OH-Induced Oxidative Cleavage of Dimethyl Disulfide in the Presence

Article pubs.acs.org/JPCA

OH-Induced Oxidative Cleavage of Dimethyl Disulfide in the Presence of NO Andrzej 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 reaction in the presence of •NO. Infrared low temperature argon matrix studies combined with ab initio calculations allowed us to identify cis-CH3SONO, which evidences the formation of the CH3SO• and CH3SH molecules in the course of the CH3SSCH3 + •OH reaction. Ab initio/quantum chemical topology calculations revealed details of the oxidative cleavage of dimethyl disulfide, which is a complex multistep process involving an alteration of S−O and S−S covalent bonds as well as a hydrogen atom transfer. The ability of delocalization of the unpaired electron density by sulfur atoms and a formation of a hydrogen bond by CH3SO• and CH3SH are the factors which seem to explain antiradical properties of DMDS.

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products other than CH3S• and CH3SOH are also important. Butkovskaya and Setser studied the DMDS + •OH reaction using the IR chemiluminescence experimental technique.14 The comparison of the IR emission spectra from the DMDS + • OH(•OD) reactions with those obtained from the CH3SH + • OH(•OD) reaction indicated two decomposition channels of the CH3S(OH)SCH3 adduct with a branching ratio of ≥3 in favor of CH3SH + CH3SO• versus CH3S• + CH3SOH products. The authors concluded that the DMDS + •OH reaction mainly proceeds via addition of a hydroxyl radical to one of the sulfur atoms, followed by migration of the H atom to the other sulfur atom, and, then, by the scission of the S−S bond.

INTRODUCTION Oxidation processes of naturally emitted reduced sulfur compounds have received a lot of attention, which has been primarily due to their importance in the chemistry of the Earth’s atmosphere. 1−5 Dimethyl disulfide, CH 3 SSCH 3 (DMDS), is the least important of the reduced sulfur compounds in terms of the amount released to the atmosphere; it accounts for not more than a few percent of the global flux of sulfur compounds. However, it is of special interest as it contains two sulfur atoms and a weak S−S bond which can be photolyzed by λ = 248 nm radiation leading to two CH3S• radicals.6−8 A major sink for DMDS in the troposphere is the CH3SSCH3 + •OH reaction, which has been the subject of numerous experimental kinetic gas phase studies.9−14 Earlier studies performed by Wine et al.9 indicated CH3S• and CH3SOH to be the primary products of the reaction. This fact, together with the negative temperature dependence of the reaction rate coefficient and lack of pressure dependence of the rate constant, has led to the suggestion that the reaction proceeds via formation of a DMDS−OH adduct which rapidly decomposes to CH3S• and CH3SOH.9

CH3SSCH3 + •OH → CH3S(OH)SCH3 → CH3SO• + CH3SH

To our knowledge this result has not been confirmed experimentally so far by other techniques. The results from the recent computational study on the mechanism of the gas-phase DMDS + •OH reaction performed by Wang et al.16 are in accord with the experimental results obtained by Butkovskaya and Setser.14 The authors considered five a priori possible reaction channels leading to different products:

CH3SSCH3 + •OH → CH3S(OH)SCH3 → CH3SOH + CH3S•

(I)

The final products of the •OH + DMDS reaction in an air + • NO atmosphere also have been interpreted in terms of reaction I.15 Later, Domine and Ravishankara used laserinduced fluorescence and photoionization mass spectrometry to investigate the reaction.11,12 The studies confirmed that CH3S• and CH3SOH are formed but led to conclusion that © 2013 American Chemical Society

(II)

CH3SH + CH3SO

(1)

CH3S + CH3SOH

(2)

Received: January 9, 2013 Revised: June 28, 2013 Published: August 16, 2013 8263

dx.doi.org/10.1021/jp4047837 | J. Phys. Chem. A 2013, 117, 8263−8273

The Journal of Physical Chemistry A

Article

H 2O + CH 2S + CH3S

(3)

CH3 + CH3SSOH

(4)

CH4 + CH3SSO

(5)

identification of the intermediate species.23 We studied the CH3SSCH3 + HONO system in low temperature argon matrixes; the •OH and •NO radicals were generated from the photolysis of nitrous acid.24,25 The irradiation applied (λ ≥ 345 nm) to the molecules entrapped in the matrix produced •OH and •NO radicals in their ground electronic states. CH3SSCH3 was not affected by the applied radiation. The identification of an elusive cis-methylsulfenyl nitrite molecule, cis-CH3SONO, interesting itself, as a product of the above-mentioned reaction provides strong, independent evidence for the formation of the CH3SO• and CH3SH products of the CH3SSCH3 + •OH reaction. The mechanism of •OH-induced oxidative cleavage of CH3SSCH3 is proposed on the basis of ab initio calculations supported by the topological analysis of electron density and electron localization function of the relevant molecules (a quantum chemical topology approach). It seems that formation of hydrogen bonds during the process and a hydrogen atom transfer are indispensable not only for the reaction of DMDS and •OH radical, but also it may have some relevance to protecting properties in biological systems.

The path leading to products 1 is favored over path 2 with respect to thermodynamics and kinetics. Processes leading to formation of 3 or 5 are thermodynamically permissible but have high energy barriers and are less favorable in terms of kinetics. The formation of products 4 is thermodynamically unfavorable.16 The calculations indicate that in the initially formed CH3S(OH)SCH3 adduct H-shift occurs from the •OH group to the other sulfur atom followed by dissociation into CH3SO• and CH3SH species. Reference 16, however, does not explain properly the mechanism of the reaction leading to products 1, as some important steps have been overlooked. Moreover, the process has been discussed only in terms of the potential energy profile and thermodynamics. The authors have not discussed the evolution of a bonding situation or an electron density 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 reports suggesting that sulfenic acids are among the most potent classes of peroxyl radical trapping agents (as the ultimate antioxidants).17,18 The chemistry of organosulfur compounds occurring in natural products such as garlic and other members of the Allium family (e.g., onion or leek) has recently attracted much attention due to their therapeutic potential attributed to antioxidant activity or reaction with thiol-containing proteins.19 The main sulfur compound occurring in garlic is alliin (Sallylcysteine sulfoxide), which is easily transformed in the presence of alliinase enzyme into allicin (diallylthiosulfinate, CH2CHCH2S(O)SCH2CHCH2).20 Vaidya et al.17 suggested recently that the antioxidant activity ascribed to allicin (and garlic in general) is actually due to 2-propenesulfenic acidthe decomposition product of allicin:

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EXPERIMENTAL AND THEORETICAL METHODS Infrared 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 of gaseous HONO; in some experiments NH415NO2 has been applied to produce HO15NO. The HONO/Ar or HO15NO/Ar mixtures were prepared in the same way as previously described.25 HONO/HO15NO (and NH3) was evaporated from a small glass tube containing NH4NO2 (NH415NO2) maintained at ca. 15 °C placed in the vacuum vessel of the cryostat ca. 20 cm from the sample holder. DMDS/Ar mixtures were deposited directly into the cryostat in such a way that they mixed 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. The resultant concentration of HONO/Ar was estimated to vary in the range 1/800−1/300. The overall DMDS/HONO/Ar concentrations of the studied matrixes were estimated to be n/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 (Air Products, Displex 202). After the infrared spectra of the initial deposit were recorded, the sample was subjected to the radiation of a 200 W medium pressure mercury lamp (Philipps CS200W2). A 5 cm water filter served to reduce the amount of infrared radiation reaching the matrix, and a glass longwavenumber filter was applied to cut off the radiation with λ < 345 nm. The electronic spectra of nitrous acid and DMDS have been extensively studied.8,26−30 The ultraviolet absorption spectrum of HONO consists of a diffuse structured band in the 300−390 nm region and a broad structureless band from 270 nm to below 180 nm. The first band corresponds to electronic excitation to the HONO first excited state (Ã 1A″ ← X1A′). Photodissociation of the HONO Ã 1A″ state produces OH(X2Π) + NO(X2Π) radicals with a nearly unit quantum yield. Therefore, the applied radiation generates •OH and •NO radicals in the matrix cage. The electronic spectrum of DMDS exhibits bands in the range below 300 nm, and DMDS is left intact by the applied radiation. The spectra were registered after 30, 60, 120, 180, and 240 min and, in some cases, also after 300

CH 2CHCH 2SOH + •OOR → CH 2CHCH 2SO• + HOOR

(III)

These results were recently confirmed by Galano et al.18 Vaidya et al. concluded also that the mechanism of reaction III is 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 by the sulfur atom due to a large delocalization of unpaired electron between S and terminal O (50:50 in comparison to peroxyl radicals, 70:30 between terminal and nonterminal oxygen atoms). As we shall show in this paper, there is an interesting similarity between the DMDS + •OH and 2propenesulfenic acid + •OOR reactions. In this paper we present a detailed theoretical study of the CH3SSCH3 + •OH reaction in the ground electronic state supplemented by the experimental study of the CH3SSCH3 + • OH reaction in the presence of •NO. The identification of the CH3SSCH3 + •OH reaction product was confirmed indirectly by its ability to recombine with the •NO radical. The matrix isolation technique has been proved to be extremely useful in 8264



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and 360 min of matrix irradiation. After the irradiation process was completed, the samples were annealed to 30 K for 10 min and then cooled again to 11 K, and the spectra of the annealed matrixes were recorded. All spectra (resolution 0.5 cm−1) were registered at 11 K in a reflection mode with a Bruker 113v FTIR 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 tubes before the DMDS/Ar mixtures were prepared. NH4NO2 was prepared according to ref 31; NH415NO2 was prepared in the same way as NH4NO2 but using Na15NO2 instead NaNO2. Computational Methods. All electronic structure calculations have been performed with the Gaussian 09 package of programs.32 Stationary points on the potential energy surface, harmonic and anharmonic frequencies, and minimum energy paths (intrinsic reaction coordinate approach) were calculated using B3LYP hybrid density functional and the aug-cc-pVTZ basis set. The basis set is large enough to keep a basis set superposition error (BSSE) small, and it was used recently to study similar systems.63 Dissociation energies of weak molecular complexes were corrected for the basis set superposition error (BSSE) using the counterpoise correction scheme proposed by Boys and Bernardi.33 All relative energies of structures considered in this paper were corrected for harmonic zero-point vibrational energy. Anharmonic frequencies have been calculated using the generalized second-order vibrational perturbation theory,64−66 as implemented in the Gaussian package. Calculations have been performed for molecules in their ground electronic states. Information on the nature of interactions has been obtained from topological analysis of the electron density and the electron localization function (ELF). We successfully used the quantum chemical topology approach to analyze the bonding situation in radical systems.34−37 Bader’s theory of “atoms in molecules” (AIM)38 provides a precise definition of a bond. In this theory the classification of the interactions is based on the analysis of the electron density (ρ), Laplacian (Δρ), total energy density (H), and delocalization index (DI) at the (3,−1) bond critical point, which is a saddle point of electron densitya minimum along the interatomic path and a maximum along the perpendicular directions. The AIM allows one to partition a molecular space into atomic basins and provides a route for consistent description of its properties such as atomic populations or spin densities. In order to classify the interactions in studied compounds, the topological analysis of the electron density has been performed at the B3LYP/aug-cc-pVTZ level of theory using the AIMAll package.39 The topological analysis of the Becke and Edgecombe electron localization function40 η(r) has been proposed by Silvi and Savin.41 η(r) can range from 0, in those places where the probability of finding same-spin electrons close together is high, to 1, where electrons are alone or form pairs of antiparallel spins. The topological analysis of η(r) provides the partition of the molecular space into core and valence attractor basins (for which one can calculate, for example, average population or volume). Analysis of η(r) has been performed using TopMod programs.42

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 is calculated versus the total energy of isolated CH3SSCH3 and HO•. For transition structures the imaginary frequency does not contribute to the zero-point vibration correction.

7.7 kcal mol−1). Subsequent decomposition of the S−S bond leads to the minimum M2, where the newly formed CH3SOH molecule and CH3S• radical form a weakly hydrogen bonded complex (Edis = 5.4 kcal mol−1). The energy barrier for this process is 7.4 kcal mol−1. The next step consists of the transfer of the H atom involved in the O−H···S bridge to form the minimum M3, where CH3SH and CH3SO• form another hydrogen bonded complex. The energy barrier for this stage is only 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 calculated also single point energies for all structures in Figure 1 using the frozen-core coupled cluster singles-and-doubles method (CCSD) to compare them with B3LYP results. Data in Table S1 (Supporting Information) indicate that both methods lead to the same topology of the potential energy surface. Potential energy distribution analysis performed for the Hessian matrix calculated for the TS1 structure reveals that the eigenvector of the imaginary eigenvalue (i421.0 cm−1) is quite complicated and involves a few internal coordinates. Dominant contributions are from the S−S distance and the O−S1 distance but HOS1S2 and CSSC torsion angles are also involved. In Figure 3 we present the minimum energy path (intrinsic reaction coordinate calculations) linking minima M1 and M2 along with the evolution of the geometric parameters involved

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RESULTS AND DISCUSSION Mechanism of •OH-Induced Oxidative Cleavage of CH3SSCH3. The two-step process involves three minima linked by two transition states (Figures 1 and 2). In the minimum M1 the •OH radical is loosely bound to the S1 sulfur atom (Edis = 8265



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

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

with the transition normal mode. The reaction is triggered by the shortening of the O−S distance from 2.32 Å in M1 to 1.91 Å in TS12, which leads to the increase of the total energy (Figure 3; Table S2, Supporting Information). Then the S−S bond dissociates, with the change of the interatomic distance from 2.07 Å in M1 to 2.19 Å in TS12 and finally to 2.88 Å in M2. Elongation of the S−S distance is the main factor leading to 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 the process 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 and 87.3°, 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 the formation of the OH···S2 bond (Figure 1, M2). At the final stage of the reaction, when S−S is strongly elongated, we observe the essential change of the CSSC torsion angle, which is accompanied by an almost negligible change of the total energy 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 the previous one, and involves the alteration of bonds in the O− H···S bridge. In Figure 4 we plotted the minimum energy path for the hydrogen atom transfer between a methylsulfenic acid 8266



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V(C,S) basins prove the covalent C−S bonds. In the vicinity of sulfur atoms and the oxygen atom we observe basins representing lone electron pairs, which are V1(S1), V2(S1), V1(S2), V2(S2), V1(O), and V2(O). The ELF value applied for plotting the isosurface (0.84) leads to localization domains containing 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 electron density of M1. All bonded atoms are linked by the bond paths with the (3,−1) bond critical points appearing along the bond paths. The topological analysis of the electron localization function calculated for minimum M1 reveals disynaptic valence basin V(S,S) located between S atoms (Figure 5) and populated with 1.47 e (Table S2, Supporting Information), which is consistent with the presence of a covalent bond. The electron density ρ calculated at the (3,−1) S−S bond critical point is reasonably large (0.147 au) and the Laplacian of the electron density Δρ is negative (−0.124 au), which indicates that the bond is of shared type, as expected for a covalent bond. The delocalization index DI (interpreted in a bonding situation as a bond order) indicates that 1.275 electron pairs are on average shared by two sulfur topological atoms. The electron energy density H is negative, as expected for a covalent bond, but small (−0.08 au). This value, together with a moderate value of |Δρ| and a moderate population of the V(S,S) basin may suggest that, although the bond is dominated by a shared interaction, it may easily 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 equilibrium distance, the V(S,S) basin splits into two monosynaptic basins V3(S1) and V3(S2) (Figure 6a; Figure S3, Supporting Information). The monosynaptic basins at TS12 are populated with 0.89 e and 0.39 e and quickly disappear just at the distance 2.25 Å. It indicates that in the vicinity of the transition state the covalent bond S−S disappears, or at least changes its nature.

molecule and methylthiyl radical together with O−H and S−H distance evolution. The initial energy increase is a consequence of the S2 atom approaching the OH group (the S···H distance decreases from 2.40 Å in M2 to 1.66 Å in TS23; Table S3, Supporting Information). In this step of the H atom transfer the 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 2.38 Å in M3. The solid elongation of the O−H distance accompanied by the formation of the S−H bond (S−H distance is 1.35 Å in M3) leads to an essential drop in the total energy of the system. The hydrogen atom transfer between O and S atoms leads to further elongation of the S−S distance from 2.88 Å in M2 to 4.45 Å in M3. Bond Evolution in •OH-Induced Oxidative Cleavage of CH3SSCH3: Quantum Chemical Topology Analysis. In Figure 5 we present ELF localization domains for the minimum

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

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

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. 8267



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Figure 6b indicates that upon the covalent O−S bond formation we observe a systematic increase of the DI(S1,O) delocalization index as well as the electron density calculated at the (3,−1) critical point on the interatomic line linking the S1 and O atoms. Due to the essential difference in the electronegativity values of the S and O atoms, we observe an increasing charge difference between the S1 and O topological atoms when the structure transforms from M1 to M2 (Figure 6c). Mechanism of the Hydrogen Atom Transfer CH3SOH···SCH3 → CH3SO···HSCH3. As has been mentioned before, the second step of the •OH-induced oxidative cleavage of CH3SSCH3 consists of H atom transfer between O and S2 atoms in a hydrogen bonded complex formed by the methanesulfenic acid and •SCH3 radical to form CH3SO··· HSCH3. Indeed, the topological analysis of the electron density reveals the (3,−1) critical points, which proves the presence of OH···S bond in the minimum M2 as well as O···HS bond in the minimum M3 (Figure S1, Supporting Information). The evolution of the electron density and ELF in the course of H atom transfer has been discussed in detail in the Supporting Information. The importance of the sulfur atom for the stability of sulfenic radicals has been recently discussed in the context of the peroxy radical trapping activity of garlic.17 The S atom stabilizes sulfenic radicals by delocalizing the unpaired electron onto itself.17 Interestingly, we observe a similar effect in structures M1 and M2 (Table S4, Supporting Information). In the first case (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 the final product of the HO•-induced oxidative cleavage of CH3SSCH3, the unpaired spin distribution is 0.55:0.41 between the 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 one proposed by Wang et al.16 The authors recognized correctly the importance of the H atom transfer. They suggest, however, that the reaction between DMDS and •OH is triggered by the barrierless formation of the structure marked by us as M2, where the O−S covalent bond has been formed and the S−S covalent bond is broken (Figures 1 and 2). In this way they missed structures M1 and TS12 on the reaction path, which are related to a formation of an adduct by DMDS and •OH with the subsequent change of O−S and S−S bond properties. Identification of CH3SONO. If DMDS reacts with an •OH radical following the mechanism discussed here, CH3SO• and CH3SH should be the products. The radical might be difficult to detect, in particular, because it is produced in the presence of CH3SSCH3 and CH3SH. The common method used for the identification of unstable species, IR matrix isolation spectroscopy, may encounter difficulties due to the fact that the CH3 group frequencies of CH3SO• are expected to have values close to those corresponding to DMDS ones. We may expect, however, that CH3SO•, when formed in an argon matrix in the presence of another radical, such as •NO, will form a stable product (CH3SONO) that will provide evidence for CH3SO• formation. Following this idea, we performed a low matrix

The fact that the S−S covalent bond breaks at the distance only slightly longer than in equilibrium M1 geometry indicates that in CH3S(OH)SCH3 molecule the bond is weak, which is consistent 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 (Figure S1, Supporting Information); however, the nature of the bond is expected to be entirely different from in the minimum M1. Indeed, the electron density at the bond critical point drops to 0.034 au; the Laplacian of the electron density is small and positive (0.051 au). The bond order (delocalization index), however, still preserves a reasonable value of 0.47. The numbers are consisted with a weak bond dominated by a closed shell interaction, which suggests the ionic character of the bond. Indeed, atomic charges calculated for topological S atoms indicate an essential charge transfer (Q(S1) = +0.68, Q(S2) = −0.18) compared to covalently bonded atoms in the minimum M1 (Q(S1) = +0.19, Q(S2) = +0.03). Topological atomic charges (Table S4, Supporting Information) indicate the oxidative character of the S−S bond cleavage with the electron transfer 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 the value of the delocalization index. Both values decrease monotonically with the increase of the S−S distance. Figure 6c illustrates the above-mentioned charge transfer between S atoms in the course of the S−S bond elongation. We observe also an essential spin transfer: in the minimum M1 the spin density is located mainly (but not entirely) within the O atom (•OH radical) whereas in the minimum M2 the spin density is located mainly (but not entirely) within the S2 atom, which proves 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 the minimum M1 the bond is characterized by parameters typical for closed shell interaction. The electron density at the O−S bond critical point is small (0.056 au), the Δρ is positive (0.128 au), and the energy density H is close to 0. Although the value of the bond order (delocalization index) proves that the atoms exchange on average 0.62 electron pair, the topological analysis of ELF reveals no disynaptic basin representing the S−O bond. A typical picture of the closed shell bond is consistent with its calculated dissociation energy (Edis = 7.7 kcal mol−1) and the charge 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 nature from closed shell ionic in the minimum M1, to shared covalent in the minimum M2. At the transition state the topology of ELF reveals the appearance of the disynaptic V(S,O) basin populated with 0.26 e, which proves the formation of a covalent bond. With the subsequent shortening of the O−S distance the population of the basin quickly increases, reaching the value of 1.06 at the minimum M2 (Figure 6; Table S2, Supporting Information). Although the value is smaller than for typical single covalent bonds, the parameters calculated at the (3,−1) bond critical point confirm its covalent nature. The electron density is reasonably large (0.204 au), Δρ is negative (−0.129 au), and the energy density H is negative (−0.245 au). The bond order (delocalization index) calculated for the O−S bond is 1.113. The polar character of the S−O bond is reflected in the atomic contribution to the V(S,O) basin (0.26 e from S and 0.79 e from O). 8268



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experiment, where DMDS reacts with irradiated HONO, which is 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 this system.43 The study indicated that both trans- and cis-HONO isomers form hydrogen bonded complexes with DMDS as evidenced by a strong perturbation of nitrous acid vibrations. The ν(NO) of trans-HONO was found to be ca. 18 cm−1 red-shifted after complex formation; the full analysis of the DMDS/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···transHONO complexes strongly diminished or disappeared (after prolonged irradiation). The bands due to the trans-HONO monomer were also decreasing but to a lesser extent than the bands due to the complexes. The intensities of the cis-HONO monomer and its complexes were only slightly affected by irradiation, which can be explained by the fact that recombination of the •OH and •NO radicals in solid argon leads to cis isomer and not to trans isomer.44 Simultaneously, in the spectra of irradiated matrixes a set of new bands appeared; their wavenumbers are collected in Table S5 in the Supporting Information. The spectra of the DMDS/HONO/Ar matrixes after deposition and after irradiation are presented in Figure 7. Figure 7 shows the three most characteristic regions in the spectra of the diluted and concentrated DMDS/HONO/Ar matrixes and in the spectrum of the DMDS/HO15NO/Ar matrix. For comparison the spectrum of the irradiated CH3SH/ HONO/Ar matrix is also presented.25 Figure 7B clearly demonstrates that the photoproducts are formed at the expense of DMDS···trans-HONO complex; the band D due to the complex disappears (in the spectrum of diluted matrix) or strongly 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/Ar matrixes and literature data. They correspond to nitrogen monoxide, NO, 45−47 nitrogen dioxide, NO245,46,48 and asymmetric dinitrogen trioxide, N2O3.45,46,48−50 At higher matrix concentrations weak bands due to N2O45,48,51 and (NO)245,46,48,52−54 were also observed. All the absorptions assigned to nitrogen oxides exhibit the appropriate isotopic shifts in DMDS/HO15NO experiments as indicated in Table S5 in the Supporting Information. However, the most conspicuous absorption observed in the spectra of irradiated DMDS/ HONO/Ar matrixes and characteristic for this system is a very intense broad band in the 1740−1690 cm−1 region with three subpeaks at 1718.5, 1713.0, and 1706.0 cm−1 (the latter observed as a shoulder). Figure 7B demonstrates that the absorption appears already in the spectra of diluted DMDS/ HONO/Ar matrixes; however, its intensity strongly grows in the concentrated ones. No corresponding band was identified in the spectra of the irradiated CH3SH/HONO/Ar matrixes. The characteristic absorption is accompanied by broad bands at ca. 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 of CH3SH.25,55 In the spectra of the DMDS/HO15NO/Ar matrixes the most characteristic absorption is shifted to the 1710−1660 cm−1 region and the three subpeaks appear at 1689.0, 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 and the sensitivity of the 1750−1690 cm−1 band on the 15N

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 radiation for 240 min. ☆, †, ◆, bands assigned to CH3SONO, CH3SH, and CH3SO; △, ○, ◇, bands due to NO, N2O3, and (NO)2. N, D, bands due to ν(NO) of HONO and its complex with DMDS, respectively; M, band due to CH3 rocking mode of CH3SH. ∗, bands due to ammonia contamination.

substitution led us to the conclusion that the characteristic band might be due to methylsulfenyl nitrite, CH3SONO, the product of the CH3SO• + •NO reaction. The calculations performed for CH3SONO confirmed this hypothesis. It has to be remembered that ammonia is also present in the matrixes as contamination; however, being trapped in a different matrix cage from the DMDS···HONO complex, it does not participate in the matrix photochemistry 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 that the CH3SONO molecule can adopt two stable structures, namely cis and trans isomers (Figure S8, Supporting Information). Geometric parameters of both isomers are 8269



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Table 1. Calculated and Observed Wavenumbers (cm−1) and Calculated Intensities (km mol−1) of the Most Intense Vibrations of the cis- and trans-CH3SONO Moleculesa cis 14

calcd calcd expt expt expt calcd calcd calcd calcd calcd a

h a

h a h a h

trans 15

N

14

N

15

N

N

ν

I

ν

I

ν

I

ν

I

assignmt

1816.1 1777.0 1718.5 1713.0 1706.0 823.6 797.2

306

1784.0 1750.2 1689.0 1683.5 1676.0 816.4 790.8

293

1818.8 1794.1

417

1786.3 1761.6

401

ν (NO)

29

901.3 884.0 600.6 592.3 415.8

42

892.4 876.7 596.0 591.2 413.3

35

δ(ONO)

238

ν(S−O)

145

ν(N−O)

31

314.5

146

311.3

144

246 145

h, harmonic; a, anharmonic.

Table 2. Comparison of Experimental Wavenumbers (cm−1) of the NO Stretching Modes of Peroxynitrites XOONO (X = H, Cl) and Nitrites XONO (X = CH3S, Cl, F, CH2N) species

νexp

cis-CH3SONO HOONO ClOONO cis-ClONO trans-ClONO

1718.5 1703.6 1717.6 1710.5 1754.9

15

N shift

−29.5 −29.8

lit.

species

νexp

this work 57 58 59 60

FONO CH2NONO CH2NONO···H2O CH2NONO···H2O

1716.4 1697.5 1704.4 1720.1

15

N shift

−28.8

lit. 60, 61 56 56 56

band is expected to occur, there are absorptions due to HONO and its complexes that probably coincide with the δ(ONO) band.43 We cannot provide an ultimate explanation of the fact that only the cis isomer is present in the low temperature matrix under experimental conditions. However, it is worth noting that recombination of the •OH and •NO radicals, originating from HONO dissociation in low temperature matrixes, leads also to the formation of the cis-HONO isomer and not to the transHONO isomer.44 This may be a direct influence of the matrix, most probably, the shape of the matrix cage which triggers the formation of the cis isomer. The broadness of the 1740−1690 cm−1 absorption is probably due to the fact that CH3SONO is formed in the same cage as CH3SH and the two molecules form complexes of different structures. The three subpeaks at 1718.5, 1713.0, and 1706 cm−1 exhibiting very similar 15N isotopic shifts (ca. 29.5 cm−1, see Table 1) are most probably due to the complexes of different structures. However, the experiments do not provide sufficient data to study and conclude on the structures of the complexes.The results of ab initio calculations reported in the Supporting Information (Table S9, Figures S9−S11) indicate that cis-CH3SONO and CH3SH can indeed form three intermolecular complexes of similar stability. According to calculations published by Wang et.al,16 the alternative reaction channels leading to products different from CH3SO• + CH3SH are unfavorable due to thermodynamic or kinetic factors. We calculated the IR spectra of the possible products of reactions of the radicals originating from the alternative channels of CH3SSCH3 + •OH with the •NO radical. We did calculations also for CH3SSCH2NO•, a possible product of •NO recombination with CH3SSCH2•. The latter radical is a hypothetical product of subtraction of H atom by • OH from DMDS, the alternative process not considered in ref 16. The results of the calculations reported in Table S10

reported in Table S6 (Supporting Information). The relative energy of isomers (E tot (trans-CH 3 SONO) − E tot (cisCH3SONO)) calculated at 0 K is small and amounts to 0.43 kcal mol−1. The zero-point vibrational correction (harmonic approximation) does not change this value significantly (ΔEZPE‑corrected = 0.45 kcal mol−1). Although this value is in favor of the experimentally observed cis-CH3SONO, it does not exclude the presence of the trans isomer. The anharmonic corrections marginally influence the zero-point vibrational energies of the isomers (cis-CH3SONO: ZPVEharm = 29.89 kcal 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 all calculated harmonic wavenumbers, their intensities, and anharmonic wavenumbers are collected. The calculations predict for both cis- and trans-CH3SONO isomers two very intense bands due to the two NO stretching modes (cis: 1777 cm−1, 306 km mol−1; 314.5 cm−1, 146 km mol−1; trans: 1794.1 cm−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 bending vibration (cis: 797.2 cm−1, 31 km mol−1; trans: 884.0 cm−1, 42 km mol−1). In addition, for the trans isomer, one very intense band due to S−O stretching vibration is predicted to occur at 592.3 cm−1 with intensity 246 km mol−1. The intensities of all other vibrations of the two isomers are equal to or less than 12 km mol−1. As discussed earlier, in the studied spectral region (4000−550 cm−1) appears one very intense product band whose frequency and 15N isotopic shift correspond to cisCH3SONO (Table 1). No new absorption was identified in the 700−550 cm−1 spectral region, which excludes the presence of the trans-CH3SONO isomer in the matrix. The attempt to identify the δ(ONO) vibration of cis-CH3SONO was not successful; however, in the 950−750 cm−1 region, where the 8270



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CH3SSCH3. In the first step we observe an essential alteration of O−S and S−S bond properties of the CH3S(OH)SCH3 adduct followed by the spatial rearrangement of the produced CH3SOH and CH3S• moieties which form an OH···S hydrogen bond. The O−S bond is formed initially due to the closed shell (electrostatic) interaction, but finally changes its nature and forms the polar covalent bond. The opposite process is observed for the S−S bond. Covalent at the beginning of the reaction, the bond becomes weaker and finally sulfur atoms are bonded due to closed shell (electrostatic) interactions. Decomposition and formation of ELF V(S,O) and (S,S) basins coincide 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 hydrogen bonded CH3 SOH···SCH 3 to form the CH 3SO···HSCH3 complex. With the progress of the reaction the covalent O− H bond changes its chemical properties and becomes a hydrogen O···H one. Opposite to that, the weak H···S hydrogen bond becomes stronger and finally forms a covalent H−S bond. Again, the qualitative changes in ELF basin patterns coincide with the transition state. The oxidative character of the CH3SSCH3 cleavage is clear from the evolution of the S1 atom charge (the one attacked by OH radical in the first step of the reaction). When the reaction proceeds through a sequence of stationary points M1 → TS12 → M2 → TS23 → M3, we observe a sequence of atomic charges: +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 the antiradical properties of organic disulfides, the properties which seem important in biological systems. In this process, the reactive and potentially harmful •OH radical is transformed into the CH3SO• one, which is supposed to be less reactive than •OH itself due to an essential delocalization of the unpaired spin density between the S and O atoms. Moreover, the newly formed CH3SO• is entrapped in a hydrogen bonded complex, which is an extra factor limiting its reactivity. All these findings prompt a hypothesis that CH3SO• is more likely to remain “dormant” in a hydrogen bonded complex and to recombine finally with other radicals present in the reaction environment rather than attack and destroy other molecules. We suspect that the mechanism of the •OH-induced oxidative cleavage of CH3SSCH3 remains valid for other R1SSR2 organic disulfides. This interesting reaction needs further theoretical and experimental study. In particular, it should be answered how R1 and R2 groups influence the mechanism 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 protecting biological systems against the •OH radical.

(Supporting Information) clearly indicate that the position of the NO stretching band in the recombination products originating from four of the five alternative channels are entirely different from the experimental NO frequency. This implies that CH3SNO, CH3NO, and CH3SSCH2NO are not present in our experiment, which allows us to rule out the reaction channels 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, as shown in Table 2, the identified ν(NO) wavenumber of cismethylsulfonyl nitrite and its 15N shift match well the corresponding wavenumbers and 15N shifts of peroxynitrites57,58 and nitrites.56,59−61 Second, the band identified at 1064.0 cm−1 is tentatively assigned to CH3SO•, which is a substrate for the CH3SONO product. The SO stretching mode of CH3SO• was recently identified at 1071 ± 1 cm−1 in the gas phase by the time-resolved Fourier transform infrared absorption technique;62 it was the only mode identified for this radical. The frequency of the 1064.0 cm−1 band matches the experimental frequency of the CH3SO• absorption as well as the calculated OS 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 with 100% yield and some amount of CH3SO• is expected to be present in the irradiated matrix. Identification of CH3SONO and CH3SO• provides strong, independent evidence that the primary products of the CH3SSCH3 + •OH reaction in solid argon are CH3SH and CH3SO•. Two bands were identified for CH3SH; a very broad diffuse absorption at ca. 2550 cm−1 is assigned to the SH stretch, and a broad band at ca. 1092 cm−1 is attributed to CH3 rocking (see Figure 7A,C). The latter band is ca. 24 cm−1 blueshifted with respect to CH3 rocking of the CH3SH monomer (1068.3 cm−1). This relatively large shift is probably due to the interaction of CH3SH with the molecules trapped in the same cage which can be CH3SO• or CH3SONO. The mode is very sensitive to complexation: in the CH3SH···NH3 complex it is ca. 20 cm−1 shifted toward higher frequencies.25 Moreover, one of the CH3 stretches is observed as a shoulder on the strong absorption of DMDS. The bands due to the other CH3SH vibrations probably coincide with the CH3SSCH3 absorptions as the frequencies of the CH3 groups in the two molecules are very close.

■

CONCLUSION The exposure of the CH3SSCH3···HONO complexes trapped in the argon matrixes to λ > 345 nm radiation leads to formation of cis-methylsulfonyl nitrite (cis-CH3SONO) and methanethiol (CH3SH) as the photoproducts. The CH3SONO molecule has been identified for the first time on the basis of the 15N isotopic substitution and comparison of the infrared spectra with the theoretical ones. The molecule, interesting itself, as a product of the DMDS + •OH reaction in the presence of •NO provides strong independent evidence for the mechanism of this reaction. The photoinduced formation of CH3SONO + CH3SH from CH3SSCH3 + HONO reactants is a complicated multistep process. The crucial stage, studied here using ab initio methods, consists of two-step •OH-induced oxidative cleavage of

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ASSOCIATED CONTENT

S Supporting Information *

Molecular graphs of structures relevant for •OH-induced cleavage of CH3SSCH3; atomic charges and AIM/ELF properties for selected bonds in M1, M2, and M3 minima and transition structures; evolution of the electron density and ELF in the course of H atom transfer; molecular structures, calculated IR spectra, potential energy distribution and band assignments of cis and trans-CH3SONO; wavenumbers of the product bands observed in the spectra of the irradiated DMDS/ HONO(HO15NO)/Ar matrixes and their assignments; calcu8271



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lated NO stretching frequency in complexes formed by cisCH3SONO and CH3SH; spectroscopic properties of the alternative products of the reaction between CH3SSCH3 and OH in the presence of NO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.B.); zofia.mielke@ chem.uni.wroc.pl (Z.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Wrocław Network and Supercomputing Center (WCSS) for providing computer time and facilities.

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ABBREVIATIONS

DMDS, dimethyl disulfide, CH3SSCH3

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