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M.E. Tucceri et al. / Chemical Physics Letters 404 (2005) 232–236
the products of the reaction between FS(O2)OO(O2)SF
and CO [3] indicates that the reaction FSO2 +
FSO3 ! F2SO2 + SO3 (which is exothermic in
58.2 kcal molꢀ1 [11,19,29] but probably an activated
process) does not occur. On the other hand, due to the
fact that reaction FSO2 + FSO3 ! SO2 + FS(O2)OF is
endothermic in 10.9 kcal molꢀ1 [11,19], we conclude that
the only reaction between FSO2 and FSO3 radicals is the
barrierless recombination (1) (see Section 3.2). From the
slope of the second-order kinetic decay plot illustrated in
the insert of Fig. 1, a ratio k1/r(FSO3)l = 1.82 ·
107 cm sꢀ1 was obtained. Then, the value k1 = 6.6 · 10
ꢀ11 cm3 moleculeꢀ1 sꢀ1 was derived. In the following
we analyze the eventual occurrence of secondary reac-
tions. The microsecond timescale of the experiments
indicates that other FSO3 loss processes are unimpor-
tant. In fact, the low values measured for the limiting
high-pressure rate coefficients of reactions (2) and (3):
way. In average, the inclusion of reactions (5a) and (5b)
and r(FSO2) increases k1 by 8–13% while an upper limit
for k5 of about 2 · 10ꢀ13 cm3 moleculeꢀ1 sꢀ1 was ob-
tained. The small increase in k1 lies within our stated
error limits of about 17% which results from the above-
mentioned uncertainty in r(FSO3), and an estimated sig-
nal noise of about 10–15%. The obtained r(FSO2) ꢁ 4 ·
19ꢀ19 cm2 moleculeꢀ1 is in reasonable agreement with
the quantum chemistry predictions discussed in Section
3.2. Nevertheless, in the absence of a realistic experimen-
tal value for r(FSO2), and due to the fact that the uncer-
tainties on the derived k1 values are small, only the
reaction (1) was considered in the present analysis.
The derived rate coefficients were found independent
of total pressure and were therefore averaged to give
the limiting high-pressure rate coefficient,
k1; 1 ¼ ð6:5 ꢂ 1:1Þ ꢃ 10ꢀ11 cm3 moleculeꢀ1 sꢀ1
:
This value can be compared with those measured for
other recombination reactions of the FSO3 radical in
the high pressure region. It is similar to the rate coeffi-
cient for the reactions of FSO3 with F atoms,
7.6 · 10ꢀ11 cm3 moleculeꢀ1 sꢀ1 [6] and Cl atoms,
6.0 · 10ꢀ11 cm3 moleculeꢀ1 sꢀ1 [8]. However, k1, 1 is lar-
ger than the rate coefficient measured for the reaction of
FSO3 with FC(O)O radicals of 1.0 · 10ꢀ12 cm3 mole-
culeꢀ1 sꢀ1 [10,11], and even markedly larger than that
determined for the reaction (2) of 4.5 · 10ꢀ14 cm3 mole-
culeꢀ1 sꢀ1 [7,9].
FSO3 þ FSO3 þ M ! FSðO2ÞOOðO2ÞSF þ M
FSO3 þ CO þ M ! FSðO2ÞOCO þ M
ð2Þ
ð3Þ
of (4.5 0.2) · 10ꢀ14 cm3 moleculeꢀ1 sꢀ1 [7,9] and
(4.3 0.9) · 10ꢀ17 cm3 moleculeꢀ1 sꢀ1 [11] (activation
energy of 6.8 kcal molꢀ1 [3,5]) preclude their participa-
tion in the reaction mechanism. On the other hand, no
experimental kinetic data are available for reactions
(4a)–(4d)
FSO2 þ CO þ M ! FSðOÞOCO þ M
FSO2 þ CO ! FSO þ CO2
ð4aÞ
ð4bÞ
ð4cÞ
ð4dÞ
3.2. Quantum chemical calculations and SACM/CT
limiting high-pressure rate coefficients
FSO2 þ CO þ M ! FðO2ÞSCO þ M
FSO2 þ CO ! SO2 þ FCO
The ab initio results detailed in Section 3.2 demon-
strate that any of these processes play a role here. Finally,
the FSO2 consumption by the self-reaction processes
(5a) and (5b):
All quantum chemical calculations were carried out
using the GAUSSIAN 98 program package [30]. The ener-
getics of reactions (4a)–(4d) was calculated using the
G3(MP2)B3 model chemistry (mean accuracy slightly
larger than 1 kcal molꢀ1) [31]. The large values obtained
for the activation energies of reactions (4a) and (4b) of
25.1 kcal molꢀ1 (imaginary frequency m# = 147i cmꢀ1
)
FSO2 þ FSO2 þ M ! FðO2ÞSSðO2ÞF þ M
FSO2 þ FSO2 ! F2SO2 þ SO2
ð5aÞ
ð5bÞ
and 19.8 kcal molꢀ1 (m# = 885i cmꢀ1) suggest very low
rate coefficients for these processes. The calculations
show that reaction (4c) is exothermic in only
0.83 kcal molꢀ1 and, thus, the weakly bound F(O2)SCO
adduct initially formed is not stabilized at all. The
abstraction reaction (4d) was found to be endothermic
in 4.3 kcal molꢀ1. This fact and the abovementioned ab-
sence of FCO allow us to discard this process. There-
fore, the quantum chemical calculations indicate that,
under the present experimental conditions, the FSO2
radical does not react with CO.
was considered. For this, a numerical simulation of the
detected absorption signals with the mechanism formed
by reactions (1), (5a) and (5b) was performed. The calcu-
lations lead to a fit identical to the depicted in the insert
of Fig. 1 when the values [FSO3]t = 0 = 1.2 · 1015 cm3
moleculeꢀ1
,
k1 = 6.6 · 10ꢀ11 cm3 moleculeꢀ1 sꢀ1 and
k5 = k5a + k5b = 1.5 · 10ꢀ13 cm3 moleculeꢀ1 sꢀ1 are used.
Further simulations including r(FSO2) as a free param-
eter, lead to [FSO3]t = 0 = 1.1 · 1015 cm3 moleculeꢀ1, k1 =
To complement the above numerical simulations, the
absorption spectrum of FSO2 is studied using time
dependent DFT calculations [32]. To this end, fully opti-
mized geometries were first calculated using the BeckeÕs
three-parameters functional [33] with the correlation
7.3 · 10ꢀ11 cm3 moleculeꢀ1 sꢀ1
,
k5 = 1.6 · 10ꢀ13 cm3
moleculeꢀ1 sꢀ1 and r(FSO2) = 4.2 · 19ꢀ19 cm2 mole-
culeꢀ1. Ten recorded signals with CO pressures ranging
from about 130 to 800 mbar were analyzed in a similar