Ion chemistry of sulfuryl fluoride: An experimental and theoretical study on gas-phase reactions involving neutral and ionized SO2F2

Article abstract of DOI:10.1016/j.ijms.2013.06.001

The gas phase ion chemistry of sulfuryl fluoride is studied by ion trap mass spectrometry and ab initio calculations. Reactions of ions of atmospheric relevance with neutral SO₂F₂ mainly result in SO ₂F₂ depletion by dissociative electron transfer. In few cases, a different reaction mechanism involving F-abstraction is invoked, since dissociative ion products are observed despite the electron transfer channel being endothermic. Ab initio calculations revealed a nearly perfect distonic structure for the molecular SO₂F₂⁺ ion, whose capability of activating strong HX bonds (X = C, N, O) is ascribable to the high spin density located on the oxygen atoms, in line with literature reports. Among the ions produced by electron ionization of SO₂F₂, the FSO_x⁺ (x = 0-2) ions are capable of activating the HNH₂ bond of ammonia. Theoretical investigation revealed that NH ₃ activation by SF⁺ requires a triplet to singlet conversion along the reaction pathway. This conversion is expected to be fast, the conceivable reaction rate determining step being the subsequent intramolecular hydrogen migration.

Full text of DOI:10.1016/j.ijms.2013.06.001

International Journal of Mass Spectrometry 354–355 (2013) 46–53
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
Ion chemistry of sulfuryl fluoride: An experimental and theoretical
study on gas-phase reactions involving neutral and ionized SO2F2
Paola Antoniottia, Paola Benzia, Lorenza Opertia, Roberto Rabezzanaa,∗
,
Stefano Boroccib, Maria Giordanib, Felice Grandinettib,∗∗
a Dipartimento di Chimica, Università degli Studi di Torino, Via P. Giuria, 7, 10125 Torino, Italy
b Dipartimento per la Innovazione nei sistemi Biologici, Agroalimentari e Forestali (DIBAF), Università della Tuscia, L.go dell’Università, s.n.c.,
01100 Viterbo, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The gas phase ion chemistry of sulfuryl fluoride is studied by ion trap mass spectrometry and ab ini-
tio calculations. Reactions of ions of atmospheric relevance with neutral SO2F2 mainly result in SO2F2
depletion by dissociative electron transfer. In few cases, a different reaction mechanism involving F-
abstraction is invoked, since dissociative ion products are observed despite the electron transfer channel
being endothermic. Ab initio calculations revealed a nearly perfect distonic structure for the molecular
Received 1 March 2013
Received in revised form 24 May 2013
Accepted 4 June 2013
Available online 17 June 2013
+
SO2F2 ion, whose capability of activating strong H X bonds (X = C, N, O) is ascribable to the high spin
In memory of Detlef Schröder, a brilliant
scientist and a lovable person.
density located on the oxygen atoms, in line with literature reports. Among the ions produced by electron
+
ionization of SO2F2, the FSOx (x = 0–2) ions are capable of activating the H NH2 bond of ammonia. The-
oretical investigation revealed that NH3 activation by SF+ requires a triplet to singlet conversion along
the reaction pathway. This conversion is expected to be fast, the conceivable reaction rate determining
step being the subsequent intramolecular hydrogen migration.
Keywords:
Sulfuryl fluoride
Gas phase ion chemistry
Ab initio calculations
Ion trap mass spectrometry
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
potential of 4780 (100 years time horizon) [7]. A continued atmo-
spheric monitoring of SO2F2 was therefore recommended [8,10].
Sulfuryl fluoride (SO2F2) is a gaseous compound extensively
used to fumigate buildings, ships, vehicles, and wood products [1].
It is also employed in post-harvest treatments as a replacement
of the ozone-depleting methyl bromide [2,3], phased-out under
the Montreal Protocol (it is however worth mentioning that, in the
context of reassessment of fluoride risk for human health, the use
of SO2F2 in stored and processed food commodities, and in food
processing and handling facilities is currently under scrutiny [4,5]).
Once released in the atmosphere, SO2F2 is inert or only marginally
reactive toward OH, O3, and O(1D), and is not photolyzed by low-
energy radiations [6–9]. The only sink is the hydrolysis in the
ocean, and this results in a relatively long globally-averaged atmo-
spheric lifetime of 36 11 years [10]. Consistently, in the last three
decades, the atmospheric mixing ratio of SO2F2 increased by 5 1%
per year up to a 2008 mean global concentration of ca. 1.4 parts
per trillion [10]. This accumulation is of concern, as SO2F2 is a
potent greenhouse gas [6–8], with an estimated global warming
While inert toward neutral atmospheric oxidants, SO2F2 could
be destroyed by ionic species present in the upper atmosphere. Lab-
oratory studies have already shown that SO2F2 efficien−tly reacts
with simple nucleophiles such as O−, OH−, CH3O−, NH2 [11,12],
and F2− [13] by substitution and/or addition–elimination reactions.
Reactions with positive ions of atmospheric interest are instead
still uncovered. As a matter of fact, only limited information is
still available on the positive ion chemistry of SO2F2. Sullivan and
Beauchamp noted so far [14] that ionized SO2F2 does not undergo
self condensation, but they observed that SO2F2+ reacts with SO2FCl
and SO2Cl2 by charge transfer and F-displacement. They noted
+
also that FSO2 reacts with SO2FCl by chloride-abstraction. The
+
+
complexes of SO2F2 with CH3 and t-C4H9 were subsequently
investigated by high-pressure mass spectrometry [15,16], and the
protonation of SO2F2 was also so far investigated [17,18], and
recently revisited [19]. Based on these considerations, and stimu-
latedalsobyourcontinuinginterest forthe gas-phaseionchemistry
of fluorinated inorganic compounds [20–24], we decided to explore
the reactions of SO2F2 with a series of positive ions, including
species of atmospheric interest, as well as the reactions of ionized
SO2F2 with exemplary substrates such as D2, H2O, CH4, and NH3.
Our experimental and theoretical results will be discussed in the
present article.
∗
Corresponding author. Tel.: +39 011 6707587; fax: +39 011 2367587.
Corresponding author. Tel.: +39 0761 357126; fax: +39 0761 357111.
E-mail addresses: roberto.rabezzana@unito.it (R. Rabezzana), fgrandi@unitus.it
∗∗
(F. Grandinetti).
1387-3806/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ijms.2013.06.001

P. Antoniotti et al. / International Journal of Mass Spectrometry 354–355 (2013) 46–53
47
Table 1
2. Experimental and computational details
Product ions, suggested neutral products, phenomenological rate constants at 333 K
(k; 10−10 cm3 molecule−1 s−1), reaction efficiencies (k/kcoll), and reaction enthalpiesa
Sulfuryl fluoride was purchased from ABCR at 99% purity grade,
and was used without further purification. The reactant ions were
formed by electron ionization (EI) of H2O (OH+, H2O+ and H3O+),
(ꢀrH298 ; kJ mol−1) for the reactions of positive ions (M+) with SO2F2. The recombi-
◦
nation energy (RE) of M+ is in eV. The ionization energy of SO2F2 is 13.04 eV.
◦
Reagent ion (RE)
Product ions
Neutral
k
k/kcoll
ꢀrH298
N
2 (N2+), N2O (NO+ and N2O+), CO2 (CO+ and CO2+), CH4 (CH4+ and
products
CH5+), NH3 (NHn+; n = 1–4), Ng (Ng+; Ng = Ne, Ar, Kr, Xe), and SO2F2
H3O+ (6.27)
NO+ (9.26)
SF+ (10.1)
No reaction
No reaction
FSO+
No reaction
No reaction
SO2F2H+
(S+, SO+, SF+, SO2+, FSO+, FSO2+ and SO2F2+). To avoid filament burn-
ing in the ion source, O+ and O2 were obtained from a N2/O2 = 4:1
+
F2SO
OH
0.45 0.035
−84.3b
−12.6d
+
mixture.
O2 (12.07)
Xe+ (12.13/13.44)c
H2O+ (12.621)
N2O+ (12.89)
OH+ (13.017)
O+ (13.62)
The experiments were performed with
a Finnigan ITMS
9.6
0.50
instrument (Austin, TX, USA) maintained at 333 K. The chosen
temperature value was the lowest possible to avoid ion thermal
excitation and, at the same time, to get a constant temperature
during the experiment. In fact, if a lower T value is set, the onset
of the ionization filament during mass spectra acquisition deter-
mines an increase of the ion trap temperature. Reagent gases and
buffer helium were introduced into the trap at typical pressures of
ca. 5.0 × 10−7 and ca. 4 × 10−4 Torr (1 Torr = 133 Pa), respectively,
as measured by a Bayard Alpert ion gauge. The nominal values
were corrected for different sensitivity toward different gases [25],
and for a calibration factor, which depends on the geometry of the
instrument [26]. The qz values of the reactant ions, determined
by setting the low-mass cut-off value, were selected so to ensure
that the qz value of any product ion (m/z ratio higher and lower
than the precursor ion) falls well within the 0.15–0.908 range. This
avoids ion loss due to ion ejection (qz > 0.908) or to low trapping
fields (qz < 0.15) [27]. Sample gas pressures were chosen in order to
achieve appreciable signal-to-noise (S/N) ratios and, on the other
hand, to get an ion density inside the trap not high enough to pro-
duce space-charge effects. Moreover, the ion trap instrument is
supplied with an automatic gain control (AGC) system [28], which
allows to determine the optimal ionization time in order to avoid
excessive ion density. Fig. 1 reports the exemplary plot of the actual
ion counts of the reactant Kr+ ion versus the scan number, which
points out the low experimental dynamic range. This experimental
drawback does not allow to get definitive rate constants and, in the
case of conceivable two-state reactivity, as for the Kr+ ion (see next
section), does not permit to distinguish the contribution of each
reactant electronic state.
In the ITMS, the ionizing electron energy is a function of both
the amplitude of the rf voltage applied to the ring electrode dur-
ing ionization and the phase of the rf as the electrons enter the
trap. Therefore, no single electron energy can be assigned. How-
ever, simulations of the electron energy have shown [29] that for a
low-mass cut-off value of 10 u (rf amplitude of 112 V0-p), the aver-
age electron energy varies from 6 to 55 eV over one cycle of the
rf. The low-mass cut-off value was selected in order to achieve
an appreciable signal-to-noise (S/N) ratio and, at the same time,
to avoid formation of electronically-excited ions following ioniza-
tion. In agreement with literature reports [30], we assume that
the ion temperature is the one of the helium buffer gas, and that
the ion temperature is relatively insensitive to the qz value used
for the ions. The reaction sequences and the rate constants were
determined by selective ion storage of the reactant ions performed
by the apex method (superimposition of dc and rf voltages). This
avoids the presence of interference ions and allows the detection
of product ions with appreciable S/N ratios. The scan modes and
the methods used for the data processing were described previ-
ously [26]. Assuming the usual uncertainties in measuring absolute
pressures with the Bayard Alpert ion gauge, the phenomenological
rate constants (average of at least two determinations) are expected
to be accurate within 30%.
No reaction
SO2F2H+
O
OF
O
CO2
Kr
CFO
CO
N2 + F
N2
Ar + F
Ar
Ne + 2F
Ne + O + F
8.6
4.4
13
9.7
9.1
4.2
8.4
7.6
2.6
0.43
0.22
0.64
0.71
0.82
0.26
0.52
0.47
0.16
0.85
0.12
0.15
0.38
−121.5d
−40.8b
−55.8
+
FSO2
+
SO2F2
SO2F2
+
+
CO2 (13.78)
−71.2
+
Kr+ (14.00/14.67)c
CO+ (14.01)
SO2F2
−92.6
+0.4b
+
FSO2
+
SO2F2
−94.0
−10.5b
−245.2
−27.9b
−262.6
−268.8
−305.2b
+
+
N2 (15.58)
FSO2
+
SO2F2
Ar+ (15.76)
Ne+ (21.56)
FSO2
12
+
+
SO2F2
1.7
2.8
7.1
+
SO2
FSO+
a
Unless stated otherwise, all thermochemical data are taken from ref. [37]. The
enthalpies of formation of the cations are the sum of the enthalpies of formation
and the ionization energies of the corresponding neutrals.
b
+
Enthalpies of formation of FSO+ and of FSO2 taken from ref. [45].
c
2
2
The two values refer to the P3/2 (lower energy) and P1/2 states arising from
spin–orbit coupling (ref. [38]).
d
Enthalpy of formation of SO2F2H+ calculated from the proton affinity of SO2F2
taken from ref. [19] and from the enthalpies of formation of SO2F2 and H+.
hybrid functional [32,33] and the coupled cluster (full electrons),
including the contribution from single and double substitut-
ions, and an estimate of connected triples, CCSD(T,full) [34]. The
geometries were fully optimized at the B3LYP/6-311++G(d,p) level
of theory, and accurate electronic energies were obtained by
CCSD(T,full)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) single-point
calculations. Any located critical point was characterized as an
energy minimum or a transition structure (TS) by calculating
its B3LYP/6-311++G(d,p) analytical frequencies, and any TS was
unambiguously related to its interconnected energy minima by
intrinsic reaction coordinate (IRC) calculations [35]. The zero-point
vibrational energies were calculated [36] using the B3LYP/6-
311++G(d,p) unscaled frequencies. The crossing structures involved
in the reactions between 3SF+ and SO2F2, and 3SF+ and NH3 were
located at the B3LYP/6-311++G(d,p) level of theory by a series of
grid-scans of the triplet and singlet surfaces at around the two con-
nected isomers until the singlet and triplet energy differed by less
than 10−6 a.u.
3. Results and discussion
3.1. Reactions of positive ions with SO2F2
The products of the presently investigated reactions between
positive ions (M+) and SO2F2 are given in Table 1 (only product
ions with abundances higher than 2% with respect to the reactants
were included).
Unless stated otherwise, all thermochemical data are taken from
the NIST compilation [37]. For Kr+ and Xe+, the two quoted recombi-
nation energy values refer to the two states 2P3/2 (lower energy) and
2P1/2 arising from spin-orbit coupling [38]. While for Ne+ and Ar+
the energy difference between these two terms is too small to result
in differences in reactivity in principle detectable by ITMS, the low
dynamic range of the present experiments did not allow to detect
The calculations were performed with the Gaussian03 set of
programs [31], using the standard internal 6-311++G(d,p) and 6-
311++G(2d,2p) basis sets. The employed methods were the B3LYP

48
P. Antoniotti et al. / International Journal of Mass Spectrometry 354–355 (2013) 46–53
Fig. 1. Kinetic plot of the reaction of the Kr+ ion in a ionized Kr/SO2F2 mixture. Absolute ion counts are plotted against the number of scans (each scan corresponds to a
0.0002 s increase of reaction time).
different reactivities either in the cases of Kr+ and Xe+. The reac-
tion efficiencies are the ratio between the phenomenological rate
constants (k) and the collision rate constants (kcoll), calculated by
the Parametrized Trajectory Theory of Su and Chesnavich [39]. The
dipole moment of SO2F2 was taken from compiled data [40], and
its polarizability (5.342 × 10−24 cm3) was calculated by the method
of Miller [41].
The reactant ions are listed in Table 1 in order of increasing RE.
This allows to appreciate that the observed reaction paths are in
general affected by the difference (ꢀE) between the RE of M+ and
the IE of SO2F2, 13.04 eV. For NO+, O2+, Xe+, and N2O+, the negative
values of ꢀE prevent the electron transfer (ET) reaction, and their
collision with SO2F2 is unreactive. The ꢀEs of H2O+ and OH+ are still
negative, but the proton affinity (PA) of SO2F2, 618.0 kJ mol−1 [19] is
higher than that of OH (593.2 kJ mol−1) and O (485.2 kJ mol−1), and
both H2O+ and OH+ react with SO2F2 by an efficient proton transfer.
The PA of H2O, 691 kJ mol−1, is instead definitely higher than that of
SO2F2, and H3O+ (RE = 6.27 eV) is overall unreactive toward SO2F2.
SF+ is the only fragment from the EI of SO2F2 [42–44] which
reacts with the parent molecule. The negative value of the corre-
sponding ꢀE prevents the ET, but the collision with SO2F2 produces
FSO+ by the reaction
(15.76 eV), the ET is accompanied by the formation of FSO2+, which
+
is ascribed to the loss of a F atom from the molecular ion SO2F2
.
This is in fact the fragmentation of lowest energy from ionized
SO2F2 [42–44], and the corresponding appearance energy (AE) was
measured so far as 15.1 0.2 eV [42], and subsequently refined as
15.07 0.07 eV [43].
The reactions of O+ and CO+ with SO2F2 both produce SO2F2
+
and FSO2+. The REs of O+ (13.62 eV) and CO+ (14.01 eV) are however
lower than 15.07 eV, and no dissociative ET is in principle expected.
The observed FSO2+ are best explained by the exothermic or nearly
thermoneutral F-atom abstractions:
O
+ + SO2F2 → FSO2+ + OF
(2)
(3)
CO+ + SO2F2 → FSO2+ + FCO
Alternatively, one could assume that, during the EI of SO2F2, the
first excited states O+(2D) and CO+(2ꢂ) are formed, and their REs
of 16.94 [38] and 16.32 eV [47], respectively, allow the formation of
+
FSO2 by dissociative ET. This possibility is however discarded by
two arguments. First, at the relatively high pressures of the ITMS
SF+ + SO2F2 → FSO+ + F2SO
(1)
The process is exothermic by more than 80 kJ mol−1 but only
less efficient (0.035). SF+ has a triplet ground state (3˙−), which
1
+
is more stable than the 1ꢁ and
˙ excited states by 74.5 and
159.0 kJ mol−1, respectively [44]. Both FSO+ [45] and F2SO [46] have
instead a singlet ground state, and reaction (1) is therefore spin-
forbidden. According to the theoretical calculations (see Fig. 2), the
process is a O-atom abstraction by 3SF+ from SO2F2, which involves
the formation of a 3[FS O(O)SF2]+ complex (exothermic by nearly
59 kJ mol−1), the conversion into a slightly less stable singlet com-
plex 1[FS O(O)SF2]+, and its eventual dissociation into FSO+ + F2SO
passing through a singlet transition structure (TS). Both the cross-
ing and the TS are high-energy structures, nearly degenerate with
the entrance channel. This suggests that the low efficiency of reac-
tion (1) likely reflects the combined effect of intersystem crossing
and decomposition of the singlet intermediate 1[FS O(O)SF2]+.
Both CO2+ (RE = 13.78 eV) and Kr+ (RE = 14.00 and 14.67 eV) react
Fig. 2. CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) potential energy profile
for the reaction between 3SF+ and SO2F2 (bond lengths in Å and bond angles in
degrees).
with SO2F2 by an efficient ET. For N2 (RE = 15.58 eV) and Ar+
+

P. Antoniotti et al. / International Journal of Mass Spectrometry 354–355 (2013) 46–53
49
Table 2
Product ions, suggested neutral products, phenomenological rate constants at 333 K
(k; 10−10 cm3 molecule−1 s−1), reaction efficiencies (k/kcoll), and reaction enthalpiesa
ꢀrH298 (kJ mol−1) for the reactions observed in SO2F2/M mixtures (M = D2, CH4,
◦
NH3).
◦
Reagents
Product ions
Neutral
k
k/kcoll
ꢀrH298
products
M = D2
SO2F2++ D2
M = CH4
SO2F2D+
SO2H+
D
0.46
7.9
0.043
0.75
−107.7b
−69.7c
SO2+ + CH4
M = NH3
S+ + NH3
SO+ + NH3
SF+ + NH3
CH3
+
NH3
NH3
NH3
S
SO
FS
HF
SO2
NH2
HF
FSO2
HF
14
6.9
5.1
3.4
0.59
0.31
0.23
0.15
0.65
0.024
0.043
0.44
0.15
0.044
−28.1
−21.6
+
+
−2.89
+
Fig. 3. Mass spectrum of selected SO2F2 ion in a D2O/SO2F2 mixture, after 40 ms
H2NS+
1215d
reaction time.
SO2+ + NH3
NH3
14
−220.0
−54.0c
916.1e,f
−70.9g,e
882.6h
−286.6
+
SO2H+
0.52
0.91
9.1
3.2
0.91
FSO+ + NH3
FSO2+ + NH3
H2NSO+
The most noticeable observation is certainly the ability of the
+
NH3
+
+
molecular ion SO2F2 to activate the robust bonds of D2, H2O, and
H2NSO2
NH3
SO2F2+ + NH3
SO2F2
+
CH4 by hydrogen-atom transfer (HAT) reactions:
a
Unless stated otherwise, all thermochemical data are taken from ref. [37]. The
enthalpies of formation of the cations are the sum of the enthalpies of formation
and the ionization energies of the corresponding neutrals.
F2SO2+ + D2 → F2SO2D+ + D
F2SO2+ + H2O → F2SO2H+ + OH
F2SO2+ + CH4 → F2SO2H+ + CH3
(4)
(5)
(6)
b
Enthalpy of formation of SO2F2H(D)+ calculated from the proton affinity of SO2F2
taken from ref. [19] and from the enthalpies of formation of SO2F2 and H(D)+.
c
Enthalpy of formation of SO2H+ taken from ref. [59].
d
Upper limit to the enthalpy of formation of H2NS+.
e
f
+
Enthalpies of formation of FSO+ and of FSO2 taken from ref. [45].
The ionic product is assigned as the O-protonated (deuterated)
F2SO2. According to G2 calculations [19], the (HF)-(F)SO2+ isomer is
in fact less stable than the F2S(O)-OH+ isomer by 104.6 kJ mol−1. The
formation of F2S(O)-OH+ is also expected from the electronic struc-
ture of F2SO2+. Thus, as shown in Fig. 4, the F2SO2+ radical cation has
a positive charge essentially located on the S atom, and a spin den-
sity essentially located on the O atoms. Interestingly, with respect
to SO2+, the presence of the two F atoms enhances the degree
Upper limit to the enthalpy of formation of H2NSO+.
g
h
Enthalpies of formation of FSO and FSO2 taken from ref. [60].
+
Upper limit to the enthalpy of formation of H2NSO2
.
environment, excited ions are in general expected to be effectively
quenched by unreactive collisions with the neutrals. Secondly, for
O+, as mentioned in the Experimental Section, the cation was gen-
erated from the EI of a O2/N2 mixture. Under these conditions,
+
of localization of the spin density, and F2SO2 is indeed a nearly
any excited O+(2D) is expected to form not only SO2F2 and FSO2
,
+
+
but also N2+. This additional product was, however, not detected.
Our suggested occurrence of reaction (2) is also consistent with
Morris et al. [48] and with Howle et al. [49], who postulated the
formation of the OF radical from the reactions of O+ with several
fluorocompounds. In particular, the processes observed by Morris
et al. [48] resulted exothermic for ground-state O+ only assuming
the formation of OF.
Finally, the reaction of SO2F2 with Ne+ produces FSO+ and
SO2+. The AEs of these ions from ionized SO2F2 were measured,
respectively, as 18.62 [43] and 18.76 eV [44], and these values are
consistent with the RE of Ne+ of 21.56 eV.
3.2. Reactions in SO2F2/D2, SO2F2/H2O, and SO2F2/CH4 mixtures
Further insights into the ion chemistry of SO2F2 were obtained
by studying the reactions occurring in binary mixtures of SO2F2 and
D2, H2O, CH4, and NH3. The obtained results are given in Table 2.
+
Fig. 3 reports the mass spectrum of isolated SO2F2 ion in a
D2O/SO2F2 mixture, after 40 ms reaction time. Apart from the abun-
dant product at m/z = 104, the spectrum shows a weak signal at
m/z = 20, indicating electron transfer between D2O and SO2F2+. The
low product abundance is ascribable to conceivable partial ion loss
due to boundary conditions. In fact, the qz values of reactant and
product ions are quite far: if we want to store the reactant ion
within the lower boundary of the stability diagram, the product ion
is consequently stored too close to the upper limit of the stability
diagram, and partial ion loss likely occurs. The incomplete detec-
tion of one of the product ions give rise to unpredictable errors in
the determination of formation rate constant of the other product
ion, which was therefore not calculated.
Fig. 4. B3LYP/6-311++G(d,p) optimized geometries (Å and◦), and Mulliken atomic
+
+
charges (e, bold), and spin densities (e, italics) of the radical ions SO2 and F2SO2
,
and of the doublet F2S(O)
O
NH+.

50
P. Antoniotti et al. / International Journal of Mass Spectrometry 354–355 (2013) 46–53
6.58 × 10−10 cm3 molecule−1 s−1 [50]. The mechanism outlined by
theoretical calculations [50] is also strictly similar to that reported
+
in Fig. 5 for reaction (6). Thus, SO2 attacks CH4 and directly forms
SO2H+ and CH3 by dissociation of a [OS OH CH3]+ intermediate.
Likewise the results of our ITMS experiments, in the FT-ICR exper-
+
iments the sole product observed from the reaction between SO2
and CH4 was SO2H+. However, under the conditions of TQMS, a
small fraction of CH3SO2 was detected too. Despite the opera-
+
tive pressures of ITMS (≈10−4 Torr) are closer to those employed
in TQMS with respect to those of FT-ICR (typically ≈10−8 Torr), we
failed to observe the higher mass product. Evidently, the neutral
gas pressure employed in ITMS is not sufficiently high to trap the
initial encounter complex [OS OH CH3]+ for a time long enough
+
to allow its isomerization and eventual rupture into CH3SO2 and
the H atom according to the mechanism outlined in Ref. [50].
As recently pointed out by Schwarz et al. [51], a high spin density
on one of the oxygen atoms of metal or metal-free open-shell oxide
cations is required for direct HAT with CH4. They noted in particular
+
for SO2 that, even if in the ground state the spin density is almost
uniformly shared among the atoms (see Fig. 4), the approach of CH4
conceivably triggers an intramolecular spin-density transfer, so to
place most of the spin density on one oxygen atom. Our observation
of reaction (6) confirms that an appreciable spin density on the O
atom(s) of cationic oxides is indeed a favorable requisite for the
occurrence of a HAT with CH4. Interestingly, a higher spin density is
apparently not necessarily related with a higher reaction efficiency.
Thus, the O spin density of SO2F2+, 0.536 (see Fig. 4), is higher than
+
that of SO2+, 0.329, but the HAT with CH4 by SO2 is more efficient
+
than that of SO2F2 (0.75 vs. 0.43, see Table 2). The trigger event is
probably fast, and the reaction rate essentially depends on the rate
of dissociation of the reaction intermediate.
Fig. 5. CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) potential energy profiles
for the reactions between SO2F2 and H2O and CH4 (bond lengths in Å and bond
angles in degrees).
+
3.3. Reactions in SO2F2/NH3 mixtures
The reactions observed in the SO2F2/NH3 mixture revealed addi-
tional features of the ion chemistry of SO2F2. While NH2+, NH3+ and
perfect distonic ion. The S atom bears a positive charge of 1.204 e
and a negligible spin density, and the O atoms bear an overall spin
density of 1.072 e and a negligible electronic charge.
NH4 are unreactive toward SO2F2, NH+ undergoes the formation
+
of various ionic products according to the reactions:
Despite being thermochemically favored by nearly 108 kJ mol−1
,
NH+ + F2SO2 → F2SO2+ + NH
→ F2SO+ + HNO
(7a)
(7b)
(7c)
(7d)
reaction (4) is only less efficient. Instead, the relatively high
product abundance of reaction (5) (Fig. 3) points to a sim-
ple HAT with no kinetic barrier. This suggestion was confirmed
by the CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) calcula-
tions, which unraveled the potential energy profiles shown in
→ HNO+ + F2SO
→ NO+ + FSO + HF
+
Fig. 5. F2SO2 directly attacks the H atom of H2O, and forms the
tightly bound [F2S(O) OH OH]+, structurally assigned as a com-
The determination of the rate constants of (7a–d) was prevented
by the fact that it was not possible to simultaneously store both the
reactant and all the product ions; in fact, in order to efficiently store
plex between F2SO(OH)+ and OH. This is suggested in particular
˚
by the long H OH distance of 1.139 A, and by the nearly unitary
spin density on the OH group. The [F2S(O) OH OH]+ intermediate
the reactant NH+ ion, the heavier product ions (F2SO2 and F2SO+)
+
directly dissociates into F2SO(OH)+ and OH.
consequently fall to a qz value too close to the lower boundary of the
stability diagram and, therefore, their partial loss is very probable
to occur.
The potential energy profile of reaction (6), also shown in Fig. 5,
+
is strictly similar to that outlined for reaction (5). F2SO2 directly
attacks the H atom of CH4, and forms the [F2S(O) OH CH3]+ inter-
mediate, which dissociates into F2SO (OH)+ and CH3. Compared
with [F2S(O) OH OH]+, the separation between F2SO (OH)+ and
The occurrence of (7a) is consistent with a RE of NH+, 13.10 eV,
which is slightly higher than the IE of SO2F2 (13.04 eV). This process
is conceivably a long-range ET, which does not imply the formation
of a long-lived intermediate. On the other hand, the occurrence of
reactions (7b–d), and the nature of the observed products, suggest
the formation of a tightly-bound complex arising from the interac-
tion of NH+ with an oxygen atom of SO2F2, which in turn dissociates
so to form the observed species with N O bonds. To explore this
possibility by theoretical calculations, one has to take into account
the peculiar situation of NH+, which has a first excited electronic
state (a4˙−) which is only 3.9 kJ mol−1 above the ground electronic
state X2ꢂ [52]. These two states are overlapping, and display a
strong mutual perturbation by a-X rovibronic transitions. In any
case, following the conceptually simplest approach, we explored
˚
CH3 is more pronounced (the H CH3 distance is as long as 1.569 A,
˚
and the O H distance is as short as 1.117 A), and the dissociation
energy of [F2S(O) OH CH3]+, 58.6 kJ mol−1, is consistently lower
than that of [F2S(O) OH OH]+, 95.4 kJ mol−1
.
Likewise F2SO2+, SO2 reacts with CH4 and forms OS OH+
and CH3. This process was first observed by de Petris et al.
[50], and its details were investigated by Fourier-transform ion
cyclotron resonance (FT-ICR), triple quadrupole mass spectrom-
etry (TQMS), and theoretical calculations. Our measured rate
constant of 7.9 × 10−10 cm3 molecule−1 s−1 is in good agreement,
within the combined uncertainties, with the previous estimate of
+

P. Antoniotti et al. / International Journal of Mass Spectrometry 354–355 (2013) 46–53
51
Fig. 6. CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) potential energy profiles for the reactions between 3SF+, 1FSO+, and 1FSO2+, and NH3 (bond lengths in Å and bond
angles in degrees).
the interaction of both the doublet and the quartet NH+ with
the O atom of SO2F2 (at the CCSD(T)/6-311++G(2d,2p)//B3LYP/6-
311++G(d,p) level of theory, the two states are nearly degenerate).
is however exceedingly lower than that of the reaction with CH4
(0.75), and this is ascribable to the strength of the H NH2 bond
which is higher than that of the H CH3 bond. Interestingly, the SF+,
NH+ resulted more stable than the
FSO+ and FSO2 cations are able to activate NH3 by the alternative
+
Thus, the doublet F2S(O)
O
corresponding quartet by nearly 190 kJ mol−1, and bound with
respect to the reactants by more than 350 kJ mol−1. In addition,
paths described by the equations
SF+ + NH3 → H2NS+ + HF
FSO+ + NH3 → H2NSO+ + HF
FSO2+ + NH3 → H2NSO2+ + HF
(8)
(9)
the optimized geometry of the doublet F2S(O)
O
NH+, shown in
Fig. 4, offers a reasonable explanation for the occurrence of reac-
tions (7b)–(7d). The dissociation along the relatively long S O bond
(10)
+
˚
(1.568 A) can in fact result into the alternative pairs F2SO /HNO
(reaction (7b)) and HNO+/F2SO (reaction (7c)), and the extrusion of
HF opens the formation of the products of reaction (7d).
These reactions eventually result in the functionalization of a
H
NH2 bond by SOx (x = 0–2), and are thermochemically driven
Due to the relatively low value of the IE of NH3 (10.07 eV), the
ions obtained from the EI of SO2F2 mostly react with NH3 by ET. For
S+, which was thermalized by unreactive collisions with N2 [53], the
observed path is consistent with that reported in the literature [54],
and includes in particular the by far prevailing ET accompanied by
the formation of low amounts of H2NS+ (less than 2%, not included
by the formation of a quite stable HF. Under the assumption that
reactions (8)–(10) are exothermic or at least thermoneutral, it is
possible to estimate an upper limit to the unknown enthalpies of
formation of the ionic products (see Table 2). From the mechanistic
point of view, the experience with previously investigated reac-
tions between singlet-state fluorocations and NH3 [55–58] suggests
that reactions (9) and (10) occur on the singlet surface passing
+
+
in Table 2). Both F2SO2 and SO2 react with NH3 by ET, but only
+
+
+
SO2 activates also the H NH2 bond by a HAT (once again, F2SO2
through tightly-bound FSOx NH3 complexes (x = 1 or 2), which
eventually undergo the extrusion of HF. Less information is instead
is less reactive than SO2+). The efficiency of this process (0.024)

52
P. Antoniotti et al. / International Journal of Mass Spectrometry 354–355 (2013) 46–53
still available on analogous reactions involving triplet-state fluo-
rocations such as SF+. We therefore explored the potential energy
profile of reaction (8), and extended also the study to reactions (9)
and (10). As shown in Fig. 6, reaction (8) commences by the addi-
tion of 3SF+ to the N atom of NH3, with formation of a 3[FS NH3]+
complex which is stable by nearly 135 kJ mol−1. The corresponding
singlet structure 1[FS NH3]+ is more stable than the triplet by more
than 140 kJ mol−1, and the conversion from triplet to singlet occurs
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˚
by a shortening of the S N distance by ca. 0.33 A, and by a opening
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S
N angle by ca. 50◦. The crossing structure is less sta-
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.
As expected, the potential energy profiles of reactions (9) and
(10) are qualitatively strictly similar to those outlined previously for
other reactions between singlet-state simple fluorocations and NH3
+
[55–58]. The singlet FSOx (x = 1, 2) attack the N atom of NH3 and
form the tightly-bound complexes FSOx NH3+, which overcome
+
the barriers corresponding to the transition structures TS FSOx
,
+
and isomerize into complexes between OxS NH2 and HF, which
eventually dissociate with no barrier. Passing from FSO+ to FSO2
,
+
the energy defect of the reaction, namely the difference between
+
the entrance channel FSOx+ + NH3 (x = 1, 2) and the TS FSOx sub-
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4. Concluding remarks
In the present paper, the first comprehensive study of gas
phase positive ion/molecule reactions involving sulfuryl fluoride
is presented. From the fundamental point of view, the positive ion
chemistry of sulfuryl fluoride displays some interesting features.
+
The molecular ion SO2F2 is able to activate strong covalent bonds
of D2, H2O, and CH4. The occurrence of these processes is consis-
tent with the electronic structure of SO2F2+, which features the spin
density on the oxygen atoms: this is considered a fundamental req-
+
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+
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to activate the N H bond of NH3, which is functionalized by SOx
(x = 0–2) by an intramolecular H-atom migration within the initial
ion–dipole complex, followed by extrusion of HF. The present ini-
tial results about SO2F2 gas phase positive ion chemistry look quite
intriguing and, in order to deepen the investigation of the most
interesting aspects, further work is already scheduled on mixtures
of SO2F2 with various hydrides.
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The authors thank the Università della Tuscia, the Università
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