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The Trifluoromethoxy Sulfuryl Radical CF3OSO2.

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Communications
Atmospheric Chemistry
A gas mixture of CF3OC(O)OOCF3, SO2, and Ar
(1:10:1000) was thermolyzed under low pressure and subsequently quenched as an inert-gas matrix. The first reaction
step corresponds to the thermal decay of the peroxide
[Eq. (1)], which is also observed in the absence of SO2.[5]
The Trifluoromethoxy Sulfuryl Radical,
CF3OSO2C**
Stefan von Ahsen* and Joseph S. Francisco
CF3 OCπOήOOCF3 ! 2 CF3 OCώCO2
Sulfur oxides like SO2 and SO3 play a major role in the
chemistry of the atmosphere such as in the production of acid
rain and smog. In addition, the heterogeneous chemistry of
H2SO4 in stratospheric clouds is a topical research area in
atmospheric chemistry. Much is known about the conversion
of SO2 into H2SO4 in homogeneous oxidation reactions in the
atmosphere. However, little is known about the interaction of
SOx species with CF3Oy radicals in the atmosphere. Since
chlorofluorocarbons (CFCs) cause ozone depletion in the
stratosphere, these materials are being replaced by alternatives such as hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs). Both CFCs and their replacements are
known to be sources of CF3 and CF3Oy radicals.[1] It is
commonly accepted that the main fate of CF3O radicals is
their reaction with NO or CH4.[2] While methane is present in
the lower atmosphere with a mixing ratio of around 1800 ppb
(parts per billion, molecules per 109 molecules) and NO
reaches only concentrations of around 1 ppb, the reaction of
CF3OC with nitrogen monoxide is roughly 2000 times faster
than its reaction with methane.[3, 4] A coupling between the
chemistry of CF3Oy and SOx in atmospheric processes has not
been addressed in the literature although the SO2 mixing ratio
ranges from 1 to 1000 ppb depending on the air quality and is
hence not too far away from the methane concentration.[3, 4]
To our knowledge no kinetic data for the reaction of CF3OC
with SO2 have been published. We present here the first direct
spectroscopic identification of the CF3OSO2 radical isolated
in an inert-gas matrix generated by reaction of CF3O radicals
with SO2. Thus we demonstrate the existence of the first link
between CnFmOy species and SOx in the atmosphere.
[*] Dr. S. von Ahsen
Bergische Universitt Wuppertal
Fachbereich C/Anorganische Chemie
42097 Wuppertal (Germany)
Fax: (+ 49) 202-439-3052
E-mail: von.ahsen@freenet.de
Prof. Dr. J. S. Francisco
Department of Chemistry, Earth and Atmospheric Sciences
Purdue University
1393 H. C. Brown Building
West Lafayette, IN 47907 (USA)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie. J.S.F. thanks the
Alexander von Humboldt Foundation for a research award for senior
U.S. scientists. The experiments were carried out in the laboratories
of Prof. Dr. H. Willner at the Universities of Duisburg (Germany)
and Wuppertal (Germany). We want to express special thanks for
his support and for helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3330
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
π1ή
In the IR spectra of the thermolysis products isolated in
matrix the absorptions of CF3OC are present in addition to the
bands of SO2, CO2, and COF2 and weak absorptions of a
previously unknown species. The absorptions of the unidentified compound strongly increase when the matrix is
annealed at 35 K while the CF3OC absorptions[5] disappear.
Hence, in the matrix and in the gas phase or during the
condensation process CF3O radicals react. Due to the lack of
IR bands of CF3OOCF3 in the product spectrum, recombination of CF3O radicals can be ruled out. Neither the formation
of FSO2C was observed, as no IR absorption in the typical SF
stretching region occurs, nor the generation of COF2 was
detected during annealing. Hence, the transfer of fluorine
from CF3OC to SO2 can be excluded. The fluorophosgene
present in the matrix was formed during the thermolysis by
secondary reactions of CF3OC with the walls of the reaction
vessel. Finally only an interaction of CF3OC with SO2 according to Equation (2) is consistent with the observations.
CF3 OCώSO2 πώMή ! CF3 OSO2 C πώMή
π2ή
Although it is reasonable to assume that the new species is
CF3OSO2C, and the recorded IR spectrum is reproduced by
quantum chemical calculation very well (see below), we
needed additional experimental proof of the existence of the
new radical. For this purpose the annealed and recooled
(16 K) product matrix, which does not contain any CF3O
radicals, was irradiated with UV light (l > 280 nm). While the
absorptions of the new formed compound disappear, fluorophosgene and SO2 form in the photochemical reaction. The
fluorine atoms that presumably form are not detectable by IR
spectroscopy. An explanation is that UV light induces the
back-reaction of CF3OSO2 formation, and the resulting CF3O
radicals are photolyzed to give COF2 and fluorine atoms in a
second step. Since CF3O photolysis with light of l > 280 nm is
known to be slow,[5] a more favored way to fluorophosgene
and FC formation is that excited CF3OSO2 dissociates into SO2
and excited CF3OC, which subsequently decays into COF2 and
FC, yielding a net reaction according to Equation (3).
CF3 OSO2 Cώhn ! SO2 ώCOF2 ώF C
π3ή
The formation of SO3 and CF3C was not observed. This is in
contrast to the behavior of the CF3OCO radical, which mainly
photodissociates into CO2 and CF3C.[6] In the case of CF3OCOC,
generation of CO2 and CF3C is an exothermic process, while
the dissociation of CF3OSO2C into CF3C and SO3 is calculated
to be endothermic. It is interesting to note that also no
formation of FSO2C (from FC+SO2) was detected, and the
FSO2 radical remains?to our knowledge?undiscovered.
DOI: 10.1002/anie.200453925
Angew. Chem. Int. Ed. 2004, 43, 3330 ?3333
Angewandte
Chemie
A difference IR spectrum from before and after photolysis of the matrix is shown in Figure 1. The absorptions of the
photolabile compound, which decrease in the course of the
photolysis, are depicted with positive extinction values while
Figure 1. IR spectrum of the thermolysis products of a CF3OC(O)OOCF3/SO2/Ar mixture isolated as a matrix after annealing
(upper trace). The depicted spectrum is the difference before and after
irradiation of the reaction products with UV light. The bands that disappear on irradiation point upwards; the bands corresponding to the
photolysis products point downwards. Those assigned to CF3OSO2C are
marked with an asterisk (*).The simulated IR spectrum (lower trace) is
down shifted by 0.2 absorbance units.
those of the photolysis products correspond to negative
bands. In the region of the SO2 stretching frequencies around
1350 and 1150 cm1 incompensation occurs, but the net
production of SO2 is evident from the increasing intensity of
the SO2 deformation band. The bands assigned to CF3OSO2C
are marked with an asterisk (*). Nevertheless, some absorptions that also vanish upon UV photolysis are not due to
CF3OSO2C as they did not clearly grow during the preceding
annealing period. The origin of these bands remains unclear.
We observed 11 absorption bands that increase during
annealing of the thermolysis product matrix and vanish upon
UV irradiation of the matrix. In Table 1 the assignment of
these bands to the CF3OSO2 radical and the description of the
IR modes are listed. The mode descriptions are based on the
calculated data and can be only approximate due to strong
vibrational coupling.
According to the calculations CF3OSO2C exhibits C1
symmetry (close to Cs), and all 18 fundamentals are IR
active. The normal modes can roughly be described as seven
stretching, nine deformation (five from the CF3O unit, three
from the SO2 unit, and one COS angle deformation), and two
torsion modes. In addition, the IR spectrum confirms the
connectivity to be CF3OSO2 ; an isomer like CF3OOSO would
display an OO stretching absorption (expected between 800
and 900 cm1) and an unusual high frequency of the S=O
stretching mode, both of which were not found experimentally. The use of isotopically labeled S18O2 resulted in isotopic
shifts of the observed absorptions that are in agreement with
Angew. Chem. Int. Ed. 2004, 43, 3330 ?3333
Table 1: Vibrational data of CF3OSO2 isolated in SO2-doped Ar matrix.
Argon matrix
n? [cm1] Irel.[b]
B3LYP/6-311 + +
B3LYP/6-31 + G(d) Description
G(3df,3pd)
(approx.)[a]
1
1
1
1
n? [cm ] I [km mol ] n? [cm ] I [km mol ]
1325
1266
1227
1178
40
55
50
100
1322
1237
1204
1163
176
320
351
304
1087
87
926
26
922
69
908
35
741
13
720
674
92
1.0
693
658
77
0.48
553
22
615
543
7.5
67
602
522
6.9
72
514
470
[d]
8
506
464
28
24
478
444
28
26
417
321[e]
4
1
410
313
303
147
15
1.6
3.0
0.65
400
292
279
140
21
2.0
4.3
0.93
79
0.47
70
0.42
49
0.55
40
0.69
1091 360
1251
1223
1201
1168
349
190
401
474
1046 183
na(SO2)
na(CF3)
na(CF3)
ns(CF3)/
ns(SO2)/
n(CO)
ns(SO2)/
n(CO)/
ns(CF3)
ns(CF3)/
n(CO)
n(OS)
d(CF3)/
d(COS)
d(CF3)
d(CF3)/
g(SO3)[c]
d(SO2)
d(SO2)/
d(CF3)
1(CF3)
d(SO2)
1(CF3)
d(COS)/
1(CF3)
t(FCOS)/
t(COSO2)
t(COSO2)/
t(FCOS)
[a] Vibration types: n = stretching, d,1 = deformation; t = torsion, g = outof-plane. [b] Relative integrated intensities. [c] Although the OSO2 group is
not planar, the S atom moves nearly perpendicular to the plain of the three
O atoms. [d] Overlapping with d(SO2). [e] From a F-IR spectrum not shown
here.
values predicted by DFT calculations. The assignment
involves only one isomer of CF3OSO2C, although the appearance of two torsional modes may indicate the existence of
more than one rotamer. In the calculations the dihedral angle
b(FCOS) leads, if varied from + 1808 to 1808 during relaxed
scans, to three identical minimum-energy structures, each
with one fluorine atom of the CF3 group nearly trans to the
OS bond. The calculated energy barrier is 5.9 kJ mol1. The
COSO dihedral angle was varied from 1808 (a Cs configuration with a CF3-O-S(O)-O structure and all oxygen atoms
in the symmetry plane) to + 1808. Two minima were found
with C1 symmetry, identical energy, and structure, which
describe two enantiomeric forms of the radical. In all
calculations the two terminally bound oxygen atoms at the
sulfur atom avoid becoming symmetry-equivalent, forcing the
radical into C1 symmetry instead of Cs. Hence, CF3OSO2C
should exist only in one distinguishable isomeric form.
The calculated structure (B3LYP/6-311 + + G(3df,3pd))
of one enantiomer is shown in Figure 2. The CF, CO, and
S=O bond lengths are in the expected range. The length of the
OS single bond, 1.686 G, is also not unusual. The C-O-S
angle is calculated to be 120.48. Some structural parameters
and calculated charges and spin densities are listed in Table 2.
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3331
Communications
DGB = 13.9 kJ mol1. The negative free enthalpy value
indicates a spontaneous generation of the trifluoromethoxy
sulfuryl radical from CF3OC and sulfur dioxide not only in the
matrix but also in the atmosphere.
Based on the now-proven existence of CF3OSO2C further
reaction pathways for CnFmOy radicals as well as for SO2 must
exist. In analogy to the reactions of the related CF3OCO
radical,[6, 8?10] conversion of the sulfuryl radical into a peroxy
radical [Eq. (4)] should be the most important reaction of the
CF3OSO2 radical.
Figure 2. Calculated [B3LYP/6-311 + + G(3df,3pd)] structure of
CF3OSO2C.
CF3 OSO2 CώO2 πώMή ! CF3 OSπOή2 OOC πώMή
Table 2: Calculated [B3LYP/6-311 + + G(3df,3pd)] structural parameters
and properties of CF3OSO2C.
Property[a]
Coordinate
r(OS) [D]
r(CO) [D]
r(CF) [D]
r(S=O) [D]
a(COS) [8]
a(FCO) [8]
a(OS=O) [8]
b(FCOS) [8]
b(COS=O) [8]
Symmetry: C1
1.686
1.380
1.327
1.336
1.324
1.437
1.441
120.4
111.7
111.6
106.9
102.6
109.0
164.1
175.9
q(S) [e]
q(O) [e]
q(F) [e]
q(=O) [e]
q(C) [e]
1s(S) [a.u.]
1s(=O) [a.u.]
1s(O) [a.u.]
DEdiss[b] [kJ mol1]
DHdiss[b] [kJ mol1]
DGdiss[b] [kJ mol1]
+0.81
0.66
0.41
0.42
0.40
0.34
0.34
+1.76
0.47
0.22
0.22
0.11
The calculated spin density distribution, which indicates
the delocalization of the unpaired electron within the
complete SO2 moiety, corresponds to about 0.47 atomic
units (a.u.) on the S atom, 0.22 a.u. on each of the doublebonded O atoms, and about 0.1 a.u. on the bridging O atom.
This results are in agreement with published data from a
previous ESR experiment, in which a spectrum was assigned
to CF3OSO2 radicals proposed to arise from CF3OS(O)2Cl
and (CH3)3Si radicals by Cl atom abstraction in toluene at low
temperatures.[7]
A relaxed-scan calculation on the SO distance at the
B3LYP/6-31 + G(d) level of theory results in an anharmonic
potential, which is roughly described by a Morse function.
The potential has a depth of 51.2 kJ mol1; with the expanded
basis set 6-311 + + G(3df,3pd) the potential depth increases
to 71.9 kJ mol1. At higher levels of theory (including more
polarization and diffuse functions) the OS bond strength and
the dissociation enthalpy (dissociation into CF3O and SO2)
increase. For the formation of CF3OSO2C according to
Equation (2) at standard conditions the following thermodynamic values were calculated: DHB = 63.4 kJ mol1 and
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
In general, peroxy radicals are reduced by trace gases in
the atmosphere such as NO to yield the corresponding oxy
radicals. In addition, the conversion of SO2 (SIV) into a sulfur
species with sulfur in the + 6 oxidation state by the reaction
of CF3O radicals may, in principle, enhance or accelerate
H2SO4 production from SO2.
The CF3OSO2 radical may also be an intermediate in the
formation of CF3OS(O)2X compounds, where X = OH,
halogen, or OR. These compounds include some very stable
and very acidic species. Hence, the CF3OSO2 radical may
contribute to the formation of superacids in the atmosphere.
Up until now the occurrence of superacids in the atmosphere
and the possible consequences have not been considered.
Therefore future work must address the impact of fluorocarbon sulfur compounds on atmospheric chemistry.
Experimental Section
+71.9
+63.4
+13.9
[a] r = bond length, q = charge, 1s = spin density. [b] Dissociation:
CF3OSO2C!CF3OC+SO2.
3332
π4ή
CF3OC(O)OOCF3 was prepared as a by-product of the synthesis of
CF3OC(O)OOC(O)OCF3 described in the literature.[9] Dry SO2 was
prepared by reaction of commercial SOCl2 (99 %, Merck, Darmstadt,
Germany) with MgO powder (> 98 %, Riedel de Haen, Seelze,
Germany) in fivefold excess at room temperature over two days. For
the isotopic labeling, a mixture of S16O2, S18O16O, and S18O2 was
prepared by hydrolysis of SCl2 with H216O/H218O mixture (where
elemental sulfur and HCl are formed as well). All compounds were
purified by trap-to-trap condensation in a glass vacuum line. The
matrix experiments used a gas mixture of CF3OC(O)OOCF3, SO2,
and argon (99.9999 %, Messer-Griesheim, Krefeld, Germany)
(1:10:1000), which was prepared and stored in a stainless-steel
vacuum line which was connected to the thermolysis nozzle by a
stainless-steel capillary. The thermolysis device was mounted directly
in front of the matrix support and held under high vacuum. The nozzle
was heated to 633 K while the matrix support was cooled to 16 K. In a
typical experiment 1 mmol of the mixture was deposited in 15 min.
The sample was then annealed to 35 K for 5 min and recooled to 16 K.
Photolysis experiments were carried out with a 150-W Xe highpressure lamp (AMKO, Tornesch, Germany) in combination with cutoff filters (Schott, Mainz, Germany) with an irradiation time of 2 min.
Details of the matrix apparatus are given elsewhere.[11]
IR spectra were recorded with a FT-IR spectrometer (IFS66v/S,
Bruker, Ettlingen, Germany) with an apodized resolution of 0.5 cm1,
and 64 scans were coadded. For measurements in the middle-IR
range a KBr beamsplitter, a KBr window, and a DTGS detector were
used; in the far-IR range a Ge-covered 6-mm Mylar foil beamsplitter,
a PE window, and a DTGS detector were used. UV spectra were
recorded with a Lambda 900 spectrometer (Perkin-Elmer, Norwalk
CT, USA) with a step size of 1 nm (1 s integration time, slid width
1 nm) and the use of a quartz cable optic (Hellma, Jena, Germany).
All spectra were detected in reflection.
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Angew. Chem. Int. Ed. 2004, 43, 3330 ?3333
Angewandte
Chemie
All calculations were performed using the Gaussian98 software
package.[12] For accurate structures, energies, and vibrational frequencies the density functional[13] method B3LYP[14, 15] with a 6-311 +
+ G(3df,3dp) basis set was used. Relaxed-scan calculations along one
internal coordinate were conducted using the smaller 6-31 + G(d)
basis set and the minimum-energy structures were checked by
geometry optimization calculations using the larger basis set. The
simulation of the IR spectrum of CF3OSO2 is based on the calculated
[B3LYP/6-311 + + G(3df,3dp)] band positions and intensities. A
Gaussian band shape with a full width at half-maximum of 10 cm1
was applied. The results of the relaxed-scan calculations are found in
Figures S1?S4 in the Supporting Information.
Received: February 4, 2004 [Z53925]
.
Keywords: atmospheric chemistry · density functional
calculations · IR spectroscopy · matrix isolation · radicals
[1] J. S. Francisco, M. M. Maricq, Adv. Photochem. 1995, 21, 79.
[2] J. S. Francisco, M. M. Maricq, Acc. Chem. Res. 1996, 29, 391.
[3] E. Uherek, Atmospheric trace gas mixing ratios, Max Planck
Institut fMr Chemie, Mainz, Germany, 2004 (www.atmosphere.mpg.de/enid/25i.html, 2004).
[4] Kinetic data from various studies: NIST webbook (http://
kinetics.nist.gov) and references therein.
[5] G. A. ArgMello, H. Willner, J. Phys. Chem. A 2001, 105, 3466.
[6] S. von Ahsen, J. Hufen, H. Willner, J. S. Francisco, Chem. Eur. J.
2001, 8, 1189.
[7] C. Chatgilialoglu, D. Griller, S. Rossini, J. Org. Chem. 1989, 54,
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[9] G. A. ArgMello, H. Willner, F. E. Malanca, Inorg. Chem. 2000,
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[10] M. A. Burgos Paci, G. A. ArgMello, P. GarcNa, H. Willner, Int. J.
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[11] H. SchnOckel, H. Willner in Infrared and Raman Spectroscopy.
Methods and Applications (Ed.: B. Schrader), VCH, Weinheim,
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[12] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. J. A. Montgomery,
R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D.
Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
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Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B.
Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M.
Head-Gordon, E. S. Replogle, J. A. Pople, GAUSSIAN 98 Rev.
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[13] W. Kohn, L. J. Sham, Phys. Rev. 1965, 140, 1133.
[14] A. D. Becke, Phys. Rev. A 1988, 38, 3098.
[15] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
Angew. Chem. Int. Ed. 2004, 43, 3330 ?3333
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