OH-Induced Oxidative Cleavage of Dimethyl Disulfide in thePresence of NOAndrzej Bil,* Katarzyna Grzechnik, Krzysztof Mierzwicki, and Zofia Mielke*

Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland

*S Supporting Information

ABSTRACT: We report the results of the theoretical study of•OH-induced oxidative cleavage of dimethyl disulfide (DMDS)and the experimental study of the CH3SSCH3 +

•OH reactionin the presence of •NO. Infrared low temperature argon matrixstudies combined with ab initio calculations allowed us toidentify cis-CH3SONO, which evidences the formation of theCH3SO

• and CH3SH molecules in the course of theCH3SSCH3 + •OH reaction. Ab initio/quantum chemical topology calculations revealed details of the oxidative cleavage ofdimethyl disulfide, which is a complex multistep process involving an alteration of S−O and S−S covalent bonds as well as ahydrogen atom transfer. The ability of delocalization of the unpaired electron density by sulfur atoms and a formation of ahydrogen bond by CH3SO

• and CH3SH are the factors which seem to explain antiradical properties of DMDS.

■ INTRODUCTION

Oxidation processes of naturally emitted reduced sulfurcompounds have received a lot of attention, which has beenprimarily due to their importance in the chemistry of theEarth’s atmosphere.1−5Dimethyl disulfide, CH3SSCH3(DMDS), is the least important of the reduced sulfurcompounds in terms of the amount released to the atmosphere;it accounts for not more than a few percent of the global flux ofsulfur compounds. However, it is of special interest as itcontains two sulfur atoms and a weak S−S bond which can bephotolyzed by λ = 248 nm radiation leading to two CH3S

•

radicals.6−8

A major sink for DMDS in the troposphere is theCH3SSCH3 + •OH reaction, which has been the subject ofnumerous experimental kinetic gas phase studies.9−14 Earlierstudies performed by Wine et al.9 indicated CH3S

• andCH3SOH to be the primary products of the reaction. This fact,together with the negative temperature dependence of thereaction rate coefficient and lack of pressure dependence of therate constant, has led to the suggestion that the reactionproceeds via formation of a DMDS−OH adduct which rapidlydecomposes to CH3S

• and CH3SOH.9

+

→

→ +

•

•

CH SSCH OH

CH S(OH)SCH

CH SOH CH S

3 3

3 3

3 3 (I)

The final products of the •OH + DMDS reaction in an air +•NO atmosphere also have been interpreted in terms ofreaction I.15 Later, Domine and Ravishankara used laser-induced fluorescence and photoionization mass spectrometryto investigate the reaction.11,12 The studies confirmed thatCH3S

• and CH3SOH are formed but led to conclusion that

products other than CH3S• and CH3SOH are also important.

Butkovskaya and Setser studied the DMDS + •OH reactionusing the IR chemiluminescence experimental technique.14 Thecomparison of the IR emission spectra from the DMDS +•OH(•OD) reactions with those obtained from the CH3SH +•OH(•OD) reaction indicated two decomposition channels ofthe CH3S(OH)SCH3 adduct with a branching ratio of ≥3 infavor of CH3SH + CH3SO

• versus CH3S• + CH3SOH

products. The authors concluded that the DMDS + •OHreaction mainly proceeds via addition of a hydroxyl radical toone of the sulfur atoms, followed by migration of the H atom tothe other sulfur atom, and, then, by the scission of the S−Sbond.

+

→

→ +

•

•

CH SSCH OH

CH S(OH)SCH

CH SO CH SH

3 3

3 3

3 3 (II)

To our knowledge this result has not been confirmedexperimentally so far by other techniques. The results from therecent computational study on the mechanism of the gas-phaseDMDS + •OH reaction performed by Wang et al.16 are inaccord with the experimental results obtained by Butkovskayaand Setser.14 The authors considered five a priori possiblereaction channels leading to different products:

+CH SH CH SO3 3 (1)

+CH S CH SOH3 3 (2)

Received: January 9, 2013Revised: June 28, 2013Published: August 16, 2013

Article

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+ +H O CH S CH S2 2 3 (3)

+CH CH SSOH3 3 (4)

+CH CH SSO4 3 (5)

The path leading to products 1 is favored over path 2 withrespect to thermodynamics and kinetics. Processes leading toformation of 3 or 5 are thermodynamically permissible but havehigh energy barriers and are less favorable in terms of kinetics.The formation of products 4 is thermodynamically unfavor-able.16 The calculations indicate that in the initially formedCH3S(OH)SCH3 adduct H-shift occurs from the •OH groupto the other sulfur atom followed by dissociation into CH3SO

•

and CH3SH species. Reference 16, however, does not explainproperly the mechanism of the reaction leading to products 1,as some important steps have been overlooked. Moreover, theprocess has been discussed only in terms of the potentialenergy profile and thermodynamics. The authors have notdiscussed the evolution of a bonding situation or an electrondensity transfer in the course of the reaction.A deeper understanding of the mechanism of reaction II

becomes very important in the light of the recent reportssuggesting that sulfenic acids are among the most potent classesof peroxyl radical trapping agents (as the ultimate antiox-idants).17,18 The chemistry of organosulfur compoundsoccurring in natural products such as garlic and other membersof the Allium family (e.g., onion or leek) has recently attractedmuch attention due to their therapeutic potential attributed toantioxidant activity or reaction with thiol-containing proteins.19

The main sulfur compound occurring in garlic is alliin (S-allylcysteine sulfoxide), which is easily transformed in thepresence of alliinase enzyme into allicin (diallylthiosulfinate,CH2CHCH2S(O)SCH2CHCH2).

20 Vaidya et al.17 suggestedrecently that the antioxidant activity ascribed to allicin (andgarlic in general) is actually due to 2-propenesulfenic acidthedecomposition product of allicin:

+

→ +

•

•

CH CHCH SOH OOR

CH CHCH SO HOOR2 2

2 2 (III)

These results were recently confirmed by Galano et al.18

Vaidya et al. concluded also that the mechanism of reaction IIIis based on a proton-coupled electron transfer (PCETP)21,22

and involves the following steps:

′ + → ′ ···

→ ′ ··· ···

→ ′ +

• •

• ‡

•

R SOH OOR (R SOH OOR)

[(R SO H OOR) ]

R SO HOOR (IV)

The authors postulated that the R′SO• radical is stabilized bythe sulfur atom due to a large delocalization of unpairedelectron between S and terminal O (50:50 in comparison toperoxyl radicals, 70:30 between terminal and nonterminaloxygen atoms). As we shall show in this paper, there is aninteresting similarity between the DMDS + •OH and 2-propenesulfenic acid + •OOR reactions.In this paper we present a detailed theoretical study of the

CH3SSCH3 + •OH reaction in the ground electronic statesupplemented by the experimental study of the CH3SSCH3 +•OH reaction in the presence of •NO. The identification of theCH3SSCH3 +

•OH reaction product was confirmed indirectlyby its ability to recombine with the •NO radical. The matrixisolation technique has been proved to be extremely useful in

identification of the intermediate species.23 We studied theCH3SSCH3 + HONO system in low temperature argonmatrixes; the •OH and •NO radicals were generated from thephotolysis of nitrous acid.24,25 The irradiation applied (λ ≥ 345nm) to the molecules entrapped in the matrix produced •OHand •NO radicals in their ground electronic states. CH3SSCH3was not affected by the applied radiation. The identification ofan elusive cis-methylsulfenyl nitrite molecule, cis-CH3SONO,interesting itself, as a product of the above-mentioned reactionprovides strong, independent evidence for the formation of theCH3SO

• and CH3SH products of the CH3SSCH3 + •OHreaction.The mechanism of •OH-induced oxidative cleavage of

CH3SSCH3 is proposed on the basis of ab initio calculationssupported by the topological analysis of electron density andelectron localization function of the relevant molecules (aquantum chemical topology approach). It seems that formationof hydrogen bonds during the process and a hydrogen atomtransfer are indispensable not only for the reaction of DMDSand •OH radical, but also it may have some relevance toprotecting properties in biological systems.

■ EXPERIMENTAL AND THEORETICAL METHODSInfrared Matrix Isolation Studies. The DMDS/Ar

mixtures were prepared by the standard manometric technique;the concentration of the mixtures varied in the range 1/100−1/1000. Crystalline ammonium nitrite was used as a source ofgaseous HONO; in some experiments NH4

15NO2 has beenapplied to produce HO15NO. The HONO/Ar or HO15NO/Armixtures were prepared in the same way as previouslydescribed.25 HONO/HO15NO (and NH3) was evaporatedfrom a small glass tube containing NH4NO2 (NH4

15NO2)maintained at ca. 15 °C placed in the vacuum vessel of thecryostat ca. 20 cm from the sample holder. DMDS/Ar mixtureswere deposited directly into the cryostat in such a way that theymixed with HONO (NH3) vapor inside the vacuum chamber.The matrix concentration was varied by changing the DMDS/Ar concentration and the flow rate of the gaseous mixture. Theresultant concentration of HONO/Ar was estimated to vary inthe range 1/800−1/300. The overall DMDS/HONO/Arconcentrations of the studied matrixes were estimated to ben/m/1200 (n = 1, 3, 4, 6, 12; m = 1.5, 3, 4).The gas mixtures were sprayed onto a gold-plated copper

mirror held at 17 K by a closed cycle helium refrigerator (AirProducts, Displex 202). After the infrared spectra of the initialdeposit were recorded, the sample was subjected to theradiation of a 200 W medium pressure mercury lamp (PhilippsCS200W2). A 5 cm water filter served to reduce the amount ofinfrared radiation reaching the matrix, and a glass long-wavenumber filter was applied to cut off the radiation with λ <345 nm. The electronic spectra of nitrous acid and DMDS havebeen extensively studied.8,26−30 The ultraviolet absorptionspectrum of HONO consists of a diffuse structured band inthe 300−390 nm region and a broad structureless band from270 nm to below 180 nm. The first band corresponds toelectronic excitation to the HONO first excited state (A1A″ ←X1A′). Photodissociation of the HONO A1A″ state producesOH(X2Π) + NO(X2Π) radicals with a nearly unit quantumyield. Therefore, the applied radiation generates •OH and •NOradicals in the matrix cage. The electronic spectrum of DMDSexhibits bands in the range below 300 nm, and DMDS is leftintact by the applied radiation. The spectra were registered after30, 60, 120, 180, and 240 min and, in some cases, also after 300

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and 360 min of matrix irradiation. After the irradiation processwas completed, the samples were annealed to 30 K for 10 minand then cooled again to 11 K, and the spectra of the annealedmatrixes were recorded. All spectra (resolution 0.5 cm−1) wereregistered at 11 K in a reflection mode with a Bruker 113vFTIR spectrometer using a liquid N2 cooled MCT detector.DMDS was commercially available (Pfaltz & Bauer); it was

distilled and kept over molecular sieves (4 Å) in sample tubesbefore the DMDS/Ar mixtures were prepared. NH4NO2 wasprepared according to ref 31; NH4

15NO2 was prepared in thesame way as NH4NO2 but using Na15NO2 instead NaNO2.Computational Methods. All electronic structure calcu-

lations have been performed with the Gaussian 09 package ofprograms.32 Stationary points on the potential energy surface,harmonic and anharmonic frequencies, and minimum energypaths (intrinsic reaction coordinate approach) were calculatedusing B3LYP hybrid density functional and the aug-cc-pVTZbasis set. The basis set is large enough to keep a basis setsuperposition error (BSSE) small, and it was used recently tostudy similar systems.63 Dissociation energies of weakmolecular complexes were corrected for the basis setsuperposition error (BSSE) using the counterpoise correctionscheme proposed by Boys and Bernardi.33 All relative energiesof structures considered in this paper were corrected forharmonic zero-point vibrational energy. Anharmonic frequen-cies have been calculated using the generalized second-ordervibrational perturbation theory,64−66 as implemented in theGaussian package. Calculations have been performed formolecules in their ground electronic states.Information on the nature of interactions has been obtained

from topological analysis of the electron density and theelectron localization function (ELF). We successfully used thequantum chemical topology approach to analyze the bondingsituation in radical systems.34−37 Bader’s theory of “atoms inmolecules” (AIM)38 provides a precise definition of a bond. Inthis theory the classification of the interactions is based on theanalysis of the electron density (ρ), Laplacian (Δρ), totalenergy density (H), and delocalization index (DI) at the (3,−1)bond critical point, which is a saddle point of electrondensitya minimum along the interatomic path and amaximum along the perpendicular directions.The AIM allows one to partition a molecular space into

atomic basins and provides a route for consistent description ofits properties such as atomic populations or spin densities. Inorder to classify the interactions in studied compounds, thetopological analysis of the electron density has been performedat the B3LYP/aug-cc-pVTZ level of theory using the AIMAllpackage.39 The topological analysis of the Becke and Edge-combe electron localization function40 η(r) has been proposedby Silvi and Savin.41 η(r) can range from 0, in those placeswhere the probability of finding same-spin electrons closetogether is high, to 1, where electrons are alone or form pairs ofantiparallel spins. The topological analysis of η(r) provides thepartition of the molecular space into core and valence attractorbasins (for which one can calculate, for example, averagepopulation or volume). Analysis of η(r) has been performedusing TopMod programs.42

■ RESULTS AND DISCUSSIONMechanism of •OH-Induced Oxidative Cleavage of

CH3SSCH3. The two-step process involves three minima linkedby two transition states (Figures 1 and 2). In the minimum M1the •OH radical is loosely bound to the S1 sulfur atom (Edis =

7.7 kcal mol−1). Subsequent decomposition of the S−S bondleads to the minimum M2, where the newly formed CH3SOHmolecule and CH3S

• radical form a weakly hydrogen bondedcomplex (Edis = 5.4 kcal mol−1). The energy barrier for thisprocess is 7.4 kcal mol−1. The next step consists of the transferof the H atom involved in the O−H···S bridge to form theminimum M3, where CH3SH and CH3SO

• form anotherhydrogen bonded complex. The energy barrier for this stage isonly 3.4 kcal mol−1.Data in Figure 2 have been obtained using the B3LYP

functional coupled to the aug-cc-pVTZ basis set. We calculatedalso single point energies for all structures in Figure 1 using thefrozen-core coupled cluster singles-and-doubles method(CCSD) to compare them with B3LYP results. Data in TableS1 (Supporting Information) indicate that both methods leadto the same topology of the potential energy surface.Potential energy distribution analysis performed for the

Hessian matrix calculated for the TS1 structure reveals that theeigenvector of the imaginary eigenvalue (i421.0 cm−1) is quitecomplicated and involves a few internal coordinates. Dominantcontributions are from the S−S distance and the O−S1 distancebut HOS1S2 and CSSC torsion angles are also involved. InFigure 3 we present the minimum energy path (intrinsicreaction coordinate calculations) linking minima M1 and M2along with the evolution of the geometric parameters involved

Figure 1. Mechanism of •OH-induced cleavage of CH3SSCH3:minima and transition structures.

Figure 2. Characteristic points on the reaction path CH3SSCH3 +HO• → CH3SO

• + CH3SH. The relative energy of each structure iscalculated versus the total energy of isolated CH3SSCH3 and HO

•. Fortransition structures the imaginary frequency does not contribute tothe zero-point vibration correction.

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with the transition normal mode. The reaction is triggered bythe shortening of the O−S distance from 2.32 Å in M1 to 1.91Å in TS12, which leads to the increase of the total energy(Figure 3; Table S2, Supporting Information). Then the S−Sbond dissociates, with the change of the interatomic distancefrom 2.07 Å in M1 to 2.19 Å in TS12 and finally to 2.88 Å inM2. Elongation of the S−S distance is the main factor leadingto the decrease of the total energy of the system (Figure 3).The final O−S distance in M2 is 1.66 Å.The evolution of OS and SS bond distances is accompanied

by essential changes in the HOS1S2 and CSSC torsion angles(Figure 3, lower panel). The HOS1S2 angle describes theprocess of the rotation of the H(O) atom along the O−S1 axis.In structures M1 and TS12 the values of the angle are 95.7 and87.3°, which refers to the situation where the O−H bond is,

roughly, perpendicular to the O−S−S plane (Figure 1, M1,TS12). In minimum M2 atoms S1, S2, O, and H are in plane(HOSS torsion angle = −1.7°), which is the consequence of theformation of the OH···S2 bond (Figure 1, M2). At the finalstage of the reaction, when S−S is strongly elongated, weobserve the essential change of the CSSC torsion angle, whichis accompanied by an almost negligible change of the totalenergy of the system (Figure 3). The angle changes from 93.5°in M1 (close to the value for an isolated CH3SSCH3) to 98.5°in TS12 and finally to 158.6° in the minimum M2.The second step of the •OH-induced oxidative cleavage of

DMDS (M2 → M3) is structurally much simpler than theprevious one, and involves the alteration of bonds in the O−H···S bridge. In Figure 4 we plotted the minimum energy pathfor the hydrogen atom transfer between a methylsulfenic acid

Figure 3. Evolution of selected geometric parameters along the minimum energy path linking minima M1 and M2. Point 0 refers to the transitionstructure TS12.

Figure 4. Evolution on selected geometric parameters along the minimum energy path linking minima M2 and M3 (the second step of HO•-inducedcleavage of CH3SSCH3). Point 0 refers to the transition structure TS23.

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molecule and methylthiyl radical together with O−H and S−Hdistance evolution. The initial energy increase is a consequenceof the S2 atom approaching the OH group (the S···H distancedecreases from 2.40 Å in M2 to 1.66 Å in TS23; Table S3,Supporting Information). In this step of the H atom transferthe O−H distance remains almost intact and is close to 0.98 Å.Just before the transistion state the O−H starts to elongate(Figure 4), reaching the value of 1.15 Å in TS23 and finally 2.38Å in M3. The solid elongation of the O−H distanceaccompanied by the formation of the S−H bond (S−Hdistance is 1.35 Å in M3) leads to an essential drop in the totalenergy of the system. The hydrogen atom transfer between Oand S atoms leads to further elongation of the S−S distancefrom 2.88 Å in M2 to 4.45 Å in M3.Bond Evolution in •OH-Induced Oxidative Cleavage

of CH3SSCH3: Quantum Chemical Topology Analysis. InFigure 5 we present ELF localization domains for the minimum

M1. The isosurface ELF(r) = 0.84 reveals several valence basinshaving chemical interpretations. Disynaptic V(C,H) basinsrepresent six C−H bonds, the V(O,H) basin represents theelectron pair involved in the O−H bond, and two disynaptic

V(C,S) basins prove the covalent C−S bonds. In the vicinity ofsulfur atoms and the oxygen atom we observe basinsrepresenting lone electron pairs, which are V1(S1), V2(S1),V1(S2), V2(S2), V1(O), and V2(O). The ELF value applied forplotting the isosurface (0.84) leads to localization domainscontaining both electron pairs centered on a given atom, e.g.,V1(S1)UV2(S1).The molecular graph representing a bonding situation in the

minimum M1 is plotted in Figure S1 (Supporting Information).The graph reflects the topological features of the electrondensity of M1. All bonded atoms are linked by the bond pathswith the (3,−1) bond critical points appearing along the bondpaths.The topological analysis of the electron localization function

calculated for minimum M1 reveals disynaptic valence basinV(S,S) located between S atoms (Figure 5) and populated with1.47 e (Table S2, Supporting Information), which is consistentwith the presence of a covalent bond. The electron density ρcalculated at the (3,−1) S−S bond critical point is reasonablylarge (0.147 au) and the Laplacian of the electron density Δρ isnegative (−0.124 au), which indicates that the bond is of sharedtype, as expected for a covalent bond. The delocalization indexDI (interpreted in a bonding situation as a bond order)indicates that 1.275 electron pairs are on average shared by twosulfur topological atoms. The electron energy density H isnegative, as expected for a covalent bond, but small (−0.08 au).This value, together with a moderate value of |Δρ| and amoderate population of the V(S,S) basin may suggest that,although the bond is dominated by a shared interaction, it mayeasily change its character while the S−S distance increases.Indeed, just before reaching the transition state TS12, when

the S−S distance is only 0.096 Å longer than the equilibriumdistance, the V(S,S) basin splits into two monosynaptic basinsV3(S1) and V3(S2) (Figure 6a; Figure S3, SupportingInformation). The monosynaptic basins at TS12 are populatedwith 0.89 e and 0.39 e and quickly disappear just at the distance2.25 Å. It indicates that in the vicinity of the transition state thecovalent bond S−S disappears, or at least changes its nature.

Figure 5. Localization domains of CH3S(OH)SCH3 (minimum M1).The isosurface plotted for ELF(r) = 0.84.

Figure 6. Evolution on selected parameters representing the bonding situation in the vicinity of the transition structure linking minima M1 and M2(the first step of HO•-induced cleavage of CH3SSCH3). Point 0 refers to the transition structure TS12.

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The fact that the S−S covalent bond breaks at the distance onlyslightly longer than in equilibrium M1 geometry indicates thatin CH3S(OH)SCH3 molecule the bond is weak, which isconsistent with the low energy barrier of the process.The presence of the (3,−1) bond critical point in the

minimum M2 indicates that the S−S bond still exists (FigureS1, Supporting Information); however, the nature of the bondis expected to be entirely different from in the minimum M1.Indeed, the electron density at the bond critical point drops to0.034 au; the Laplacian of the electron density is small andpositive (0.051 au). The bond order (delocalization index),however, still preserves a reasonable value of 0.47. Thenumbers are consisted with a weak bond dominated by aclosed shell interaction, which suggests the ionic character ofthe bond. Indeed, atomic charges calculated for topological Satoms indicate an essential charge transfer (Q(S1) = +0.68,Q(S2) = −0.18) compared to covalently bonded atoms in theminimum M1 (Q(S1) = +0.19, Q(S2) = +0.03). Topologicalatomic charges (Table S4, Supporting Information) indicate theoxidative character of the S−S bond cleavage with the electrontransfer from the atom S1 to S2 and O.Figure 6b presents the evolution of ρ calculated at the (3,−1)

point on the interatomic line linking two S atoms as well as thevalue of the delocalization index. Both values decreasemonotonically with the increase of the S−S distance. Figure6c illustrates the above-mentioned charge transfer between Satoms in the course of the S−S bond elongation. We observealso an essential spin transfer: in the minimum M1 the spindensity is located mainly (but not entirely) within the O atom(•OH radical) whereas in the minimum M2 the spin density islocated mainly (but not entirely) within the S2 atom, whichproves the radical character of the newly formed CH3S

• moiety.The evolution of the O−S bond presents the pattern

opposite to the one discussed above for the S−S bond. In theminimum M1 the bond is characterized by parameters typicalfor closed shell interaction. The electron density at the O−Sbond critical point is small (0.056 au), the Δρ is positive (0.128au), and the energy density H is close to 0. Although the valueof the bond order (delocalization index) proves that the atomsexchange on average 0.62 electron pair, the topological analysisof ELF reveals no disynaptic basin representing the S−O bond.A typical picture of the closed shell bond is consistent with itscalculated dissociation energy (Edis = 7.7 kcal mol−1) and thecharge distribution in topological O and S1 atoms (Q(O) =−0.91, Q(S1) = +0.19).Presented in Figure 6 the evolution of the above-discussed

parameters shows that the O−S interaction changes its naturefrom closed shell ionic in the minimum M1, to shared covalentin the minimum M2. At the transition state the topology ofELF reveals the appearance of the disynaptic V(S,O) basinpopulated with 0.26 e, which proves the formation of a covalentbond. With the subsequent shortening of the O−S distance thepopulation of the basin quickly increases, reaching the value of1.06 at the minimum M2 (Figure 6; Table S2, SupportingInformation). Although the value is smaller than for typicalsingle covalent bonds, the parameters calculated at the (3,−1)bond critical point confirm its covalent nature. The electrondensity is reasonably large (0.204 au), Δρ is negative (−0.129au), and the energy density H is negative (−0.245 au). Thebond order (delocalization index) calculated for the O−S bondis 1.113. The polar character of the S−O bond is reflected inthe atomic contribution to the V(S,O) basin (0.26 e from S and0.79 e from O).

Figure 6b indicates that upon the covalent O−S bondformation we observe a systematic increase of the DI(S1,O)delocalization index as well as the electron density calculated atthe (3,−1) critical point on the interatomic line linking the S1and O atoms. Due to the essential difference in theelectronegativity values of the S and O atoms, we observe anincreasing charge difference between the S1 and O topologicalatoms when the structure transforms from M1 to M2 (Figure6c).

Mechanism of the Hydrogen Atom TransferCH3SOH···SCH3 → CH3SO···HSCH3. As has been mentionedbefore, the second step of the •OH-induced oxidative cleavageof CH3SSCH3 consists of H atom transfer between O and S2atoms in a hydrogen bonded complex formed by themethanesulfenic acid and •SCH3 radical to form CH3SO···HSCH3. Indeed, the topological analysis of the electron densityreveals the (3,−1) critical points, which proves the presence ofOH···S bond in the minimum M2 as well as O···HS bond in theminimum M3 (Figure S1, Supporting Information). Theevolution of the electron density and ELF in the course of Hatom transfer has been discussed in detail in the SupportingInformation.The importance of the sulfur atom for the stability of sulfenic

radicals has been recently discussed in the context of the peroxyradical trapping activity of garlic.17 The S atom stabilizessulfenic radicals by delocalizing the unpaired electron ontoitself.17 Interestingly, we observe a similar effect in structuresM1 and M2 (Table S4, Supporting Information). In the firstcase (structure M1, where •OH is loosely bound to DMDS)the unpaired spin distribution is 0.64:0.34 between the O atom(•OH radical) and the S1 atom (DMDS). In structure M2,where CH3SOH forms a hydrogen bonded complex with•SCH3, the unpaired spin distribution is 0.70:0.25 between S2(in methylthiyl radical) and S1 (in methanesulfenic acid) atoms.In the complex CH3SO···HSCH3 (structure M3), which is thefinal product of the HO•-induced oxidative cleavage ofCH3SSCH3, the unpaired spin distribution is 0.55:0.41 betweenthe S and O atoms in the CH3SO

• radical.The mechanism of the oxidative cleavage of DMDS revealed

in this paper is a process much more complicated than the oneproposed by Wang et al.16 The authors recognized correctly theimportance of the H atom transfer. They suggest, however, thatthe reaction between DMDS and •OH is triggered by thebarrierless formation of the structure marked by us as M2,where the O−S covalent bond has been formed and the S−Scovalent bond is broken (Figures 1 and 2). In this way theymissed structures M1 and TS12 on the reaction path, which arerelated to a formation of an adduct by DMDS and •OH withthe subsequent change of O−S and S−S bond properties.

Identification of CH3SONO. If DMDS reacts with an •OHradical following the mechanism discussed here, CH3SO

• andCH3SH should be the products. The radical might be difficultto detect, in particular, because it is produced in the presence ofCH3SSCH3 and CH3SH. The common method used for theidentification of unstable species, IR matrix isolation spectros-copy, may encounter difficulties due to the fact that the CH3group frequencies of CH3SO

• are expected to have values closeto those corresponding to DMDS ones. We may expect,however, that CH3SO

•, when formed in an argon matrix in thepresence of another radical, such as •NO, will form a stableproduct (CH3SONO) that will provide evidence for CH3SO

•

formation. Following this idea, we performed a low matrix

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experiment, where DMDS reacts with irradiated HONO, whichis a source of •OH and •NO radicals.The spectra of the DMDS + HONO trapped in solid Ar

matrixes are in accord with the spectra reported earlier for thissystem.43 The study indicated that both trans- and cis-HONOisomers form hydrogen bonded complexes with DMDS asevidenced by a strong perturbation of nitrous acid vibrations.The ν(NO) of trans-HONO was found to be ca. 18 cm−1

red-shifted after complex formation; the full analysis of theDMDS/HONO/Ar spectra is reported in ref 43.When the DMDS/HONO/Ar matrixes were exposed to the

λ > 345 nm radiation, all absorptions due to the DMDS···trans-HONO complexes strongly diminished or disappeared (afterprolonged irradiation). The bands due to the trans-HONOmonomer were also decreasing but to a lesser extent than thebands due to the complexes. The intensities of the cis-HONOmonomer and its complexes were only slightly affected byirradiation, which can be explained by the fact thatrecombination of the •OH and •NO radicals in solid argonleads to cis isomer and not to trans isomer.44 Simultaneously, inthe spectra of irradiated matrixes a set of new bands appeared;their wavenumbers are collected in Table S5 in the SupportingInformation. The spectra of the DMDS/HONO/Ar matrixesafter deposition and after irradiation are presented in Figure 7.Figure 7 shows the three most characteristic regions in thespectra of the diluted and concentrated DMDS/HONO/Armatrixes and in the spectrum of the DMDS/HO15NO/Armatrix. For comparison the spectrum of the irradiated CH3SH/HONO/Ar matrix is also presented.25 Figure 7B clearlydemonstrates that the photoproducts are formed at the expenseof DMDS···trans-HONO complex; the band D due to thecomplex disappears (in the spectrum of diluted matrix) orstrongly diminishes (in the spectrum of concentrated matrix),whereas the bands due to photoproducts grow.The identification of most photoproducts is straightforward

on the basis of their occurrence in the irradiated HONO/Armatrixes and literature data. They correspond to nitrogenmonoxide, NO,45−47 nitrogen dioxide, NO2

45,46,48 andasymmetric dinitrogen trioxide, N2O3.

45,46,48−50 At highermatrix concentrations weak bands due to N2O

45,48,51 and(NO)2

45,46,48,52−54 were also observed. All the absorptionsassigned to nitrogen oxides exhibit the appropriate isotopicshifts in DMDS/HO15NO experiments as indicated in Table S5in the Supporting Information. However, the most conspicuousabsorption observed in the spectra of irradiated DMDS/HONO/Ar matrixes and characteristic for this system is a veryintense broad band in the 1740−1690 cm−1 region with threesubpeaks at 1718.5, 1713.0, and 1706.0 cm−1 (the latterobserved as a shoulder). Figure 7B demonstrates that theabsorption appears already in the spectra of diluted DMDS/HONO/Ar matrixes; however, its intensity strongly grows inthe concentrated ones. No corresponding band was identifiedin the spectra of the irradiated CH3SH/HONO/Ar matrixes.The characteristic absorption is accompanied by broad bands atca. 2550 cm−1 and at ca. 1092, 1064 cm−1 (see Figure 7A,C)that occur in the region of the ν(SH) and γ(CH3) vibrations ofCH3SH.25,55 In the spectra of the DMDS/HO15NO/Armatrixes the most characteristic absorption is shifted to the1710−1660 cm−1 region and the three subpeaks appear at1689.0, 1683.5, and 1676.0 cm−1, respectively. The 2550, 1092,and 1064 cm−1 bands are not sensitive to isotopic substitution.The appearance of the absorptions in the CH3SH regions andthe sensitivity of the 1750−1690 cm−1 band on the 15N

substitution led us to the conclusion that the characteristic bandmight be due to methylsulfenyl nitrite, CH3SONO, the productof the CH3SO

• + •NO reaction. The calculations performed forCH3SONO confirmed this hypothesis. It has to be rememberedthat ammonia is also present in the matrixes as contamination;however, being trapped in a different matrix cage from theDMDS···HONO complex, it does not participate in the matrixphotochemistry as demonstrated in our earlier studies on CO/HONO/Ar,24 CH2NOH/HONO/Ar,

56 and CH3SH/HONO/Ar systems.25

The DFT/B3LYP/aug-cc-pVTZ calculations revealed thatthe CH3SONO molecule can adopt two stable structures,namely cis and trans isomers (Figure S8, SupportingInformation). Geometric parameters of both isomers are

Figure 7. The (A) 3000−2400 cm−1, (B) 1900−1650 cm−1, and (C)1250−1000 cm−1 regions in the spectra of matrixes: DMDS/HONO/Ar = 1/2/1000 (a, b); DMDS/HONO/Ar = 3/2/600 (c, d); DMDS/HO15NO/Ar = 3/2/600 (e); CH3SH/HONO/Ar = 3/1/600 (f).Spectra a and c were recorded after matrix deposition, and spectra b, d,e, and f were recorded after matrix exposure to λ > 345 nm radiationfor 240 min. ☆, †, ◆, bands assigned to CH3SONO, CH3SH, andCH3SO; △, ○, ◇, bands due to NO, N2O3, and (NO)2. N, D, bandsdue to ν(NO) of HONO and its complex with DMDS, respectively;M, band due to CH3 rocking mode of CH3SH. ∗, bands due toammonia contamination.

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reported in Table S6 (Supporting Information). The relativeenergy of isomers (Etot(trans-CH3SONO) − Etot(cis-CH3SONO)) calculated at 0 K is small and amounts to 0.43kcal mol−1. The zero-point vibrational correction (harmonicapproximation) does not change this value significantly(ΔEZPE‑corrected = 0.45 kcal mol−1). Although this value is infavor of the experimentally observed cis-CH3SONO, it does notexclude the presence of the trans isomer. The anharmoniccorrections marginally influence the zero-point vibrationalenergies of the isomers (cis-CH3SONO: ZPVEharm = 29.89kcal mol−1, ZPVEanh = 29.50 kcal mol−1; trans-CH3SONO:ZPVEharm = 29.91 kcal mol−1, ZPVEanh = 29.50 kcal mol−1).In Table 1 the wavenumbers corresponding to the most

intense vibrations of cis- and trans-CH3SONO are presented,whereas in Tables S7 and S8 in the Supporting Information allcalculated harmonic wavenumbers, their intensities, andanharmonic wavenumbers are collected. The calculationspredict for both cis- and trans-CH3SONO isomers two veryintense bands due to the two NO stretching modes (cis: 1777cm−1, 306 km mol−1; 314.5 cm−1, 146 km mol−1; trans: 1794.1cm−1, 417 km mol−1; 415.8 cm−1, 145 km mol−1; see Table 1)and one band of medium intensity due to the ONO bendingvibration (cis: 797.2 cm−1, 31 km mol−1; trans: 884.0 cm−1, 42km mol−1). In addition, for the trans isomer, one very intenseband due to S−O stretching vibration is predicted to occur at592.3 cm−1 with intensity 246 km mol−1. The intensities of allother vibrations of the two isomers are equal to or less than 12km mol−1. As discussed earlier, in the studied spectral region(4000−550 cm−1) appears one very intense product bandwhose frequency and 15N isotopic shift correspond to cis-CH3SONO (Table 1). No new absorption was identified in the700−550 cm−1 spectral region, which excludes the presence ofthe trans-CH3SONO isomer in the matrix. The attempt toidentify the δ(ONO) vibration of cis-CH3SONO was notsuccessful; however, in the 950−750 cm−1 region, where the

band is expected to occur, there are absorptions due to HONOand its complexes that probably coincide with the δ(ONO)band.43

We cannot provide an ultimate explanation of the fact thatonly the cis isomer is present in the low temperature matrixunder experimental conditions. However, it is worth noting thatrecombination of the •OH and •NO radicals, originating fromHONO dissociation in low temperature matrixes, leads also tothe formation of the cis-HONO isomer and not to the trans-HONO isomer.44 This may be a direct influence of the matrix,most probably, the shape of the matrix cage which triggers theformation of the cis isomer.The broadness of the 1740−1690 cm−1 absorption is

probably due to the fact that CH3SONO is formed in thesame cage as CH3SH and the two molecules form complexes ofdifferent structures. The three subpeaks at 1718.5, 1713.0, and1706 cm−1 exhibiting very similar 15N isotopic shifts (ca. 29.5cm−1, see Table 1) are most probably due to the complexes ofdifferent structures. However, the experiments do not providesufficient data to study and conclude on the structures of thecomplexes.The results of ab initio calculations reported in theSupporting Information (Table S9, Figures S9−S11) indicatethat cis-CH3SONO and CH3SH can indeed form threeintermolecular complexes of similar stability.According to calculations published by Wang et.al,16 the

alternative reaction channels leading to products different fromCH3SO

• + CH3SH are unfavorable due to thermodynamic orkinetic factors. We calculated the IR spectra of the possibleproducts of reactions of the radicals originating from thealternative channels of CH3SSCH3 + •OH with the •NOradical. We did calculations also for CH3SSCH2NO

•, a possibleproduct of •NO recombination with CH3SSCH2

•. The latterradical is a hypothetical product of subtraction of H atom by•OH from DMDS, the alternative process not considered in ref16. The results of the calculations reported in Table S10

Table 1. Calculated and Observed Wavenumbers (cm−1) and Calculated Intensities (km mol−1) of the Most Intense Vibrationsof the cis- and trans-CH3SONO Moleculesa

cis trans14N 15N 14N 15N

ν I ν I ν I ν I assignmt

calcd h 1816.1 306 1784.0 293 1818.8 417 1786.3 401 ν (NO)calcd a 1777.0 1750.2 1794.1 1761.6expt 1718.5 1689.0expt 1713.0 1683.5expt 1706.0 1676.0calcd h 823.6 31 816.4 29 901.3 42 892.4 35 δ(ONO)calcd a 797.2 790.8 884.0 876.7calcd h 600.6 246 596.0 238 ν(S−O)calcd a 592.3 591.2calcd h 314.5 146 311.3 144 415.8 145 413.3 145 ν(N−O)

ah, harmonic; a, anharmonic.

Table 2. Comparison of Experimental Wavenumbers (cm−1) of the NO Stretching Modes of Peroxynitrites XOONO (X = H,Cl) and Nitrites XONO (X = CH3S, Cl, F, CH2N)

species νexp15N shift lit. species νexp

15N shift lit.

cis-CH3SONO 1718.5 −29.5 this work FONO 1716.4 −28.8 60, 61HOONO 1703.6 −29.8 57 CH2NONO 1697.5 56ClOONO 1717.6 58 CH2NONO···H2O 1704.4 56cis-ClONO 1710.5 59 CH2NONO···H2O 1720.1 56trans-ClONO 1754.9 60

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(Supporting Information) clearly indicate that the position ofthe NO stretching band in the recombination productsoriginating from four of the five alternative channels are entirelydifferent from the experimental NO frequency. This impliesthat CH3SNO, CH3NO, and CH3SSCH2NO are not present inour experiment, which allows us to rule out the reactionchannels leading to CH3S

• + CH3SOH, H2O + CH2S + CH3S•,

•CH3 + CH3SSOH, and CH3SSCH2• + H2O as the products of

the reaction between DMDS and •OH.There is additional evidence that strongly confirms the

formation of CH3SONO in the studied matrixes. First, asshown in Table 2, the identified ν(NO) wavenumber of cis-methylsulfonyl nitrite and its 15N shift match well thecorresponding wavenumbers and 15N shifts of peroxyni-trites57,58 and nitrites.56,59−61 Second, the band identified at1064.0 cm−1 is tentatively assigned to CH3SO

•, which is asubstrate for the CH3SONO product. The SO stretchingmode of CH3SO

• was recently identified at 1071 ± 1 cm−1 inthe gas phase by the time-resolved Fourier transform infraredabsorption technique;62 it was the only mode identified for thisradical. The frequency of the 1064.0 cm−1 band matches theexperimental frequency of the CH3SO

• absorption as well asthe calculated OS stretching frequency (νharm 1049.3 cm−1,intensity 42 km mol−1, νanh 1036.5 cm−1).Although the photoproducts of the DMDS + HONO

reaction, namely the CH3SO• and •NO radicals, are trapped in

the same cage, their recombination does not proceed with100% yield and some amount of CH3SO

• is expected to bepresent in the irradiated matrix.Identification of CH3SONO and CH3SO

• provides strong,independent evidence that the primary products of theCH3SSCH3 + •OH reaction in solid argon are CH3SH andCH3SO

•. Two bands were identified for CH3SH; a very broaddiffuse absorption at ca. 2550 cm−1 is assigned to the SHstretch, and a broad band at ca. 1092 cm−1 is attributed to CH3rocking (see Figure 7A,C). The latter band is ca. 24 cm−1 blue-shifted with respect to CH3 rocking of the CH3SH monomer(1068.3 cm−1). This relatively large shift is probably due to theinteraction of CH3SH with the molecules trapped in the samecage which can be CH3SO

• or CH3SONO. The mode is verysensitive to complexation: in the CH3SH···NH3 complex it isca. 20 cm−1 shifted toward higher frequencies.25 Moreover, oneof the CH3 stretches is observed as a shoulder on the strongabsorption of DMDS. The bands due to the other CH3SHvibrations probably coincide with the CH3SSCH3 absorptionsas the frequencies of the CH3 groups in the two molecules arevery close.

■ CONCLUSIONThe exposure of the CH3SSCH3···HONO complexes trappedin the argon matrixes to λ > 345 nm radiation leads toformation of cis-methylsulfonyl nitrite (cis-CH3SONO) andmethanethiol (CH3SH) as the photoproducts. The CH3SONOmolecule has been identified for the first time on the basis ofthe 15N isotopic substitution and comparison of the infraredspectra with the theoretical ones. The molecule, interestingitself, as a product of the DMDS + •OH reaction in thepresence of •NO provides strong independent evidence for themechanism of this reaction.The photoinduced formation of CH3SONO + CH3SH from

CH3SSCH3 + HONO reactants is a complicated multistepprocess. The crucial stage, studied here using ab initio methods,consists of two-step •OH-induced oxidative cleavage of

CH3SSCH3. In the first step we observe an essential alterationof O−S and S−S bond properties of the CH3S(OH)SCH3adduct followed by the spatial rearrangement of the producedCH3SOH and CH3S

• moieties which form an OH···S hydrogenbond. The O−S bond is formed initially due to the closed shell(electrostatic) interaction, but finally changes its nature andforms the polar covalent bond. The opposite process isobserved for the S−S bond. Covalent at the beginning of thereaction, the bond becomes weaker and finally sulfur atoms arebonded due to closed shell (electrostatic) interactions.Decomposition and formation of ELF V(S,O) and (S,S) basinscoincide with the transition state on the minimum energy path.The next step of the •OH-induced oxidative cleavage of

CH3SSCH3 consists of the H atom transfer in hydrogenbonded CH3SOH···SCH3 to form the CH3SO···HSCH3complex. With the progress of the reaction the covalent O−H bond changes its chemical properties and becomes ahydrogen O···H one. Opposite to that, the weak H···Shydrogen bond becomes stronger and finally forms a covalentH−S bond. Again, the qualitative changes in ELF basin patternscoincide with the transition state.The oxidative character of the CH3SSCH3 cleavage is clear

from the evolution of the S1 atom charge (the one attacked byOH radical in the first step of the reaction). When the reactionproceeds through a sequence of stationary points M1 → TS12→ M2 → TS23 → M3, we observe a sequence of atomiccharges: +0.19 → +0.42 → +0.68 → +0.84 → +1.09.The mechanism proposed here of •OH-induced oxidative

cleavage of CH3SSCH3 may help in understanding theantiradical properties of organic disulfides, the propertieswhich seem important in biological systems. In this process,the reactive and potentially harmful •OH radical is transformedinto the CH3SO

• one, which is supposed to be less reactivethan •OH itself due to an essential delocalization of theunpaired spin density between the S and O atoms. Moreover,the newly formed CH3SO

• is entrapped in a hydrogen bondedcomplex, which is an extra factor limiting its reactivity. All thesefindings prompt a hypothesis that CH3SO

• is more likely toremain “dormant” in a hydrogen bonded complex and torecombine finally with other radicals present in the reactionenvironment rather than attack and destroy other molecules.We suspect that the mechanism of the •OH-induced

oxidative cleavage of CH3SSCH3 remains valid for otherR1SSR2 organic disulfides. This interesting reaction needsfurther theoretical and experimental study. In particular, itshould be answered how R1 and R2 groups influence themechanism of the reaction, the relative enthalpy of the product,and the energy barriers.Further computational study may help in designing

molecules with increased antiradical properties protectingbiological systems against the •OH radical.

■ ASSOCIATED CONTENT*S Supporting InformationMolecular graphs of structures relevant for •OH-inducedcleavage of CH3SSCH3; atomic charges and AIM/ELFproperties for selected bonds in M1, M2, and M3 minimaand transition structures; evolution of the electron density andELF in the course of H atom transfer; molecular structures,calculated IR spectra, potential energy distribution and bandassignments of cis and trans-CH3SONO; wavenumbers of theproduct bands observed in the spectra of the irradiated DMDS/HONO(HO15NO)/Ar matrixes and their assignments; calcu-

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lated NO stretching frequency in complexes formed by cis-CH3SONO and CH3SH; spectroscopic properties of thealternative products of the reaction between CH3SSCH3 andOH in the presence of NO. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: abil@elrond.chem.uni.wroc.pl (A.B.); zofia.mielke@chem.uni.wroc.pl (Z.M.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge the Wrocław Network and Super-computing Center (WCSS) for providing computer time andfacilities.

■ ABBREVIATIONS

DMDS, dimethyl disulfide, CH3SSCH3

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