The Methylsulfonyloxyl Radical, CH 3 SO 3

Spectroscopic Characterization and Photochemistry of the Atmospherically Relevant Methanesulfenic Acid

Methanesulfenic acid, CH3SOH, is a fleeting intermediate in the ·OH-initiated oxidation reactions of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere. Herein, we report the characterization and photochemistry of CH3SOH in Ar- and N2-matrices at 10 K. The characterization of CH3SOH with matrix-isolation IR and UV-vis spectroscopy is supported by D and 13C isotope labeling experiments and quantum chemical calculations. In line with the observed absorption at 260 nm for CH3SOH, its photolysis at 254 nm leads to dissociation by yielding the novel water complex H2CS···H2O, which exhibits a five-membered ring structure with intermolecular S···HO and CH···O hydrogen bonding interactions. Upon further irradiation at 193 nm, the H2CS···H2O complex undergoes dehydrogenation to form CS···H2O, which can further convert to HC(O)SH under irradiation at 254 nm. When the photolysis of CH3SOH was performed in an O2-doped Ar-matrix, methanesulfonic acid (MSA, CH3SO3H) was obtained as the oxidation product.

Atmospheric Gas-Phase Formation of Methanesulfonic Acid

Despite its impact on the climate, the mechanism of methanesulfonic acid (MSA) formation in the oxidation of dimethyl sulfide (DMS) remains unclear. The DMS + OH reaction is known to form methanesulfinic acid (MSIA), methane sulfenic acid (MSEA), the methylthio radical (CH3S), and hydroperoxymethyl thioformate (HPMTF). Among them, HPMTF reacts further to form SO2 and OCS, while the other three form the CH3SO2 radical. Based on theoretical calculations, we find that the CH3SO2 radical can add O2 to form CH3S(O)2OO, which can react further to form MSA. The branching ratio is highly temperature sensitive, and the MSA yield increases with decreasing temperature. In warmer regions, SO2 is the dominant product of DMS oxidation, while in colder regions, large amounts of MSA can form. Global modeling indicates that the proposed temperature-sensitive MSA formation mechanism leads to a substantial increase in the simulated global atmospheric MSA formation and burden.

Infrared Spectroscopic and Theoretical Investigations of Group 13 Oxyfluorides OMF2 and OMF (M=B, Al, Ga, In)

Group 13 oxyfluorides OMF2 were produced by the reactions of laser-ablated group 13 atoms M (M=B, Al, Ga and In) with OF2 and isolated in excess neon or argon matrices at 5 K. These molecules were characterized by matrix-isolation infrared spectroscopy and isotopic substitution experiments in conjunction with quantum-chemical calculations. The calculations indicate that the OMF2 molecules have a 2 B2 ground state with C2v symmetry. The computed molecular orbitals and spin densities show that the unpaired electron is mainly located at the terminal oxygen atom. Oxo monofluorides OMF were only observed in solid argon matrices and exhibit a linear structure in the singlet ground state. The M-O bonding in the OMF molecules can be rationalized as highly polar multiple bonds based on the calculated bond lengths and natural resonance theory (NRT) analyses. In particular, the molecular orbitals of OBF exhibit the character of a triple bond B-O resulting from two degenerate electron-sharing π bonds and an O→B dative σ bond formed by the oxygen 2p lone pair which donates electron density to the boron empty 2p orbital.

Fluoromethylsulfinyl radicals: spectroscopic characterization and photoisomerization via intramolecular hydrogen shift

Two new sulfinyl radicals, CHF₂SO˙ and CH₂FSO˙, have been generated in the gas phase through homolytic cleavage of the weak S-S bonds in disulfane oxides CHF₂S(O)SCF₃ and CH₂FS(O)SCF₃ by high-vacuum flash pyrolysis (HVFP) at ca. 500 °C. The IR spectroscopy characterization of the two fluoromethylsulfinyl radicals in solid N₂ (10 K), Ar (10 K), and Ne (3 K) matrices reveals the presence of two conformers for CHF₂SO˙ (gauche and cis) and one conformer for CH₂FSO˙ (gauche). Upon 266 nm laser irradiation, these radicals undergo both isomerization and decomposition in the matrices. In addition to the dominant formation of the elusive oxathiyl radicals CHF₂OS˙ (gauche and cis) and CH₂FOS˙ (gauche) via 1,2-alkyl migration, two higher-energy carbon-centered radicals ˙CF₂SOH and ˙CHFSOH bearing similar molecular structures to hydroperoxyalkyl radicals (˙QOOH) form via intramolecular 1,3-hydrogen shift in the two sulfinyl radicals. Additionally, the involvement of 1,3-hydrogen shift in CHF₂OS˙ and CH₂FOS˙ is also indicated by the observation of the fragmentation species. The identification of these radicals by matrix-isolation IR and UV-vis spectroscopy is aided by the quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory. The stability of the isomers of the two sulfinyl radicals CHF₂SO˙ and CH₂FSO˙ has been discussed according to the experimental observations and also based on the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) calculated energy profiles.

Spectroscopic characterization of two peroxyl radicals during the O2-oxidation of the methylthio radical

The atmospheric oxidation of dimethyl sulfide (DMS) yields sulfuric acid and methane sulfonic acid (MSA), which are key precursors to new particles formed via homogeneous nucleation and further cluster growth in air masses. Comprehensive experimental and theoretical studies have suggested that the oxidation of DMS involves the formation of the methylthio radical (CH₃S•), followed by its O₂-oxidation reaction via the intermediacy of free radicals CH₃SO_x• (x = 1-4). Therefore, capturing these transient radicals and disclosing their reactivity are of vital importance in understanding the complex mechanism. Here, we report an optimized method for efficient gas-phase generation of CH₃S• through flash pyrolysis of S-nitrosothiol CH₃SNO, enabling us to study the O₂-oxidation of CH₃S• by combining matrix-isolation spectroscopy (IR and UV-vis) with quantum chemical computations at the CCSD(T)/aug-cc-pV(X + d)Z (X = D and T) level of theory. As the key intermediate for the initial oxidation of CH₃S•, the peroxyl radical CH₃SOO• forms by reacting with O₂. Upon irradiation at 830 nm, CH₃SOO• undergoes isomerization to the sulfonyl radical CH₃SO₂• in cryogenic matrixes (Ar, Ne, and N₂), and the latter can further combine with O₂ to yield another peroxyl radical CH₃S(O)₂OO• upon further irradiation at 440 nm. Subsequent UV-light irradiation (266 nm) causes dissociation of CH₃S(O)₂OO• to CH₃SO₂•, CH₂O, SO₂, and SO₃. The IR spectroscopic identification of the two peroxyl radicals CH₃SOO• and CH₃S(O)₂OO• is also supported by ¹⁸O- and ¹³C-isotope labeling experiments.

Capture and Reactivity of an Elusive Carbon-Sulfur Centered Biradical

The initial oxidation product of dimethyl sulfide in the marine boundary layer, the methyl thiomethyl radical, has remained elusive. A structurally analogous biradical with one radical center in the α-position to a sulfur atom could now be obtained by UV irradiation of p-nitrobenzaldehyde dithiane isolated in solid dinitrogen (N₂) or Ar at cryogenic temperatures. A spin-forbidden reaction with triplet dioxygen (³O₂) does not occur. The dithiane of o-nitrobenzaldehyde rather undergoes a series of rearrangements under the same conditions, resulting in overall photodeprotection.

Atmospherically Relevant Radicals Derived from the Oxidation of Dimethyl Sulfide

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 quantum-mechanical calculations on the generation and structural elucidation of CH₃SO_x (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 CH₃SO_x (x = 0-3).

Capture of SO3isomers in the oxidation of sulfur monoxide with molecular oxygen

Two SO₃isomers,cis-OSOO and cyclic OS(O)O, were formed in the oxidation of SO and trapped in cryogenic matrices for spectroscopic characterization.