Photochemistry of SO2/CI2/O2 gas mixtures: Synthesis of the new peroxide CISO2OOSO2CI

Article abstract of DOI:10.1021/ic802143p

Photochemically induced gas-phase reactions of CI₂/SO ₂/O₂ mixtures at -78 °C have been investigated on a preparative scale. The sulfuryl chlorides CISO₂(OSO₂) _nCI, n = 0,1, or 2, along wit

Full text of DOI:10.1021/ic802143p

Inorg. Chem. 2009, 48, 1906-1910
Photochemistry of SO2/Cl2/O2 Gas Mixtures: Synthesis of the New
Peroxide ClSO2OOSO2Cl
Rosana M. Romano,*,† Carlos O. Della Ve´dova,†,‡ Helmut Beckers,§ and Helge Willner§
CEQUINOR (UNLP-CONICET), Departamento de Qu´ımica, Facultad de Ciencias Exactas,
UniVersidad Nacional de La Plata, 47 esq. 115, (1900) La Plata, Argentina, Laboratorio de
SerVicios a la Industria y al Sistema Cient´ıfico (UNLP-CIC-CONICET), Departamento de
Qu´ımica, Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, Camino Centenario e/
505 y 508, (1897) Gonnet, Argentina, and Anorganische Chemie, Bergische UniVersita¨t
Wuppertal, Gauꢀstr. 20, D-42097 Wuppertal, Germany
Received November 9, 2008
Photochemically induced gas-phase reactions of Cl2/SO2/O2 mixtures at -78 °C have been investigated on a
preparative scale. The sulfuryl chlorides ClSO2(OSO2)nCl, n ) 0, 1, or 2, along with SO3 and the hitherto unknown
bis(chlorosulfuryl)peroxide, ClSO2OOSO2Cl, have been obtained. The new peroxide has been purified by trap-to-
trap distillation and conclusively characterized by gas-phase and matrix IR, as well as Raman, spectroscopy. At
room temperature, the peroxide decomposes to form quantitatively Cl2 and SO3. DFT calculations suggest that
ClSO2OOSO2Cl occurs at ambient temperatures in at least two different rotamers, whereas in matrix-isolation
experiments in Ar at 15 K only the most stable form has been found. The mechanism of the formation of the
peroxide via the ClSO2OO• peroxy radical and the supposed involvement of the ClSO3• intermediate is discussed.
Introduction
2 SO2+O2 f 2 SO3
(5)
The ClSO3• radical and its dimer are possible intermediates
in eq 4. They could play an important role in atmospheric
chemistry, particularly in regions of high Cl2 and SO2
abundances.4 Reactions 1-4 can also account for chemical
processes in the Venus stratosphere similar to the O2-
destroying reactions reported for CO/O2/Cl2 mixtures. Both
processes have been suggested to explain the unexpectedly
low oxygen content of the Venus stratosphere.3,5
To the best of our knowledge, the ClSO3• radical has not
been reported. In contrast, the analogous FSO3• radical, in
equilibrium at ambient temperature with FSO2OOSO2F, has
been the subject of several investigations during the past 50
years.6 F atoms, generated by photolysis of F2, were shown
to react with SO3 to give FSO2OOSO2F in high yield.7
The catalytic or photolytic treatment of SO2/Cl2 mixtures
affords well-known routes for the industrial synthesis of
sulfuryl chloride Cl2SO2.1,2 However, in the presence of
oxygen, for example in the atmospheric degradation of SO2,
the Cl2/SO2 photolysis reaction sets in train a catalytic
oxidation cycle (eqs 1-4) for the conversion of SO2 to SO3
(eq 5).3
Cl2 + hν f 2 Cl•
(1)
(2)
(3)
(4)
2 Cl • +2 SO2 f 2 ClSO2•
2 ClSO2 • +2 O2 f 2 ClSO2OO•
2 ClSO2OO • f Cl2+2 SO3+O2
* To whom correspondence should be addressed. E-mail: romano@
quimica.unlp.edu.ar.
(4) Uherek, E. Atmospheric Trace Gas Mixing Ratios; Max Planck Institut
fu¨r Chemie: Mainz, Germany;www.atmosphere.mpg.de.
(5) Pernice, H.; Garcia, P.; Willner, H.; Francisco, J. S.; Mills, F. P.; Allen,
M.; Yung, Y. L. Proc. Nation. Acad. Sci. U.S.A. 2004, 28, 14007–
1410.
(6) Beckers, H.; Willner, H.; Grote, D.; Sander, W. J. Chem. Phys. 2008,
128. 084501 and references cited therein. .
† CEQUINOR (UNLP-CONICET), Departamento de Qu´ımica, Facultad
de Ciencias Exactas, Universidad Nacional de La Plata.
‡ Laboratorio de Servicios a la Industria y al Sistema Cient´ıfico (UNLP-
CIC-CONICET), Departamento de Qu´ımica, Facultad de Ciencias Exactas,
Universidad Nacional de La Plata.
§
Anorganische Chemie, Bergische Universita¨t Wuppertal.
(1) Horner, L.; Podschus, G. Angew. Chem. 1953, 65, 534–535.
(2) Schumacher, H. J.; Schott, C. Z. Physik. Chem. 1944, 193, 343–366.
(3) Demore, W. B.; Leu, M.-T.; Smith, R. H.; Yung, Y. L. Icarus 1985,
63, 347–353.
(7) (a) Gambaruto, M.; Sicre, J. E.; Schumacher, H. J. J. Fluorine Chem
1975, 5, 175–179. (b) Staricco, E. H.; Sicre, J. E.; Schumacher, H. J.
Z. Physik. Chem. 1962, 35, 122–128. (c) Schumacher, H. J.; Basualdo,
W. H.; Schumacher, H. J. Z. Physik. Chem. 1965, 47, 57–64.
1906 Inorganic Chemistry, Vol. 48, No. 5, 2009
10.1021/ic802143p CCC: $40.75  2009 American Chemical Society
Published on Web 02/04/2009

New Peroxide ClSO2OOSO2Cl
energy minimum geometries for which no imaginary vibrational
frequency was found.
However, the chloro analogue ClSO2OOSO2Cl has not been
reported so far.
Results and Discussion
Experimental Section
Photochemical Reactions between Cl2, SO2, and O2. In
preliminary experiments, gaseous mixtures of SO3 and Cl2
were irradiated with light at wavelengths >320 nm (Heraeus
TQ 150, Z2 lamp) at -25 °C, but apparently no reaction
took place. In contrast, irradiation of gaseous mixtures of
Cl2, SO2, and O2 (λ > 320 nm) at -78 °C yielded, together
with the known products Cl2SO2, SO3, ClSO2OSO2Cl,
and ClSO2OSO2OSO2Cl, the hitherto unknown peroxide
ClSO2OOSO2Cl. The product distribution was dependent on
(i) the proportions of the reactants, and (ii) the conditions
during workup because the peroxide was found to be
thermally rather unstable. The highest yield of the peroxide
was obtained from mixtures of Cl2/SO2/O2 ≈ 1:2:10 at a total
pressure of 550 Torr measured at ambient temperature.
At dry ice temperature, the pressure decreased to 380 Torr,
and further to 340 Torr during irradiation. Volatile
products were distilled into three traps held at -90, -120,
and -196 °C. Excess oxygen was pumped off, whereas
Cl2SO2 was trapped at -120 °C, and the less volatile
compounds remained in the reactor at -78 °C. In a further
distillation, using traps at -70, -110, and -196 °C, the
reactor was allowed to warm up slowly to room temperature.
Subsequently, ClSO2OOSO2Cl was evaporated and retained
in the -70 °C trap, together with some Cl2SO2 and
ClSO2OSO2Cl. ClSO2OSO2OSO2Cl was found as a residue
in the photoreactor. The raw product was purified by several
trap-to-trap distillations from -20 °C using traps at -50,
-80, and -196 °C. Pure ClSO2OOSO2Cl was finally
collected in the -50 °C trap as a colorless solid. When the
peroxide was allowed to reach room temperature, it decom-
posed giving SO3 and Cl2 in a molar ratio of 2:1.
Chemicals. The chemicals Cl2 (99.8%), SO2 (99.98%), SO3,
Cl2SO2 were obtained from commercial sources and purified by
trap-to-trap distillation, whereas CCl4, Ar (6.0), and O2 (5.1) were
used as received. ClSO2OSO2Cl was synthesized through the
thermal reaction between CCl4 and SO3 according to the reported
procedure.8
Photochemical Reactions. Volatile materials were manipulated
in a glass vacuum line equipped with a capacitance pressure gauge
(MKS Baratron 221 AHS-1000, Wilmington, MA, USA), three
U-traps, and valves with PTFE stems (Young, London, UK). The
vacuum line was connected via a flexible stainless steel tube to a
5.5 L glass photoreactor equipped inside with a UV lamp (Heraeus
TQ 150 Z2) in a double-walled glass insert cooled by water. The
vacuum line was also connected to an IR gas cell (optical path
length 200 mm, with 0.5 mm thick Si wafers as windows) in the
sample compartment of the FTIR instrument. This arrangement
made it possible to follow the course of the reaction during the
synthesis and to monitor, at a later stage, the improvement in the
purification process. The products were stored in glass ampoules
under liquid nitrogen. The ampoules were opened and flame-sealed
by means of an ampule key.9 Different amounts of the gaseous
reactants were mixed in the photoreactor held at dry ice temperature,
and the pressure was monitored during the reactions. The reaction
mixtures were irradiated until no pressure decrease was observed.
The photolysis products were separated by trap-to-trap condensation,
and the progress was monitored by gas-phase IR spectroscopy.
Spectroscopy. IR spectra of the gaseous samples were recorded
with a resolution of 2 cm-1 in the range 4000-400 cm-1 with a
Bruker Vector 25 spectrometer, and the Raman spectra of liquid
samples with a Bruker-Equinox 55 FRA 106/S FT-Raman spec-
trometer equipped with a 1064 nm Nd:YAG laser. IR spectra of
Ar matrices were recorded in reflectance mode with the Bruker
IFS 66v spectrometer with transfer optics. An MCT (DTGS)
detector, together with a KBr/Ge beam splitter, was used in the
region 5000-650 (400) cm-1. One-hundred scans were added for
All of the volatile products were identified by their IR and
Raman spectra. The infrared bands of Cl2SO2 were compared
with those of a commercial sample.12 ClSO2OSO2Cl was
identified by the IR spectrum of the vapor and the Raman
spectrum of the liquid. Because we observed small differ-
ences between our spectra and those reported previously,13
the identity of ClSO2OSO2Cl was confirmed by the spectrum
of an authentic sample.8 Monomeric SO3 was detected in
the spectra with apodized resolutions of 0.5 (2) cm-1
.
Matrix Isolation. A few milligrams of pure ClSO2OOSO2Cl
were transferred to a small U-trap connected to the inlet nozzle of
the matrix apparatus. This nozzle made of quartz consisted of a
tube with an internal diameter of 4 mm and an outlet opening of 1
mm diameter. Its temperature could be raised by heating over a
length of 20 mm. A stream of Ar (2 mmol h-1) was directed over
the sample held at -65 °C, and the resulting gas mixture was
condensed on the matrix support (a rhodium-plated copper mirror)
held at 15 K. Details of the matrix apparatus are given elsewhere.10
Theoretical Calculations. Quantum chemical calculations were
performed using the Gaussian 03 program system.11 Geometry
optimizations were sought using standard gradient techniques by
simultaneous relaxation of all of the geometrical parameters. The
calculations of vibrational properties were carried out at potential
(11) Frisch, M. J. ; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Jr., Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K. Fukuda, R.; Hasegawa,
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Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
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Ferdinand Enke: Stuttgart, 1975; 389.
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and Applications; Schrader, B., Ed.; VCH: Weinheim, 1994; 297.
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Inorganic Chemistry, Vol. 48, No. 5, 2009 1907

Romano et al.
the gas phase by its IR spectrum, whereas γ-SO3 was
identified by comparison of the Raman spectrum of a solid
sample sealed in a glass tube with the reported details.14
Proposed Reaction Mechanism. Starting from photo-
chemically produced chlorine atoms according to eq 1,
different alternative routes to the observed products can be
proposed. The borosilicate glass used for the photo reactor
transmits light of wavelengths >320 nm. Thus, photode-
composition of the reactions products (e.g., sulfuryl chlorides)
is expected to be negligible.15,16 Chlorine atoms are known
to react with sulfur dioxide to give sulfuryl dichloride,
presumably via the short-lived ClSO2 radical, as depicted in
eqs 2 and 6.3,17
the cycle represented in eqs 1-5, this second cycle also
proceeds with complete regeneration of the Cl• atoms initially
consumed.
ClSO2OOSO2Cl f 2 Cl • +2 SO3
(8)
The compounds ClSO2(OSO2)nCl, n ) 1 or 2, are thought
to be formed in secondary reactions via the reduction of
ClSO2OOSO2Cl by SO2. In a kinetic study of the analogous
reaction between FSO2OOSO2F and SO2, FSO2(OSO2)2F was
shown to be formed.27
The ClSO3• radical could be formed during the stepwise
thermal decomposition of the peroxide according to eq 9 but
it decomposes rapidly according to eq 10.
ClSO2OOSO2Cl f 2 ClSO3•
2 ClSO3 • f 2 Cl • +2 SO3
(9)
ClSO2 • + Cl • f Cl2SO2
(6)
The initial formation of ClSO2• has been recently proved
by laser irradiation at 355 nm of Ar matrices containing Cl2
and SO2.18 However, in the presence of excess of molecular
oxygen, reaction (6) will be efficiently suppressed because
O2 competes with the Cl• atom to give the peroxy radical
ClSO2OO• (eq 3). There is a large number of precedents
where similar peroxy radicals are formed in the presence of
O2.19-23
It is safe to conclude that the peroxy radicals ClSO2OO•
are involved in the formation of all of the products identified
apart from Cl2SO2. The low-temperature recombination of
ClSO2OO• radicals (eq 7) is the most feasible route leading
to the novel peroxide ClSO2OOSO2Cl. A similar reaction
pathway has been recently reported for several formylperoxy
radical derivatives.24-26
(10)
Unsuccessful attempts to produce the ClSO3• radical by
pyrolysis of ClSO2OOSO2Cl and subsequent matrix isolation
of the products can thus be rationalized.
Dissociation of ClSO2OOSO2Cl was found to be irrevers-
ible. Thermal decomposition to SO3 and Cl2 takes place in
the gas phase according to a first-order rate law with a half-
lifetime of 15 min at ambient temperature. The decomposi-
tion of ClSO2OOSO2Cl in the liquid phase follows the same
channel, as revealed by the growth of bands of both SO3
and Cl2 during the measurements of the Raman spectrum. It
is interesting to note that the ClSO3• radical behaves similarly
28
to the chlorinated alkoxy radicals ClCO2 and CF2ClO,29
which decompose rapidly by chlorine extrusion.
Vibrational Spectra of Bis(chlorosulfuryl)peroxide. The
IR spectrum of bis(chlorosulfuryl)peroxide, ClSO2OOSO2Cl,
was measured for gaseous and Ar matrix-isolated samples,
whereas the Raman spectrum was recorded for the neat
liquid. Figure 1 shows the IR spectrum of ClSO2OOSO2Cl
vapor and the Raman spectrum of the liquid. Experimental
wavenumbers and calculated IR and Raman spectra of the
conformer predicted to be most stable (theoretical calcula-
tions below) are listed in Table 1. A tentative assignment of
some of the more intense bands is also given in Table 1.
However, the DFT calculations predict rather strong coupling
between deformations and S-Cl stretching modes that
prevent any simple descriptive assignment of these low-lying
modes.
The FTIR spectrum of pure ClSO2OOSO2Cl isolated in a
solid Ar matrix is shown in Figure 2. The expected 35/37Cl
isotopic splitting of the S-Cl stretching bands appears to
be obscured by matrix site splittings of the absorptions. In
further experiments using a heated spray-on nozzle, an
additional absorption occurred at 1385.1 cm-1 that was
readily assigned to SO3.30,31 At 95 °C, the decomposition
2 ClSO2OO • f ClSO2OOSO2Cl + O2
(7)
Apart from SO3, that may be formed via the catalytic cycle
(eqs 1-5) outlined above, Cl2SO2 and ClSO2OOSO2Cl are
the main products of the Cl2/SO2/O2 system. Note that the
low stability of ClSO2OOSO2Cl eventually results in a
second chemical cycle (eqs 1-3, 7, and 8) that gives rise to
the oxidation of SO2 to form SO3 via the unstable
ClSO2OOSO2Cl that decomposes according to eq 8. As in
(14) Khanna, R. K.; Pearl, J. C.; Dahmani, R. Icarus 1995, 115, 250–257.
(15) Uthman, U. A.; Demlein, P. J.; Allston, T. D.; Withiam, M. C.;
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J. Chem. Soc., Faraday Trans. 2 1982, 78, 1323–1330.
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J. Am. Chem. Soc. 1985, 107, 1838–1845.
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2843–2846.
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1908 Inorganic Chemistry, Vol. 48, No. 5, 2009

New Peroxide ClSO2OOSO2Cl
Figure 2. FTIR spectrum in the spectral region 1600-400 cm-1 of
ClSO2OOSO2Cl isolated in an Ar matrix.
Table 2. Relative Energies ∆E (in kcal mol-1) of Different Conformers
of ClSO2OOSO2Cl Calculated at the B3P86/6-31+G* Approximation
τ1
τ2
τ
∆E
structure O-O-S-Cl O-O-S-Cl S-O-O-S [kcal mol-1
]
G-G-
G+G-
G+G+
AG-
AG+
AA
73.3
-75.1
-73.4
-170.4
179.8
73.3
69.5
-73.4
72.8
-75.3
179.1
111.0
130.5
153.5
116.5
133.0
120.6
0.00
1.09
1.70
2.74
3.07
5.05
Figure 1. Vapor-phase FTIR spectrum (bottom; resolution: 2 cm-1
pressure: 0.1 mbar, and optical path length of 20 cm) and Raman spectrum
(top; liquid at room temperature, excitation at 1064 nm, 150 mW) of
ClSO2OOSO2Cl.
,
179.1
Table 1. Experimental (Vapor-Phase IR, Liquid Raman and Ar Matrix
IR) and Calculated (B3P86/6-31+G*) Wavenumbers (in cm-1) and
Relative Intensities of ClSO2OOSO2Cl
indicating that only bands of the most stable rotamer are
likely to appear in the IR spectrum of the matrix-isolated
sample and that fast relaxation to the most stable form took
place during low-temperature deposition.
IR and Raman spectra of ClSO2OOSO2Cl were calculated
using the B3P86 functional that has been shown in prelimi-
nary calculations to reproduce satisfactorily the vibrational
spectra of several compounds containing the -SO2- moiety.
The B3P86/6-31+G* approximation was used throughout.
Prior to the vibrational calculations, the most-stable con-
former and possible conformational equilibria have been
studied theoretically.
Assuming a nonplanar (gauche) S-O-O-S structure,
several conformers are feasible, depending on the orientations
of each of the -SO2Cl groups. Relaxed potential energy
curves, in which the O-O-S-Cl torsional angles were
scanned in steps of 10°, yield six minima that are designated
as G-G-, G+G-, G+G+, AG-, AG+, and AA, where G or A
correspond to gauche or anti O-O-S-Cl torsional angles,
and the positive and negative signs refer to an anticlockwise
and clockwise rotation, respectively. The structures of these
six conformers, each of them occurring as enantiomeric pairs,
were optimized by simultaneous relaxation of all of the
geometrical parameters. All of them correspond to stable
structures for which no imaginary frequencies occur.
Table 2 presents the relative energy differences of the six
conformers together with their optimized torsional angles.
The most stable structure according to the B3P86/6-31+G*
approximation corresponds to the G-G- form (Figure 3),
followed by the G+G- conformer that is predicted to lie
approximately 1 kcal mol-1 to higher energy. This result is
Raman
IR (gas)a (liquid)a IR (Ar matrix)a,b B3P86/6-31+G*c assignment
I (IR)d I (Ra)d
1450(m)
1463(vs) 1401(w)
1464.5(m)
1454.8(s)
1438.3(w)
1220.3(sh)
1216.7(s)
1199.9(s)
808(vw)
1443 14
1435 48
30
8
νasSO2
νasSO2
1234(sh)
1222(s) 1212(vs)
1205(s)
1195 9
1180 16
904 <1
100
<1
52
νsSO2
νsSO2
νO-O
817(sh)
807(vs)
720(w)
711(vs) 698(w)
755 <1
664 100
580 25
570 37
16
3
41
2
2
5
νS-O
νS-O
δSO2
δSO2
FSO2
FSO2
708.2(vs)
611.7(vs)
599.1(vs)
557.3(w)
521.1(w)
503.5(w)
485.3(s)
613(s)
600(vs)
543
515
7
3
488(m)
449
9
5
419(vs)
391 <1
387 <1
350 <1
342 <1
36
25
8
13
2
νSCl
νSCl
371(w)
347(vw)
316(vw)
314
1
273 <1
263 <1
181 <1
124 <1
<1
11
3
8
3
281(w)
203(vw)
143(w)
84(w)
281(w)
142(w)
57
49
39
<1
<1
<1
1
1
a Relative intensities in parentheses: w weak, m medium, s strong, vs
very strong, sh shoulder. b Most-abundant matrix site. c Most-stable rotamer.
d Predicted relative intensities.
of ClSO2OOSO2Cl was almost complete, with the SO3 signal
being the most intense feature in the IR spectrum. No clear
signs attributable to a second conformer of the peroxide were
observed in the spectra taken at different nozzle temperatures,
Inorganic Chemistry, Vol. 48, No. 5, 2009 1909

Romano et al.
approximation. The results for the three most stable forms
in the experimental range (above 400 cm-1) is presented in
Table S1 of the Supporting Information. The theoretical
spectra of the radicals ClSO2OO · and ClSO3 · were also
calculated with the same approximation (Tables S2 and S3
of the Supporting Information). In light of these theoretical
results, we can conclude that there are no clear signs in the
experimental spectra of any of these species.
Acknowledgment. C.O.D.V. and R.M.R. thank the Con-
sejo Nacional de Investigaciones Cient´ıficas y Te´cnicas
(CONICET) (PIP 4695), the Agencia Nacional de Promocio´n
Cient´ıfica y Tecnolo´gica (PICT 33878), the Comisio´n de
Investigaciones Cient´ıficas de la Provincia de Buenos Aires
(CIC), the Facultad de Ciencias Exactas, Universidad Na-
cional de La Plata, and the DAAD for financial support. H.B.
and H.W. acknowledge support from the Deutsche Fors-
chungsgemeinschaft (DFG) and the Fonds der Chemischen
Industrie.
Figure 3. B3P86/6-31+G* molecular model of the most-stable conformer
G-G- of ClSO2OOSO2Cl.
in agreement with the structure reported for FSO2OOSO2F.32
The electron diffraction pattern produced by gaseous
FSO2OOSO2F was found to be consistent with either an
equilibrium between G-G- and G+G+ or a mixture of the
G-G- and G+G- forms.
Supporting Information Available: Theoretical wavenumbers
(in cm-1) of the G-G- G+G- and G+G+ forms of ClSO2OOSO2Cl
and of ClSO3 and ClSO2OO the radical above 400 cm-1 calculated
with the B3P86/6-31+G* approximation. This material is available
free of charge via the Internet at http://pubs.acs.org.
The vibrational spectra of the different conformers of
ClSO2OOSO2Cl were simulated with the B3P86/6-31+G*
(32) Hagen, K.; Hedberg, K.; Gard, G.; Aubke, F. J. Mol. Struct. 2001,
567-568, 1–10.
IC802143P
1910 Inorganic Chemistry, Vol. 48, No. 5, 2009