamps (Ushio, UER-20 and Asahi spectra, Max-
302UV) were employed as VUV and UV irradiation sources, respec-
tively. Some short-wavelength cut filters (Asahi spectra) were used
to select an irradiating UV region. Other experimental details were
reported in Refs. [25,26].
Quantum chemical calculations were performed using GAUSSIAN
03 program [27] on TSUBAME, a supercomputer at Tokyo Institute
of Technology. The CCSD/aug-cc-pVDZ level calculation was per-
formed, when high-level quantum chemical calculations were re-
quired to optimize the geometries of HOOH-type molecules and
estimate their vibrational frequencies.
BrOBr, BrOOBr, and HOOBrO etc.), which are possibly yielded from
(HBr)2–O2 and HBr–(O2)2 in a Ne matrix cage, do not correspond to
the observed spectral bands. In addition, the IR spectrum of the
matrix-isolated sample prepared under the experimental mixing
ratio of HBr/O2/Ne = 1/1/1000 suggests that monomers and
small-sized complexes like HBr–O2 and (HBr)2 are dominant chem-
ical species in the matrix cage.
A group of two bands (here denoted as a) at 819.8 and
1356.9 cmꢁ1 exhibit a common time behavior upon UV irradiation,
and another group of bands at 823.4, 825.1 and 1102.3 cmꢁ1 (de-
noted as b) also exhibit a common but different behavior. This sug-
gests that the bands a and b should be assigned to different product
species. In the 700–1600 cmꢁ1 region, the CCSD/aug-cc-pVDZ
level calculation predicts that HOOBr has intense bands at 867.9
and 1414.7 cmꢁ1, which are close to the bands a observed at 819.8
and 1356.9 cmꢁ1, respectively. The bands calculated at 805.5 and
1164.7 cmꢁ1for HOBrO seem to correspond to the bands b observed
at 825.1 and 1102.3 cmꢁ1, respectively. The 825.1 cmꢁ1 band in b
accompanied by a side band at 823.4 cmꢁ1 with a similar intensity
is assigned to the Br–O stretching mode of HOBrO. Table 1 summa-
rizes the observed and calculated wavenumbers for the VUV
photoproducts from HBr/16O2. Isotope-labeled experiments have
confirmed these assignments, as discussed below.
3. Results and discussion
3.1. VUV photolysis products from HBr/16O2
Figure 2a shows a difference IR spectrum associated with the
240-min VUV irradiation (k = 155–195 nm, kmax = 172 nm) of
HBr/O2/Ne, with a mixing ratio of 1/1/1000, where bands increasing
with the VUV irradiation time were assigned to photoproducts.
According to Refs. [7,14–17,25,28], most of the increasing bands in
Figure 2a were assignable to known species; O3 (1038.6 and
1039.8 cmꢁ1), HOO (1098.0, 1391.6 and 1394.7 cmꢁ1), BrOO (split-
ting bandsat1476.0, 1478.9, 1481.1, 1484.2and 1487.1 cmꢁ1), HOBr
(1163.1 and 1164.0 cmꢁ1) and OBrO (846.3 and 848.5 cmꢁ1), though
we could not detect a band around 730 cmꢁ1 due to BrO. Then we
focused our attention on the other unidentified increasing bands:
three pairs of doublet bands at (819.8–821.5), (823.4–825.1) and
(1102.3–1103.3) cmꢁ1, and a multiplet of bands at (1354.4, 1356.4,
3.2. VUV photolysis products from DBr/16O2 and HBr/18O2
Figure 3 shows the difference spectra associated with the VUV
photolysis of (a) DBr/16O2, (b) HBr/16O2, and (c) HBr/18O2. In Fig-
ure 3a, some VUV photoproducts from DBr/16O2 are identified as
O3, DOBr, DOO, BrOO, and OBrO by comparison with previous re-
ports [14–17], while several new bands have been observed at
819.8, 820.8, 826.8, 828.2 and 1012.2 cmꢁ1. The VUV photoprod-
ucts of HBr/18O2 shown in Figure 3c are identified as 18O3,
H18OBr, Br18O18O, 18OBr18O, and H18O18O, while new bands at
775.7, 784.9, 786.8, 1099.0 and 1350.6 cmꢁ1 are assigned to either
HOOBr or HOBrO species, as discussed below.
1356.9, 1357.6, 1358.6) cmꢁ1
.
On the analogy of the VUV photolysis system of HCl/O2/Ne [7],
we expected the formation of HOOBr and HOBrO isomers in the
present HBr/O2/Ne system. Their optimized structures and relative
energies are illustrated in Figure 1, and their simulated IR bands
are shown in Figure 2b. The calculated IR bands seem to be located
around the unassigned bands observed in Figure 2a. The simulated
IR spectra of other candidate photoproducts (HOOOBr, HOOOOBr,
Table 2 lists the experimental and theoretical wavenumbers for
HOOBr and HOBrO, together with their isotope-labeled species.

N. Akai et al. / Chemical Physics Letters 499 (2010) 117–120
119
Table 1
Observed vibrational wavenumbers (in cmꢁ1) of VUV photoproducts from HBr/O2 in a Ne matrix and their assignments.
Observed
Calculatedb
HOOBr
Assignment
This studya
Reported
HOBrOc
316.4
385.4
616.4
867.9
245.8 (245.3)
359.0 (359.0)
589.4 (588.1)
OOBr/OBrO bend.
wag.
O–Br str.
O–O str.
819.8/821.5
823.4
825.1
846.3
848.5
(803.8)
805.5
81Br–O str.
79Br–O str.
O81BrO
O79BrO
O3
843.0d
845.3d
1038.6/1039.8
1098.0
1038.6/1039.8e
1101.3f
HOO
1164.7 (1164.7)
3767.8 (3767.8)
HOBr bend.
1102.3/1103.3
1163.1/1164.0
1164g
HOBr
1414.7
3740.9
HOO bend.
1354.4/1356.4/1356.9/1357.6/1358.6
1374.7
Unidentified
HOO
1391.5e
1485.1d
1391.6/1394.7
BrOO
1476.0/1478.9/1481.1/1484.2/1487.1
O–H str.
a
b
c
d
e
f
Underlined wavenumbers are the strongest bands among the blended ones.
Calculated at the CCSD/aug-cc-pVDZ level.
Calculated wavenumbers for HO79BrO and (HO81BrO).
In Ar matrix, Ref. [15].
In Ne matrix, Ref. [7].
In Ar matrix, Ref. [28].
g
In Ar matrix, Ref. [16].
the isotope shifts
49.6 cmꢁ1 for H18O18OBr, which seem to be consistent with the
820.8 cmꢁ1 band ( = +1.0 cmꢁ1) in DBr/16O2 and the 775.7 cmꢁ1
band (
= 44.1 cmꢁ1) in Figure 3c. Opposite red- and blue-isotope
D
are estimated to be 1.0 cmꢁ1 for DOOBr and
D
D
shifts are recognized in the observation and calculation of the O–O
stretching mode for HOOBr and DOOBr, and a similar problem has
also been discussed in the wavenumbers of HOOCl and DOOCl cal-
culated at the CCSD(T)/cc-aug-pVDZ level [7]. This discordance is
probably originated from theoretical difficulty that the vibrational
wavenumbers of HOOH-type molecules drastically depend on the
levels of theory from which the optimized structures are derived
[29–31].
The 1356.9 cmꢁ1 band shown in Figures 2 and 3b is assigned to
the HOO bending mode of HOOBr. The calculated wavenumber
shifts from the normal species are 370.4 and 7.1 cmꢁ1 for DOOBr
and H18O18OBr, respectively, which agree with the experimental
values of 344.7 cmꢁ1 for DBr/16O2 and 6.3 cmꢁ1 for HBr/18O2 as
shown in Table 2.
The bands observed at 823.4 and 825.1 cmꢁ1 are assigned to the
81Br–16O and 79Br–16O stretching modes of HO81BrO and HO79BrO,
respectively. The isotope shifts for these bands are calculated to be
1.0 cmꢁ1 for DO79BrO (0.9 cmꢁ1 for DO81BrO) and 38.1 cmꢁ1 for
H18O79Br18O. The corresponding experimental values are
+3.1 cmꢁ1 for DO79BrO and 38.3 cmꢁ1 for H18O79Br18O.
Figure 3. Comparisons of IR spectra recorded with the VUV photolysis of three
isotopomer systems: (a) DBr/16O2/Ne, (b) HBr/16O2/Ne and (c) HBr/18O2/Ne.
The 819.8 cmꢁ1 band shown in Figures 2 and 3b is assigned to the
O–O stretching mode of HOOBr. According to the CCSD calculation,
Table 2
Experimental and theoretical wavenumbers (in cmꢁ1) of HOOBr and HOBrO, together with their isotope-labeled species.
Vibrational modes
HBr/16O2
Observed
HBr/18O2
DBr/16O2
Calculated
Observed (shift)a
Calculated (shift)a
Observed (shift)a
Calculated (shift)a
HOOBr
O–O stretch
HOO bend
819.8
1356.9
867.9
1414.7
775.7 (ꢁ44.1)
1350.6 (ꢁ6.3)
818.4 (ꢁ49.6)
1407.6 (ꢁ7.1)
820.8 (1.0)
1012.2 (ꢁ344.7)
867.0 (ꢁ1.0)
1044.3 (ꢁ370.4)
HOBrO
81Br–O stretch
79Br–O stretch
HOBr bend
823.4
825.1
1102.3
803.8
805.5
1164.7
784.9 (ꢁ38.5)
786.8 (ꢁ38.3)
1099.0 (ꢁ3.3)
765.6 (ꢁ38.2)
767.3 (ꢁ38.1)
1161.1 (ꢁ3.7)
826.8 (3.4)
828.2 (3.1)
819.8 (ꢁ282.5)
802.9 (ꢁ0.9)
804.5 (ꢁ1.0)
852.2 (ꢁ312.5)
a
Isotope shift from the corresponding HBr/16O2 species.

120
N. Akai et al. / Chemical Physics Letters 499 (2010) 117–120
The 1102.3 cmꢁ1 band is assigned to the HOBr bending mode of
HOBrO. The calculated isotope shifts, 312.5 cmꢁ1 for DOBrO and
3.7 cmꢁ1 for H18OBr18O, are consistent with the experimental val-
ues of 282.5 and 3.3 cmꢁ1, respectively.
In the DBr/O2 experiment, the following three bands appear in a
narrow region between 820 and 830 cmꢁ1: i.e., the O–O stretching
(DOOBr), Br–O stretching (DOBrO), and DOBr bending (DOBrO)
modes. However, they are distinguishable because of their unique
band shapes: doublet splitting due to the 79Br and 81Br isotopes,
relative intensities, and widths. The above-mentioned isotope-la-
beled experiments strongly support that the chemical species of
HOOBr and HOBrO are really generated in the VUV photolysis of
HBr/O2 in a Ne matrix and that the vibrational assignments are
reasonable.
Figure 4. Depletion spectra of HOOBr and HOBrO upon (a) the 385 nm light
irradiation for 180 min and (b) the subsequent 350 nm light irradiation for 180 min.
Bands marked with asterisks are unidentified.
3.3. Photoreactivity of HOOBr and HOBrO
The details of the production mechanisms of HOOBr and HOBrO
from HBr/O2 upon the VUV irradiation in a Ne matrix will not be
discussed here. Briefly, both reactants of HBr and O2 can photo-dis-
sociate to prepare H, Br and two O atoms in a Ne matrix cage
[32,33], and then these atoms are combined possibly leading to
formation of HOOBr and HOBrO. The less stable HBrO2 isomer
shown in Figure 1 has not been observed in the present study. As
in the case of the HCl/O2 system, the primary products of the pres-
ent HBr/O2 system are four-atomic molecules of HOOBr and
HOBrO. The produced atoms are not likely to be sufficiently ener-
getic to escape from the matrix cage. Triatomic products such as
HOO, BrOO, HOBr and OBrO seem to be secondary products from
HOOBr and HOBrO. Judging from the geometrical structures
(Figure 1), HOO and BrOO are probably formed from HOOBr, while
HOBr and OBrO are produced possibly from HOBrO. In accordance
with the report in Ref. [17], the photo-isomerization of OBrO to
BrOO has been observed under the irradiation of visible light
(k P 400 nm); The OBrO bands at 846.3 and 848.5 cmꢁ1 decreased
in intensity, while the BrOO multiple bands around 1480 cmꢁ1
increased.
In the HCl/O2 system, the photolysis thresholds of HOOCl and
HOClO are found to exceed 365 and 405 nm, respectively, which
has enabled individual observation of their spectra by wave-
length-selected UV irradiation [7]. As for the present HOOBr and
HOBrO cases, we have not selectively photolyzed each species,
but we have differentiated the decomposition rates of two species
by controlling the irradiation UV wavelength. By irradiating the
products with 385 nm light, the intensities of the 1102.3, 825.1
and 823.4 cmꢁ1 bands assigned to HOBrO decreased more rapidly
than the 1356.9 and 819.8 cmꢁ1 bands assigned to HOOBr, as
shown in Figure 4a. The band intensities of HOBrO were mostly
diminished after the irradiation at 385 nm. Subsequent irradiation
at 350 nm following the 385 nm illumination depleted HOOBr
dominantly, as shown in Figure 4b.
BrO and OH radicals cannot be detected in a matrix cage if they are
yielded from HOOBr upon UV irradiation.
Acknowledgment
This work was financially supported in part by Grant-in-Aid for
Young Scientists (No. 22750010) and for Scientific Research on Pri-
ority Area (No. 22018005) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
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Although triatomic species of HOO, BrOO, HOBr and OBrO were
expected to be the photoproducts of VUV as well as UV photolyses
of HOOBr and HOBrO, their IR bands were not detectable upon the
UV irradiation. Nor was isomerization between HOOBr and HOBrO
identified. Though the energy of 350 nm radiation is high enough
to photolyze both HOOBr and HOBrO, it would not be sufficient
to remove fragment atoms out of the matrix cage. When UV light
yields two fragments such as HOO and Br from HOOBr or HOBr
and O produced from HOBrO, they would immediately be recom-
bined to hold the reactants in the matrix cage. For the same reason,