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A computational investigation of the atmospheric reaction CH3O2 + ClO
Chemical Physics 358 (2009) 230–234
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A computational investigation of the atmospheric reaction CH3O2 + ClO Agnie M. Kosmas *, Evangelos Drougas 1 Division of Physical Chemistry, Department of Chemistry, University of Ioannina, 451 10 Dourati, Greece
a r t i c l e
i n f o
Article history: Received 13 November 2008 Accepted 11 February 2009 Available online 14 February 2009 Keywords: Methylperoxy radicals Chlorine monoxide Chlorine peroxide Methyl hypochlorite Computational investigation
a b s t r a c t The computational characterization of the reaction between methylperoxy radicals and ClO is reinvestigated. The calculations suggest a plausible mechanism for the description of the two important channels observed experimentally, leading to CH3O + ClOO (1a) and CH3OCl + O2 (1b) products. More specifically, the proper reevaluation of the exothermicity of channel (1a) using spin-restricted methods, describes satisfactorily the experimental evidence about the significance of this channel as the most important pathway. The methyl hypochlorite production, channel (1b), is also shown to be significant and thermodynamically possible through either the singlet or the triplet surface. Finally, the channel CH2O + HOOCl (1e) is also investigated and found to be thermodynamically accessible, with HOOCl product readily dissociating into HCl and O2. Ó 2009 Elsevier B.V. All rights reserved.
CH3 O2 þ ClO ! CH3 OCl þ O2
1. Introduction
1
Halogen monoxide radicals, XO (X = Cl, Br), have long been known to react with a variety of atmospheric species like HOx, CH3Ox, NOx (x = 1, 2) regenerating active halogen atoms. Consequently, they have been widely recognized to play a key role in atmospheric chemistry [1], affecting the composition of the marine boundary layer air and the ozone-depleting cycles in the lower stratosphere. The reaction of ClO with the methylperoxy radical in particular, is an interesting such system [2–9] with an important impact on stratospheric ozone chemistry and the CH4 oxidation chain. The earlier experimental investigations had suggested that the main channel is the formation of methoxy radicals and chlorine peroxide, readily decomposing into molecular oxygen and active atomic chlorine [2,3]. 1
CH3 O2 þ ClO ! CH3 O þ ClOODH298 ¼ 0:1 1:6ð1:8Þ kcal mol ! CH3 O þ Cl þ O2
ð1aÞ
Indeed, as only production of active atomic chlorine was detected in the older studies [2,3] and no other species such as CH3Cl, CH3OCl, OClO, and O3 were found among the reaction products at the time, the initial conclusion drawn was that the reaction proceeds by (85 ± 15)% through channel (1a). However, other thermodynamically accessible channels have been also considered and investigated in subsequent studies [4,9] * Corresponding author. E-mail address: [email protected] (A.M. Kosmas). 1 Present address: Institute of Physical and Theoretical Chemistry, University of Essen, 45117 Essen, Germany. 0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2009.02.005
DH298 ¼ 41:3 0:7 ð42:5Þ kcal mol ! CH3 Cl þ O3
1
DH298 ¼ 12:4 0:7 ð3:3Þ kcal mol ! CH3 O þ OClO 1
DH298 ¼ 0:1 2:4 ð1:8Þ kcal mol ! CH2 O þ HOOCl ! CH2 O þ HCl þ O2
DH298 ¼ 75:0 0:7 ð76:4Þ kcal mol
ð1bÞ ð1cÞ ð1dÞ ð1eÞ
1
Reaction enthalpy values, DH298, are taken from Helleis et al. [4] with those in parentheses from Daële et al. [9]. Thus, later experimental measurements [4–9] were able to identify methyl hypochloride, CH3OCl, among the reaction products, indicating that channel (1b) may also be a potentially important pathway with a considerable branching ratio at the low temperatures prevalent in the polar winter and early springtime stratosphere. The large discrepancies however, observed in the measured rate constant values of channels (1a) and (1b) [9], have created severe uncertainties about the real significance of each pathway. Finally, formaldehyde formation through channel (1e) was also discussed as possible although the detection of CH2O in the reaction products has been attributed primarily to the fast secondary reaction [9]
CH3 O þ ClO ! CH2 O þ HOCl
ð2Þ
The only computational investigation of reaction (1) with particular emphasis on the quantum mechanical characterization of (CH3ClO3) isomers, the most important energy minima in the potential energy surface of reaction (1), has been reported from our
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laboratory [10]. In that work several drawbacks have been observed, for example the main channel (1a) was calculated to present a relatively significant critical energy contrary to the experimental evidence [2,4,9]. The secondary channel (1b) leading to methyl hypochlorite CH3OCl formation, was shown to be possible only through the triplet surface. Channels (1c) and (1e) have not been investigated at all. In the current work, a careful and thorough reexamination of the reaction pathways of the system CH3O2 + ClO is carried out. The present analysis has brought about a number of interesting changes and new features that are described in detail in Section 3 along with the current calculations. The new results also allow the suggestion of a plausible new mechanism for the overall description of reaction (1).
chlorine dioxide, OClO, is satisfactorily treated by both spin-unresticted and spin-restricted calculations which yield almost identical optimized parameters and vibrational frequencies in excellent agreement with experiment [17]. All calculations have been performed using the Gaussian 98 series of programs [20].
2. Computational details
Table 1 Total electronic (Hartrees) and relative energies (kcal mol1) for the most important molecular species of the reaction CH3O2 + ClO at the G2MP2 level.
Geometry optimizations have been carried out at the MP2(full)/ 6-31G(d) level of theory and harmonic vibrational frequencies have been obtained at the same level. The transition states were identified by one imaginary frequency as first-order saddle points. The energy refinement has been performed at the G2MP2 level of theory using the MP2(full)/6-31G(d) optimized structural parameters. Two species have attracted particular attention in the present calculations, the proper energy computation of the 1Dg state of the O2 molecule and the theoretical characterization the ClOO radical. As mentioned in several studies of the reactions XO + HO2 (X = halogen atom) [13–15], the correct energy of the 1Dg state of the O2 molecule must be studied using complex orbital techniques. The computation of (1Dg) O2 at the MP2 level of theory in combination with the 6-31G(d) basis set and using complex orbitals produce the correct experimental energy gap of 22.5 kcal mol1 between singlet and triplet molecular oxygen [16]. The second important point is the characterization of the ClOO radical which requires specific care, presenting a high sensitivity to spin-unrestricted and spin-restricted methods and some undesirable basis set effects mentioned several times in the literature [17–19]. Beltrán et al. [17], using B3LYP, found that the calculation of structural parameters and vibrational frequencies of ClOO is sensitive to the open-shell scheme: the ClO distance was calculated larger with the spin-unrestricted approximation (2.257 Å) than with the spin-restricted method (1.929 Å), which is a more realistic value closer to the experimental result. The OO distance was found to behave in the opposite way (1.193 Å and 1.215 Å, respectively). As a consequence, the OO stretching frequency was overestimated by the ROB3LYP scheme and underestimated by UB3LYP while the opposite trends were observed for the ClO stretching and ClOO bending frequencies. Recently, Suma et al. [18] have tested a variety of high computational methods to investigate the anomalously weak Cl–O bond in the chlorine peroxide radical with striking results. They found that single configuration methods such as B3LYP and RCCSD(T) combined with the aug-ccpVTZ and aug-cc-pVQZ basis set were found to give moderate Cl–O bond lengths in the range 1.935–2.025 Å, consistent with the experimental evidence. On the contrary, the multiconfigurational CASSCF method gave a fairly long Cl–O bond, larger than 3 Å and a very poor binding energy [18]. The most reliable results with respect to the experimental evidence (in parenthesis) were given by the MRSDCI+Q calculations in the CBS limit that produced 2.109 Å (2.084 Å) for the Cl–O bond distance and a binding energy of 4.26 kcal mol1 (4.69 ± 0.10 kcal mol1) [18]. In the present work, the ClOO calculation has been carried out at the ROMP2/6311+G(3df) level that gave a Cl–O bond distance of 2.142 Å [19] and an energy difference 0.6 kcal mol1 with respect to reactants. It is interesting to note here that the isomeric C2v structure, i.e.,
3. Results and discussion The energy results at the G2MP2 level are summarized in Table 1 while Fig. 1 displays the structures of the transition states, TS1, TS2 and TS9 at the MP2(full)/6-31G(d) level of theory, that have been determined in the present work, along with their geometrical
Species
Eh
DE
CH3O2 + ClO 1 CH3OOOCl 3 CH3OOOCl CH3OOClO CH3OClO2 TS1„[CH3O2 + ClO ? CH3OClO2] TS2„[CH3OClO2 ? CH3OCl + 1O2] TS3„[1CH3OOOCl ? CH3OCl + 1O2] TS4„[CH3OOClO ? CH3OCl + 1O2] TS5„[1CH3OOOCl ? CH3Cl + O3] TS6„[3CH3OOOCl ? CH3OCl + 3O2] TS7„[1CH3OOOCl ? CH3OOClO] TS8„[CH3OOClO ? CH3OClO2] TS9„[1CH3OOOCl ? CH2O + HOOCl] CH3O + ClOOb CH3O + OClO CH3OCl + 1O2 CH3OCl + 3O2 CH3Cl + O3 CH2O + HOOCl
724.67876 724.71172 724.68831 724.69194 724.72439 724.68242 724.70113 724.62513 724.64243 724.62898 724.68187 724.65123 724.68599 724.67671 724.67780 724.67419 724.71206 724.74725 724.70554 724.77342
0.0 20.7 6.0 8.3 28.6 (16.5)a 2.3 14.0 33.6 22.8 31.2 2.0 17.3 4.5 1.3 0.6 2.9 20.9c 43.0 16.8 59.4
a b c
Result at the QCISD(T)/6-311G(2df, 2p) level [10]. Result at the ROMP2/6-311+G(3df) level. Result at the MP2/6-31G(d) level using complex orbitals.
Fig. 1. Structures of transition states TS1„[CH3O2 + ClO ? CH3OClO2], TS2„[CH3OClO2 ? CH3OCl + 1O2] and TS9„[1CH3OOOCl ? CH2O + HOOCl] optimized at the MP2/6-31G(d) theory level.
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Table 2 TS1, TS2, TS9 harmonic frequencies (cm1) at the MP2/6-31G(d) level of theory. TS1 TS2 TS9
477, 11, 90, 243, 394, 446, 512, 956, 1151, 1224, 1260, 1278, 1551, 1597, 1639, 3136, 3185, 3250 251, 4, 22, 31, 37, 72, 436, 786, 1161, 1267, 1317, 1619, 1636, 1649, 1998, 3233, 3310, 3332 2582, 72, 128, 251, 395, 426, 548, 811, 917, 931, 1093, 1289, 1477, 1546, 1861, 1952, 2171, 3061
parameters. Table 2 lists the vibrational frequencies of the transition states TS1, TS2 and TS9 and the overall reaction energy profile is depicted in Fig. 2. The energetics results for the two nascent association adducts, i.e., the methoxy chloroperoxide, in both the singlet, and the triplet state, 1CH3OOOCl and 3CH3OOOCl, and the methoxy chlorite, CH3OOClO, as well as for the other two isomeric forms, CH3OClO2 and CH3ClO3 are shown to follow interesting trends (Table 1). 1 CH3OOOCl and CH3OClO2 are calculated to be quite stable with respect to reactants and competing in stability. At the G2MP2 level of theory, 1CH3OOOCl is found to lie at 20.7 kcal mol1 below reactants and methyl chlorate, CH3OClO2, at 28.6 kcal mol1 , i.e. more stable than the former by 7.9 kcal mol1. Previous calculations at the QCISD(T)/6-311G(2df, 2p) level [10] have also indicated the high stability of these two isomers but they have predicted that 1 CH3OOOCl is more stable than CH3OClO2 by 4.2 kcal mol1. The second association minimum, CH3OOClO, is found less stable than 1 CH3OOOCl and CH3OClO2 but still lower than the reactant energy level, thus, making another possible reaction intermediate. The final isomer CH3ClO3 is a very unstable structure, quite higher than the other isomers and the reactants CH3O2 + ClO and thus, irrelevant for the reaction mechanism. Isomerization among the various reaction intermediates is also of interest. The transition state TS7 for the isomerization 1CH3OOOCl M CH3OOClO is located quite high at 17.3 kcal mol1 relative to the reactants CH3O2 + ClO. The large energy barrier that separates the two isomers makes the interconversion process highly improbable and thus, each association minimum once formed, follows its own fate upon dissociation. The isomerization CH3OOClO M CH3OClO2 however, is feasible through transition state TS8 located at 4.5 kcal mol1 lower than the reactants and determines the most probable fate of methoxy chlorite. An important new feature in the current work is the finding that methyl chlorate, CH3OClO2, although not a nascent association minimum, may also be formed as a reaction intermediate on the singlet surface directly from the reactants, opposite to what has been previously assumed [10]. Thus, CH3OClO2 may be formed either from CH3O2 + ClO through the newly determined transition state TS1 or through isomerization of the methoxy chlorite isomer,
Fig. 2. Energy profile at the G2MP2 level for the reaction CH3O2 + ClO.
CH3OOClO, via transition state TS8. TS1 displays a loose structure resulting from the considerable elongation of the peroxy bond in the CH3O2 radical to 2.95 Å and the approach of the Cl atom to the terminal O atom (Fig. 1). It possesses an imaginary frequency 477i s1 and closely resembles a weak interaction adduct between the methoxy radical and chlorine dioxide, OClO. It is located at 2.3 kcal mol1 below CH3O2 + ClO, meaning no intervening barrier to methyl chlorate formation. TS8 is also located below reactants as already said, presenting no barrier for the isomerization CH3OOClO M CH3OClO2. Hence, the current calculations suggest that methyl chlorate may be formed as a reaction intermediate through either TS1 or TS8 and may participate in the mechanism of the reaction CH3O2 + ClO, contrary to what has been originally assumed [10]. As seen in Fig. 2, several reaction pathways take place upon dissociation of the intermediates 1CH3OOOCl, CH3OOClO and CH3OClO2 on the singlet PES and through 3CH3OOOCl on the triplet. The most favourable energetically reaction channel involves the decomposition of 1CH3OOOCl into CH3O + ClOO with its exothermicity reevaluated to a more favourable value following the proper characterization of the ClOO species, as described in the Computational section. The process is now found to be about thermoneutral with the dissociation products placed just 0.6 kcal mol1 above reactants, much lower than 5.9 kcal mol1, the value originally suggested [10]. Consequently, channel (1a) is properly described computationally to be the most probable reaction pathway, in agreement with the experimental evidence [2,4,9]. The analogous reaction channel (1d) resulting from the dissociation of the CH3OOClO intermediate and leading to CH3O + OClO products has also been considered in the present work. It is found to be less favourable than (1a), located at 2.9 kcal mol1 above reactants and reflecting the small but existing stability difference between ClOO and OClO of the order of 2 kcal mol1 [17]. The CH3OCl + (1D)O2 products (channel (1a)) may be formed either from 1CH3OOOCl or CH3OOClO through the transition state configurations denoted TS3 and TS4, respectively, [10]. TS3 and TS4 are located considerably high with respect to the reactants and the large energy barriers involved make the corresponding routes entirely improbable to be considered as a plausible mechanism leading to CH3OCl formation on the single potential energy surface. Hence, the computational examination rules out the possibility that the production of CH3OCl observed experimentally, may originate from the decomposition of 1CH3OOOCl and CH3OOClO association complexes via low energy barriers, as suggested by Helleis et al. [4]. The situation is quite analogous with that of the CH3O2 + BrO reaction where the formation of CH3OBr + (1D)O2 products through the CH3OOOBr and CH3OOBrO intermediates has also been found to involve high energy barriers [12]. It is also certain that CH3OCl cannot be formed in a direct fashion i.e., by a simple metathesis mechanism. Consequently, its detection as a reaction product is a clear indication that a complex mechanism must operate for channel (1b). As reported, the examination of the triplet surface has provided a mechanism that could explain the production of CH3OCl observed experimentally [10]. The study of the triplet surface has led to a thermodynamically accessible reaction pathway yielding methyl hypochlorite and triplet molecular oxygen, CH3OCl + (3R)O2 , through the 3CH3OOOCl association intermediate and the transition state TS6 (Fig. 2). However, the
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possibility for CH3OCl formation on the singlet surface has been considered as an interesting issue that needed further examination. In view of these points, an exhaustive search has been carried out in the present work to explore a feasible pathway leading to methyl hypochlorite production through CH3OClO2, also a possible reaction intermediate as emphasized in the previous paragraph. The persistent investigation has yielded good results and the structure TS2 has been determined, formed from the large extension of both Cl–O bonds and the approach of the two O atoms to form the O2 molecule. It is located at 14.0 kcal mol1 below reactants with an imaginary frequency of 251i cm1 and represents the transition state for the dissociation of methyl chlorate to CH3OCl + (1D)O2. In addition to the determination of TS2, the relocation of the CH3OCl + (1D)O2 products at 20.9 kcal mol1 below reactants, i.e., 4.1 kcal mol1 lower than originally reported [10] assists in the improvement of the thermodynamics and places the products clearly below TS2. Consequently, the following scheme
CH3 O2 þ ClO ! TS1 ! CH3 OClO2 ! TS2 ! CH3 OCl þ ð1 DÞO2
CH3 O2 þ ClO ! CH3 OOClO ! TS8 ! CH3 OClO2 ! TS2 ð4Þ
In fact, the energy results show that isomerization of CH3OOClO to CH3OClO2 is the most probable fate for the methoxy chlorite intermediate, since its dissociation to CH3O + OClO (channel 1c) involves a higher critical energy than the isomerization to CH3OClO2 (Table 1). This fact may explain why OClO was never detected among the reaction products. Based on Eqs. (3) and (4) the new mechanism suggested is completed and interprets the methyl hypochlorite production on the singlet surface, channel 1b, as resulting from the decomposition via TS2 of methyl chlorate, CH3OClO2, which may be formed as a reaction intermediate either directly from the reactants through transition state TS1 or from the isomerization of CH3OOClO through TS8. The reinvestigation of the feasible reaction channels is completed with the examination of the following pathway, originating from the dissociation of the methoxy peroxide minimum and presenting the largest exothermicity:
CH3 O2 þ ClO ! 1 CH3 OOOCl ! TS9 ! CH2 O þ HOOCl
CH3O2 + IO reaction [21]. Nevertheless, a recent experimental study of the reaction CH3O2 + BrO [22] has not detected bromine atoms, a finding which has been interpreted as an evidence for a small probability of stabilization of the CH3OOOBr intermediate and the formation of the decomposition products CH2O + HOOBr. Instead, yield of methyl bromide, CH3Br, has been discussed as taking place to a limited degree [22]. The analogous pathway in the CH3O2 + ClO reaction leading to the formation of CH3Cl + O3 species (channel 1c) has been examined [10]. These products may result from the dissociation of 1CH3OOOCl through the transition state configuration TS5 and are thermodynamically more stable than the reactants. However, the large critical energy associated with TS5 makes this channel quite improbable. Thus, the two systems CH3O2 + ClO and CH3O2 + BrO are shown to present interesting similarities and differences and one cannot easily transfer conclusions from one system to the other. One thing is clear: Further experimental studies are necessary to establish the reaction products and the relevant branching ratios for both reactions CH3O2 + XO (X = Cl, Br) in order to effectively assist in the understanding and the clarification of the kinetics of these important atmospheric systems.
ð3Þ
provides a thermodynamically feasible channel that leads to methyl hypochlorite formation on the singlet surface and may be clearly suggested as a plausible mechanism to explain the experimental evidence [4]. The determination of TS2 gives also a new significance to the CH3OOClO M CH3OClO2 unimolecular rearrangement through the transition state TS8. The methoxy chlorite nascent association intermediate, CH3OOClO, provides an alternative route to methyl hypochlorite formation by isomerization via transition state TS8 to CH3OClO2 which can further dissociate into CH3OCl + (1D)O2, as described in reaction (3)
! CH3 OCl þ ð1 DÞO2
233
ð5Þ
HOOCl presumably will produce Cl + HO2 and HCl + O2 [11] which are the most probable species accompanying CH2O formation [4,9]. TS9 displays a rectangular geometry with the participation of a methylic H that approaches the middle O atom in 1CH3OOOCl. It possesses an imaginary frequency of 2582i cm1 and is located at 22.0 kcal mol1 above 1CH3OOOCl but only 1.3 kcal mol1 above the reactants CH3O2 + ClO. So this should be in principle, a feasible pathway analogous to CH2O + HOOBr in the corresponding CH3O2 + BrO reaction [11], which however, requires stabilization of methoxy chloroperoxide. A similar thermodynamically feasible pathway leading to CH2O + HOOOI has been computed for the
4. Summary In the following we summarize the new features and the changes studied in the present work in comparison to the earlier study of the reaction CH3O2 + ClO [10]: (a) The exothemicity of channel (1a) is reevaluated following the improved characterization of ClOO, at just 0.6 kcal mol1 above reactants relative to the 5.9 kcal mol1 originally estimated [10]. This allows the proper identification of this channel as the most important pathway, in agreement with the experimental evidence [2–9]. (b) The formation of methyl chlorate isomer as a reaction intermediate is found to be possible not only from the CH3OOClO isomerization [10] but also directly from the reactants through the newly determined transition state TS1 located at 2.3 kcal mol1. (c) The exothermicity of channel (1b) is reevaluated following energy at the corrected calculation of (1D)O2 20.9 kcal mol1, i.e., lower by 4.1 kcal mol1 than originally reported [10]. (d) Production of CH3OCl + (1D)O2 (channel (1b)) is found to be thermodynamically feasible on the singlet surface resulting from the dissociation of CH3OClO2 through the newly determined transition state TS2 located at 14.0 kcal mol1. (e) Channel (1e) is investigated in the current work and it is found to be thermodynamically accessible resulting from the decomposition of CH3OOOCl intermediate through the newly determined transition state TS9 located at just 1.3 kcal mol1. (f) Channel (1d) is examined in the current work and it is found to be located at 2.9 kcal mol1 above reactants, i.e., less probable than channels (1a), (1b), and (1e). (g) Finally, the decomposition processes CH3OOOCl ? CH3OCl + CH3OOOCl ? CH3Cl + O3 and CH3OOClO ? (1D)O2, CH3OCl + (1D)O2 have been reconfirmed to involve high energy barriers and are considered irrelevant for the reaction mechanism. We conclude that the present calculations describe satisfactorily channel (1a) as the most probable reaction channel. They also suggest a plausible mechanism for the secondary channel (1b) based on points (b), (c) and (d) above, which explains the formation of methyl hypochlorite, CH3OCl, on the singlet surface, in
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addition to that already reported on the triplet surface through the 3 CH3OOOCl isomer [10]. The new mechanism, as expressed by Eqs. (3) and (4), involves the intermediate formation of methyl chlorate as a reaction intermediate either directly from the reactants through the newly determined TS1 or from the isomerization of CH3OOClO via TS8, and the decomposition of CH3OClO2 into CH3OCl + (1D)O2 through the newly determined TS2. We must emphasize the importance of this mechanism, because it provides a thermodynamically feasible pathway leading to methyl hypochlorite production on the singlet surface. Finally, channel (1e) leading to CH2O + HOOCl formation is studied in the present work and it is found to be thermodynamically accessible. References [1] B.J. Finlayson-Pitts, J.N. Pitts Jr., Chemistry of the Upper and Lower Atmosphere, Academic Press, London, 2000. [2] F.G. Simon, J.P. Burrows, W. Schneider, G.K. Moortgat, P.J. Crutzen, J. Phys. Chem. 93 (1989) 7807. [3] W.B. DeMore, J. Geophys. Res. 96 (1991) 4995. [4] F. Helleis, J.N. Crowley, G.K. Moortgat, J. Phys. Chem. 97 (1993) 11464. [5] R.D. Kenner, K.R. Ryan, L.C. Plumb, Geophys. Res. Lett. 20 (1993) 1571. [6] A.S. Kukui, T.P.W. Jungkamp, R.S. Schindler, Ber. Bunsenges. Phys. Chem. 98 (1994) 1298. [7] F. Helleis, J.N. Crowley, G.K. Moortgat, Geophys. Res. Lett. 21 (1994) 1795.
[8] P. Biggs, C.E. Canosa-Mas, J.M. Frachebound, D.E. Shallcross, R.P. Wayne, Geophys. Res. Lett. 22 (1995) 1221. [9] V. Daële, G. Poulet, J. Chim. Phys. 93 (1996) 1081. [10] E. Drougas, A.F. Jalbout, A.M. Kosmas, J. Phys. Chem. A (2003) 11386. [11] R. Sumathi, S.D. Peyerimhoff, Phys. Chem. Chem. Phys. 1 (1999) 5429. [12] S. Guha, J.S. Francisco, J. Chem. Phys. 118 (2003) 1779. [13] D. Buttar, D.M. Hirst, J. Chem. Soc. Faraday Trans. 90 (1994) 1811. [14] N. Kaltsoyannis, D.M. Rowley, Phys. Chem. Chem. Phys. 4 (2002) 419. [15] E. Drougas, A.M. Kosmas, J. Phys. Chem. A 109 (2005) 3887. [16] See for example, D. Kovalev, E. Cross, N. Künzner, F. Koch, V.Y. Timoshenko, M. Fujii, Phys. Rev. Lett. 89 (2002) 137401. [17] A. Beltrán, J. Andrés, S. Noury, B. Silvi, J. Phys. Chem. A 103 (1999) 3078. [18] K. Suma, Y. Sumiyoshi, Y. Endo, J. Chem. Phys. 121 (2004) 8351. [19] D.K. Papayannis, A.M. Kosmas, V.S. Melissas, J. Phys. Chem. A 105 (2001) 2209. [20] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, Jr., R.E.Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, C.M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 98, Revision A.9, Gaussian Inc., Pittsburgh, PA, 1998. [21] E. Drougas, A.M. Kosmas, J. Phys. Chem. A 111 (2007) 3402. [22] S. Enami, T. Yamanaka, T. Nakayama, S. Hashimoto, M. Kawasaki, D.E. Shallcross, Y. Nakano, T. Ishiwata, J. Phys. Chem. A 111 (2007) 3342.
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Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
A computational investigation of the atmospheric reaction CH3O2 + ClO Agnie M. Kosmas *, Evangelos Drougas 1 Division of Physical Chemistry, Department of Chemistry, University of Ioannina, 451 10 Dourati, Greece
a r t i c l e
i n f o
Article history: Received 13 November 2008 Accepted 11 February 2009 Available online 14 February 2009 Keywords: Methylperoxy radicals Chlorine monoxide Chlorine peroxide Methyl hypochlorite Computational investigation
a b s t r a c t The computational characterization of the reaction between methylperoxy radicals and ClO is reinvestigated. The calculations suggest a plausible mechanism for the description of the two important channels observed experimentally, leading to CH3O + ClOO (1a) and CH3OCl + O2 (1b) products. More specifically, the proper reevaluation of the exothermicity of channel (1a) using spin-restricted methods, describes satisfactorily the experimental evidence about the significance of this channel as the most important pathway. The methyl hypochlorite production, channel (1b), is also shown to be significant and thermodynamically possible through either the singlet or the triplet surface. Finally, the channel CH2O + HOOCl (1e) is also investigated and found to be thermodynamically accessible, with HOOCl product readily dissociating into HCl and O2. Ó 2009 Elsevier B.V. All rights reserved.
CH3 O2 þ ClO ! CH3 OCl þ O2
1. Introduction
1
Halogen monoxide radicals, XO (X = Cl, Br), have long been known to react with a variety of atmospheric species like HOx, CH3Ox, NOx (x = 1, 2) regenerating active halogen atoms. Consequently, they have been widely recognized to play a key role in atmospheric chemistry [1], affecting the composition of the marine boundary layer air and the ozone-depleting cycles in the lower stratosphere. The reaction of ClO with the methylperoxy radical in particular, is an interesting such system [2–9] with an important impact on stratospheric ozone chemistry and the CH4 oxidation chain. The earlier experimental investigations had suggested that the main channel is the formation of methoxy radicals and chlorine peroxide, readily decomposing into molecular oxygen and active atomic chlorine [2,3]. 1
CH3 O2 þ ClO ! CH3 O þ ClOODH298 ¼ 0:1 1:6ð1:8Þ kcal mol ! CH3 O þ Cl þ O2
ð1aÞ
Indeed, as only production of active atomic chlorine was detected in the older studies [2,3] and no other species such as CH3Cl, CH3OCl, OClO, and O3 were found among the reaction products at the time, the initial conclusion drawn was that the reaction proceeds by (85 ± 15)% through channel (1a). However, other thermodynamically accessible channels have been also considered and investigated in subsequent studies [4,9] * Corresponding author. E-mail address: [email protected] (A.M. Kosmas). 1 Present address: Institute of Physical and Theoretical Chemistry, University of Essen, 45117 Essen, Germany. 0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2009.02.005
DH298 ¼ 41:3 0:7 ð42:5Þ kcal mol ! CH3 Cl þ O3
1
DH298 ¼ 12:4 0:7 ð3:3Þ kcal mol ! CH3 O þ OClO 1
DH298 ¼ 0:1 2:4 ð1:8Þ kcal mol ! CH2 O þ HOOCl ! CH2 O þ HCl þ O2
DH298 ¼ 75:0 0:7 ð76:4Þ kcal mol
ð1bÞ ð1cÞ ð1dÞ ð1eÞ
1
Reaction enthalpy values, DH298, are taken from Helleis et al. [4] with those in parentheses from Daële et al. [9]. Thus, later experimental measurements [4–9] were able to identify methyl hypochloride, CH3OCl, among the reaction products, indicating that channel (1b) may also be a potentially important pathway with a considerable branching ratio at the low temperatures prevalent in the polar winter and early springtime stratosphere. The large discrepancies however, observed in the measured rate constant values of channels (1a) and (1b) [9], have created severe uncertainties about the real significance of each pathway. Finally, formaldehyde formation through channel (1e) was also discussed as possible although the detection of CH2O in the reaction products has been attributed primarily to the fast secondary reaction [9]
CH3 O þ ClO ! CH2 O þ HOCl
ð2Þ
The only computational investigation of reaction (1) with particular emphasis on the quantum mechanical characterization of (CH3ClO3) isomers, the most important energy minima in the potential energy surface of reaction (1), has been reported from our
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laboratory [10]. In that work several drawbacks have been observed, for example the main channel (1a) was calculated to present a relatively significant critical energy contrary to the experimental evidence [2,4,9]. The secondary channel (1b) leading to methyl hypochlorite CH3OCl formation, was shown to be possible only through the triplet surface. Channels (1c) and (1e) have not been investigated at all. In the current work, a careful and thorough reexamination of the reaction pathways of the system CH3O2 + ClO is carried out. The present analysis has brought about a number of interesting changes and new features that are described in detail in Section 3 along with the current calculations. The new results also allow the suggestion of a plausible new mechanism for the overall description of reaction (1).
chlorine dioxide, OClO, is satisfactorily treated by both spin-unresticted and spin-restricted calculations which yield almost identical optimized parameters and vibrational frequencies in excellent agreement with experiment [17]. All calculations have been performed using the Gaussian 98 series of programs [20].
2. Computational details
Table 1 Total electronic (Hartrees) and relative energies (kcal mol1) for the most important molecular species of the reaction CH3O2 + ClO at the G2MP2 level.
Geometry optimizations have been carried out at the MP2(full)/ 6-31G(d) level of theory and harmonic vibrational frequencies have been obtained at the same level. The transition states were identified by one imaginary frequency as first-order saddle points. The energy refinement has been performed at the G2MP2 level of theory using the MP2(full)/6-31G(d) optimized structural parameters. Two species have attracted particular attention in the present calculations, the proper energy computation of the 1Dg state of the O2 molecule and the theoretical characterization the ClOO radical. As mentioned in several studies of the reactions XO + HO2 (X = halogen atom) [13–15], the correct energy of the 1Dg state of the O2 molecule must be studied using complex orbital techniques. The computation of (1Dg) O2 at the MP2 level of theory in combination with the 6-31G(d) basis set and using complex orbitals produce the correct experimental energy gap of 22.5 kcal mol1 between singlet and triplet molecular oxygen [16]. The second important point is the characterization of the ClOO radical which requires specific care, presenting a high sensitivity to spin-unrestricted and spin-restricted methods and some undesirable basis set effects mentioned several times in the literature [17–19]. Beltrán et al. [17], using B3LYP, found that the calculation of structural parameters and vibrational frequencies of ClOO is sensitive to the open-shell scheme: the ClO distance was calculated larger with the spin-unrestricted approximation (2.257 Å) than with the spin-restricted method (1.929 Å), which is a more realistic value closer to the experimental result. The OO distance was found to behave in the opposite way (1.193 Å and 1.215 Å, respectively). As a consequence, the OO stretching frequency was overestimated by the ROB3LYP scheme and underestimated by UB3LYP while the opposite trends were observed for the ClO stretching and ClOO bending frequencies. Recently, Suma et al. [18] have tested a variety of high computational methods to investigate the anomalously weak Cl–O bond in the chlorine peroxide radical with striking results. They found that single configuration methods such as B3LYP and RCCSD(T) combined with the aug-ccpVTZ and aug-cc-pVQZ basis set were found to give moderate Cl–O bond lengths in the range 1.935–2.025 Å, consistent with the experimental evidence. On the contrary, the multiconfigurational CASSCF method gave a fairly long Cl–O bond, larger than 3 Å and a very poor binding energy [18]. The most reliable results with respect to the experimental evidence (in parenthesis) were given by the MRSDCI+Q calculations in the CBS limit that produced 2.109 Å (2.084 Å) for the Cl–O bond distance and a binding energy of 4.26 kcal mol1 (4.69 ± 0.10 kcal mol1) [18]. In the present work, the ClOO calculation has been carried out at the ROMP2/6311+G(3df) level that gave a Cl–O bond distance of 2.142 Å [19] and an energy difference 0.6 kcal mol1 with respect to reactants. It is interesting to note here that the isomeric C2v structure, i.e.,
3. Results and discussion The energy results at the G2MP2 level are summarized in Table 1 while Fig. 1 displays the structures of the transition states, TS1, TS2 and TS9 at the MP2(full)/6-31G(d) level of theory, that have been determined in the present work, along with their geometrical
Species
Eh
DE
CH3O2 + ClO 1 CH3OOOCl 3 CH3OOOCl CH3OOClO CH3OClO2 TS1„[CH3O2 + ClO ? CH3OClO2] TS2„[CH3OClO2 ? CH3OCl + 1O2] TS3„[1CH3OOOCl ? CH3OCl + 1O2] TS4„[CH3OOClO ? CH3OCl + 1O2] TS5„[1CH3OOOCl ? CH3Cl + O3] TS6„[3CH3OOOCl ? CH3OCl + 3O2] TS7„[1CH3OOOCl ? CH3OOClO] TS8„[CH3OOClO ? CH3OClO2] TS9„[1CH3OOOCl ? CH2O + HOOCl] CH3O + ClOOb CH3O + OClO CH3OCl + 1O2 CH3OCl + 3O2 CH3Cl + O3 CH2O + HOOCl
724.67876 724.71172 724.68831 724.69194 724.72439 724.68242 724.70113 724.62513 724.64243 724.62898 724.68187 724.65123 724.68599 724.67671 724.67780 724.67419 724.71206 724.74725 724.70554 724.77342
0.0 20.7 6.0 8.3 28.6 (16.5)a 2.3 14.0 33.6 22.8 31.2 2.0 17.3 4.5 1.3 0.6 2.9 20.9c 43.0 16.8 59.4
a b c
Result at the QCISD(T)/6-311G(2df, 2p) level [10]. Result at the ROMP2/6-311+G(3df) level. Result at the MP2/6-31G(d) level using complex orbitals.
Fig. 1. Structures of transition states TS1„[CH3O2 + ClO ? CH3OClO2], TS2„[CH3OClO2 ? CH3OCl + 1O2] and TS9„[1CH3OOOCl ? CH2O + HOOCl] optimized at the MP2/6-31G(d) theory level.
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Table 2 TS1, TS2, TS9 harmonic frequencies (cm1) at the MP2/6-31G(d) level of theory. TS1 TS2 TS9
477, 11, 90, 243, 394, 446, 512, 956, 1151, 1224, 1260, 1278, 1551, 1597, 1639, 3136, 3185, 3250 251, 4, 22, 31, 37, 72, 436, 786, 1161, 1267, 1317, 1619, 1636, 1649, 1998, 3233, 3310, 3332 2582, 72, 128, 251, 395, 426, 548, 811, 917, 931, 1093, 1289, 1477, 1546, 1861, 1952, 2171, 3061
parameters. Table 2 lists the vibrational frequencies of the transition states TS1, TS2 and TS9 and the overall reaction energy profile is depicted in Fig. 2. The energetics results for the two nascent association adducts, i.e., the methoxy chloroperoxide, in both the singlet, and the triplet state, 1CH3OOOCl and 3CH3OOOCl, and the methoxy chlorite, CH3OOClO, as well as for the other two isomeric forms, CH3OClO2 and CH3ClO3 are shown to follow interesting trends (Table 1). 1 CH3OOOCl and CH3OClO2 are calculated to be quite stable with respect to reactants and competing in stability. At the G2MP2 level of theory, 1CH3OOOCl is found to lie at 20.7 kcal mol1 below reactants and methyl chlorate, CH3OClO2, at 28.6 kcal mol1 , i.e. more stable than the former by 7.9 kcal mol1. Previous calculations at the QCISD(T)/6-311G(2df, 2p) level [10] have also indicated the high stability of these two isomers but they have predicted that 1 CH3OOOCl is more stable than CH3OClO2 by 4.2 kcal mol1. The second association minimum, CH3OOClO, is found less stable than 1 CH3OOOCl and CH3OClO2 but still lower than the reactant energy level, thus, making another possible reaction intermediate. The final isomer CH3ClO3 is a very unstable structure, quite higher than the other isomers and the reactants CH3O2 + ClO and thus, irrelevant for the reaction mechanism. Isomerization among the various reaction intermediates is also of interest. The transition state TS7 for the isomerization 1CH3OOOCl M CH3OOClO is located quite high at 17.3 kcal mol1 relative to the reactants CH3O2 + ClO. The large energy barrier that separates the two isomers makes the interconversion process highly improbable and thus, each association minimum once formed, follows its own fate upon dissociation. The isomerization CH3OOClO M CH3OClO2 however, is feasible through transition state TS8 located at 4.5 kcal mol1 lower than the reactants and determines the most probable fate of methoxy chlorite. An important new feature in the current work is the finding that methyl chlorate, CH3OClO2, although not a nascent association minimum, may also be formed as a reaction intermediate on the singlet surface directly from the reactants, opposite to what has been previously assumed [10]. Thus, CH3OClO2 may be formed either from CH3O2 + ClO through the newly determined transition state TS1 or through isomerization of the methoxy chlorite isomer,
Fig. 2. Energy profile at the G2MP2 level for the reaction CH3O2 + ClO.
CH3OOClO, via transition state TS8. TS1 displays a loose structure resulting from the considerable elongation of the peroxy bond in the CH3O2 radical to 2.95 Å and the approach of the Cl atom to the terminal O atom (Fig. 1). It possesses an imaginary frequency 477i s1 and closely resembles a weak interaction adduct between the methoxy radical and chlorine dioxide, OClO. It is located at 2.3 kcal mol1 below CH3O2 + ClO, meaning no intervening barrier to methyl chlorate formation. TS8 is also located below reactants as already said, presenting no barrier for the isomerization CH3OOClO M CH3OClO2. Hence, the current calculations suggest that methyl chlorate may be formed as a reaction intermediate through either TS1 or TS8 and may participate in the mechanism of the reaction CH3O2 + ClO, contrary to what has been originally assumed [10]. As seen in Fig. 2, several reaction pathways take place upon dissociation of the intermediates 1CH3OOOCl, CH3OOClO and CH3OClO2 on the singlet PES and through 3CH3OOOCl on the triplet. The most favourable energetically reaction channel involves the decomposition of 1CH3OOOCl into CH3O + ClOO with its exothermicity reevaluated to a more favourable value following the proper characterization of the ClOO species, as described in the Computational section. The process is now found to be about thermoneutral with the dissociation products placed just 0.6 kcal mol1 above reactants, much lower than 5.9 kcal mol1, the value originally suggested [10]. Consequently, channel (1a) is properly described computationally to be the most probable reaction pathway, in agreement with the experimental evidence [2,4,9]. The analogous reaction channel (1d) resulting from the dissociation of the CH3OOClO intermediate and leading to CH3O + OClO products has also been considered in the present work. It is found to be less favourable than (1a), located at 2.9 kcal mol1 above reactants and reflecting the small but existing stability difference between ClOO and OClO of the order of 2 kcal mol1 [17]. The CH3OCl + (1D)O2 products (channel (1a)) may be formed either from 1CH3OOOCl or CH3OOClO through the transition state configurations denoted TS3 and TS4, respectively, [10]. TS3 and TS4 are located considerably high with respect to the reactants and the large energy barriers involved make the corresponding routes entirely improbable to be considered as a plausible mechanism leading to CH3OCl formation on the single potential energy surface. Hence, the computational examination rules out the possibility that the production of CH3OCl observed experimentally, may originate from the decomposition of 1CH3OOOCl and CH3OOClO association complexes via low energy barriers, as suggested by Helleis et al. [4]. The situation is quite analogous with that of the CH3O2 + BrO reaction where the formation of CH3OBr + (1D)O2 products through the CH3OOOBr and CH3OOBrO intermediates has also been found to involve high energy barriers [12]. It is also certain that CH3OCl cannot be formed in a direct fashion i.e., by a simple metathesis mechanism. Consequently, its detection as a reaction product is a clear indication that a complex mechanism must operate for channel (1b). As reported, the examination of the triplet surface has provided a mechanism that could explain the production of CH3OCl observed experimentally [10]. The study of the triplet surface has led to a thermodynamically accessible reaction pathway yielding methyl hypochlorite and triplet molecular oxygen, CH3OCl + (3R)O2 , through the 3CH3OOOCl association intermediate and the transition state TS6 (Fig. 2). However, the
A.M. Kosmas, E. Drougas / Chemical Physics 358 (2009) 230–234
possibility for CH3OCl formation on the singlet surface has been considered as an interesting issue that needed further examination. In view of these points, an exhaustive search has been carried out in the present work to explore a feasible pathway leading to methyl hypochlorite production through CH3OClO2, also a possible reaction intermediate as emphasized in the previous paragraph. The persistent investigation has yielded good results and the structure TS2 has been determined, formed from the large extension of both Cl–O bonds and the approach of the two O atoms to form the O2 molecule. It is located at 14.0 kcal mol1 below reactants with an imaginary frequency of 251i cm1 and represents the transition state for the dissociation of methyl chlorate to CH3OCl + (1D)O2. In addition to the determination of TS2, the relocation of the CH3OCl + (1D)O2 products at 20.9 kcal mol1 below reactants, i.e., 4.1 kcal mol1 lower than originally reported [10] assists in the improvement of the thermodynamics and places the products clearly below TS2. Consequently, the following scheme
CH3 O2 þ ClO ! TS1 ! CH3 OClO2 ! TS2 ! CH3 OCl þ ð1 DÞO2
CH3 O2 þ ClO ! CH3 OOClO ! TS8 ! CH3 OClO2 ! TS2 ð4Þ
In fact, the energy results show that isomerization of CH3OOClO to CH3OClO2 is the most probable fate for the methoxy chlorite intermediate, since its dissociation to CH3O + OClO (channel 1c) involves a higher critical energy than the isomerization to CH3OClO2 (Table 1). This fact may explain why OClO was never detected among the reaction products. Based on Eqs. (3) and (4) the new mechanism suggested is completed and interprets the methyl hypochlorite production on the singlet surface, channel 1b, as resulting from the decomposition via TS2 of methyl chlorate, CH3OClO2, which may be formed as a reaction intermediate either directly from the reactants through transition state TS1 or from the isomerization of CH3OOClO through TS8. The reinvestigation of the feasible reaction channels is completed with the examination of the following pathway, originating from the dissociation of the methoxy peroxide minimum and presenting the largest exothermicity:
CH3 O2 þ ClO ! 1 CH3 OOOCl ! TS9 ! CH2 O þ HOOCl
CH3O2 + IO reaction [21]. Nevertheless, a recent experimental study of the reaction CH3O2 + BrO [22] has not detected bromine atoms, a finding which has been interpreted as an evidence for a small probability of stabilization of the CH3OOOBr intermediate and the formation of the decomposition products CH2O + HOOBr. Instead, yield of methyl bromide, CH3Br, has been discussed as taking place to a limited degree [22]. The analogous pathway in the CH3O2 + ClO reaction leading to the formation of CH3Cl + O3 species (channel 1c) has been examined [10]. These products may result from the dissociation of 1CH3OOOCl through the transition state configuration TS5 and are thermodynamically more stable than the reactants. However, the large critical energy associated with TS5 makes this channel quite improbable. Thus, the two systems CH3O2 + ClO and CH3O2 + BrO are shown to present interesting similarities and differences and one cannot easily transfer conclusions from one system to the other. One thing is clear: Further experimental studies are necessary to establish the reaction products and the relevant branching ratios for both reactions CH3O2 + XO (X = Cl, Br) in order to effectively assist in the understanding and the clarification of the kinetics of these important atmospheric systems.
ð3Þ
provides a thermodynamically feasible channel that leads to methyl hypochlorite formation on the singlet surface and may be clearly suggested as a plausible mechanism to explain the experimental evidence [4]. The determination of TS2 gives also a new significance to the CH3OOClO M CH3OClO2 unimolecular rearrangement through the transition state TS8. The methoxy chlorite nascent association intermediate, CH3OOClO, provides an alternative route to methyl hypochlorite formation by isomerization via transition state TS8 to CH3OClO2 which can further dissociate into CH3OCl + (1D)O2, as described in reaction (3)
! CH3 OCl þ ð1 DÞO2
233
ð5Þ
HOOCl presumably will produce Cl + HO2 and HCl + O2 [11] which are the most probable species accompanying CH2O formation [4,9]. TS9 displays a rectangular geometry with the participation of a methylic H that approaches the middle O atom in 1CH3OOOCl. It possesses an imaginary frequency of 2582i cm1 and is located at 22.0 kcal mol1 above 1CH3OOOCl but only 1.3 kcal mol1 above the reactants CH3O2 + ClO. So this should be in principle, a feasible pathway analogous to CH2O + HOOBr in the corresponding CH3O2 + BrO reaction [11], which however, requires stabilization of methoxy chloroperoxide. A similar thermodynamically feasible pathway leading to CH2O + HOOOI has been computed for the
4. Summary In the following we summarize the new features and the changes studied in the present work in comparison to the earlier study of the reaction CH3O2 + ClO [10]: (a) The exothemicity of channel (1a) is reevaluated following the improved characterization of ClOO, at just 0.6 kcal mol1 above reactants relative to the 5.9 kcal mol1 originally estimated [10]. This allows the proper identification of this channel as the most important pathway, in agreement with the experimental evidence [2–9]. (b) The formation of methyl chlorate isomer as a reaction intermediate is found to be possible not only from the CH3OOClO isomerization [10] but also directly from the reactants through the newly determined transition state TS1 located at 2.3 kcal mol1. (c) The exothermicity of channel (1b) is reevaluated following energy at the corrected calculation of (1D)O2 20.9 kcal mol1, i.e., lower by 4.1 kcal mol1 than originally reported [10]. (d) Production of CH3OCl + (1D)O2 (channel (1b)) is found to be thermodynamically feasible on the singlet surface resulting from the dissociation of CH3OClO2 through the newly determined transition state TS2 located at 14.0 kcal mol1. (e) Channel (1e) is investigated in the current work and it is found to be thermodynamically accessible resulting from the decomposition of CH3OOOCl intermediate through the newly determined transition state TS9 located at just 1.3 kcal mol1. (f) Channel (1d) is examined in the current work and it is found to be located at 2.9 kcal mol1 above reactants, i.e., less probable than channels (1a), (1b), and (1e). (g) Finally, the decomposition processes CH3OOOCl ? CH3OCl + CH3OOOCl ? CH3Cl + O3 and CH3OOClO ? (1D)O2, CH3OCl + (1D)O2 have been reconfirmed to involve high energy barriers and are considered irrelevant for the reaction mechanism. We conclude that the present calculations describe satisfactorily channel (1a) as the most probable reaction channel. They also suggest a plausible mechanism for the secondary channel (1b) based on points (b), (c) and (d) above, which explains the formation of methyl hypochlorite, CH3OCl, on the singlet surface, in
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addition to that already reported on the triplet surface through the 3 CH3OOOCl isomer [10]. The new mechanism, as expressed by Eqs. (3) and (4), involves the intermediate formation of methyl chlorate as a reaction intermediate either directly from the reactants through the newly determined TS1 or from the isomerization of CH3OOClO via TS8, and the decomposition of CH3OClO2 into CH3OCl + (1D)O2 through the newly determined TS2. We must emphasize the importance of this mechanism, because it provides a thermodynamically feasible pathway leading to methyl hypochlorite production on the singlet surface. Finally, channel (1e) leading to CH2O + HOOCl formation is studied in the present work and it is found to be thermodynamically accessible. References [1] B.J. Finlayson-Pitts, J.N. Pitts Jr., Chemistry of the Upper and Lower Atmosphere, Academic Press, London, 2000. [2] F.G. Simon, J.P. Burrows, W. Schneider, G.K. Moortgat, P.J. Crutzen, J. Phys. Chem. 93 (1989) 7807. [3] W.B. DeMore, J. Geophys. Res. 96 (1991) 4995. [4] F. Helleis, J.N. Crowley, G.K. Moortgat, J. Phys. Chem. 97 (1993) 11464. [5] R.D. Kenner, K.R. Ryan, L.C. Plumb, Geophys. Res. Lett. 20 (1993) 1571. [6] A.S. Kukui, T.P.W. Jungkamp, R.S. Schindler, Ber. Bunsenges. Phys. Chem. 98 (1994) 1298. [7] F. Helleis, J.N. Crowley, G.K. Moortgat, Geophys. Res. Lett. 21 (1994) 1795.
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