Reactions of the methylsulfinyl radical [CH3(O)S] with oxygen (3O2) in solid argon

Article abstract of DOI:10.1039/c5cc02168e

The atmospherically highly relevant methylsulfinyl radical (CH₃(O)S) readily reacts with molecular triplet oxygen in cryogenic argon matrices containing small amounts of ³O₂. Comparison of experimental and computed IR- and UV/Vis spectra, including isotope exchange, show that the initially formed ³O₂ adduct has the structure of a peroxyl radical (CH₃(O)SOO), which upon irradiation with UV light isomerizes to the methylsulfonoxyl radical (CH₃SO₃). The latter transforms into the methansulfonic acid radical (CH₂SO₃H) by irradiation with visible light. During the matrix photolysis small amounts of SO₃ and the methyl radical were detected indicating competitive direct photodissociation.

Full text of DOI:10.1039/c5cc02168e

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Reactions of the methylsulfinyl radical [CH3(O)Sꢀ]
with oxygen (3O2) in solid argon†
Cite this:DOI: 10.1039/c5cc02168e
Hans Peter Reisenauer,*a Jarosław Romanski,b Grzegorz Mloston*b and
´
´
Peter R. Schreinera
Received 13th March 2015,
Accepted 18th May 2015
DOI: 10.1039/c5cc02168e
www.rsc.org/chemcomm
The atmospherically highly relevant methylsulfinyl radical (CH3(O)Sꢀ)
readily reacts with molecular triplet oxygen in cryogenic argon matrices
containing small amounts of 3O2. Comparison of experimental and
computed IR- and UV/Vis spectra, including isotope exchange, show
that the initially formed 3O2 adduct has the structure of a peroxyl
radical (CH3(O)SOOꢀ), which upon irradiation with UV light isomerizes
to the methylsulfonoxyl radical (CH3SO3ꢀ). The latter transforms into
the methansulfonic acid radical (ꢀCH2SO3H) by irradiation with visible
light. During the matrix photolysis small amounts of SO3 and the methyl
radical were detected indicating competitive direct photodissociation.
Scheme 2 Generation of 1 and 2 starting with allylmethylsulfoxide (3) and
allylmethylsulfone (4), respectively.
A recent study reports the high vacuum flash pyrolysis
(HVFP) of allylmethylsulfoxide (3) generating the highly stabilized
p-radical 1 isolated in an Ar matrix at 10 K.5 Similarly, 2 was trapped
starting with allylmethylsulfone (4) (Scheme 2).6
The study showed that 2 in comparison with 1 is significantly
less stable and easily undergoes partial decomposition in the gas
phase leading to the formation of SO2 and the methyl radical. To
date little is known about the reactions of 1 and there are only a
few kinetic studies of its reactions with O2, O3, and NO2.7 In the case
of the reaction with 3O2, the formation of an adduct with the
structure of the methylsulfinylperoxyl radical (5) as the key inter-
mediate was postulated (Scheme 3).8 However, to the best of our
knowledge, no spectroscopic data for 5 have been presented to date.
The fundamental question whether the formation of 5 by
addition of 1 to molecular oxygen is an exo- or an endothermic
process has not yet been unambiguously answered by theory. The
DH1 values computed at ab initio and density functional theory (DFT)
levels with large basis sets vary considerably; G2(MP2) computations
indicate slightly exothermic (À2.5 kcal molÀ1) formation of 5.9
The main goal of the present work was to determine if this
reaction takes place at all and to examine the structure of the product
formed. Equally important for the understanding of atmospheric
chemistry is an investigation of the photochemistry of the initially
formed oxygen adduct. The matrix isolation technique currently is
the best method to provide reliable answers to both questions.
Volatile sulfur compounds such as hydrogen sulfide and
dimethylsulfide (Me2S and DMS) produced from natural plankton
are emitted into the atmosphere and are converted into a plethora
of sulfur products via radical processes. As part of the so-called
‘sulfur cycle’ they are of great importance for the atmospheric
sulfur balance1 with DMS as the major component of biogenic
emissions.2 The impact of sulfur compounds on the climate is
explained based on the ‘CLAW’ hypothesis.3 DMS and other volatile
sulfur compounds undergo cascade conversions in the atmosphere
eventually leading to sulfuric acid and methanesulfonic acid
(Scheme 1).3c The methylsulfinyl radical (1) and the methylsulfonyl
radical (2) are recognized as important intermediates involved in
these processes.4
Scheme 1 Postulated reaction cascade for dimethyl sulfide oxidation to
sulfuric acid and methylsulfonic acid in the atmosphere.
a Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58,
D-35392 Giessen, Germany. E-mail: reisenauer@org.chemie.uni-giessen.de;
Fax: +49 641-9934309; Tel: +49 641-9934380
b Department of Organic and Applied Chemistry, University of Lodz, Tamka 12,
91-493 Lodz, Poland. E-mail: gmloston@uni.lodz.pl; Fax: +48 42 6655162;
Tel: +48 42 6355761
† Electronic supplementary information (ESI) available: Table, experimental,
additional IR spectra and computational details. See DOI: 10.1039/c5cc02168e
Scheme 3 Most relevant [CH3SO3]ꢀ radical species 5–8.
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1391/1383 cmÀ1, 527/521 cmÀ1, and 489 cmÀ1 11
.
The fate of the
methyl radical as the co-product of the photodissociation reaction
remains unclear. Neither its IR bands nor those of the methyl-
peroxyl radical12 as the expected reaction product with excess
3O2 could be found, most probably because their specific
intensities are much lower than that of SO3. The structure of
Scheme 4 Generation of the methylsulfinyl radical (1) by HVFP of pre-
cursor 3 and its reaction with oxygen in solid argon at 10 K.
Following the previously described protocol for the generation the second product was deduced on the basis of its UV/Vis
of 1,3 a sample of the precursor 3 was pyrolized at 600 1C and the spectrum revealing a very strong absorption band in the range
stream of products was co-condensed at 10 K with a large excess of of 510–350 nm with a distinct vibrational fine structure along
argon containing ca. 0.5, 1.0 or 5.0 vol% of oxygen, respectively with a much weaker broad structureless absorption between ca.
(Scheme 4). In experiments with 0.5 and 1.0 vol% of 3O2 only 700 nm and 530 nm (Fig. 2, dashed line). Similar bands were
partial conversion of 1 and the allyl radical (9) was observed. In reported in a laser flash photolysis experiment for the initial
the IR spectra the characteristic absorptions located at 800 cmÀ1 photodissociation product of bis(methylsulfonyl)peroxide and
(for 6) and at 1068 cmÀ1 (for 1) were still present. In addition a assigned to 6.13 Moreover, similar bands were observed for the
new, intense band found at 1191 cmÀ1 provided evidence for the product obtained from the same precursor after HVFP with
formation of a new product. However, in the case of experiments subsequent isolation in noble gas matrices.14
3
performed with 5 vol% O2 content, the conversions were com-
In good accordance with the experimental UV/Vis spectra,
plete and none of the bands of 1 or 9 were found in the pyrolysate. TD-UB3LYP computations of the excitation energies (ESI,†
Two new sets of characteristic absorptions indicate the formation Table S3) provide two weak absorption bands at 388 and
of two new products. Based on previously reported data10 and on 366 nm for 5 and a very intense band at 476 nm for 6, whereas
the basis of results of our present studies on the reaction of the the photochemical final product 7 should have no absorption
allyl radical generated by HVFP of 1,5-hexadiene with molecular above 315 nm. For the unobserved isomer 8 the computations
oxygen, one of them was readily identified as the allyldioxyl predict absorptions of medium intensity at 375 and 361 nm
radical (10) (Fig. 1). Another new product with an intense band that are close to those of 5. It might well be that 8 forms upon
located at 1191.1 cmÀ1 and less intense at 1295.5 and 686.3 cmÀ1
respectively, was identified as 5. Conceivable alternative structures remains below the detection limit.
,
irradiation, but it is photolabile and therefore its concentration
resulting from bonding to the O-atom do not represent energy
minima (ESI,† Fig. S1).
The most intense IR bands attributed to 6 were observed at
757, 1334, and 1441 cmÀ1 (Fig. 3, negative bands). They coincide well
The UV/Vis spectrum of the matrix material showed a broad with those reported for argon matrix isolated 6.14 Comparison of the
absorption maximum at lmax = 394 nm (Fig. 2, full line). Based on experimental IR bands with computed IR spectra of 6 was not
this observation, photolyses (Hg high pressure lamp, monochro- helpful in supporting the structural assignment. Especially the
mator, Dl = 10 nm) were carried out with light at l = 366 nm or theoretically predicted strong band for the symmetric SO3 stretching
alternatively at l = 405 nm. In both cases, disappearance of the vibrations (a1) has no counterpart in the experimental spectrum.
initial IR- and UV/Vis absorption bands attributed to 5 was Due to the special electronic situation of C3v symmetric 6 with a
observed, whereas those of 10 remained unchanged. The products degenerate singly occupied highest molecular orbital that is subject
formed thereby were identified as SO3 and 6. The presence of to Jahn–Teller distortion to Cs symmetry, one may expect strong
small amounts of SO3 is evident from its characteristic IR bands at vibronic coupling of the energy levels and anharmonic potentials.
Fig. 1 FTIR spectra of pyrolysis of precursor 2 (A) and 1,5-hexadiene (*) (B) isolated in an Ar matrix containing 5.0 vol% 3O2; pyrolysis products of 2
isolated in an Ar matrix in the absence of 3O2 (C).
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Fig. 2 UV/Vis spectra of pyrolysis of precursor 2 isolated in an Ar matrix containing 5.0 vol% 3O2.
The electronic situation of 6 is similar to that of the highly the expected decomposition products such as the methoxy radical
symmetric NO3 radical for which it was shown that standard DFT and SO2 could be detected.
and ab initio methods completely fail in predicting the vibrational
frequencies due to vibronic coupling and highly anharmonic rotation around the S–O single bond. The results of DFT and
potentials.15 ab initio computations show that they differ by 0.5 kcal molÀ1
Structure 5 is expected to exist as a mixture of rotamers by
Prolonged irradiation of the matrix with light at l = 435 nm (see data for 5 in ESI†). The computed IR spectrum of equal
showed the disappearance of 6, and no absorption maximum amounts of two representative rotamers fits well to the experimental
could be found in the UV/Vis spectrum above 300 nm. The IR spectrum (Fig. 3). The most intense band observed at 1191 cmÀ1 is
spectra of the new product can be assigned to C-centered radical 7. attributed to the SQO double bond stretch. Apparently, within the
The absorption band located at 3569 cmÀ1 unambiguously proves applied experimental spectral resolution the positions of the n(SQO)
the formation of the –S(O)2OH group. This position is very close to absorption bands are identical for both rotamers.
OH-stretching vibration of argon matrix isolated H2SO4 at 3567 cmÀ1
The formation of 5 as the initial product in the reaction of 1
and CH3SO3H at 3579 cmÀ1, respectively.16 In addition, absorptions with O2 is strongly supported by additional isotopic labelling
at 1403, 1391, 1383, 1189, 1184, 842, 827, 784 cmÀ1, partially split by experiments. In particular, the observed spectral shifts in the IR
matrix effects, can also be attributed to 7 (ESI,† Fig. S3). Upon spectrum of the 18O-labelled product were key for the spectral
labelling two of the three oxygen atoms of 7 by 18O, two distinct OH assignments (Fig. 4; in ESI,† Tables S1 and S2).
3
stretching bands at 3570.0 and 3559.0 cmÀ1 with an intensity ratio of
The most intense IR band of 5 displayed only a very small
1 : 2 were observed (ESI,† Fig. S4) as expected for the postulated isotopic band shift (+0.2 cmÀ1) when triplet 18O2 was used instead
structure. Apparently, the thermodynamically most stable isomeric of triplet 16O2. This IR band could be attributed unambiguously to a
radical 8 does not form in the matrix under photolysis. Neither 8 nor pure SQO stretching vibration. Furthermore, the labelling
Fig. 3 Comparison of the computed IR spectra of two conformers of 5 (UB3LYP/6-311+G(3df,3pd) unscaled, above) and the experimental difference
spectrum of the 366 nm photolysis of the 3O2 adduct 5 (below); positive bands vanish, negative bands appear upon irradiation.
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Fig. 4 Isotopical band shifts of the [18O2]-methylsulfinyldioxy radical ([18O2]-5); radicals 5 were obtained by reaction of methylsulfinyl radical (1) with
triplet 16O2 (A) and triplet 18O2 (B).
experiment provides evidence that a pair of weak bands at 1100 and thanks National Science Center (Poland) for financial support
1082 cmÀ1 has to be assigned to the O–O stretching vibrations of (Project Meastro-3; Dec-2012/06/A/ST5/00219).
two rotamers of 5. The experimental band shifts of À61.0 and
À58.0 correspond well to the isotopic shifts (ESI,† Tables S1 and
Notes and references
S2). Moreover, the expected band shifts were also observed for the
S–O stretching vibration (600 cmÀ1 and 555 cmÀ1 versus 575 cmÀ1
and 534 cmÀ1, respectively). The observed IR band shifts after
deuteration of the methyl group (ESI,† Fig. S5) also agree very well
with these band assignments. Whereas, in accordance with DFT
computation (ESI,† Fig. S6), the positions of the n(SQO) and the
n(O–O) remain almost unchanged, and large bathochromic shifts
were observed for the d(CH3) and r(CH3) vibrations.
1 J. E. Lovelock, J. Maggs and R. A. Rasmussen, Nature, 1972, 237,
452–453.
2 T. S. Bates, B. K. Lamb, A. Guenther, J. Dignon and R. E. Stoiber,
J. Atmos. Chem., 1992, 14, 315–337.
3 (a) R. J. Charlson, J. E. Lovelock, M. O. Andreae and S. G. Warren, Nature,
1987, 326, 655–661; (b) G. P. Ayers and J. M. Cainey, Environ. Chem.,
2007, 4, 366–374; (c) I. Faloona, Atmos. Environ., 2009, 43, 2841–2854.
4 (a) G. S. Tyndall and A. R. Ravishankara, Int. J. Chem. Kinet., 1991,
23, 483–527; (b) J. Barnes, J. Hjorth and N. Mihalopoulos, Chem.
Rev., 2006, 106, 940–975.
The present study shows that atmospherically highly
relevant radical 1 generated in the gas phase by high-vacuum
flash pyrolysis reacts spontaneously with molecular oxygen in a
solid argon matrix under cryogenic conditions. Obviously, the
formation of the oxygen adduct is a nearly barrierless, exothermic
process. The structure of initially formed peroxyl radical 5 was
proven by comparison of computed with experimental IR and UV
spectra as well as through extensive isotopic labelling experi-
ments. Upon irradiation with near UV light, adduct 5 undergoes
photochemical dissociation into SO3 and a methyl radical or
isomerization to 6. The latter is also not stable against irradiation
with visible light and eventually transforms to 7. These results
underscore earlier postulates regarding the existence of a reactive
oxygen adduct of the methylsulfinyl radical. Furthermore, our
´
´
5 H. P. Reisenauer, J. Romanski, G. Mloston and P. R. Schreiner,
Chem. Commun., 2013, 49, 9467–9469.
6 H. P. Reisenauer, P. R. Schreiner, J. Romanski and G. Mloston,
J. Phys. Chem. A, 2015, 119, 2211–2216.
7 (a) G. S. Tyndall and A. R. Ravishankara, J. Phys. Chem., 1989, 93,
2426–2435; (b) F. Yin, D. Grosjean, R. C. Flagan and J. H. Seinfeld,
J. Atmos. Chem., 1990, 11, 365–399.
8 S. B. Barone, A. A. Turnipseed and A. R. Ravishankara, Faraday
Discuss., 1995, 100, 39–54.
9 Z. Salta, A. M. Kosmas and A. Lesar, Comput. Theor. Chem., 2012,
1001, 67–76.
10 E. G. Baskir and O. M. Nefedov, Russ. Chem. Bull., 1996, 45, 99–106.
11 (a) V. E. Bondybey and J. H. English, J. Mol. Spectrosc., 1985, 109,
221–228; (b) A. Givan, A. Loewenschuss, C. J. Nielsen and
M. Rozenberg, J. Mol. Struct., 2007, 830, 21–34.
12 (a) P. Ase, W. Bock and A. Snelson, J. Phys. Chem., 1986, 90,
2099–2109; (b) S. Nandi, S. J. Blanksby, X. Zhang, M. R. Nimlos,
D. C. Dayton and G. B. Ellison, J. Phys. Chem. A, 2002, 106, 7547–7556.
´
´
results demonstrate the importance of photochemical trans- 13 H.-G. Korth, A. G. Neville and J. Lysztyk, J. Phys. Chem., 1990, 94,
8835–8839.
formations in atmospheric oxidation processes of volatile organic
sulfur compounds. These photochemical pathways have not yet
14 X. Zeng, H. Beckers, J. S. Francisco, H. Willner, to be submitted
(private communication).
been considered in the discussions related to the atmospheric 15 (a) J. F. Stanton, Mol. Phys., 2009, 107, 1059–1075; (b) J. F. Stanton,
J. Gauss and R. J. Bartlett, J. Phys. Chem., 1991, 94, 4084–4087;
(c) J. F. Stanton, J. Chem. Phys., 2007, 126, 134309.
16 A. Givan, A. Loewenschuss and C. J. Nielsen, J. Mol. Struct., 2005,
sulfur cycle.
This work was supported by the DAAD Partnership of the
University of Lodz and the Justus-Liebig University. G. M.
748, 77–90.
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