Neon matrix-isolation infrared spectrum of HOOCl measured upon the VUV-light irradiation of an HCl/O2 mixture

Article abstract of DOI:10.1016/j.cplett.2009.06.064

Vacuum ultraviolet (VUV) photolysis of an HCl/O₂ mixture has been carried out to identify HOOCl by the Ne matrix-isolation infrared spectroscopy with the aid of quantum chemical calculations. Newly-observed IR bands at 823, 1363 and 3542 cm^-1 are assigned to those of HOOCl on the basis of the isotope shifts with DOOCl and the CCSD(T)/aug-cc-pVDZ calculation. The observed dependence of band intensities on the time of VUV photolysis indicates that HOOCl and HOClO are the photoproducts formed in an early stage. In addition, HOOCl is found to decompose to form HCl in the UV photolysis (λ ≥ 365 nm), which consists with the S₁-S₀ energy separation estimated by the TDDFT method.

Full text of DOI:10.1016/j.cplett.2009.06.064

Chemical Physics Letters 477 (2009) 70–74
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Chemical Physics Letters
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Neon matrix-isolation infrared spectrum of HOOCl measured upon the
VUV-light irradiation of an HCl/O2 mixture
*
Takeo Yoshinobu, Nobuyuki Akai, 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) photolysis of an HCl/O2 mixture has been carried out to identify HOOCl by the
Ne matrix-isolation infrared spectroscopy with the aid of quantum chemical calculations. Newly-
observed IR bands at 823, 1363 and 3542 cmꢀ1 are assigned to those of HOOCl on the basis of the isotope
shifts with DOOCl and the CCSD(T)/aug-cc-pVDZ calculation. The observed dependence of band intensi-
ties on the time of VUV photolysis indicates that HOOCl and HOClO are the photoproducts formed in an
early stage. In addition, HOOCl is found to decompose to form HCl in the UV photolysis ðk P 365 nmÞ,
which consists with the S1–S0 energy separation estimated by the TDDFT method.
Received 24 March 2009
In final form 18 June 2009
Available online 24 June 2009
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
tion, where HOOCl is expected to play an important role as a reac-
tion intermediate. According to Burrows et al. [12], the
Photochemistry of molecules including chloride atom is known
to be important for atmospheric chemistry, especially in relation to
the ozone depletion in the stratosphere. In one of the monumental
studies of Molina and Rowland, they found that the chlorine atom
produced by the UV irradiation of chlorofluorocarbons (CFCs) cat-
alytically consumed ozone molecules [1]. A multitude of theoreti-
cal and experimental studies for ozone depletion have since been
reported [2,3] after their model of the photochemical ozone
destruction. Short-lived species composed of chlorine and oxygen
atoms have been studied extensively to promote better under-
standing of the chlorine cycles in the atmosphere by IR, UV and
fluorescence spectroscopy [4–11]. For example, Johnsson et al.
measured the IR spectrum of HOClO and HClO2 produced by the
addition reaction of hydrogen atom with OClO in Ar matrices and
determined their geometries with the aid of quantum chemical
calculations [6]. The IR spectra of various related species such as
ClOO radical [7], HOOClO2 [8], ClOOCl [9], ClOx [10] have been
measured, and continual efforts have since been made to identify
the postulated reaction intermediates either theoretically or
mechanistically.
atmospheric OH + ClO reaction proceeds through the formation of
the HOOCl intermediate as
OH þ ClO ! ðHOOClÞ ! HOO þ Cl
OH þ ClO ! ðHOOClÞ ! HCl þ O2
ð1Þ
ð2Þ
Since such branching reaction pathways may contribute to under-
standing of the quantitative material balance in the atmospheric
chlorine cycles, several theoretical studies have been made on the
potential diagram of the HO + ClO ? HOOCl reaction [11–17]. How-
ever, no experimental information has yet been reported on the
chemical properties, photo-reactivity and fundamental spectra of
HOOCl, because it has never been identified experimentally.
The chemical species generated by the reaction of H + OClO
have so far been assumed to be HOClO and HClO2. The chemical
transformation from OClO to OOCl seems hardly feasible; in fact,
no experimental data of HOOCl have yet been available in this
reaction system. Hence we have chosen HCl/O2 reactants in the
present study to photochemically generate HOOCl, where the
VUV photolysis of HCl/O2 and DCl/O2 mixtures in a low-tempera-
ture Ne matrix has been carried out to assign the IR spectra of
HOOCl and DOOCl.
Chlorous acid, which is one of the expected intermediates com-
posed of H, Cl and O atoms in the atmosphere, has three isomers
shown in Fig. 1 [11]. Two less stable isomers, HOClO and HClO2,
have been detected in matrix experiments [6], but the most stable
isomer, HOOCl, has never been identified. This situation seems
unusual, because HOOCl is estimated to be more stable than HOClO
by as much as 65.3 kJ molꢀ1 at the CCSD(T)/TZ2P level [12]. Several
reaction models have been proposed for stratospheric ozone deple-
2. Experimental and calculation methods
Hydrogen chloride (HCl) was synthesized from NaCl (Kanto
Chemical) and H2SO4 (Wako Pure Chemical Industries), and DCl
from D2SO4 (Cambridge Isotopes Laboratories) [18]. Oxygen gas
(Taiyo Nissan, 99.999%) was used without further purification.
The samples were diluted with excess Ne gas (Spectra Gases,
99.9999%), and then mixed gases of HCl/O2/Ne or DCl/O2/Ne were
* Corresponding author. Fax: +81 3 5734 2224.
E-mail address: kshibuya@chem.titech.ac.jp (K. Shibuya).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.06.064

T. Yoshinobu et al. / Chemical Physics Letters 477 (2009) 70–74
71
3. Results and discussion
rHO = 0.98 Å
rOO = 1.44 Å
3.1. Products from the VUV photolysis of HCl/O2
Cl
H
rOCl = 1.78 Å
O
Fig. 2a shows a difference IR spectrum associated with the
180 min VUV irradiation (k = 155–195 nm, kmax = 172 nm) of HCl/
O2/Ne = 1/1/2000. Almost all the increasing bands due to the
photoproducts, i.e., those caused by the irradiation, are assigned
to several species of reported IR spectra. For example, the bands
of 969.9, 978.3, 1173.2 and 3556.7 cmꢀ1 are assigned to HOClO
[6], and those of 1238.9 and 3607.8 cmꢀ1 can be assigned to HOCl
[21]. Table 1 summarizes these results, together with the other ob-
served bands. No band to be assigned to HClO2 is detectable in this
spectrum. Three intense bands at 823.1 cmꢀ1 (with a side band at
821.1 cmꢀ1), 1362.5 cmꢀ1 (with multiple side bands at 1359.6,
1361.4, 1361.9 and 1363.6 cmꢀ1) and 3542.4 cmꢀ1 (with a side
band at 3543.6 cmꢀ1) remain unidentified. The observed band
splittings are ascribed to the matrix-site effect, because these side
bands were weakened by annealing the matrix sample at 10 K.
Fig. 3a plots the dependence of the intensities of these three
bands on the irradiation time. Their time profiles exhibit a uniform
trend, which suggests that they are possibly due to an identical
photoproduct species. Fig. 3b shows the time profiles of the band
intensities of HOClO and HOCl, which implies that HOClO is the
photoproduct formed in an early stage, while HOCl produced with
an induction time is probably formed in a later stage of the photo-
chemical sequence.
∠HOO = 100.9°
∠OOCl = 108.4°
∠HOOCl = 88.5°
O
HOOCl
(0)
HClO2
(327.5)
HOClO
(76.0)
Fig. 1. Three isomers of chlorous acid. Optimized geometrical parameters of HOOCl
and relative energies among the three isomer in kJ molꢀ1, shown in parentheses, are
calculated at the CCSD(T)/aug-cc- pVDZ level.
introduced into a vacuum chamber through a stainless steel pipe
and deposited on a CsI plate cooled by a closed-cycle helium refrig-
erator (Iwatani, Cryomini) at ꢁ6 K.
Infrared spectra of the matrix isolated samples were measured
with an FTIR spectrophotometer (Jeol, SPX200ST). The spectral res-
olution was 0.5 or 0.25 cmꢀ1, and the accumulation number was
100. A xenon lamp (Ushio, UER-20-172A) and a super-high-pres-
sure mercury lamp (Ushio, H500) were used as VUV and UV irradi-
ation sources, respectively, and sharp-cut filters (Asahi spectra)
were used to select visible and UV wavelengths. Other experimen-
tal details have been reported in Ref. [19].
A further analysis of the mechanism of these VUV photochem-
ical reactions is complicated, because the photolysis of HCl and
O2, matrix-cage effects, multi-reaction pathways, etc. must be
properly taken into account.
3.2. Assignment of HOOCl as the main photoproduct
Quantum chemical calculations were performed using the GAUS-
SIAN 03 program [20] on TSUBAME, a supercomputer installed in
Tokyo Institute of Technology. The CCSD(T)/aug-cc-pVDZ calcula-
tion was performed to optimize molecular geometries and esti-
mate their vibrational frequencies, together with their intensities
derived by the B3LYP/aug-cc-pVTZ level calculation. The MP2/
aug-cc-pVDZ level calculation with vibrational anharmonicity
was performed for vibrational analyses. The method of time-
dependent density functional theory (TDDFT) at the B3LYP/aug-
cc-pVTZ level was applied to estimation of vertical transition
energies.
As candidates for the main photoproduct, which are composed
of H, Cl and O atoms, we have chosen HOOCl, ClO radical/anion,
HOOOCl, HOOOOCl, etc. and calculated their vibrational frequen-
cies. Since it has been known that popular B3LYP methods cannot
satisfactorily reproduce the vibrational frequencies of molecules
composed of several Cl and O atoms, the theory levels including
multi-configuration reference functions are required for accurate
estimation [22–24]. Therefore, the CCSD(T)/aug-cc-pVDZ level
has been used for their vibrational frequencies and the B3LYP/
aug-cc-pVTZ level for their IR intensities. By the following compar-
ison of the three observed wavenumbers with their calculated re-
HOOCl
HOClO
*
HOClO
*
*
HOCl
OClO
O3
ClOO
HOO
HOCl
HOOCl
HOClO
Fig. 2. IR spectra of VUV photoproducts from HCl/O2. (a) A difference IR spectrum associated with the 180 min VUV irradiation of HCl/O2, and (b) a composite spectrum of
HOOCl and HOClO simulated at CCSD(T)/aug-cc-pVDZ for vibrational wavenumbers and B3LYP/aug-cc-pVTZ for their intensities. The bands marked with asterisks are
assigned to H2O impurity and daggers to unknown species.

72
T. Yoshinobu et al. / Chemical Physics Letters 477 (2009) 70–74
Table 1
IR band assignments of VUV photoproducts from HCl/O2 in Ne matrix.
Obs.
Calc.
Refs.c
Assignment
ꢀ1
ꢀ1a
ꢀ1
/cm
/cm
Int/km molꢀ1b
v
~
v
~
v
~
/cm
821.1/823.1
969.9
978.3
808
896
904
47
83
85
O–O stretching of HOOCl
HO37ClO
965.0d
973.9d
HO35ClO
1029.0
1038.6/1039.8
1095.4
1107.2
1173.2
1238.9
Unknown
O3
971
940
951
1152
1254
235
96
98
48
44
1038.7/1039.9e
1095.9f
O37ClO
1107.6f
O35ClO
HOClO
HOCl
1176.9d
1239g
1282.0
Unknown
HOO bending of HOOCl
HOO
Unknown
ClOO
Unknown
Unknown
O–H stretching of HOOCl
H2O impurity
HOClO
1359.6/1361.4/1361.9/1362.5/1363.6
1391.5
1397.6
1438.1/1438.5/1439.6/1441.2/1442.3
1456.3
1458.2
3542.4/3543.6
3546.3/3548.6/3555.4
3556.7
1389
1425
44
49
1388.9h
1438.6f
1495
3690
200
44
3700
3742
80
78
3527.1d
3581g
3607.8
HOCl
a
Calculated at the CCSD(T)/aug-cc-pVDZ level.
Calculated at the B3LYP/aug-cc-pVTZ level.
Data reported in the literature d–h.
In Ar matrix, Ref. [6].
In Ne matrix, Ref. [19].
In Ne matrix, Ref. [25].
In Ar matrix, Ref. [21].
In Ar matrix, Ref. [26].
b
c
d
e
f
g
h
sults, the best agreement between calculation and experiment is
found to be a hitherto unidentified species, HOOCl.
theoretically predicted to have comparable IR intensities. The
823.1 cmꢀ1 band is accompanied by
satellite band at
a
The six vibrational wavenumbers of this HOOCl species are cal-
culated to be 356, 410, 623, 808, 1389 and 3690 cmꢀ1 without a
scaling factor. Though Francisco et al. reported that HOOCl had
three stable conformers with skew, trans and cis geometries [11],
our calculation indicates that only the skew geometry shown in
Fig. 1 is located at the energy minimum, whereas the trans and
cis conformers are on the saddle points. Our optimized geometrical
parameters agree well with the reported values estimated by a
CASSCF calculation [24]. Three observed bands at 823.1, 1362.5
and 3542.4 cmꢀ1 correspond to the O–O stretching (808 cmꢀ1 from
our calculation), the HOO bending (1389 cmꢀ1) and the O–H
stretching (3690 cmꢀ1), respectively. Fig. 2b shows a simulated
spectrum of an HOOCl/HOClO mixture, where the relative IR inten-
sities of HOOCl and HOClO are adjusted so that the simulated spec-
trum reproduces the observed one (Fig. 2a) satisfactorily.
Table 1 summarizes the observed and calculated wavenumbers,
together with those for the other photoproducts. The experimental
bands assigned to OClO consist with the other experimental values
reported in Ref. [25]. Note that the three vibrational bands of HOO-
Cl measured in the present study agree with the reported values of
818, 1361 and 3559 cmꢀ1, which were predicted from the experi-
mental data of HOOH and ClOOCl by Lee and Rendell [13]. Thus,
we conclude that the bands observed at 823.1, 1362.5 and
3542.4 cmꢀ1 can be assigned to the O–O stretching, HOO bending
and O–H stretching modes of HOOCl, respectively. The photochem-
ical reactions to form HOOCl and HOClO are likely to proceed
through the OH and ClO intermediates as follows:
821.1 cmꢀ1 with comparable intensity, but the observed HOO
bending band is still twice as intense as the O–O stretching band.
A similar disagreement in the intensities is also recognized for HO-
ClO in Fig. 2. Despite the limit of accuracy in the theoretical predic-
tion of the IR intensity, the group of 823.1, 1362.5 and 3542.2 cmꢀ1
bands can thus be assigned to those of HOOCl.
Fig. 3 shows that the concentrations of HOClO and HOOCl reach
plateaus at irradiation times of 100 and 200 min, respectively,
which are controlled by the rates of creation and annihilation of
the HO2Cl isomers.
3.3. Assignment of deuterated photoproducts
In order to confirm the identification and band assignments de-
scribed above, we performed a VUV photolysis experiment using
DCl/O2/Ne instead of HCl/O2/Ne; see Supplementary material. The
isotope red-shifts of HOOCl toward DOOCl were calculated to be
1 cmꢀ1 for the O–O stretching, 364 cmꢀ1 for the HOO bending and
1002 cmꢀ1 for the OH stretching. In the VUV photolysis of DCl/O2/
Ne we observed new bands at 823.1, 1021.6 and 2614.8 cmꢀ1, which
correspond to the isotope red-shifts of 0, 341 and 928 cmꢀ1, respec-
tively. The agreement between the observed and calculated isotope
shifts seems reasonable. The vibrational calculation considering
anharmonicity at the MP2/aug-cc-pVDZ level leads to a better
agreement regarding the isotope shifts (5, 342 and 911 cmꢀ1
,
respectively) and the IR intensity patterns. Thus, the photoproducts
responsible for the 823.1, 1021.6 and 2614.8 cmꢀ1 bands observed
in the VUV photolysis of DCl/O2 are assigned to DOOCl. In turn, this
observation has confirmed the assignment of the HOOCl bands.
The formation rates of HOOCl and HOClO are found to be essen-
tially equal to those of DOOCl and DOClO, respectively, in the VUV
photolysis. This observation indicates that the diffusion process of
the H/D atom is not the rate-determining step of reaction (3).
HCl þ O2 þ hvVUVðꢁ 172 nmÞ ! ½OH þ ClOꢂ ! HOOCl ðand HOClOÞ
ð3Þ
Fig. 2 shows a comparison of the observed spectrum with the
theoretical calculation. One finds that the 1362.5 cmꢀ1 band is
three times as strong as the 823.1 cmꢀ1 band, whereas they are

T. Yoshinobu et al. / Chemical Physics Letters 477 (2009) 70–74
73
vation. Upon the UV light (k P 365 nm) irradiation after the photo-
bleaching of HOClO by the visible light (k P 405 nm), we recorded
the IR difference spectrum of Fig. 4. This difference spectrum
clearly shows that the UV light (k P 365 nm) photolyzes HOOCl
to form HCl and O2.
a
1.0
0.8
0.6
0.4
0.2
0.0
HOOCl þ hvUVðꢁ 365 nmÞ ! HCl þ O2
ð4Þ
The wavelength thresholds for the photolysis of the isomers corre-
spond with the TDDFT calculation, which estimates the first elec-
tronic transition wavelengths of HOOCl and HOClO to be 357 and
413 nm, respectively.
cm -1
823.1
1362.5
3542.2
The branching reaction pathways to HOO + Cl and HCl + O2 in
reactions (1) and (2) are known to be important for atmospheric
chemistry. As mentioned in Introduction, HOOCl was once postu-
lated as an intermediate of the OH+OCl reaction. However, HOOCl
has never been detected experimentally, although it is expected to
be the most stable isomer. HOClO is a better-known isomer that
exists as a chemically stable molecule, although the latter isomer
is less stable by 76 kJ molꢀ1; in fact, the substance called ‘chlorous
acid’ usually denotes HOClO. This unusual feature is consistent
with our present study, where the photochemical isomerization
between HOOCl and HOClO was not observed upon the UV light
(k P 365 nm) or visible light (k P 405 nm) irradiation.
0
100
200
300
400
500
b
1.0
0.8
0.6
0.4
0.2
0.0
HOClO
Acknowledgments
cm -1
978
1173
3557
1239
3608
The authors are grateful to Dr. Takashi Imamura (National Insti-
tute for Environmental Studies) for his helpful comments. This
work was financially supported in part by a Grant-in-Aid for Scien-
tific Research on Priority Areas (Nos. 17073006 and 20050009)
from the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan.
HOCl
0
60
120
180
240
Appendix A. Supplementary material
Fig. 3. Plots of IR band intensities against VUV irradiation time. (a) The 823.1,
1362.5 and 3542.4 cmꢀ1 bands are assigned to HOOCl. (b) The 978.3, 1173.2 and
3556.7 cmꢀ1 bands are assigned to HOClO, and the 1238.9 and 3607.8 cmꢀ1 bands
to HOCl. The intensity of each band is normalized to its maximum intensity.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2009.06.064.
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3.4. UV photolysis of HOOCl intermediate
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800
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