Atmospherically Relevant Radicals Derived from the Oxidation of

Article Cite This: Acc. Chem. Res. 2018, 51, 475−483

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Atmospherically Relevant Radicals Derived from the Oxidation of Dimethyl Sulfide Artur Mardyukov* and Peter R. Schreiner* Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany CONSPECTUS: The large number and amounts of volatile organosulfur compounds emitted to the atmosphere and the enormous variety of their reactions in various oxidation states make experimental measurements of even a small fraction of them a daunting task. Dimethyl sulfide (DMS) is a product of biological processes involving marine phytoplankton, and it is estimated to account for approximately 60% of the total natural sulfur gases released to the atmosphere. Ocean-emitted DMS has been suggested to play a role in atmospheric aerosol formation and thereby cloud formation. The reaction of ·OH with DMS is known to proceed by two independent channels: abstraction and addition. The oxidation of DMS is believed to be initiated by the reaction with ·OH and NO3· radicals, which eventually leads to the formation of sulfuric acid (H2SO4) and methanesulfonic acid (CH3SO3H). The reaction of DMS with NO3· appears to proceed exclusively by hydrogen abstraction. The oxidation of DMS consists of a complex sequence of reactions. Depending on the time of the day or altitude, it may take a variety of pathways. In general, however, the oxidation proceeds via chains of radical reactions. Dimethyl sulfoxide (DMSO) has been reported to be a major product of the addition channel. Dimethyl sulfone (DMSO2), SO2, CH3SO3H, and methanesulfinic acid (CH3S(O)OH) have been observed as products of further oxidation of DMSO. Understanding the details of DMS oxidation requires in-depth knowledge of the elementary steps of this seemingly simple transformation, which in turn requires a combination of experimental and theoretical methods. The methylthiyl (CH3S·), methylsulfinyl (CH3SO·), methylsulfonyl (CH3SO2·), and methylsulfonyloxyl (CH3SO3·) radicals have been postulated as intermediates in the oxidation of DMS. Therefore, studying the chemistry of sulfur-containing free radicals in the laboratory also is the basis for understanding the mechanism of DMS oxidation in the atmosphere. The application of matrix-isolation techniques in combination with quantummechanical calculations on the generation and structural elucidation of CH3SOx (x = 0−3) radicals is reviewed in the present Account. Experimental matrix IR and UV/vis data for all known species of this substance class are summarized together with data obtained using other spectroscopic techniques, including time-resolved spectroscopy, electron paramagnetic resonance spectroscopy, and others. We also discuss the reactivity and experimental characterization of these species to illustrate their practical relevance and highlight spectroscopic techniques available for the elucidation of their geometric and electronic structures. The present Account summarizes recent results regarding the preparation, characterization, and reactivity of various radical species with the formula CH3SOx (x = 0−3).

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INTRODUCTION Atmospheric chemistry studies chemical reactions controlling the formation and decay of chemical species from anthropogenic and natural emissions.1,2 A large number volatile organosulfur compounds (VOSCs) emitted to the atmosphere have important environmental functions, being involved in global warming, acid precipitation, and cloud formation.3−6 Dimethyl sulfide (DMS, (methylsulfanyl)methane, (methylthio)methane), the most abundant biogenic VOSC, is emitted from the oceans by oceanic phytoplankton in large quantities (24−27 Tg of S year−1).7 DMS is produced from decomposition of algal materials such as the osmoticum dimethylsulfoniopropionate ((CH 3 ) 2 S + CH 2 CH 2 COO − , DMSP) in both aerobic and anaerobic environments by enzymatic cleavage of the S−C bond.4,8,9 The marine emissions come almost exclusively in the form of DMS, whereas the emissions from land derive from a variety of chemical species, including H2S, DMS, methanethiol, and others.4 DMS oxidation products and sulfate aerosol formation in the upper © 2018 American Chemical Society

troposphere and lower stratosphere significantly influence the Earth’s radiation budget. DMS and other VOSCs undergo cascade conversions, eventually leading to the formation of sulfuric acid (H2SO4) and methanesulfonic acid (CH3SO3H, MSA),10−12 both of which are key contributors to cloud condensation nuclei (CCN).13,14 According to kinetic and theoretical models, the oxidation of DMS in the atmosphere (initiated by reaction with ·OH during the day and NO3· during the night) occurs through stepwise formation of radical species CH3SOx (x = 0−3).15−18 The OHinitiated oxidation of DMS proceeds either through the formation of a DMS−OH adduct or through the formation of the (methylthio)methyl radical (CH 3SCH2·) via H abstraction;19,20 various experimental methods have been used to unravel the mechanisms of these reactions.2,21−23 Received: October 26, 2017 Published: February 2, 2018 475

DOI: 10.1021/acs.accounts.7b00536 Acc. Chem. Res. 2018, 51, 475−483

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Accounts of Chemical Research

Figure 1. DMS oxidation under marine boundary layer conditions.

Both laboratory and field observations suggest that SO2 is a major product of DMS oxidation.24−27 In the addition channel, the formation of a DMS−OH adduct has been observed in the gas phase28 and in solution,29,30 thus confirming the existence of a stable complex with a two-center−three-electron bonding motif. Comparison of the gaseous and aqueous behaviors of the DMS−OH adduct reveals that aqueous solvation strongly stabilizes the S−O bond against dissociation into DMS and ·OH. The Gibbs free energy of solvation of DMS−OH was calculated to be −12 ± 3 kcal mol−1.31 According to density functional theory (DFT) and ab initio computations, the dissociation enthalpy of the S−O bond in the DMS−OH adduct is in the range of 9.0 kcal mol−1.32,33 Under NOx-free conditions, the DMS−OH adduct reacts with O2 to form dimethyl sulfoxide (DMSO).34 Dimethyl sulfone (DMSO2), SO2, MSA, and methanesulfinic acid (CH3S(O)OH) have been observed as products of further oxidation of DMSO.35,36 Despite the maturity of the field, a large discrepancy still exists between the experimental measurements and simulations;37−39 the DMS oxidation mechanisms devised in model simulations often differ from field observations.13,24,25 Several experimental studies employed higher NOx concentrations and obtained results differing from those observed at the much lower NOx concentrations encountered in the atmosphere.40,41 Consequently, there are large variations in product distributions, and it is not possible to make reliable quantitative predictions of the DMS oxidation products for specific sets of atmospheric conditions.16 Studies of five different DMS oxidation mechanisms were compared with nine different field measurements, but no single mechanism reproduced the observations and predictions of MSA formation, indicating that the mechanism of DMS oxidation is very complex and poorly understood.37,38 It is evident that no single mechanism is capable of giving satisfactory results in rationalizing the product distributions in DMS oxidation under field and laboratory conditions. For a complete understanding of the DMS oxidation mechanism, it is crucial to acquire detailed knowledge of the elementary steps that may provide general guidelines for understanding atmospherically relevant processes. Figure 1 shows the proposed radical-initiated DMS oxidation mechanism; aspects of the depicted radicals are covered in recent authoritative reviews.2,21,42

Here we present the current state of the art of DMS radical oxidation, including the characterization of some elusive sulfur radical species, highlighting their relevance for a variety atmospheric processes. Most of the radicals described here were characterized by IR, UV/vis, and electron paramagnetic resonance (EPR) spectroscopic methods, providing detailed insights into the electronic structures, geometries, and electron spin distributions. Equally important for understanding atmospheric chemistry is the photochemistry of the initially formed volatile sulfur adducts and oxidation intermediates.

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DISCUSSION

Methylthiyl Radical (CH3S·) and Methylthiyl Peroxy Radical (CH3SOO·)

Although only indirect evidence is available, the methylthiyl radical (CH3S·) is believed to be one of the key intermediates in the OH-initiated oxidation of DMS.21 The reactions of CH3S· are important in the transformations of DMS to SO2, MSA, and H2SO4.43 Therefore, the methylthiyl radical has been investigated in many studies using a variety of spectroscopic methods, including electronic absorption,44−46 laser-induced fluorescence (LIF),47,48 fluorescence depletion,49 and infrared spectroscopy50 in combination with the matrix-isolation technique. With a C3v-symmetric structure and an X̃ 2E ground electronic state, the CH3S· radical has three nondegenerate vibrational modes (ν1−ν3) and three doubly degenerate vibrational modes (ν4−ν6).50 Matrix isolation of CH3S· was pioneered by Jacox in 1983 through the reaction of fluorine radicals with CH3SH in an argon matrix.51 The IR band at 3515 cm−1 was assigned to HF bonded to CH3S·; no IR bands of the X̃ 2E state of CH3S· were reported in the mid-IR region.51 The vibrational frequencies of CH3S· were derived from dispersed fluorescence and photoelectron spectra.52,53 The IR spectrum of CH3S· was produced via in situ photodissociation of the precursors CH3SH, CH3SCH3, and CH3SSCH3 isolated in solid p-H2 (Scheme 1).50 The IR spectrum of CH3S· allowed the identification of all three Jahn− Scheme 1. Generation of CH3S· and Isolation in Solid p-H2

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CH3SO· are the most abundant primary radicals. CH3SO· is proposed to form in a barrierless radical−radical reaction of CH3S· with CH3SOO· or CH3OO· to form the corresponding peroxides (CH3SOOSCH3 or CH3SOOCH3) with up to 70 kcal mol−1 of excess energy. The weak peroxy bonds in these peroxides then dissociate to give corresponding oxy radicals (CH3SO· and CH3O·).60 This is in line with the formation of CH3SO· via secondary reaction after photoirradiation of the flowing mixture of CH3SOO (vide infra).61 On the basis of the experimental kinetic data, the major atmospheric loss of CH3S· is through its reactions with O2 and O3.57,62 The rate coefficients for the reactions of CH3S· with · NO2 and O3 were measured by LIF spectroscopy.56,62 The major channel with ·NO2 is the generation of the methylsulfinyl (CH3SO·) and ·NO radicals.54,63 The reaction with ·NO2 is particularly important at elevated NOx concentrations. The rate of formation of the addition product, methylthio nitrite (CH3SNO2), was determined as (8 ± 5) × 10−12 cm3 molecule−1 s−1 in photoreactor experiments.64 Th reaction mechanism and kinetics of the gas-phase reaction of CH3S·(2A′) with O3(1A1) on the lowest doublet electronic state surface was investigated with the G3MP2 method.65 The major products at lower temperatures are CH3SO· and O2. Hydrogen abstraction from CH3S· by O3 to form CH2S and HO3· is not important at lower temperatures.65 The reaction of CH3S· with O3 was investigated as a function of temperature (259−381 K) over the pressure range of 25−300 Torr using laser photolysis/laser-induced fluorescence techniques. The obtained rate constant of (1.02 ± 0.30) × 10−12 cm3 molecule−1 s−1 is pressure-independent.62 The CH3S· + O3 reaction was investigated at 300 K in a discharge flow tube reactor coupled to a photoionization mass spectrometer.57 Independent of pressure, the major product was CH3SO· (15 ± 4%) between 0.7 and 2.2 Torr He, suggesting that the reaction with O3 is a major CH3S· removal process in the atmosphere.

Teller vibrational modes. The vibrational assignment of IR bands at 2898.4 (a1), 1400.0 (a1), 1056.6 (e), and 771.1 cm−1 (a1) are in agreement with computations taking into account Jahn−Teller distortions and spin−orbit interactions.50 The reactions of CH3S· with O2, O3, and NO2 are also of relevance to atmospheric chemistry and were studied experimentally and theoretically.54−57 Ravishankara and coworkers determined the rate constant for the reaction of CH3S· with O2(3P) to be (7−18) × 10−14 cm3 molecule−1 s−1 in the range 216−250 K55 and reported the equilibrium constant for the reaction CH3S· + O2 → CH3SOO· as (1.62−79.90) × 10−18 cm3 molecule−1 at 298 K. The reaction of CH3S· with O2 results in a weakly bound adduct, i.e., the methylthiylperoxy radical (CH3SOO·), which has a bond dissociation energy (BDE) of 11 kcal mol−1.55 Several theoretical investigations of the CH3S· + O2 reaction were reported.58−60 In 2006, Zhu and Bozzelli60 used DFT and ab initio computations to determine the thermochemical parameters, reaction paths, and kinetic barriers in the CH3S· + O2 reaction. At the G3MP2 level, the formation of CH3SOO· from CH3S· + O2 has no barrier (ΔH298), and the conversion of anti-CH3SOO· to syn-CH3SOO· has a small barrier of ∼0.1 kcal mol−1 (Figure 2). According to the computations, syn-

Figure 2. Energies (in kcal mol−1) for the reaction of CH3S· with O2 at 298 K. Values were adopted from ref 49, in which all of the barriers were computed at the G3MP2 level, except those labeled with # and *, which were computed at the CBS-QB3 and G2 levels, respectively.

Methylsulfinyl Radical (CH3SO·) and Methylsulfinyl Peroxy Radical (CH3S(O)OO·)

The methylsulfinyl radical has been postulated as an intermediate in the oxidation of organosulfur compounds in the atmosphere.2,66,67 CH3SO· is highly reactive, and lowtemperature or time-resolved spectroscopy is necessary to characterize this species.61,68 The EPR spectrum of CH3SO· was observed upon UV irradiation of CH3SH in aqueous as well as organic matrices at 77 K in the presence of O2(3P).69 According to computational70 and experimental data,71 in the ground state the odd electron in CH3SO· is localized in a π* orbital. Spin population analysis indicates that the unpaired electron is localized almost equally on the oxygen and sulfur atoms, with a spin densities of 0.58 and 0.41 on sulfur and oxygen, respectively.69 According to CCSDT(Q)/cc-pV(5+d)Z computations, the ground and first excited states of CH3SO· are X̃ 2A″ and Ã 2A′ with S−O distances of 1.499 and 1.652 Å, respectively.70 The X̃ and Ã adiabatic energy difference was predicted to be 45.1 kcal mol−1;70 the rotational barrier around the C−S bond is only 0.9 kcal mol−1. In 2010, Chu and Lee61 reported a transient infrared spectrum of CH3SO· with a band origin at 1071 cm−1 that was assigned to the S−O stretching mode. Here the radical was generated via a secondary reaction of CH3SOO·, which was produced upon 248 nm irradiation of a gaseous mixture of CH3SSCH3 and O2(3P) at 260 K.61

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CH3SOO· (which is 0.4 kcal mol more stable than antiCH3SOO·) forms exothermically (−9.6 kcal mol−1) from CH3S· + O2. The energy difference between CH3S· + O2 and syn-CH3SOO· is consistent with the value of −11.8 kcal mol−1 reported by Turnipseed et al.55 Three decomposition channels may lead to products, including CH3SO· + O, H2CS + HO2·, and ·CH3 + SO2. Under atmospheric conditions the elimination of HO2· is most relevant, followed by the isomerization to much more stable CH3SO2·. The isomerization of synCH3SOO· to ·H2CSOOH is unimportant because of a large barrier. Chu and Lee61 used a step-scan Fourier transform spectrometer to monitor the time-resolved infrared absorption spectrum of CH3SOO· produced upon 248 nm irradiation of a gaseous mixture of CH3SSCH3 and O2 at 260 K. Two transient band origins at 1397 ± 1 and 1110 ± 3 cm−1 were assigned to the antisymmetric CH3 deformation and O−O stretching modes of syn-CH3SOO·, in agreement with the vibrations simulated on the basis of the rotational parameters of synCH3SOO· and anti-CH3SOO· computed at the B3LYP/aug-ccpVTZ and B3P86/aug-cc-pVTZ levels of theory.61 Zhu and Bozzelli60 proposed a kinetic model for the CH3SCH3 + O2 system, where CH3OO·, CH3SOO·, and 477


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SO3 and ·CH3 or isomerization to the methylsulfonoxyl radical (CH3SO3·).73 The latter is also not stable against irradiation with visible light and eventually transforms to the methanesulfonic acid radical (·CH2SO3H) (Scheme 4). These results

We recently reported68 an efficient method for the generation and matrix isolation of CH3SO· by flash vacuum pyrolysis (FVP) of allyl methyl sulfoxide (Scheme 2). The Scheme 2. Bond Dissociation Energies (in kcal mol−1) for the Formation of the Methylsulfinyl Radical

Scheme 4. Photochemistry of syn-CH3S(O)OO·

unambiguous IR assignments of CH3SO· were achieved by comparison of the experimental data with the AE-CCSD(T)/ cc-pVTZ-computed IR spectrum. The most intense S−O stretching vibration at 1068.2 cm−1 is in good agreement with the reported value at 1071 cm−1 derived from an FT-IR experiment.61 Less intense bands at 669.9 and 340.8 cm−1 were attributed to the C−S stretching vibration and the C−S−O bending mode, respectively. Moreover, 12 of the 13 IR-active fundamental bands were identified and assigned to CH3SO·. Photolysis of CH3SO· at 300 nm leads to unspecific decomposition and formation of a mixture of products such as CO, COS, H2O, and thioformic acid.68 The UV/vis spectrum of matrix-isolated CH3SO· exhibits a weak band displaying pronounced vibrational fine structure starting at 635 nm and terminating at around 450 nm (λmax ≈ 530 nm)68 and a strong, more intense band at λmax ≈ 260 nm. Both correlate well with those computed with time-dependent DFT (TD-DFT) at the TD-B3LYP/6-311+G(3df,2pd) level. The lowest-energy electronic transition, from the ground state to the first excited state (Ã 2A′ ← X̃ 2A″) at 540 nm with a low oscillator strength of 0.0005, is due to an n → π* transition. The second transition, (B̃ 2A″ ←X̃ 2A″) at 255 nm, overlaps with the third absorption band (C̃ 2A′ ← X̃ 2A″) at 250 nm, which is 2 orders of magnitude more intense (π → π* transition).68 The methylsulfinyl peroxy radical (CH3S(O)OO·) is another important atmospheric species. It forms as an intermediate in the oxidation of CH3SO· through reaction with O2(3P).72 The identity of the CH3S(O)OO· radical was first experimentally verified through its detection in low-temperature matrices.73 CH3S(O)OO· was generated by cocondensation of CH3SO· with O2(3P) in the gas phase and subsequently trapped in a solid argon matrix under cryogenic conditions.73 Computations and spectroscopic measurements confirmed that the CH3S(O)OO· radical can adopt anti and syn conformations (Scheme 3).72 High-level ab initio and DFT computations showed that

underscore the long-postulated existence of a reactive oxygen adduct of the methylsulfinyl radical.67 The isomerization of CH3S(O)OO· to CH3SO3· has been studied computationally,72 and it is associated with an activation barrier in excess of 25 kcal mol−1. Thus, thermal formation of CH3SO3· in the atmosphere is very unlikely. Furthermore, these results demonstrate the importance of photochemical transformations in atmospheric oxidation processes of VOSCs. Methylsulfonyl Radical (CH3SO2·)

The methylsulfonyl radical (CH3SO 2·) is another key intermediate in the oxidation of DMS in the atmosphere. There have been many theoretical and experimental investigations on CH3SO2· and its isomer, methoxysulfinyl radical (CH3OSO·); detailed computations on the reaction of CH3· with SO2 were reported.74−76 This reaction might proceed via two paths: one nearly barrierless to produce CH3SO2· and one with a barrier of 11−14 kcal mol−1 to give syn-CH3OSO·, which readily equilibrates with less stable anti-CH3OSO· with a barrier of ≲3 kcal mol−1 (Figure 3).74 Most importantly, CH3OSO·

Figure 3. Enthalpy profile (ΔH0 in kcal mol−1) for the reaction of CH3S· with O2(3P) at 298 K. Values were adopted from ref 61, in which relative energies and barriers were computed at the CCSD(T)/ 6-311++G(2df,p) level.

Scheme 3. Reaction of CH3SO· with O2(3P)

was predicted to be thermally stable because of substantial activation barriers for the endoergic fragmentation (∼30 kcal mol−1) into ·CH3 and SO2 and the isomerization to the higherenergy isomer CH3SO2·, which is associated with a barrier of 49.8 kcal mol−1. CH3SO2· is much less stable because of the significantly lower barrier for its dissociation into ·CH3 and SO2 (14.9 kcal mol−1).74,76,77 Several approaches for the identification of CH3SO2· and CH3OSO· were utilized. In the gas phase, both species were produced with femtosecond collisional electron transfer and detected with variable-time neutralization−reionization mass spectrometry.76 CH3SO2+ was produced by dissociative ionization of dimethyl sulfone, (CH3)2SO2, at 70 eV.75

they differ by only ∼0.5 kcal mol−1 depending upon the theoretical method used, with the syn conformer being more favorable (Scheme 3).73 As expected, the formation of the oxygen adduct is a nearly barrierless exothermic process. The structure of the initially formed peroxyl radical was established by comparison of computed and experimental IR and UV spectra and through isotopic labeling experiments. Upon irradiation with near-UV light, CH3S(O)OO· undergoes photochemical dissociation into 478


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to afford ·CH3 and SO3. Moreover, CH3SO3· is highly reactive in hydrogen abstraction reactions in acetonitrile and cyclohexene.84 The formation of CH3SO3· was also proposed in the reaction of CH3SO2· with ·NO2 using a discharge flow reactor, with a rate constant of (2.2 ± 1.1) × 10−12 cm3 molecule−1 s−1 at 298 K.85 Wang and co-workers86 studied the reaction mechanism and kinetics of the decomposition of CH3SO3· at the G3XMP2// B3LYP/6-311+G(3df,2p) level of theory. Among various decomposition channels, the decomposition of CH3SO3· into ·CH3 and SO3 via C−S bond cleavage is the most feasible pathway, with a barrier of 15.4 kcal mol−1. The intramolecular hydrogen atom shift to the carbon-centered ·CH2SO3H radical is associated with an activation barrier of 31.8 kcal mol−1. The migration of the CH3 group from the S atom to the neighboring O atom gives CH3OSO2· with a barrier of 37 kcal mol−1. Zeng and co-workers reported the isolation and IR and UV/ vis characterization of CH3SO3· in solid argon,87 generated by FVP of methylsulfonyl peroxide (CH3SO2OOSO2CH3). For C3v-symmetric CH3SO3·, IR bands were found at 1419, 1334, 1011, 962, 864, 757, and 524 cm−1. The UV/vis matrix spectrum exhibits a well-resolved band at 511 nm due to the 0 ← 0 transition (τ00) of CH3SO3·. In the optimized geometry of CH3SO3·, the S−O bond lengths (1.468 Å) are between those of single and double bonds, and the unpaired electron is delocalized equally over the three oxygen atoms (CCSD(T)/6311++G(2df,2pd)). Upon 360 nm irradiation of the matrix, a hydrogen shift from the methyl group of CH3SO3· to an oxygen atom occurs, giving the thermodynamically more stable carboncentered ·CH2SO3H radical (Scheme 6).87

CH3SO2· was generated from the photodissociation of CH3SO2Cl at 193 nm and characterized by measuring product velocities in a crossed laser−molecular beam scattering apparatus. Electronically and vibrationally excited CH3SO2· radicals undergo subsequent dissociation to ·CH3 + SO2, consistent with a computed barrier of 14.6 kcal mol−1 at the B3LYP/6-311++G(3df,2p) level.76 A transient IR spectrum of CH3SO2· was recorded upon 248 nm irradiation of CH3I and SO2 in the gas phase.77 Two bands at 1280 and 1076 cm−1 were assigned to the antisymmetric and symmetric stretching modes of SO2, respectively.77 CH3SO2· was also generated from the reaction of ·CH3 with SO2 and isolated in p-H2 matrices, resulting in the observation of five fundamental IR bands at 1416, 1272, 1071, 917, and 633 cm−1.78 Moreover, the ESR spectrum helped to identify CH3SO2· as a product of the radiolysis of H2O/DMSO mixtures. According to the observed EPR spectra and ab initio computations, in CH3SO2· the unpaired electron is localized on the SO2 moiety.79 CH3SO2· was also identified as an intermediate in the pulse radiolysis of methanesulfonyl chloride (CH 3SO2Cl) through its UV absorption at λ ≈ 320 nm.80,81 We recently reported82 an efficient method for the generation of CH3SO2· by thermolysis of allyl methyl sulfone followed by isolation in an argon matrix at 10 K (Scheme 5). Scheme 5. Bond Dissociation Energies (in kcal mol−1) for the Formation of the Methylsulfonyl Radical

CH3SO2· was characterized through the assignment of eight fundamental IR bands of its CD3 and 13CH3 isotopologues on the basis of their excellent agreement with B3LYP/aug-ccpVTZ computed harmonic vibrational frequencies, with the two most prominent IR bands at 1267 and 1067 cm−1. The challenge is that the C−S bond in CH3SO2· is very weak, and CH3SO2· undergoes partial decomposition leading to ·CH3 and SO2. Experimental and computational data (B3LYP/aug-ccpVTZ) provide a C−S bond dissociation energy of only 14 kcal mol−1. Therefore, the dissociation of CH3SO2· proceeds readily upon pyrolysis, whereas the comparable cleavage of CH3SO· to ·CH3 and SO requires higher energy.82 The reaction of CH3SO2· with O2(3P) gives the methylsulfonyl peroxy radical (CH3S(O)2OO·), which to the best to our knowledge has not yet been examined spectroscopically. Computationally,72 CH3S(O)2OO· is more stable than CH3SOO· or CH3S(O)OO· with respect to dissociation to the radical and O2(3P); its S−OO bond also is the shortest.

Scheme 6. CH3SO3· Generated from Methylsulfonyl Peroxide via Flash Vacuum Pyrolysis in an Argon Matrix and Its Subsequent Photochemical [1,3]-Hydrogen Shift Reaction

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SUMMARY AND OUTLOOK The field of reactive sulfur intermediates has been blossoming in recent years, and it impacts atmospheric and interstellar chemistry as well as other areas of science. Knowledge of the key properties of atmospheric radicals has provided valuable insights into the complex mechanisms of their formation and decomposition reactions. In this Account, we have summarized some of the key aspects of sulfur-containing radicals derived from DMS, which is highly abundant in the atmosphere. We have described the current state of knowledge of the CH3SOx (x = 0−3) radicals based on recent theoretical and experimental studies. Although new knowledge has been accumulated over the last two decades, many challenges remain in DMS oxidation chemistry. To the best of our knowledge, no spectroscopic signatures are available for the methyl thiol radical (CH3SCH2·) and its oxidation product, the methyl thiol peroxy radical (CH3SCH2OO·). Only time-resolved spectroscopy was used for kinetic measurements, which is not suitable for the spectroscopic identification of these radicals.88,89 To record the complete spectra of CH3SCH2· and to investigate its

Methylsulfonoxyl Radical (CH3SO3·)

The methylsulfonoxyl radical (CH3SO3·) is a highly reactive intermediate of fundamental importance in the industrial production of MSA from CH4 and SO3.83 The dissociation of CH3SO3· leading to SO3 is a direct pathway for H2SO4 formation. Despite the importance of CH3SO3· in atmospheric chemistry, little is known about the spectroscopy and reactivity of this species. The UV/vis transient spectrum of CH3SO3· was recorded upon 308 nm laser flash photolysis (LFP) of methylsulfonyl peroxide (CH3SO2OOSO2CH3) in acetonitrile solution;84 CH3SO3· exhibits a broad absorption band at λmax ≈ 450 nm with a lifetime of 7−20 μs.84 Upon two-photon irradiation (λmax = 480 nm), CH3SO3· undergoes photocleavage 479


Accounts of Chemical Research reactivity under the conditions of matrix isolation, more suitable precursors are necessary. Another important aspect is the study of hydrogen-bonded complexes of atmospheric radicals.90,91 Since water is an important component in the atmosphere, the hydrogen-bonded complexes with atmospheric molecules have been extensively studied.92 Despite their importance for atmospheric chemistry, the complexes between water and radicals have been much less well examined.93 Complexes of water with ·OH, ·NO, ·NO2, HCO·, and HOO· were studied experimentally and theoretically93−95 because hydrogen bonding affects their reactivities.96 For example, the presence of water enhances the rate constant of the HOO· self-reaction.97 A similar acceleration of the reaction between HOO· and ·NO2 was also observed.98,99 The catalytic effect of a single water molecule on the rate of the reaction of ·OH with acetaldehyde in the gas phase was observed as well.100 To our knowledge, there are no experimental data regarding the reaction of CH3SOx· radicals with water. Undoubtedly, there is a need for further research on the preparation and characterization of CH3SOx + H2O complexes and the effect of water on photochemical oxidation mechanisms relevant to atmospheric chemistry.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported over the years by the Deutsche Forschungsgemeinschaft and the Alexander von Humboldt Foundation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Peter R. Schreiner: 0000-0002-3608-5515 Author Contributions

The manuscript was written and the final version approved by both authors. Notes

The authors declare no competing financial interest. Biographies Artur Mardyukov received his Dr. rer. nat (2010) in Organic Chemistry from Ruhr University Bochum in the Sander group. In 2011−2014 he conducted postdoctoral studies as an Alexander von Humboldt Fellow in the Studer group in Münster. Since 2014 he has been working as a senior researcher in the Schreiner group in Giessen. His research focuses on the preparation and characterization of novel reactive intermediates. Peter R. Schreiner is Professor of Organic Chemistry and Liebig Chair at Justus-Liebig University Giessen. He studied chemistry at the University of Erlangen-Nürnberg in Germany, where he received his Dr. rer. nat. (1994) in Organic Chemistry. Simultaneously, he obtained a Ph.D. (1995) in Computational Chemistry from the University of Georgia in the USA. He completed his habilitation at the University of Göttingen (1999). He is a member of the LeopoldinaGerman National Academy of Sciences and the 2003 recipient of the Dirac Medal, and he received the Adolf von Baeyer Memorial Award in 2017. He currently serves as an editor for the Journal of Computational Chemistry, the Editor-in-Chief for Wiley Interdisciplinary Reviews: Computational Molecular Science, and an associate editor for Beilstein Journal of Organic Chemistry. 480


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