Vacuum synthesis and determination of the ionization energies of different molecular orbitals for BrOBr and HOBr

Article abstract of DOI:10.1063/1.1607310

HOBr, as a reservoir for bromine, is an important bromine species in the atmosphere, and BrOBr as a precursor of HOBr. However, far less is known about their properties in the gas phase, and no gas-phase photoelectron spectroscopy (PES) investigation is available to date. This paper discusses vacuum synthesis of pure BrOBr and HOBr, and determination of ionization energies of different molecular orbitals by the PES and both the GAUSSIAN 2 (G2) and the outer valence Green's functional (OVGF) calculations.

Full text of DOI:10.1063/1.1607310

Vacuum synthesis and determination of the ionization energies of different molecular
orbitals for BrOBr and HOBr
Zhimin Qiao, Shutao Sun, Qiao Sun, Jincai Zhao, and Dianxun Wang
Citation: The Journal of Chemical Physics 119, 7111 (2003); doi: 10.1063/1.1607310
View online: http://dx.doi.org/10.1063/1.1607310
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 119, NUMBER 14
8 OCTOBER 2003
Vacuum synthesis and determination of the ionization energies
of different molecular orbitals for BrOBr and HOBr
Zhimin Qiao, Shutao Sun, and Qiao Sun
Center of Molecular Science, Institute of Chemistry, Chinese Academy of Science,
Beijing 100080, People’s Republic of China and Graduate School of Chinese Academy of Science,
Beijing 100080, People’s Republic of China
Jincai Zhao and Dianxun Wanga)
Center of Molecular Science, Institute of Chemistry, Chinese Academy of Science,
Beijing 100080, People’s Republic of China
͑Received 12 March 2003; accepted 17 July 2003͒
Pure BrOBr and HOBr were synthesized in vacuum by heterogeneous reactions of the dried bromine
vapor and Br2 /H2O mixture vapors ͑5:1͒ with HgO, respectively, and then characterized by He I
photoelectron spectroscopy ͑PES͒ and augmented by ab initio GAUSSIAN 2 and the outer valence
Green’s functional calculations. The first PE band at 10.26 eV with vibrational spacing 550Ϯ60
cmϪ1 and the second PE band at 11.23 eV with vibrational spacing 240Ϯ60 cmϪ1 are, respectively,
assigned as ionizations of the electrons of the highest occupied molecular orbital ͑HOMO͒
(6b1(39)) and the SHOMO(13b2(38)) orbitals of BrOBr. The first PE band at 10.73 eV with
vibrational spacing 750Ϯ60 cmϪ1 and the second PE band at 11.56 eV with vibrational spacing
650Ϯ60 cmϪ1 are, respectively, assigned as ionizations of the electrons of the HOMO(6a (22))
Љ
and the SHOMO(16a (21)) orbitals of HOBr. The study does not only provide vacuum synthesis
Ј
conditions for preparing pure BrOBr and HOBr, but also provide experimental PES results along
with theoretical ionization energies of different molecular orbitals for BrOBr and HOBr. © 2003
American Institute of Physics. ͓DOI: 10.1063/1.1607310͔
I. INTRODUCTION
molecular orbitals by the PES and both the GAUSSIAN 2 ͑G2͒
and the outer valence Green’s functional ͑OVGF͒ calcula-
tions. Studies in the gas phase are more important than those
in solution for atmospheric chemistry and the synthesis of
pure BrOBr and HOBr could further promote the study of
their action in the atmosphere.
It is universally recognized in the world that halogen-
containing compounds lead to ozone depletion in the atmo-
sphere. Many chlorine-containing species are known and
have been well characterized. In comparison with the
chlorine-containing species, much less is known about
bromine-containing species, mainly because of their low
thermal stability and the difficulty of obtaining them in a
pure state.1–4 Bromine-containing species, of either biogenic
or anthropogenic origin, released at the earth’s surface have
been linked to ozone destruction in various altitude regions
of the earth’s atmosphere.5–7 Recent findings show8,9 that the
effectively catalytic removal of ozone by bromine on a per
atom basis in the polar stratosphere is higher one hundred
times than that of chlorine. Consequently, thorough and ex-
tensive investigations of bromine-containing species from
both experiments and theory are urgently needed.
II. PES MEASUREMENT
The PES experiments were performed in a double-
chamber UPS Machine-II that was built specially to detect
transient species as described elsewhere.11 The PE spectra of
BrOBr and HOBr were measured at resolutions of about 30
meV as indicated by the Arϩ(3P2/3) PE peak. The experi-
mental ionization energy (Iv in eV͒ is calibrated by simulta-
neous addition of a small amount of argon and methyl iodide
to the sample.
Ab initio ͑G2͒ and OVGF calculations. To assign the
PES bands of BrOBr and HOBr, both G2 and OVGF calcu-
lations have been performed on the ground and ionic states
corresponding to ionization of the electron of different mo-
lecular orbitals in the C2v symmetry for BrOBr and in the Cs
symmetry for HOBr. The G2 procedure has been described
in detail by Curtiss et al.12 The OVGF calculations are
shown in Ref. 13. The computed ionization energies (Ev in
eV͒ are obtained from the difference between the total en-
ergy of the resulting cation to the total energy of the ground
state of the neutral molecule.
HOBr, as a reservoir species for bromine, is an important
bromine species in the atmosphere,10 and BrOBr as a precur-
sor of HOBr. But far less is known about their properties in
the gas phase, and no gas-phase photoelectron spectroscopy
͑PES͒ investigation is available to date. In this paper, we
would like to report vacuum synthesis of pure BrOBr and
HOBr, and determination of ionization energies of different
a͒
Author to whom all correspondence should be addressed. Electronic mail:
wangdx@infoc3.icas.ac.cn
0021-9606/2003/119(14)/7111/4/$20.00
7111
© 2003 American Institute of Physics
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7112
J. Chem. Phys., Vol. 119, No. 14, 8 October 2003
Qiao et al.
III. RESULTS AND DISCUSSION
A. Vacuum synthesis of pure BrOBr
Pure BrOBr was synthesized in vacuum by heteroge-
neous reaction of the bromine vapor with HgO,14,15
2Br g͒ϩHgO͑s)→HgBr s͒ϩBrOBr͑g).
͑1͒
͑
͑
2
2
As the first step for obtaining pure BrOBr, the moisture
in bromine must previously be removed. Bromine was first
frozen at Ϫ78 °C ͑liquid N2/acetone mixture͒ to degas by a
vacuum pumping at 10Ϫ3 Torr, then dried by passing through
P2O5 ͑Aldrich͒ at 25 °C and collected in a tube with two
sealed valves. Second, 100 g of HgO ͑yellow͒ powder ͑Ald-
rich͒ were previously put in a 500 ml glass flask, which was
loosely filled with the glass wool and equipped with inlet and
outlet valves. The flask was evacuated to 10Ϫ3 Torr, and then
heated to 100 °C for 1 12 h to activate HgO. Third, the evacu-
ated flask was filled with dried bromine to about 150 Torr at
0 °C in the dark. After approximately 7 min of mixing and
reacting between HgO and Br2 , the reaction gases were
transferred into a Ϫ196 °C liquid nitrogen cooled trap by a
rotation pumping system. This procedure was repeated until
a sufficient quantity of BrOBr was obtained. The trapped
sample was then pumped at Ϫ45 °C to remove unreacted Br2
impurity. After a half hour, the brownish-green pure BrOBr
was obtained and thence kept at Ϫ55 °C to avoid BrOBr
decomposition prior to its use for PES measurements.
B. Vacuum synthesis of pure HOBr
Pure HOBr was synthesized using similar equipment and
procedures as mentioned earlier. First, the HgO sample was
prepared exactly as above. Second, the evacuated flask was
successively filled with bromine vapor up to 150 Torr and
with H2O vapor up to 30 Torr above that at 25 °C in the dark.
After approximately 10 min of mixing and reacting between
Br2 , H2O, and HgO, the reaction gases were transferred to a
Ϫ196 °C liquid nitrogen cooled trap by a rotation pumping
system. This procedure was repeated until a sufficient quan-
tity of HOBr was obtained to run a satisfactory PE spectrum.
The trapped sample was then pumped at Ϫ50 °C to remove
the Br2 /BrOBr impurity. After a half hour, the greenish-
yellow, pure HOBr was obtained and kept at Ϫ65 °C to avoid
HOBr decomposition prior to its use for PES measurements.
A ‘‘pure’’ PE spectrum of HOBr is obtained under the
condition of having strong PE signal due to H2O. Obviously,
this is attributed to the equilibrium reaction ͑2͒ for formation
of HOBr:16
FIG. 1. The complete PE spectra of BrOBr ͑a͒, HOBr ͑b͒, and a mixture of
BrOBr and HOBr ͑c͒.
the PE spectrum of HOBr, and its expanded part in the low
ionization energy region ͑Ͻ13.50 eV͒ is shown in Fig. 2͑b͒.
Figure 1͑c͒ is the PE spectrum of the products synthesized
without removing moisture in bromine and its expanded part
in the low ionization energy region ͑Ͻ13.50 eV͒ is shown in
Fig. 2͑c͒. Figures 1͑c͒ and 2͑c͒ are clearly mixture PE spec-
tra of BrOBr and HOBr. The PES bands of H2O also appear
on the ͑b͒ and ͑c͒ of Figs. 1 and 2.17
Table I gives the experimental vertical ionization ener-
gies (Iv in eV͒ and computed ionization energies (Ev in eV͒
by OVGF and G2 methods for BrOBr and HOBr, because
theoretical calculations cannot only support PES experimen-
tal results, but also predict the character of the species
studied/proposed.
BrOBr͑g)ϩH2O͑g)ꢀ2HOBr͑g).
͑2͒
Table II gives the vibrational frequencies ͑cmϪ1͒ ob-
tained in the PES experiments, theoretical calculations along
with literature values for BrOBr and HOBr. These results
further support the present assignments for the PES bands of
BrOBr and HOBr.
In Fig. 1͑a͒, four bands are evident at 10.26, 11.23,
11.73, and 13.34 eV as well as a broad band centered near
16.25 eV in the PE spectrum of BrOBr. The band at the
lowest energy 10.26 eV is the first PES band of BrOBr be-
cause its value is in excellent agreement with the reported
If the moisture in bromine was not removed, a mixture PE
spectrum of BrOBr, HOBr, and H2O was obtained ͓see the
͑c͒ of Figs. 1 and 2͔.
C. Determination of the ionization energies
of different molecular orbitals for BrOBr and HOBr
Figure 1͑a͒ is the full PE spectrum of pure BrOBr and a
corresponding expanded part in the low ionization energy
region ͑Ͻ13.50 eV͒ is given in Fig. 2͑a͒. Figure 1͑b͒ gives
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J. Chem. Phys., Vol. 119, No. 14, 8 October 2003
Ionization energies for BrOBr and HOBr
7113
TABLE I. He I experimental vertical ionization energies (Iv in eV͒, com-
puted ionization energies ͑in eV͒ by OVGF and G2 methods for the PES
bands of BrOBr(C2v) and HOBr(Cs). ͓The geometries of BrOBr and HOBr
**
were optimized at the B3LYP/6-311ϩϩg level. The calculation by the
**
OVGF method is at the 6-311ϩϩG basis set level. The molecular orbitals
associated with each ionization band are designated according to their
atomic and bonding character. Experimental vertical ionization energies (Iv
in eV͒ are given in the form of overlapping band maxima since the observed
bands are associated with varying orbital ionization numbers.
Species
BrOBr
I
v (eV)
OVGF ͑eV͒ G2 ͑eV͒
10.404 10.369
MO
Character
10.26 10.26
10.33
6b1(39)
nBr
10.40
10.47
11.11
11.14
11.17
11.20
11.23 11.23
11.26
11.113
13b2(38)
␴
Br–O
11.73
11.332
11.480
14.820
15a1(37)
5a2(36)
14a1(35)
␴
nBr
␴
Br–O
13.34
Br–O
HOBr
10.64
10.645
10.73 10.73
10.82
10.689
6a (22)
Љ
␲
Br–O
10.91
11.48
11.56 11.56
11.64
11.254
16a (21)
nBr
Ј
11.72
14.20
14.74
13.748
14.717
17.293
15a (20)
␴
␲
␴
Ј
Br–O
5a (19)
Љ
Br–O
14a (18)
Ј
Br–O
orbital. The third band at 11.73 eV may be ascribed to ion-
ization of the electrons of both 15a1 and 5a2 orbitals be-
cause the computed ionization energies 11.332 eV͑OVGF͒
for 15a1 and 11.480 eV ͑OVGF͒ for 5a2 are very close to
this PES value and because such a high intensity band could
be the result of ionization of the electrons of multiple
orbitals.17 The bands of BrOBr in the high ionization energy
region have lower intensity because photoionization cross
sections of heavy elements are low for He I radiation,17 and
the broad band usually relates to ionization of the electrons
of several orbitals ͑see Table I͒.
FIG. 2. The expanded PE spectra of BrOBr ͑a͒, HOBr ͑b͒, and a mixture of
BrOBr and HOBr ͑c͒ in the low ionization energy region ͑Ͻ13.50 eV͒.
experimental value 10.264 eV18͑a͒ and calculated values
10.23 eV.18͑b͒ The present PES value, 10.26 eV, agrees rea-
sonably well with our computed values 10.369͑G2͒ and
10.404͑OVGF͒ ͑see Table I͒. The vibrational spacing
͑550Ϯ60 cmϪ1͒ of the first band is somewhat larger than that
of the reported Br–O stretch model ͑533 cmϪ1 ͑Ref. 15͒͒ and
the calculated values ͑see Table II͒ for the neutral BrOBr.
This shows that ionization corresponding to the first band
results from removal of the electron of an antibonding or-
bital. The highest occupied molecular orbital ͑HOMO͒ 6b1
of BrOBr ͑see Table I͒ is just an antibonding ␲ orbital. The
adiabatic ionization energy (Ia) is equivalent to the vertical
ionization energy (Iv) on the first band, because the 6b1
embodies mainly the contribution of the two Br atoms,
which is similar to the lone-pair of bromine.
From the ͑b͒ of both Figs. 1 and 2, it is clearly seen that
the highest intensity peak at 12.61 eV is the first band of
H2O.17 The band at 14.74 eV with vibrational spacing
TABLE II. The vibrational frequencies ͑cmϪ1͒ of experiments, calculations,
and literature for BrOBr and HOBr. ͑The three DFT methods are all at the
**
6-311ϩϩG basis set level.͒
The calculated values
Species This expt B3P86 B3LYP B3PW91 Literature
Assign
BrOBr 550Ϯ60 526.78 509.73 523.69
240Ϯ60
533a
620b
Br–O stretch
The band at 11.23 eV with a vibrational spacing of
240Ϯ60 cmϪ1 is the second PE band of BrOBr because this
value agrees with the computed value of 11.113͑OVGF͒ and
because a long vibrational progression ͑at least 8͒ should
relate to ionization of the electron of a strong bonding 13b2
HOBr
750Ϯ60 626.38 606.03 623.20
650Ϯ60
Br–O stretch
aReference 15.
bReference 20.
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7114
J. Chem. Phys., Vol. 119, No. 14, 8 October 2003
Qiao et al.
970Ϯ60 cmϪ1 and a long vibrational progression is the sec-
ond band of H2O. The abnormally high intensity of the sixth,
seventh, and eighth vibrational peaks of the second band of
H2O is attributed to overlapping with the bands of other
species. The remaining four bands at 10.73, 11.56, 14.20,
14.74 are attributed to the PE spectrum of HOBr. The band at
the lowest energy 10.73 eV is the first PE band of HOBr
because the adiabatic ionization energy at 10.64 eV is in
excellent agreement with and thus confirms the reported ex-
perimental values 10.617 eV,19͑a͒ 10.638 eV,19͑b͒ and a calcu-
lated value 10.65 eV.19͑c͒ The present PES value is equivalent
to our G2 calculated value of 10.645 eV and matches well
the OVGF value 10.689 ͑see Table I͒. The third vibrational
peak at 10.82 eV on the first band has the highest intensity
because the ␤ peak of He I radiation for the first band at
12.61 eV of H2O overlaps on the third vibrational peak of
the first band of HOBr. The vibrational spacing 750Ϯ60
cmϪ1 is larger than the reported value of 620 cmϪ1 ͑Ref. 20͒
for the Br–O stretch model of the neutral HOBr molecule
͑see Table II͒. This shows that ionization corresponding to
the first band is the result of removal of the electron of an
antibonding orbital. This is not only consistent with the char-
tion energy side.17 That is, the PE spectrum of HOBr consists
of at least four bands at 10.73, 11.56, 14.20, and 14.74 eV.
The ͑c͒ of both Figs. 1 and 2 are PE spectra of a mixture
of BrOBr and HOBr which were synthesized without remov-
ing moisture from bromine. It further demonstrates that the
͑a͒ of Figs. 1 and 2 are the PE spectra of pure BrOBr and the
͑b͒ of Figs. 1 and 2 are the PE spectra of pure HOBr.
ACKNOWLEDGMENTS
This project was supported by the National Natural Sci-
ence Foundation of China ͑Contract Nos. 29973051 and
20073052͒ and the Basic Bureau of Chinese Academy of
Sciences ͑No. KJCXZ-HZ-01͒. Z.Q., S.S., and Q.S. thanks
the Chinese Academy of Sciences for a scholarship during
the period of this work.
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Љ
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650Ϯ60 cmϪ1 is the second PE band of HOBr. It results
from ionization of the electron of the 16a , because its value
Ј
matches the computed values of 11.254 eV ͑OVGF͒ and
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**
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15a (20) orbital. An obvious high intensity band at 14.74 eV
Ј
is the result of overlapping of the bands of both HOBr and
H2O because the band centered near 14.74 eV of H2O should
have a symmetrical shape, the same as with its low ioniza-
20 I. Barnes, V. Bastian, K. H. Becker, R. Overath, and T. Zhu, Int. J. Chem.
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