The rotational spectrum and structure of HOOOH

Article abstract of DOI:10.1021/ja0556530

Dihydrogen trioxide, HOOOH, which is a species with fundamental importance for understanding the chain formation ability of the oxygen atom, was detected in a supersonic jet by a Fourier transform microwave spectrometer with a pulsed discharge nozzle, together with double resonance and triple resonance techniques. Its precise molecular structure was determined from the experimentally determined rotational constants of HOOOH and its isotopomer, DOOOD. Many of the microwave and millimeter wave transitions can now be accurately predicted, which could be facilitated for remote sensing of the molecule to elucidate its roles in various chemical processes. Copyright

Full text of DOI:10.1021/ja0556530

Published on Web 10/06/2005
The Rotational Spectrum and Structure of HOOOH
Kohsuke Suma, Yoshihiro Sumiyoshi, and Yasuki Endo*
Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku,
Tokyo 153-8902, Japan
Received August 25, 2005; E-mail: endo@bunshi.c.u-tokyo.ac.jp
Hydrogen oxides, H2On with n ) 1 (i.e., water) and n ) 2 (i.e.,
hydrogen peroxide), are familiar species in various fields of natural
science. However, extended species (n > 2), namely, hydrogen
polyoxides, have been believed to be quite unstable due to the
repulsion between the lone-pair electrons on the oxygen atoms.
Thus, the questions, “how long can you make an oxygen chain?1”
and “if such oxygen chain molecules exist, what kind of roles do
these molecules play?” have long been fascinating chemists even
from the19th century.2 Recently, we have reported a microwave
observation of a radical species with three oxygen atoms, HO3,
and determined its precise molecular structure. HO3 is regarded as
a weak adduct of HO-O2 and is expected to be a sink of the
atmospheric OH radical.3 On the other hand, no detection of a closed
shell species, H2O3, in the gas phase has been reported yet. Its
existence and stability in the gas phase, as well as its precise
molecular structure, are still open questions. Recent theoretical
studies predict its existence in the gas phase. For example, McKay
et al. predicted that the oxygen chain could be extended infinitely
in a very low temperature gas.1 Several experiments on H2O3 in
the condensed phase have meanwhile been reported. Infrared
absorption spectra in a solid state4 and the Ar matrix5 and NMR
spectra in several organic solutions6 have shown its existence in
respective media. These reports indicate that H2O3 is far more stable
than was previously believed. Plesnicˇar et al. have recently reviewed
these experiments.6
Wentworth et al. reported that all antibodies were capable of
catalyzing the oxidation of water by singlet oxygen (1∆ O2) to
generate H2O2 and probably O3, as well, and postulated that these
antibodies carry the reaction through H2O3 as a key intermediate.7
Furthermore, H2O3 is expected to be involved in oxidation processes
that span atmospheric, environmental, and biological systems.6 It
is thus expected that precise rotational transitions could be used to
probe H2O3 and to elucidate its potential roles in these chemical
systems, if they are observed.
In the present study, we were able to detect H2O3 by Fourier
transform microwave (FTMW) spectroscopy8 and FTMW-mm-
wave double resonance9 and triple resonance spectroscopy, provid-
ing sufficient data to precisely determine its ground-state geometry.
H2O3 was produced in a pulsed discharge nozzle (PDN) by
discharging a gas mixture of O2 (10%) and Ar, passed through a
reservoir filled with a 30% hydrogen peroxide solution. Discharged
products were expanded as a supersonic jet into a FTMW
spectrometer cavity. Since H2O3 is a singlet molecule without any
hyperfine splittings and only one component of the dipole moment,
µb, has a nonzero value in the trans form (C2 symmetry), the
observable transition by our FTMW spectrometer, whose frequency
coverage is 4-40 GHz, is limited to only one, 303 - 212, at about
21 GHz, which was predicted by an ab initio calculation.10-12
Therefore, we performed double resonance spectroscopy by moni-
toring all possible candidate lines observed in the region of 21 (
2 GHz, to check which of the lines belongs to H2O3. The lines to
be checked by double resonance spectroscopy were selected
beforehand to a few lines by checking if they show a diamagnetic
behavior, are the discharge products, need both hydrogen peroxide
and O2, and do not depend on a particular buffer gas, Ar or Ne.
When one of the observed lines at 20 838.6 MHz was used as a
monitor for 303 - 212, double resonance signals at the predicted
regions, 45 (312 - 303), 79 (212 - 101), and 186 (321 - 212) GHz,
were observed. For the confirmation of the spectral carrier, a newly
developed triple resonance method was also applied to observe the
transition, 110 - 101, at 42 GHz. Details of this technique will be
reported elsewhere. In total, five rotational transitions, as shown
in Supporting Information, were observed. Neither a-type nor c-type
transitions were observed around the predicted frequencies, indicat-
ing that the dipole moment components, µa and µc, are 0 or very
small. These results support that the spectral carrier is H2O3 in the
trans form. We could not observe transitions of the cis isomer (Cs
symmetry) despite careful scans with similar experimental condi-
tions in sufficiently wide frequency regions. The transition at
20 838.6 MHz was also observed slightly weakly by discharging a
mixture gas of H2O/O2/Ar. Rotational transitions of D2O3 were thus
searched for by using the D2O/O2/Ar mixture gas. Three transitions
listed in the table in Supporting Information were observed by
FTMW spectroscopy. Using these transitions as monitors, two more
transitions were observed by the double resonance technique. In
total, five rotational transitions were observed for D2O3. Although
the dipole moments, µb, of H2O3 and D2O3 are almost identicals
about 1 D predicted by B3LYP/cc-pVQZ calculations10ssignals
of D2O3 were much weaker than expected from those of H2O3. By
irradiating the microwave parallel to the supersonic jet beam,13 small
hyperfine splittings due to the electric quadrupole moment of
deuterons were resolved. Since D2O3 has two equivalent deuterons,
which obey Bose-Einstein statistics, the rotational energy levels
consist of ortho (Itotal ) 0, 2) and para (Itotal ) 1) states. The 110-
101 transition (para) is well reproduced by the electric quadrupole
coupling constants obtained by the ab initio calculation.10
However, extra lines were observed for two other transitions 303-
12 and 211-202, as shown in the figures in Supporting Information.
2
They were observed for both ortho and para states and could not
be explained by changing the values of the electric quadrupole
coupling constants within a reasonable range. On the other hand,
no such doubling was observed for H2O3. At present, we were not
able to explain the origin of these splittings. They could be
considered to be the splittings caused by the tunneling between
the two chiral isomers through the torsional motions of OH moieties.
However, these splittings are estimated to be extremely small by
the simple WKB (Wentzel-Kramers-Brillouin) approximation
along the one-dimensional torsion vibration coordinate using a
potential energy surface obtained by the B3LYP/cc-pVTZ calcula-
tion.10 H2O3 should be considered to be a chiral molecule at the
time scale of our experimental apparatus, and no tunneling splittings
should be observed. Although parity-violation effect may also
9
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J. AM. CHEM. SOC. 2005, 127, 14998-14999
10.1021/ja0556530 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Comparison of the Rotational Constants of the cis and
trans Forms Obtained by the Ab Initio Calculation with Those
Determined Experimentally (in MHz)
The present microwave data provide us means for its detection
in the earth’s atmosphere and, conceivably, in the interstellar space.
For example, the observed transition frequency of H2O3, 20838.6
MHz, agrees well with the U-line (20 838.2 MHz) observed in the
W51 region.15 Both HOOOH and HOOO3 are considered to be the
starting point species for the investigations of the chain formation
ability of the oxygen atom. Their successful detections by the high-
resolution spectroscopic method in the gas phase have now firmly
established the stabilities of the HnO3 (n ) 1, 2) systems and opened
a possibility to investigate the existence and detection of other
catenated oxygen species. For example, observations of the
rotational transitions of species such as H2O42 and XOOOH (X )
halogen,16 methyl,17 and so on) are well expected by a similar
experimental setup. Such observations should reveal their accurate
and detailed characters and give means for the detections in various
chemical systems. These studies will lead us to the chemistry of
“oxygen chain molecules”.
HOOOH
trans
DOOOD
trans
cis
exp.
cis
exp.
A0
B0
C0
52437
10558
9297
51152
10719
9380
51149
10688
9355
44287
9549
8646
42962
9740
8770
43043
9709
8745
Acknowledgment. This work is supported by a Grant-in-Aid
for priority research entitled “Radical Chain Reactions” (Grant No.
13127101). K.S. thanks the support of the Research Fellowships
of the Japan Society for the Promotion of Science for Young
Scientists (JSPS No. 16-10895).
Figure 1. The molecular structure of HOOOH determined from the
experimental rotational constants and the vibration-rotation constants
derived by the ab initio calculations. The structural parameters predicted
by the ab initio calculations are rOH ) 0.966 Å, rOO ) 1.427 Å, θ(OOO)
) 107.0°, θ(HOO) ) 101.1°, and æ(HO-OO) ) 81.1° (dihedral angle).
Supporting Information Available: Complete refs 11 and 12 and
observed and simulated microwave spectra of D2O3 and rotational
transition frequencies, molecular constants, and energy diagrams of
H2O3 and D2O3. This material is available free of charge via the Internet
at http://pubs.acs.org.
contribute to these splittings, such an effect is too small for the
splittings to be ascribed to this effect. Clarification of these
mysterious splittings requires more extensive experimental studies
along with more accurate theoretical studies.
Five molecular constants listed in Supporting Information have
been directly determined from the observed five transitions for each
isotopomers by using Watson’s A-reduced Hamiltonian, where the
hyperfine splittings of D2O3 were neglected. The standard deviations
in the least-squares analysis become larger than the experimental
accuracy, if all of the centrifugal distortion constants are fixed to
the ab initio values.10 The negative nonzero inertial defects, ∆I )
Icc - Ibb - Iaa ) -3.143 uÅ2 for H2O3 and ∆I ) -6.007 uÅ2 for
D2O3, clearly suggest H2O3 is nonplanar. The rotational constants
agree well with those of the trans form predicted by the ab initio
calculation, as shown in Table 1, where the ab initio rotational
constants are corrected for the vibration-rotation constants.10
Differences of the rotational constants predicted for the trans form
are less than 1%. These agreements are reasonable for typical singlet
molecules and support that the observed H2O3 is in the trans form.
By assuming H2O3 is in a trans geometry, the molecular structure
shown in Figure 1 was determined from the rotational constants of
H2O3 and D2O3, whose vibration-rotation constants were corrected
by the ab initio calculation.10 The O-O bond lengths of H2O3, 1.428
Å, are much shorter than that of the radical species, (HO-OO),
1.688 Å,3 and are even shorter than that of H2O2, 1.464 Å.14 This
fact clearly indicates that, although the HO3 radical can be regarded
as a weakly bound HO-OO adduct, H2O3 is a molecule with rigid
O-O bonds, which are stronger than that of H2O2. The determined
molecular structure agrees very well with that of the ab initio
calculation. This situation is also quite different from the case of
HOOO, where results deviate significantly depending on the
calculation methods employed.3
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