Methoxysulfinyl Radical CH3OSO: Gas-Phase Generation

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Methoxysulfinyl Radical CHOSO: Gas-Phase Generation, Photochemistry, and Oxidation Qifan Liu, Zhuang Wu, Jian Xu, Yan Lu, Hongmin Li, and Xiaoqing Zeng J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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The Journal of Physical Chemistry

Methoxysulfinyl Radical CH3OSO: Gas-phase Generation, Photochemistry, and Oxidation

Qifan Liu,† Zhuang Wu,† Jian Xu,† Yan Lu,† Hongmin Li† and Xiaoqing Zeng*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou, P. R. China.

Abstract Methylsulfoxide radicals CH3SOx (x = 1–4) are key reactive sulfur species (RSS) in the atmospheric oxidation of

volatile

organic

sulfur

compounds

(VOSCs).

Through

flash

vacuum

pyrolysis

(FVP)

of

trifluoromethanesulfinic acid methyl ester CF3S(O)OCH3 at 1000 K, the methoxysulfinyl radical CH3OSO has been generated in the gas phase and subsequently characterized in cryogenic N2, Ar, and Ne matrices by IR spectroscopy. Upon 266 nm laser irradiation, CH3OSO efficiently isomerizes to the less stable methylsulfonyl radical CH3SO2 in matrices without noticeable decomposition. In the gas phase, CH3OSO reacts with O2 and yields sulfinyl peroxyl radical CH3OS(O)OO, a new member in the CH3SOx (x = 1–4) family. This radical dissociates into SO3 and CH3O with the intermediacy of the sulfonyoxyl radical CH3OSO3 under the 266 nm laser irradiation. Additionally, the photoisomerization of CF3S(O)OCH3 to sulfenic ester CF3OSOCH3 was also observed.

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Introduction Volatile organic sulfur compounds (VOSCs), produced from natural plankton and anthropogenic processes, have received considerable attention due to their potential effects on atmospheric chemistry and climate change.1-3 Dimethyl sulfide (DMS), the most abundant biogenic VOSCs, is emitted from the oceans by 24–27 Tg S yr-1.4 As part of global sulfur cycle, DMS and its oxidation products are of great significance for the atmospheric sulfur balance, the earth’s radiation budget and sulfate aerosols formation in the upper troposphere.5 The atmospheric oxidation of DMS, initiated by reaction with OH or NO3 radicals, eventually leads to the formation of sulfuric and methanesulfonic acids.5,6 As the key intermediates in the complex oxidation reactions, transient methylsulfoxide radicals CH3SOx (x = 1–4, Scheme 1) have been the targets of numerous experimental and theoretical studies.7–11

Scheme 1. Molecular structures of methylsulfoxide radicals CH3SOx (x = 1–4).

Among these radicals, CH3SO2 can exist as three distinct isomers: methylsulfonyl radical (CH3SO2, 2),8 methylthio peroxyl radical (CH3SOO, 3),12 and methoxysulfinyl radical (CH3OSO, 4).13 According to the extensive theoretical studies,8,11,14–16 CH3OSO is most stable and can adopt syn and anti 2


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conformations with the former being lower in energy by 2.0 kcal mol–1 (CCSD(T)/aug-cc-pV(T+d)Z). More importantly, CH3OSO was predicted to be thermally stable due to the substantial activation barriers for both the endoergic fragmentation (30.7 kcal mol–1) into CH3 and SO2 and the isomerization (49.8 kcal mol–1) to the higher-energy CH3SO2. Whereas, CH3SO2 is much less stable because of the significantly lower barrier (14.3 kcal mol–1) for its dissociation into CH3 and SO2. Experimentally, two methods for the generation of CH3OSO have been utilized (Scheme 2). UV light photolysis of methoxysulfinyl chloride CH3OS(O)Cl was most frequently used in producing CH3OSO radicals in solution, solid para-H2 matrix, and gas phase for the subsequent characterization with electron paramagnetic resonance (EPR),17 matrix-isolation IR spectroscopy,18 time-resolved IR spectroscopy,13 and velocity map imaging apparatus.19 The gas phase formation of CH3OSO from the neutralization of the corresponding cation was also confirmed by neutralization-reionization mass spectrometer (NRMS).14 As for the IR spectroscopy characterization, five transient IR bands in the gas phase13 and eleven IR fundamentals in para-H2 matrix18 have been observed for CH3OSO.

Scheme 2. Methods of generating gas phase CH3OSO.13,14,18

Despite the progress made in the generation and characterization of CH3OSO, the knowledge about its reactivity including the photochemistry is limited. It has been suggested that irradiation might interconvert the three isomers (Scheme 1).13 Photodissociation of CH3SO2 to CH3 and SO2 at 193 nm was observed by using velocity map imaging technique.15 Inefficient photoisomerization from CH3OSO to CH3SO2 was observed in 3



para-H2 matrix with prolonged irradiation at 239±20 nm.18 Very recently, the oxidation of sulfinyl radical CH3SO (1, Scheme 1), generated from the flash vacuum pyrolysis (FVP) of allylmethylsulfoxide, by molecular oxygen into methylsulfonyloxyl radical CH3SO3 via the intermediacy of peroxyl radical CH3S(O)OO was studied in solid Ar matrix.9 In contrast, the oxidation of CH3OSO remains unexplored, the main obstacle is probably due to the low yield of the radical through the photolysis of CH3OS(O)Cl. Therefore, it is desirable to find a more practical method for producing the thermally stable CH3OSO radicals in the gas phase for the studies of its structure and reactivity. Continuing our interest in the small reactive sulfur species (e.g., CH3SO3,20 NSO,21 and HNSO222), herein, we report a new convenient approach for the gas-phase generation of CH3OSO through FVP of trifluoromethanesulfinic acid methyl ester CF3S(O)OCH3. Then, the IR spectra of CH3OSO in cryogenic N2, Ar, and Ne matrices were obtained, the efficient photo-induced isomerization to CH3SO2 and the oxidation with O2 to a novel peroxyl species CH3OS(O)OO were observed.

Experimental section Sample preparation CF3S(O)OCH3 was synthesized from the reaction of trifluoromethylsufinyl chloride (CF3S(O)Cl) with methanol (CH3OH).23 Briefly, freshly prepared CF3S(O)Cl (228 mg, 1.5 mmol)24 was condensed (liquid nitrogen) into a reaction vessel containing CH3OH (32 mg, 1 mmol) in the vacuum line. The mixture was slowly warmed to room temperature and stirred for 12 hours. Then all the volatiles were separated by passing through three successive cold U-traps held at –80, –120, and –196 °C. Pure CF3S(O)OCH3 (103 mg, 0.7 mmol) was retained in the first trap as colourless liquid with boiling point of 73 °C. Unreacted CF3S(O)Cl was found in the last trap. The purity of the sample was checked by gas-phase IR spectroscopy.25

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Matrix-isolation experiments Matrix IR spectra were recorded on a FT-IR spectrometer (Bruker 70V) in a reflectance mode using a transfer optic. A KBr beam splitter and MCT detector were used in the mid-IR region (4000–600 cm–1). For each spectrum, 200 scans at a resolution of 0.5 cm–1 were co-added. Freshly purified CF3S(O)OCH3 was mixed with N2 or noble gases (Ar and Ne) (1:1000) in a 1 L stainless steel storage container. A small amount of the mixture was then passed through an aluminium oxide (Al2O3) furnace (o.d. 2.0 mm, i.d. 1.0 mm), which could be heated (voltage 5.0 V, current 3.3 A) over a length of ca. 25 mm by a tantalum wire (o.d. 0.4 mm, resistance ca. 1.0 Ω) prior to deposition (2 mmol/h) on the Rh-plated copper block matrix support (2.8 for Ne and 10.0 K for Ar and N2) in a high vacuum (∼10-6 Pa). To investigate the oxidation of CH3OSO, oxygen-doped pyrolysis experiments with gaseous mixture of the precursor in N2 and O2 (1:10:1000, vol%) were carried out. Photolysis was performed by using an ArF excimer laser (Gamlaser EX5/250, 3 Hz, 193 nm), a Nd3+:YAG laser (266 nm, MPL-F-266, 10 mW), and a flashlight (Boyu T648, 365 nm, 20 W).

Computational details Geometry optimization and harmonic IR frequency (unscaled) calculations were performed using DFT B3LYP,26 MPW1PW91,27 and BP8628 methods combined with the 6-311++G(3df,3pd) basis set. Accurate relative energies of the species were further calculated by the using the complete basis set CBS-QB329 and CCSD(T)30 methods, for the latter the optimized structures at the B3LYP/6-311++G(3df,3pd) level and the aug-cc-pVTZ basis set were applied. Local minima are confirmed by vibrational frequency analysis. The DFT and CCSD(T) calculations were performed using the Gaussian 0931 and Molpro 201232 software package.

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Results and discussion Flash vacuum pyrolysis of CF3S(O)OCH3 The flash vacuum pyroysis (FVP) of CF3S(O)OCH3 was carried out by passing the mixture of CF3S(O)OCH3 in N2, Ar or Ne (1:1000) through a heated Al2O3 furnace (ca. 1000 K). The resulting mixture was immediately condensed onto the cryogenic Rh-matrix for IR spectroscopy analysis. The comparison of the IR spectrum of CF3S(O)OCH3 (Figure 1A) with that of the pyrolysis products (Figure 1B) isolated in solid N2 matrix at 10.0 K clearly shows the decomposition of the precursor.

Figure 1. (A) IR spectrum CF3S(O)OCH3 in solid N2 matrix at 10.0 K. (B) IR spectrum of the flash vacuum pyrolysis products of CF3S(O)OCH3 in solid N2 matrix at 10.0 K. The IR bands of CF3S(O)OCH3 (a), CF3 (b), CH3OSO (c), SO2 (d), CH3 (e), F2CO (f), H2CO (g), H2O (h), and unknown species (*) are labeled.

Nearly 60% of CF3S(O)OCH3 decomposes at 1000 K. As a result, CF3 (b, 1249.8, 1083.7, 700.9 cm–1),33 SO2 (d, 1351.5, 1152.6 cm–1),34 CH3 (e, 611.1 cm–1),35 F2CO (f, 1940.2, 1912.6 cm–1),36 and H2CO (g, 1737.6 cm–1)37 were identified by IR spectroscopy. Additionally, a few new IR bands (c, Figure 1B) at 2958.7, 1464.0, 1145.9, 985.7, and 713.5 cm–1 were observed. The band positions are very close to those previously observed for CH3OSO (2950.4, 1465.2, 1147.8, 989.7, and 714.5 cm–1) in solid para-H2 matrix.18 Given the predominant formation of CF3 in the unimolecular decomposition of CF3S(O)OCH3 in the highly diluted N2 mixture, it is 6


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reasonable to make the assignment of these bands to CH3OSO. Further decomposition of CH3OSO occurs during the pyrolysis as evidenced by the observation of the fragments CH3 and SO2 among the decomposition products. Interestingly, traces of H2CO were found under the pyrolysis conditions, which should be also derived from the fragmentation of CH3OSO. Generation of CH3OSO from the flash vacuum pyrolysis of CF3S(O)OCH3 is fully reproducible by using Ar and Ne as carrier gases. In fact, similar decomposition has been very recently observed for CF3S(O)NCO, from which heterocumulene OSNCO and CF3 radicals were obtained via the fragmentation of the weak F3Cδ+–Sδ+ bond at ca. 1200 K.38 The observed IR bands of CH3OSO in N2, Ar, and Ne matrices are collected in Table 1 and compared with those previously reported 13,18 and calculated with the B3LYP/6-311++G(3df,3pd) method. According to the calculations, the IR spectra for the two conformers of CH3OSO differ mainly in the two stretching vibrations ν(C–O) (syn: 997 cm–1, anti: 1017 cm–1) and ν(S–O) (syn: 702 cm–1, anti: 722 cm–1) with the large calculated IR

intensities

(Table

1).

Given

the

estimated

small

energy

differences

(2.0

kcal

mol–1,

CCSD(T)/aug-cc-pV(T+d)Z) between these two conformers15 and the rather high temperature (1000 K) used for the gas phase generation of CH3OSO, it should be possible to identify the less stable anti conformer in the matrix-isolation IR spectrum, particularly considering the calculated large differences (∆ν = 20 cm–1) for the two aforementioned stretching vibrations. Close inspection of the spectrum (Figure 1B) indeed reveals the existence of weak signals around the main bands at 985.7 and 713.5 cm–1, however, they heavily overlap with the IR bands of CF3 and undecomposed CF3S(O)OCH3.

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Table 1. Observed and calculated IR frequencies (cm–1) of CH3OSO.a observed calculatedc assignmentd b this work previous work N2 Ar Ne para-H2e Gas-phasef syn-CH3OSO anti-CH3OSO 3030.9 3022.9 3027.6 3028.4 3152 (60) 3144 (6) ν(CH2)asym 3010.5 3003.3 2996.3 2999.5 2991 3120 (15) 3091 (24) ν(CH3)asym 2958.7 2950.9 2950.6 2950.4 2956 3039 (36) 3025 (47) ν(CH3)sym 1464.0 1463.2 1467.8 1465.2 1502 (11) 1499 (15) δs(CH2) 1449.1 1450.9 1454.3 1452.0 1487 (10) 1492 (9) δ(CH3) 1408.3 1417.8 1466 (1) 1470 (1) u(CH3) 1163.0 1165.3 1166.2 1165.2 1184 (8) 1190 (90) ρ(CH3) 1148.1 1151.6 1153.4 1152.1 1154 1168 (6) 1187 (8) ω(CH3) 1145.9 1146.2 1148.7 1147.8 1151 1156 (75) 1170 (1) ν(S=O) 985.7 987.1 982.3 989.7 994 997 (191) 1017 (189) ν(C–O) 713.5 709.2 716.6 714.5 702 (105) 722 (136) ν(S–O) a Full list of the calculated harmonic IR frequencies (unscaled) are given in Table S1 in the Supporting Information (SI). b Observed band positions for the intense matrix sites in N2, Ar, and Ne matrices. c Calculated frequencies with the B3LYP/6-311++G(3df,3pd) method, IR intensities (km mol–1) in parentheses. d Tentative assignment base on the calculated vibrational displacement vectors of the syn conformer. ν: stretching, δ: bend or deformation, δs: scissor, u: umbrella, ω: wag, ρ: rock, sym: symmetric, asym: antisymmetric. e Taken from reference 18. f Taken from reference 13.

Photochemistry of CH3OSO To identify the remaining IR bands of CH3OSO, the N2-matrix containing the pyrolysis products of CF3S(O)OCH3 was subjected to the irradiation of a 266 nm laser. The corresponding IR difference spectrum showing the changes of the matrix is depicted in Figure 2. All the aforementioned IR bands of CH3OSO were completely depleted. As a result, strong IR bands (j, Figure 2) at 1414.9, 1267.7, 1071.9, 920.0, and 631.7 cm–1 appear, which are reasonably assigned to the isomeric CH3SO2 by comparison with the previously reported spectrum in both para-H239 and Ar matrices (Table 2).40

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Figure 2. IR difference spectrum showing the change of N2 matrix (10.0 K) containing the flash vacuum pyrolysis products of CF3S(O)OCH3 upon the 266 nm laser irradiation. Bands for the formed and depleted species point upward and downward, respectively. The IR bands of CF3S(O)OCH3 (a), CF3 (b), CH3OSO (c), SO2 (d), F2CO (f), H2CO (g), H2O (h), FC(O) (i), CH3SO2 (j), and unknown species (*) are labeled.

Additionally, slight depletion of the IR bands for CF3S(O)OCH3 (a), CF3 (b), and SO2 (d) happened upon the laser irradiation, F2CO (f), H2CO (g), H2O (h), FCO (i), CO (2139.7 cm–1), and OCS (2050.5 cm–1) were formed. The photodecomposition of CH3SO2 into OCS, CO, and H2O was previously observed in solid Ar matrix at 10 K.40 Attempts to convert CH3SO2 back to CH3OSO were made by applying various irradiations (193 and 365 nm), however, no transformation occurred.

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Table 2. Observed and calculated IR frequencies (cm–1) of CH3SO2.a observed this workb previous work e N2 Ar Ne para-H2 Arf gas-phaseg

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calculatedc

assignmentd

3185 (0) ν(CH2)asym 3164 (1) ν(CH3)asym 3061 (0) ν(CH3)sym 1414.9 1413.9 1417.6 1416.0 1413.8 1455 (10) δs(CH2) 1447 (5) δ(CH3) 1313 (1) ν(CH3) 1267.7 1274.2 1277.8 1272.5 1267.1 1280 1271 (141) ν(SO2)asym 1071.9 1074.5 1069.3 1071.1 1067.6 1076 1076 (66) ν(SO2)sym 955 (0) ω(CH3) 920.0 915.6 917.4 917.5 915.3 934 (7) ρ(CH3) 631.7 628.0 633.2 633.8 631.3 625 (16) ν(C–S) a Full list of the calculated harmonic IR frequencies (unscaled) are given in Table S1 in the SI. b Observed band positions for the intense matrix sites in N2, Ar, and Ne matrices. c Calculated frequencies with the 6-311++G(3df,3pd) basis set, IR intensities (km mol–1) in parentheses. d Tentative assignment base on the calculated vibrational displacement vectors of the syn conformer. ν: stretching, δ: bend or deformation, δs: scissoring, u: umbrella, ω: wagging, ρ: rocking, sym: symmetric, asym: antisymmetric. e Taken from reference 39. f Taken from reference 40. g Taken from reference 8.

The selective conversion from CH3OSO to CH3SO2 enables a clear identification of all IR bands for both species in different matrices (Table 2) in the available spectral range (4000–600 cm–1) in this study. Particularly, the strong and sharp bands for the SO2 asymmetric (ν(SO2)asym) and symmetric (ν(SO2)sym) and C–S stretches (ν(C–S)) in CH3SO2 allow clear identification of the weaker bands associated with the naturally abundant cm–1, respectively. The

34

S-containing CH3SO2, corresponding to isotopic shifts of 15.5, 7.0, and 6.3

32/34

S isotopic shifts of 9.4 and 7.8 cm–1 for the two strongest IR bands of

CH3OSO at 1145.9 and 713.5 cm–1 for the S=O (ν(S=O)) and S–O (ν(S–O)) stretching vibrations were also observed. As can be seen in Figure 2, there are several weaker side bands for these two vibrations, which might be caused by either the presence of a second conformer or the perturbation with the surrounding molecules in same matrix cage. Assuming the presence of the less stable anti conformer of CH3OSO, the relative intensities of these side bands should not be significantly changed when the carrier gas was changed from N2 to Ar or Ne while maintaining the pyrolysis temperature at 1000 K. However, the obtained IR difference spectrum reflecting the photo-induced CH3OSO → CH3SO2 conversion in Ar matrix (Figure 3) clearly shows the disappearance of the side bands. The only exception at 974.2 cm–1 exhibits redshift of 12.9 cm–1

10


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comparing to the main band at 987.1 cm–1, which is significantly smaller than the calculated blueshift of 20 cm–1. Therefore, the side bands in the IR spectrum of CH3OSO in N2 matrix are more likely to be caused by weakly interacting surrounding molecules in the same matrix cage. The formation of only one conformer for CH3OSO was also observed in the photolysis of matrix-isolated CH3OS(O)Cl in para-H2 matrix at 3.2 K,18 despite that the presence of the anti conformer in the photolysis of gaseous CH3OS(O)Cl at room temperature was tentatively concluded.13

Figure 3. IR difference spectrum showing the change of Ar matrix (10.0 K) containing the flash vacuum pyrolysis products of CF3S(O)OCH3 upon the 266 nm laser irradiation. Bands for the formed and depleted species point upward and downward, respectively. The IR bands of CF3S(O)OCH3 (a), CF3 (b), CH3OSO (c), SO2 (d), H2CO (g), H2O (h), CH3SO2 (j), and unknown species (*) are labeled.

Upon the 266 nm irradiation of CH3OSO in N2, Ar, and Ne matrices, the IR bands of SO2 were only slightly perturbed (d in Figure 2 and Figure 3), no IR band for CH3 appeared at all. This is in sharp contrast to the previous observation of the decomposition of CH3OSO18 and CH3SO239 in para-H2 matrix (3.2 K) upon prolonged (3 hours) irradiation at 239±20 and 365 nm, respectively. According to the previous theoretical study,15 the barrier for the fragmentation of CH3OSO to CH3 and SO2 is lower than that for the isomerization to CH3SO2 by 19.1 kcal mol–1, and the recombination of CH3 and SO2 to CH3SO2 is barrierless. Therefore, the efficient photo-induced conversion in the more rigid N2, Ar, and 11



Ne matrices is likely due to stronger cage effect, i.e., the initially generated fragmentation intermediates (CH3 and SO2) can hardly escape from the original cage but undergo barrierless recombination to the energetically favorable CH3SO2. Nevertheless, the 1,2-CH3 shift mechanism for CH3OSO → CH3SO2 can not be completely ruled out under the irradiation condition because of the possible involvement of reactive species in the excited state.

Oxidation of CH3OSO The facile gas phase generation of CH3OSO enables the subsequent study of its oxidation by molecular oxygen for the first time. The N2-matrix IR spectrum of the flash pyrolysis (1000 K) products of CF3S(O)OCH3 in the presence of O2 (CF3S(O)OCH3:O2:N2 = 1:10:1000) is shown in Figure 4A, and the IR difference spectrum reflecting the change of the matrix upon subsequent 266 nm laser irradiation is depicted in Figure 4B.

Figure 4. (A) IR spectrum of the flash vacuum pyrolysis products of CF3S(O)OCH3 containing 10 vol% O2 in solid N2 matrix at 10.0 K; (B) IR difference spectrum showing the change of the matrix upon 266 nm irradiation. Bands for the formed and depleted species point upward and downward, respectively. The IR bands of CF3S(O)OCH3 (a), SO2 (d), F2CO (f), H2CO (g), H2O (h), CF3OO (k), CO2 (l), CO (m), CH3OS(O)OO (n), SO3 (o), CH3O (p), CH3OO (q), and unknown species (*) are labeled. (C) Calculated IR spectrum of syn-CH3OS(O)OO at the B3LYP/6-311++G(3df,3pd) level. (D) Calculated IR spectrum of anti-CH3OS(O)OO at the B3LYP/6-311++G(3df,3pd) level.

As expected, none of the IR bands for CF3 (b) and CH3OSO (c) was observed, instead the IR bands for CF3OO (k, 1306.4, 1261.1, 1174.6, and 1090.7 cm–1)41 and two broad bands (n) at 960.3 and 742.8 cm–1 appeared.

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Given the very recently observed oxidation of sulfinyl radical CH3SO to the peroxyl radical CH3S(O)OO,9 the IR spectra for the most likely candidate species CH3OS(O)OO and isomers CH3OSO3 and CH3S(O)2OO were performed (see Table 3).

Table 3. Observed and calculated IR frequencies (cm–1) of CH3OS(O)OO, CH3S(O)2OO, and CH3OSO3. CH3OS(O)OO CH3S(O)2OO CH3OSO3 Observeda calculatedb calculatedb calculatedb N2-matrix syn anti 3169 (3) 3171 (3) 3185 (2) 3174 (3) 3152 (5) 3153 (5) 3166 (2) 3149 (4) 3065 (18) 3066 (18) 3070 (1) 3063 (19) 1500 (11) 1499 (11) 1459 (6) 1501 (11) 1489 (12) 1489 (11) 1456 (22) 1490 (16) 1465 (2) 1464 (1) 1429 (206) 1474 (1) 1261.1c 1278 (61) 1260 (172) 1350 (18) 1282 (59) 1219 (148) 1186 (5) 1208 (155) 1195 (9) 1185 (6) 1178 (21) 1154 (12) 1178 (1) 1167 (6) 1168 (9) 985 (4) 1052 (119) 960.3 975 (231) 976 (238) 977 (27) 1022 (210) 724.8 727 (123) 732 (104) 749 (67) 864 (26) 571 (10) 601 (33) 598 (40) 787 (99) 490 (2) 517 (15) 505 (47) 555 (15) 356 (19) 382 (20) 468 (42) 477 (21) 271 (1) 282 (7) 369 (3) 421 (0) 239 (9) 241 (11) 317 (1) 336 (1) 206 (3) 197 (7) 286 (1) 296 (0) 126 (4) 133 (5) 221 (4) 242 (4) 92 (1) 83 (0) 186 (0) 148 (1) 61 (2) 61 (2) 97 (2) 108 (1) a Band position of the most intense matrix. b At the B3LYP/6-311++G(3df,3pd) level, the calculated IR intensities (km mol–1) are given in parentheses. c Heavily overlap with the IR band of CF3OO at 1261.1 cm–1.

According to the calculation, CH3OS(O)OO has two conformers (vide infra) with similar IR spectra (Table 3). The positions and relative intensities for the experimentally observed IR bands at 960.3 and 742.8 cm–1 fit quite well to the calculated strong IR bands for the ν(CH3–O) and ν(CH3O–S) stretching vibration modes in syn (975 and 727 cm–1, Figure 4C) and anti (976 and 732 cm–1, Figure 4D) conformers. The calculated strong IR bands for the ν(S=O) stretching vibration mode in the anti conformer of CH3OS(O)OO locates at 1260 cm–1, which could not be identified due to heavy overlap with the IR band of CF3OO at 1261.1 cm–1. Furthermore, the rather lower IR intensity for the ν(O–O) vibration mode at 1178 cm–1 renders its identification difficult. As for the syn conformer, the ν(S=O) and ν(O–O) stretches are vibrationally coupled, leading to the in-phase and 13



out-of-phase vibrations at 1278 and 1219 cm–1, respectively, both might be masked by the IR bands of CF3OO (1261.1 cm–1) and CF3S(O)OCH3 (1208.4 cm–1). The existence of additional IR band at the 1261.1 cm–1 can be unambiguously inferred by its greater intensity than that of the IR band at 1306.4 cm–1, since it is known that the observed relative intensities (parentheses) for the IR bands of CF3OO in Ne-matrix at 1306.4 (100), 1261.1 (82), 1174.6 (47), and 1090.7 cm–1 (35) steadily decrease with the frequency.41 Therefore, the frequency for the ν(S=O) stretching mode in CH3OS(O)OO (1261.1 cm–1) is blueshifted comparing to that in CH3S(O)OO (1190.8 cm–1).9 Similar blueshift occurs to the same vibration mode in CH3OSO (1146.2 cm–1, Ar-matrix) and CH3SO (1068.2 cm–1, Ar-matrix).7 To further support the tentative assignment of CH3OS(O)OO, the matrix containing the FVP products of the CF3S(O)OCH3:O2:N2 (1:10:1000) mixture was subjected to 365 nm light irradiation. Only the IR bands of CF3OO were slightly depleted, despite that efficient isomerization of CH3S(O)OO to CH3SO3 was observed under the 366 nm irradiation.9 Alternatively, the 266 nm laser irradiation diminished the IR bands of CF3OO (k) and CH3OS(O)OO (n), the IR bands for F2CO (f), H2CO (g), CO2 (l), CO (m), SO3 (o, 1394.6 cm–1),34 CH3O (p, 1457.1, 1342.1, 1042.1 cm–1),42 and FO (not labeled, 1025.4 cm–1)43 appeared in the difference spectrum (Figure 4B). Interestingly, a strong IR band at 2234.9 cm–1 (*, Figure 4B) also occurred, but it was not observed when Ar was used as the carrier gas. Unlike the photochemistry of matrix-isolated CH3S(O)OO,9 no evidence for the formation of the expected isomer CH3OSO3 (Table 3) was obtained due to the absence of new IR band in the range of 1100–900 cm–1. One plausible explanation is that this initially generated sulfonyloxyl radical CH3OSO3 dissociates into CH3O and SO3 under the irradiation conditions, and CH3O undergoes secondary photofragmentation into H2CO (g, Figure 4B).

Photoisomerization of CF3S(O)OCH3 As can be seen in Figure 4B, the sulfinyl precursor CF3S(O)OCH3 was also depleted upon the 266 nm laser irradiation. This observation is consistent with the early studies on the photochemistry of other sulfinyl compounds CF3S(O)CF344 and CF3S(O)F,45 from which the isomeric sulfenic ester CF3SOCF3 and CF3OSF were obtained after the irradiation with high-pressure mercury lamp. In order to identify the photolysis product of CF3S(O)OCH3, its photochemistry was investigated in solid N2 and Ar matrices by using the more powerful 193 nm ArF laser.

14


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The IR difference spectrum reflecting the change of the matrix-isolated CF3OSOCH3 upon the laser irradiation is shown in Figure 5A. The calculated IR spectra for the expected isomer CF3OSOCH3 in the anti (Figure 5B) and syn conformations (Figure 5C) are depicted for comparison.

Figure 5. (A) IR difference spectrum showing the change of the N2 matrix-isolated CF3S(O)OCH3 (10.0 K) upon 193 nm laser irradiation. Bands for the formed and depleted species point upward and downward, respectively. (B) Calculated IR spectrum of anti-CF3OSOCH3 at the B3LYP/6-311++G(3df,3pd) level. (C) Calculated IR spectrum of syn-CF3OSOCH3 at the B3LYP/6-311++G(3df,3pd) level.

The formation of CF3OSOCH3 can be ascertained by the occurrence of two pairs of IR bands at 994.3/917.5 and 757.1/739.4 cm–1 (Figure 5A), which are reasonably assigned to the characteristic stretching vibration modes ν(CH3–O)/ν(CF3–O) and νsym(OSO)/νasym(OSO), respectively. IR bands with similar frequencies have been found to the closely related CH3OSOCH3 (νin-phase(CH3–O)/νout-of-phase(CH3–O): 987/979 cm–1 and νsym(OSO): 731 cm–1)46, CF3SOCF3 (ν(CF3–O): 937.5 cm–1 and ν(S–O): 791.6 cm–1),44 and CF3OSF (ν(CF3–O): 934.1 cm–1 and ν(S–O): 806.9 cm–1).45 The IR bands for CF3OSOCH3 in the range of 1250–1100 cm–1 are heavily overlapped with those of CF3S(O)OCH3, which are mostly associated with the CF3 stretching and CH3 rocking modes. Computationally, the two conformers of CF3OSOCH3, with the CF3 and CH3 moieties being in either syn or anti configuration to the OSO plane, differ by 0.7 kcal mol–1. The former configuration is less

15



favorable because of steric hindrance. The preference of the anti configuration has been experimentally established for CH3OSOCH3 in both gas phase and solid state.46 Quantum chemical calculations The molecular structures for CH3OS(O)OO, CH3S(O)2OO, and CH3OSO3 were fully optimized and the results are shown in Figure 6. Their relative energies and the energies for the S–O bond dissociation to CH3OSO + O2, CH3SO2 + O2, and CH3O + SO3 were also calculated and listed in Table 4.

Figure 6. Calculated molecular structures of CH3SO4 isomers at the B3LYP/6-311++G(3df,3pd) level. Bond lengths are given in Å. Table 4. Calculated relative energies (kcal mol‒1) of CH3SO4 isomers and fragments (SO3+CH3O). B3LYPa BP86a MPW1PW9a CBS-QB3 CCSD(T)b CH3S(O)2OO 42.1 39.7 42.1 44.4 38.5 CH3OSO3 0 0 0 0 0 syn-CH3OS(O)OO 43.6 39.1 41.8 52.2 38.3 anti-CH3OS(O)OO 43.8 40.2 41.8 51.2 38.1 CH3SO2 + O2 38.7 40.3 41.5 47.3 38.3 syn-CH3OSO + O2 33.6 38.0 38.9 43.3 29.1 anti-CH3OSO + O2 35.4 39.3 40.6 45.2 31.2 CH3O + SO3 5.8 9.0 8.9 11.0 7.4 a b The 6-311++G(3df,3pd) basis set was applied. Single point energy calculations at the 6-311++G(3df,3pd) basis set with the B3LYP/6-311++G(3df,3pd) optimized structure.

Among these isomers, sulfonyloxyl radical CH3OSO3 is the global minimum. However, the S–O bond 16


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dissociation

energy

(BDE)

to

CH3O

and

SO3

CCSD(T)/6-311++G(3df,3pd)//B3LYP/6-311++G(3df,3pd)

is

merely

level.

This

7.4

kcal

accounts

for

mol‒1 the

at

the

observed

photofragmentation of CH3OS(O)OO into CH3O and SO3 in matrices, during which the isomerization from CH3OS(O)OO to CH3OSO3 should be involved. In fact, radical CH3OSO3 has been detected by EPR spectroscopy upon the radiolysis of crystalline salt CH3OSO3Na, the EPR signal is persistent even at 300 K, and its photodissociation into CH3O and SO3 was also observed.47 The two peroxyl isomers CH3S(O)2OO and CH3OS(O)OO are significantly higher in energy than CH3OSO3 by about 40 kcal mol‒1. Due to the potent involvement in the oxidation of DMS in atmosphere, CH3S(O)2OO has been frequently explored by quantum chemical calculations.48‒51 The formation of CH3S(O)2OO has been proposed in the oxidation of CH3SO2 by O2 in aqueous solution with experimentally determined rate constant of 1.1×109 M‒1 s‒1, surprisingly, its re-dissociation was found to occur very slowly.52 The thermal stability of CH3S(O)2OO is consistent with the theoretically calculated S–O bond length (1.761 Å, B3LYP/6-311++G(3df,3pd)), which is dramatically shorter than those in CH3S(O)OO (syn: 1.848 Å, anti: 1.869 Å, B3LYP/6-311+G(3df,3pd))9 and CH3OS(O)OO (syn: 1.950 Å, anti: 1.865 Å, Figure 6). The recent success of generating gaseous CH3SO2 through the pyrolysis of allylmethylsulfone40 might provide access to CH3S(O)2OO via the oxidation with molecular oxygen.

Conclusions The gas phase generation, photoisomerization, and oxidation of methoxysulfinyl radical CH3OSO (Scheme 3) have been studied by combining flash vacuum pyrolysis, matrix-isolation IR spectroscopy, and quantum chemical calculations.

Scheme 3. Generation, photoisomerization, and oxidation of CH3OSO. 17



Specifically, through flash vacuum pyrolysis of the easily accessible CF3S(O)OCH3 at 1000 K, CH3OSO is generated in the gas phase and subsequently isolated in N2, Ar and Ne matrices, as followed by characterization with IR spectroscopy. The facile gas phase generation of CH3OSO will not only open the door to further studies on its structure and reactivity, but also stimulate new experimental efforts on the search of other reactive sulfur species through the fragmentation of the weak F3Cδ+–Sδ+ bond in sulfinyl compounds. In addition, the photochemistry and oxidation of CH3OSO in cryogenic matrices were reported. Upon irradiation (266 nm), CH3OSO exclusively rearranges to its isomer CH3SO2 in cryogenic matrices. The reaction between CH3OSO and O2 in the gas phase results in the formation of a novel radical CH3OS(O)OO, which enriches the CH3SOx (x = 1–4) family. This newly discovered peroxyl radical undergoes photofragmentation into sulfur trioxide and methoxy radical upon further 266 nm laser irradiation.

ASSOCIATED CONTENT Author Information Corresponding Author *X. Q. Zeng: e-mail, [email protected]; phone, +86 512 65883583.

Notes The authors declare no competing financial interest.

Acknowledgments This research was supported by the National Natural Science Foundation of China (21422304, 21673147), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Supporting Information Calculated IR spectroscopic data, energies, and coordinates for all species discussed in the paper. This material is available free of charge via the internet at http://pubs.acs.org.

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Graphic abstract

22


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TOC Graphic 199x118mm (300 x 300 DPI)



Figure 1. (A) IR spectrum CF3S(O)OCH3 in solid N2 matrix at 10.0 K. (B) IR spectrum of the flash vacuum pyrolysis products of CF3S(O)OCH3 in solid N2 matrix at 10.0 K. The IR bands of CF3S(O)OCH3 (a), CF3 (b), CH3OSO (c), SO2 (d), CH3 (e), F2CO (f), H2CO (g), H2O (h), and unknown species (*) are labeled. 197x148mm (300 x 300 DPI)


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Figure 2. IR difference spectrum showing the change of N2 matrix (10.0 K) containing the flash vacuum pyrolysis products of CF3S(O)OCH3 upon the 266 nm laser irradiation. Bands for the formed and depleted species point upward and downward, respectively. The IR bands of CF3S(O)OCH3 (a), CF3 (b), CH3OSO (c), SO2 (d), F2CO (f), H2CO (g), H2O (h), FC(O) (i), CH3SO2 (j), and unknown species (*) are labeled. 189x136mm (300 x 300 DPI)



Figure 3. IR difference spectrum showing the change of Ar matrix (10.0 K) containing the flash vacuum pyrolysis products of CF3S(O)OCH3 upon the 266 nm laser irradiation. Bands for the formed and depleted species point upward and downward, respectively. The IR bands of CF3S(O)OCH3 (a), CF3 (b), CH3OSO (c), SO2 (d), H2CO (g), H2O (h), CH3SO2 (j), and unknown species (*) are labeled. 189x136mm (300 x 300 DPI)


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Figure 4. (A) IR spectrum of the flash vacuum pyrolysis products of CF3S(O)OCH3 containing 10 vol% O2 in solid N2 matrix at 10.0 K; (B) IR difference spectrum showing the change of the matrix upon 266 nm irradiation. Bands for the formed and depleted species point upward and downward, respectively. The IR bands of CF3S(O)OCH3 (a), SO2 (d), F2CO (f), H2CO (g), H2O (h), CF3OO (k), CO2 (l), CO (m), CH3OS(O)OO (n), SO3 (o), CH3O (p), CH3OO (q), and unknown species (*) are labeled. (C) Calculated IR spectrum of syn-CH3OS(O)OO at the B3LYP/6-311++G(3df,3pd) level. (D) Calculated IR spectrum of antiCH3OS(O)OO at the B3LYP/6-311++G(3df,3pd) level. 259x191mm (300 x 300 DPI)



Figure 5. (A) IR difference spectrum showing the change of the N2 matrix-isolated CF3S(O)OCH3 (10.0 K) upon 193 nm laser irradiation. Bands for the formed and depleted species point upward and downward, respectively. (B) Calculated IR spectrum of anti-CF3OSOCH3 at the B3LYP/6-311++G(3df,3pd) level. (C) Calculated IR spectrum of syn-CF3OSOCH3 at the B3LYP/6-311++G(3df,3pd) level. 218x161mm (300 x 300 DPI)


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Figure 6. Calculated molecular structures of CH3SO4 isomers at the B3LYP/6-311++G(3df,3pd) level. Bond lengths are given in Å. 201x137mm (300 x 300 DPI)



Scheme 1. Molecular structures of methylsulfoxide radicals CH3SOx (x = 1–4). 197x146mm (300 x 300 DPI)


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Scheme 2. Methods of generating gas phase CH3OSO.13,14,18 135x43mm (300 x 300 DPI)



Scheme 3. Generation, photoisomerization, and oxidation of CH3OSO. 105x41mm (300 x 300 DPI)


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Methoxysulfinyl Radical CH3OSO: Gas-Phase Generation

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