Article pubs.acs.org/JPCA
The OH-Initiated Oxidation of CS2 in the Presence of NO: FTIR MatrixIsolation and Theoretical Studies A. Bil,* K. Grzechnik, M. Sałdyka, and Z. Mielke* Institute of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland S Supporting Information *
ABSTRACT: We studied the photochemistry of the carbon disulfide−nitrous acid system with the help of Fourier transform infrared (FTIR) matrix isolation spectroscopy and theoretical methods. The irradiation of the CS2···HONO complexes, isolated in solid argon, with the filtered output of the mercury lamp (λ > 345 nm) was found to produce OCS, SO2, and HNCS; HSCN was also tentatively identified. The 13C, 15N, and 2H isotopic shifts as well as literature data were used for product identifications. The evolution of the measured FTIR spectra with irradiation time and the changes in the spectra after matrix annealing indicated that the identified molecules are the products of different reaction channels: OCS being a product of another reaction path than SO2 and HNCS or HSCN. The possible reaction channels between SC(OH)S/SCS(OH) radicals and NO were studied using DFT/B3LYP/aug-cc-pVTZ method. The SC(OH)S and/or SCS(OH) intermediates are formed when HONO attached to CS2 photodissociates into OH and NO. The calculations indicated that SC(OH)S radical can form with NO two stable adducts. The more stable SC(OH)S···NO structure is a reactant for a simple one-step process leading to OCS and HONS molecules. An alternative, less-stable complex formed between SC(OH)S and NO leads to formation of OCS and HSNO. The calculations predict only one stable complex between SCS(OH) radical and NO, which can dissociate along two channels leading to HNCS and SO2 or HSCN and SO2 as the end products. The identified photoproducts indicate that both SC(OH)S and SCS(OH) adducts are intermediates in the CS2 + OH + NO reaction leading to different reaction products.
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INTRODUCTION Because of its significance in atmospheric chemistry, the CS2 + OH reaction has received considerable attention. Early kinetics studies by flash photolysis−resonance fluorescence suggested that the reaction proceeds via formation of a CS2OH adduct that may dissociate into substrates or decompose to give OCS and SH radical as the main products.1−3 However, more detailed study of the reaction kinetics indicated that the adduct formed between OH and CS2 predominantly decomposes back to reactants.1,3−7 In 1982 the effect of molecular oxygen on the CS2 + OH reaction was demonstrated.8−10 Since then, numerous studies have been performed for the CS2 + OH + O2 reaction,11−22 and the following reaction scheme was employed to explain the experimental results: OH + CS2 + M ↔ CS2 OH + M (1) CS2 OH + O2 → products
predicted to form without activation barrier, while formation of the C-adduct required an activation barrier 6.4 kcal mol−1; the height of the barrier of the SCSOH→SC(OH)S isomerization was calculated to be 14 kcal mol−1. The first direct observation of CS2OH as a gaseous isolated species living at least 0.9 μs at 298 K has been reported. It was generated by electron transfer to the CS2OH+ ion in the source of a multisector mass spectrometer by suitable ion−molecule reaction.25 Reaction 2 is very complex, with more than 25 exothermic pathways.20,24 The carbon- and sulfur-containing products identified for the gaseous CS2/OH/O2 reaction include OCS, SO2, CO, and HO2.15,20 They are formed as the primary products of the CS2OH + 3O2 reaction (OCS, CO), via a rapid reaction of a primary product with O2 (HO2, SO2) or via a prompt SO intermediate (SO2). The mechanism of the CS2 + OH + 3O2 reaction has been studied by B3LYP/6-31+G(d) method.24 The calculations predicted the formation of the SCSOH adduct in the initial step, which readily adds molecular oxygen to form the SC(OO)SOH complex. A key step of the reaction is the oxygen atom transfer to the sulfur carrying the hydroxyl group, which leads directly to OCS and HOSO.
(2)
Intermediacy of CS2OH in CS2 + OH reaction was confirmed by laser-photolysis laser-induced fluorescence techniques (LP-LIF).11−15 Theoretical studies23−25 of the OH radical addition to CS2 demonstrated that OH could attach both to the carbon or sulfur atoms forming stable SC(OH)S or SCSOH adducts. The CCSD(T)/aug-cc-pVTZ calculations25 indicated that SCSOH was bound by 7.6 kcal mol−1, while SC(OH)S was bound by 31.1 kcal mol−1. The S-adduct was © XXXX American Chemical Society
Received: June 24, 2016 Revised: August 4, 2016
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at λ = 345 nm to 0.02 cm2 molecule−1 at λ = 370 nm).35 So, we can sensibly assume that CS2 molecules are not affected by radiation from the range λ > 345 nm. The spectra were registered after 15, 30, 45, and 60 min and then after every 1 h up to 540 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 matrices were recorded. Ammonia was present in the studied matrices as a contamination, but it did not participate in the photochemical processes similarly like in our earlier studied systems.28,36 All spectra (resolution 0.5 cm−1) were registered at 11 K in a reflection mode with Bruker 113v FTIR spectrometer using liquid N2-cooled MCT detector. CS2 and 13CS2 were commercially available (CS2 99.7% Merck; 13CS2 99% Sigma-Aldrich); they were thoroughly degassed prior to use. NH 4 NO 2 was prepared from (NH4)2SO4 and KNO2 according to ref 37. NH415NO2 was prepared in the same way as NH4NO2 but using Na15NO2 instead NaNO2. A source of DONO was ND4NO2, which was synthesized from (ND4)2SO4 and KNO2. (ND4)2SO4 was obtained by multiple dissolution and crystallization of ammonium sulfate from D2O. Computational Details. 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 coupled to aug-cc-pVTZ basis set. The basis set is large enough to keep a basis set superposition error (BSSE) small. This approach was used successfully to study similar systems.36,38 All relative energies of structures reported in this paper were corrected for harmonic zero-point vibrational energy. Reported dissociation energies of weak molecular complexes were corrected for the BSSE using the counterpoise correction scheme proposed by Boys and Bernardi.39 All electronic structure calculations were performed with Gaussian 09 package of programs40 for molecules in their ground electronic states. Anharmonic frequencies were calculated using the generalized secondorder vibrational perturbation theory.41−43
Almost barrierless HOSO + 3O2 reaction can readily generate HO2 + SO2 in the atmosphere.24 The kinetics gas phase studies have been also performed for the CS2OH + O3,17 CS2OH + NO,14,20 and CS2OH + NO214 reactions. They indicated that the reactions of the CS2OH adduct with the three molecules are more rapid than with O2. The rate coefficients for the reactions of CS2OH with O2, NO, and NO2 have been determined to be 3.1 × 10−14, 7.3 × 10−13, and 4.2 × 10−11 cm3 molecule−1 s−1, respectively. However, the mechanisms of the reactions have been studied neither experimentally nor theoretically. More recently the CS2 + OH reaction has been investigated in aqueous solutions.26,27 The OH radicals originated from laser-induced photolysis of H2O2 or HONO dissolved in water. The influence of temperature and pH on the reaction kinetics has been reported. In this paper we report the experimental and mechanistic study of the CS2 + OH + NO reaction. The obtained results demonstrate that the presence of NO in the matrix has a major impact on the formation of OCS from CS2 and OH radical.
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EXPERIMENTAL SECTION Infrared Matrix-Isolation Studies. The CS2/Ar mixtures were prepared by the standard manometric technique; the concentration of the mixtures varied in the range of 1/200−1/ 1500. Crystalline ammonium nitrite was used as a source of gaseous HONO. The HONO/Ar mixtures were prepared in the same way as previously described.28 HONO (and NH3) was evaporated from small glass tube containing NH4NO2 maintained at ca. 15 °C placed in the vacuum vessel of the cryostat ca. 20 cm from the sample holder. CS2/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 CS2/Ar concentration and the flow rate of the gaseous mixture. The resultant concentration of HONO/Ar was estimated to vary in the range of 1/800−1/300. The overall CS2/HONO/Ar concentrations of the studied matrices were estimated to be n/m/1200 (n = 1, 3, 4, 6, 12; m = 1.5, 3, 4). A few experiments were also performed for the following isotopically substituted systems: CS2/DONO, 13CS2/HONO, CS2/HO15NO, 13CS2/ HO15NO, 13CS2/DONO, CS2/DO15NO, and 13CS2/DO15NO. 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 spectrum of the initial deposit was 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 glass longwavenumber filter was applied to cut off the radiation with λ < 345 nm. The electronic spectra of HONO and CS2 have been extensively studied.29−35 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.29−31,35 The first band corresponds to electronic excitation to 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. Thus, the applied radiation generates OH and NO radicals in the matrix cage in their ground electronic states. CS2 has an absorption band in the near UV in the 290−380 nm region with maximum near 315 nm.32−35 In the region λ > 345 nm the absorption cross section is low (it drops from 0.35 × 10−20 cm2 molecule−1
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RESULTS AND DISCUSSION Spectra Analysis. The infrared spectra of the CS2/ HONO/Ar matrices have been reported some time ago.44 They indicated the formation of the CS2···HONO-trans and CS2···HONO-cis complexes characterized by the 3511.0, 3508.0, 3498.0, 1683.0, 806.5 cm−1 and 1629.5, 1627.0, 856.5 cm−1 bands, respectively. The present spectra recorded directly after matrix deposition correspond well to those reported earlier. When the matrix is subjected to λ > 345 nm radiation the bands due to the CS2···HONO complexes gradually diminish, and, in addition, the bands due to HONO also slightly diminish. Simultaneously, the bands characteristic for the primary and secondary HONO photolysis products appear, NO, NO2, N2O3, H2O, that were reported earlier.36,45−48 However, we did not observe the band due to the OH free radical that was observed in our earlier study.45,46,48 This is perhaps because most of the HONO present in the matrix interacts with CS2 and the OH radicals formed during complex photolysis react immediately with CS2. Our earlier studies suggest that OH originating from HONO monomer photolysis, similarly like the OH radicals formed in H2O2 photolysis, can escape the matrix cage.49 However, they will not contribute to the CS2 + OH + NO reaction. B
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(b−e, respectively), and after matrix annealing for 10 min to 30 K and recooling to 11 K (f). In Figure S1, Supporting Information, the same spectra are shown in expanded ordinate scale. In Figure 2 the spectra of irradiated matrices doped with
Additionally, a set of new bands is growing up that is characteristic for the products of the photochemical reaction that undergo the CS2···HONO complexes. The bands corresponding to reaction products can be separated into three groups, I, II, and III, on the basis of their response to matrix irradiation and matrix annealing. The bands attributed to the groups II and III appear with reasonable intensity at early stages of photolysis but are growing slower with extended photolysis time than the bands belonging to group I. Band III is less sensitive to matrix annealing than the band II. Group I involves sharp strong band at 2059.4 and a weaker one at 2049.7 with a shoulder at 2052.3 cm−1. On the basis of literature data50,51 and our earlier studies52 the 2049.7 cm−1 band can be safely assigned to the ν(CO) vibration of OCS. The performed experiments with 13CS2/HONO/Ar matrices confirm this assignment. The sharp band at 2059.4 cm−1 exhibits the same 13C isotopic shift as the 2049.7 cm−1 absorption evidencing that the band belongs to the ν(CO) vibration of the complexed OCS molecule. Figure 1 presents the spectra of the CS2/HONO/Ar matrix after deposition (a), after 40, 120, 230, and 720 min of photolysis at λ > 345 nm
Figure 2. 2300−1925 cm−1 region in the spectra of matrices 12CS2/ HO14NO/Ar, 12CS2/HO15NO/Ar, 13CS2/HONO/Ar, and 12CS2/ DO14NO of approximate concentration 3/2/600 after their exposure to λ > 345 nm radiation for 420 min x-band due to 2ν2 + ν3 mode of 13 CS261.
isotopically substituted molecules are presented. In Table 1 the identified wavenumbers of the photoproducts of the processes occurring during irradiation of the CS2···HONO complexes are collected, and their 13C, 15N, and 2H isotopic shifts are also given. Table 1. Wavenumbers (in cm−1) of the Bands Occurring after Irradiation of the CS2/HONO/Ar Matrices, Their Attribution to the Groups I, II, III, Their 13C, 15N, and 2H Isotopic Shifts (Δν = νisot − ν), and Assignment CS2/ HONO band set
Figure 1. (A) 2580−2250 cm−1, (B) 2080−1950 cm−1, and (C) 1380−1140 cm−1 regions in the spectra of matrices CS2/HONO/Ar ≈ 3/2/600 after deposition (a) and after matrix exposure to λ > 345 nm radiation for 40, 120, 230, 720 min (b−e, respectively). Spectrum (f) was recorded after annealing of matrix (e) to 30 K for 10 min. I, II, and III indicate bands assigned to the groups I, II and III; * indicates band due to the OCS monomer.
13 CS2/ HONO
CS2/ CS2/ HO15NO DONO Δν
13 CS2/ DONO
ν
Δν
Δν
assignment
I
2059.4 sa
−53.1
0
0
Δν
−53.1
I
2052.3 sh
-c
0
0
-
I
2049.7 w
−53.4
0
0
−53.4
II
1995.8 s
−49.7
−18.8
−41.7
−90.6
ν(CO), OCScomp b ν(CO), OCScomp ν(CO), OCS ν(NC), HNCScomp
III III
2286.2 w 2271.7 s
−37.5
−41.8 −38.3
0
−37.5
III
2265.0 w
−37.5
−38.3
0
−37.5
III
2254.8 w
−36.1
−39.3
0
−36.1
III
2539.8 w
0
0
-
-
II, III
1350.3 b
0
0
0
0
II, III
1345.2 s
0
0
0
0
II, III
1151.3 vw
0
0
0
0
II, III
1148.9 vw
0
0
0
0
ν(CN), HSCNcomp ν(CN), HSCNcomp ν(CN), HSCNcomp ν(SH), HSCNcomp νas(SO2), SO2comp νas (SO2), SO2comp νs(SO2), SO2comp νs(SO2), SO2comp
a s−strong, w−weak, vw−very weak, sh−shoulder, b−broad. bComp− OCS, HNCS, HSCN or SO2 molecule interacting with another molecules present in the matrix. cHyphen “-” indicates the corresponding bands were not identified in the matrix.
C
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The Journal of Physical Chemistry A In addition to the ν(CO) bands due to the free and perturbed OCS molecule, in the 2300−1950 cm−1 region other characteristic absorptions attributed to the groups II and III also appear. A sharp band at 1995.8 cm−1 is assigned to the group II, whereas a band at 2271.7 cm−1 and three weaker absorptions at 2286.2, 2265.0, and 2254.8 cm−1, occurring in the vicinity of the 2271.7 cm−1 band, are attributed to the group III. The 2271.7 and 1995.8 cm−1 absorptions appear at early stages of photolysis with reasonable intensity, being more intense than the ν(CO) absorptions; however, they grow more slowly than the 2059.4 cm−1 band I and stop growing after prolonged irradiation. The 2271.7 cm−1 band III is less sensitive to matrix annealing than the 1995.8 cm−1 absorption II; the latter band strongly decreases during annealing. The 2271.8 cm−1 band is accompanied by a weak absorption at ca. 2539.8 cm−1 in the SH stretching region; the two bands show the same response to matrix irradiation and annealing. The absorptions due to the photolysis products of the CS2··· HONO complex occurred also in the lower wavenumbers region. A sharp band occurred at 1345.2 cm−1 on a relatively broad absorption at ca. 1350.3 cm−1, and in addition two very weak bands appeared at 1151.3 and 1148.9 cm−1. The 1345.2 and 1350.3 cm−1 absorptions show the behavior of the bands due to the groups II and III; the weakness of the other two absorptions did not allow us to attribute them to the particular group II or III. The isotopic studies showed that the wavenumbers of the four bands are not sensitive to 13C, 15N, and 2H substitution. This fact and the agreement of the 1345.2/ 1350.3 and 1151.3/1148.9 cm−1 wavenumbers with the reported infrared spectra for SO2 isolated in Ar matrices53,54 prove that the four bands are due to sulfur dioxide. The assignment of the bands II and III that appear in the higher wavenumbers region will be discussed below on the basis of the results of density functional theory (DFT) calculations. The Mechanism of the CS2 + OH + NO Reaction Leading to OCS and Coproducts. The three identified groups of products (characterized by bands I, II, and III) suggest that the photochemical processes occurring in the matrix may follow various reaction channels. We tried to explain observed phenomena exploring the potential energy surface of CS2 + OH + NO system in its ground electronic state. This is justified by the facts that OH and NO radicals are produced in their ground electronic states and that CS2 remains practically unaffected by the applied radiation. The performed calculations demonstrated that CS2 in the presence of OH and NO radicals, which are the products of HONO photolysis, can form three stable structures: M1, M3, and M5. The three structures shown in Figure 3 and Figures 4−7 are relevant for understanding processes observed in the irradiated argon matrix. The most stable structures M1 and M3 are the complexes formed by NO and SC(OH)S radicals, which is consistent with earlier results for the CS2 + OH system, where SC(OH)S adduct turned out to be 23.5 kcal mol−1 more stable than the one with OH attached to any of the sulfur atoms.25 The most stable SC(OH)S···NO structure (M1) is a reactant for a simple one-step process leading to OCS and HONS molecules with the energy barrier of 8.1 kcal mol−1 (Figures 3 and 4), which is considerably less than 29 kcal mol−1 observed in the case of SC(OH)S→OCS + HS reaction.24,25 Instead of the H atom transfer from OH group to S atom and subsequent dissociation of (H)S−C bond,24 the NO group involved in a six-member ring accepts the hydrogen atom and assists the
Figure 3. Relative energies of minima and transition states for the four analyzed channels of the reaction between CS2 + OH adducts and NO. The energies are expressed in relation to the most stable complex formed by SC(OH)S and NO (structure M1) and include zero-point vibrational corrections. Path 1−black solid lines; path 2−red dashed; path 3−blue dotted; path 4−green dashed/dotted lines. All structures are depicted in Figures 4−7
Figure 4. Mechanism of the reaction between SC(OH)S adduct and NOpath 1.
Figure 5. Mechanism of the reaction between SC(OH)S adduct and NOpath 2.
dissociation of S−C bond. In the transition state TS1 the H atom transfer in the O−H−O bridge occurs simultaneously with N−S bond formation and S−C bond dissociation. The reaction products can form a weakly bonded intermolecular complex HONS···OCS (M2) (ΔEBSSE = −1.5 kcal mol−1). It cannot be excluded that proton tunneling may influence the kinetics of the reaction along path 1. An alternative complex, M3, formed by SC(OH)S and NO, with SC(OH)S···NO five-membered ring, can also act as a reactant for OCS formation following the mechanism presented in Figure 5. The first stage, through TS2 transition state, involves a direct hydrogen atom transfer in O−H−S bridge with a barrier of 36.3 kcal mol−1, which is even higher than for D
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(Table S1, Supporting Information), which may explain why the other bands were not identified. The calculated IR spectrum of trans-HSNO-OCS (Table S2, Supporting Information), which is a product of path 2, reveals a presence of two intense bands due to ν(CO) (2068.6 cm−1, Int 929 km mol−1) and ν(NO) (1654 cm−1, Int 603 km mol−1). The experimental spectra of the trans-HSNO molecule are also known.55,56 The bands characteristic for the acid may coincide with the absorptions of the other molecules present in the matrix; in particular, the strong ν(NO) absorption may be covered by the ν(NO) absorption of nitrous acid or δ(OH) band of the water molecule that was also present in the matrix as a contamination. The reported calculations indicate that the presence of NO in the matrix has a major impact on the formation of OCS from CS2 and OH radical. As mentioned in the Introduction, the CS2 + OH adduct tends to decompose into reactants rather than form the OCS + SH products. Calculated energy profile of the SC(OH)S + NO reaction allowed us to relate this phenomenon to the high energy barrier of a direct proton transfer in O−H−S bridge in SC(OH)S adduct. NO radical, which can form a complex with SC(OH)S, acts as a hydrogenatom acceptor and modifies the mechanism of the reaction lowering considerably the energy barrier, which makes the formation of OCS more favorable in terms of kinetics. The Mechanism of the CS2 + OH + NO Reaction Leading to SO2 and Coproducts. As reported in the Spectra Analysis section, in the irradiated argon matrices doped with CS2 and HONO, there are two more independent groups of bands (marked as II and III) that do not match the bands related to the formation of OCS. This is evidence that other channels compete with the main reaction. Apart from the SC(OH)S adduct, CS2 and OH radical can form also a SCSOH one, with OH attached to a sulfur atom, which is 23.5 kcal mol−1 less stable than the former adduct. Not surprisingly, SCSOH radical can form a complex M5 with NO (Figures 3, 6, and 7), which is 21.7 kcal mol−1 less stable than complex M1. Interestingly, our calculations revealed that structure M5 can act as a reactant for two processes, depicted as paths 3 and 4, in Figures 6 and 7. Path 3 ends up in products whose simulated spectra match band II. It is a two-step reaction leading to HNCS and SO2 molecules, which form a stable complex M7. The first step requires crossing the barrier of 39 kcal mol−1 (Figure 3) and involves the formation of a transition structure TS3 with a fourmembered C−N−O−S ring (Figure 6), where C−S and N−O bonds break and a new S−O bond forms. The products of this step, which are SCN radical and OSOH molecule, form a stable complex M6, where H atom is directed toward the nitrogen atom. In the subsequent step the hydrogen atom is transferred along the O−H−N bridge, which requires crossing the barrier of 2.7 kcal mol−1 (a transition structure TS4). The final products are then HNCS and SO2 weakly bonded in the complex M7. The experimental data confirm the formation of these two molecules in the CS2/HONO/Ar matrix subjected to λ > 345 nm radiation. The wavenumbers of the identified bands attributed to the group II (1995.8, 1345.2/1350.3, 1151.3/ 1148.9 cm−1) correspond well to the reported wavenumbers of the HNCS (ν(NC) = 1979 cm−1)57,58 and SO2 (νas = 1355.2 cm−1, νs = 1151.3 cm−1)53,54 molecules isolated in the argon matrices. The 1345.2/1350.3, 1151.3/1148.9 cm−1 bands are insensitive on 2H, 13C, and 15N isotopic substitutions (Table 1) as expected for the SO2 molecule. The simulated IR spectrum of the SO2···HNCS complex is presented in Table S3,
Figure 6. Mechanism of the reaction between SCS−OH adduct and NOpath 3.
Figure 7. Mechanism of the reaction between SCS−OH adduct and NOpath 4.
SC(OH)S→OCS + HS. The process triggers a barrierless dissociation of (H)S−C bond, which is illustrated by means of structures S1 and S2 in Figure 5. Newly formed SH radical recombines with NO to form HSNO (structure S3), and finally HSNO and OCS molecules form a stable intermolecular complex M4. In the light of the energy profiles of paths 1 and 2 plotted in Figure 3 we suggest that the most likely products of a reaction between SC(OH)S adduct and NO are OCS and HONS, as they are formed in the simple process that starts from the most probable SC(OH)S···NO reactant and proceeds with the lowest energy barrier. Even if some amount of OCS···HSNO was formed in a process following path 2 the product can convert to OCS···HONS. According to previous experimental study trans-HSNO rearranges to trans-HONS when subjected to λ = 365 nm radiation. In turn, the trans-HONS molecule converts back into trans-HSNO upon irradiation with λ = 610 nm.55,56 In the performed experiments both processes can occur, as the sample was irradiated with λ > 345 nm. Group I involves the ν(CO) band of the perturbed OCS molecule, which confirms that reaction along path 1 may occur in the matrix. Unfortunately, no other band belonging to the group I was identified. So, it is not possible to resolve ultimately which of HSNO or HOSN isomers is an actual product accompanying the formation of OCS. The simulated IR spectrum of cisHONS-OCS, which is a product on path 1, indicates that ν(CO) is expected to be the most intense band in the spectrum E
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The Journal of Physical Chemistry A
SO2. The 2ν1 overtone and the ν1 + 2ν2 combination modes of SO2 were observed at 2295.8 and 2179.5 cm−1,59,60 respectively. The wavenumbers of the combination mode of SO2 (2179.5 cm−1) and ν(CN) fundamental of HSCN (2182.3 cm−1) are very close, which means that Fermi resonance interaction between the two modes may occur. The resonance interaction will affect both the position of ν(CN) and its 13C and 15N isotopic shifts. In turn, the 2ν1 overtone of SO2 also lies in the vicinity of the ν1 + 2ν2, SO2, and ν(CN), HSCN, modes, and its interaction with these two modes cannot be completely excluded, particularly if ν(CN) of HSCN monomer is shifted toward higher wavenumbers after complexation with SO2. This is an example of intermolecular Fermi resonance, which has been reported for a few systems only. The first observation of the phenomenon of intermolecular Fermi resonance between molecularly distinct species concerns the resonance interaction between the triple combination mode, ν1 + ν2 + ν3, of sulfur dioxide and the fundamental CH stretching, νCH, of chloroform in a weak SO2···CHCl3 complex.62 We considered also a set of alternative products sharing CN structural motif (prominent 13 C and 15 N shifts strongly suggest CN contribution to the discussed mode), but none of them has a C−N stretching band matching both experimental frequency and experimental 13C and 15N isotope shifts (see Table S7, Supporting Information). On the basis of the above arguments we tentatively assign the 2271.7 cm−1 band to HSCN complexed with SO2. The other weak bands (2265.0, 2254.8 cm−1) appearing in the vicinity of the 2271.7 cm−1 absorption show similar 13C and 15N isotopic shifts as the stronger absorption and are probably due to site effects. So, on the basis of the calculated mechanism of the reaction along path 4 as well as on the above arguments we suggest that the band III originates in SO2···HSCN complexes. It may seem surprising that the products of path 3 and possibly also those of path 4 of the SCSOH + NO reaction were detected experimentally. Both paths start with the reactant M5, which is 21.7 kcal mol−1 less stable than the complex M1. Moreover, they require also crossing large energy barriers, which is 39.0 kcal mol−1 for path 3 and 25.1 kcal mol−1 for path 4. However, one shall remember that the OH radical originating from HONO photolysis is born hot with a substantial excess of energy. The matrices were exposed to radiation of λ > 345 nm (E < 84 kcal mol−1), and the HONO dissociation energy is equal to 49.3 kcal mol−1. The SCSOH adduct and its complex with NO are formed barrierlessly, so, the complex M5 can still have some excess of energy. Also, a large thermodynamic stability of the products of the reactions is a factor acting in favor of formation of some amount of SO2··· HNCS and SO2···HSCN. The first complex, which is 62.6 kcal mol−1 more stable than the reactant M5 and 40.9 kcal mol−1 more stable than the reference complex M1, turned out to be the most stable structure of all considered in our calculations. SO2···HSCN is some 10.6 kcal mol−1 higher in the energy scale than SO2−HNCS, but its formation is related to a lower energy barrier. The fact that bands II and III were recorded under experimental conditions together with the results from DFT calculations confirm that SCSOH isomer can play a role in CS2 + OH + NO reaction, as was suspected on the basis of CS2 + OH + O2 reaction. However, the product of the reaction is SO2 (and HSCN/HNCS), contrary to the expected SCO, as the NO radical forms a complex with SCSOH in such a way that the nitrogen atom forms a bond directly with a carbon atom.
Supporting Information; the calculated isotopic shifts of the most important bands are collected in Table S5. Calculated νas(SO2) and νs(SO2) anharmonic wavenumbers (1310.7 and 1135.4 cm−1) match well the experimental values. The most intense band in the calculated spectrum corresponds to ν(CN) of HNCS, and the calculated anharmonic ν(CN) value of 2023.5 cm−1 matches well the experimental value of 1995.8 cm−1. The ν(CN) of HNCS shifts toward higher wavenumbers after complex formation, which is in accord with the experimental data (1995.8 and 1979 cm−1 for the complex and monomer, respectively). Calculated 2H (−42.7 cm−1), 13C (−52.4 cm−1), and 15N (−20.5 cm−1) isotopic shifts of this band are also in accord with our experimental values (−41.7, −49.7, and −18.8 cm−1, respectively). So, the obtained experimental and theoretical data evidence strongly that one of the decomposition channels of the M5 complex between the SCSOH and NO radicals leads to the SO2···HNCS complex. Theoretical data indicate that the second decomposition channel (path 4) of the M5 complex leads to the HSCN and SO2 products and proceeds through two transition structures TS5 and TS6 and one intermediate minimum M8 (Figures 3 and 7). The first stage involves a proton transfer along (S)O− H−S bridge, which is related to a relatively low energy barrier of 8.4 kcal mol−1, and the subsequent formation of a product M8 with a four-membered C−N−O−S ring. The next step consists of a cleavage of N−O and C−S bonds in the ring, which leads directly to HSCN and SO2 molecules. The calculated energy barrier for this step is 17.4 kcal mol−1, and newly formed molecules seem to form a weakly hydrogenbonded complex M9. Unfortunately, the obtained experimental and theoretical data do not allow for definitive assignment of the group III bands to the SO2···HSCN complex. The spectrum of HSCN isolated in the argon matrix has been reported,58 and the ν(SH) and ν(CN) vibrations were observed at 2580.2 and 2182.3 cm−1, respectively. The ν(SH) band identified at 2539.8 cm−1 may be attributed to the ν(SH) vibration of the HSCN··· SO2 complex; the 40.4 cm−1 shift of this band with respect to the corresponding band of the HSCN monomer can be due to complex formation. The ν(SO2) bands at 1350.3/1345.2 belonging to the groups II/III also suggest formation of this complex after matrix irradiation. However, the relatively large difference between 2182.3 cm−1 wavenumber, characteristic for the HSCN monomer, and 2271.7 cm−1 wavenumber, due to the strongest band attributed to group III, brings some doubts on whether the latter band is due to HSCN. The observed 13C, 15 N, and 2H isotopic shifts for the 2271.7 cm−1 band, presented in Table 1, also do not match well the calculated ones for the HSCN molecule. Table S4 in Supporting Information presents the calculated infrared spectrum of the SO2···HSCN complex; calculated isotopic shifts of the most important bands are collected in Table S6. The calculated anharmonic ν(CN) wavenumber (2244.4 cm−1) matches well the group of experimental frequencies forming bands III (2286.2, 2271.7, 2264.9, and 2254.9 cm−1), but, as stated above, it does not match the experimental ν(CN) wavenumber for HSCN (2182.3 cm−1). The calculated 2H (0.9 cm−1), 13C (−52.4 cm−1), and 15N (−30.6 cm−1) isotopic shifts also differ from the experimental values observed for the 2271.7 cm−1 band (0.0, −36.9, and −39.4 cm−1, respectively). It is possible that the above discrepancies, between the observed and calculated wavenumbers and isotopic shifts of the 2271.7 cm−1 band, are due to Fermi resonance interaction between the fundamental CN stretch of HSCN and an overtone or combination mode of F
DOI: 10.1021/acs.jpca.6b06412 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A The calculations presented here were performed under a natural assumption that what was experimentally observed is related to electronic ground-state phenomena. It cannot be excluded, however, that potential energy surface of the system in its excited states may offer an alternative mechanism leading to observed products.
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AUTHOR INFORMATION
Corresponding Authors
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*E-mail: 3757328. *E-mail: 3757475.
CONCLUSIONS The B3LYP/aug-cc-pVTZ study of the oxidation of CS2 in the presence of OH and NO radicals indicated that SC(OH)S radical forms with NO two stable SC(OH)S···NO adducts. The more stable adduct decomposes into OCS and HONS, and the second one decomposes into OCS and HSNO. The formation of OCS is confirmed by the matrix-isolation study. In turn, the SCSOH radical forms one stable SCS(OH)···NO adduct that decomposes along two channels into SO2 and HNCS or SO2 and HSCN. The FTIR matrix-isolation studies provide strong evidence that HNCS and SO2 are the products of photochemical processes occurring in the matrix and do not exclude formation of HSCN. The calculated energy profiles of the two decomposition channels of SC(OH)S···NO suggest that the most stable SC(OH)S···NO adduct, with the weak N···S and O···H bonds formed between NO and SC(OH)S, is responsible for the production of OCS in the matrix. The decomposition reaction of this adduct is a simple one-step process with the energy barrier of 8.1 kcal mol−1, which is considerably less than 31 kcal mol−1 observed in the case of SC(OH)S→OCS + HS reaction. The presence of NO in the matrix has a major impact on the formation of OCS from CS2 and OH radical. NO radical in the most stable complex with SC(OH)S acts as a hydrogen-atom acceptor and modifies the mechanism of the reaction lowering considerably the energy barrier, which makes the formation of OCS more favorable in terms of kinetics. In the SCS(OH)···NO adduct the N atom of NO is attached to the carbon atom of CS2, and a weak C···N bond is formed. The calculated energy profiles of the two decomposition channels of this adduct indicate that both channels require crossing relatively large energy barriers, which are 39 kcal mol−1 for the channel producing SO2 and HNCS and 25 kcal mol−1 for the channel leading to formation of SO2 and HSCN. The matrix-isolation data suggest that SCSOH···NO in the matrix decomposes along the first channel and do not exclude the second channel of decomposition. The formation of SO2 as the product of the photochemical processes in the matrix indicate that SCS(OH) isomer and not only the more stable SC(OH)S one can play a role in CS2 + OH + NO reaction as was suspected for the CS2 + OH + O2 reaction.
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intense bands of the HNCS-OSO and HSCN-OSO complexes. (PDF)
[email protected]. Phone: +48 71 (A.B.) zofi
[email protected]. Phone: +48 71 (Z.M.)
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank to Polish Ministry of Science and Higher Education for financial support. The grant of computer time from the Wrocław Center for Networking and Supercomputing is gratefully acknowledged.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b06412. The spectra of the CS2/HONO/Ar matrices after deposition and after matrix irradiation (λ > 345 nm). The B3LYP/aug-cc-pVTZ calculated harmonic and anharmonic wavenumbers, and the intensities of the strongest bands of the cis-HONS-OCS, trans-HSNOOCS, HNCS-OSO, and HSCN-OSO complexes. The calculated 2H, 13C, and 15N isotopic shifts of the most G
DOI: 10.1021/acs.jpca.6b06412 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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