Computational Chemical Kinetics for the Reaction of Criegee

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Computational Chemical Kinetics for the Reaction of Criegee Intermediate CHOO with HNO and Its Catalytic Conversion to OH and HCO 2

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Putikam Raghunath, Yuan-Pern Lee, and Ming-Chang Lin J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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The Journal of Physical Chemistry

Computational Chemical Kinetics for the Reaction of Criegee Intermediate CH2OO with HNO3 and Its Catalytic Conversion to OH and HCO

†

P. Raghunath, Yuan-Pern Lee,

†

†, ‡, §,

†

* and M. C. Lin ,*

Center for Interdisciplinary Molecular Science, Department of Applied Chemistry,

National Chiao Tung University, Hsinchu 300, Taiwan ‡

Department of Applied Chemistry and Institute of Molecular Science, National Chiao

Tung University, Hsinchu 30010, Taiwan. §

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

(Received xx xxxx 2017; accepted xx xxxx 2017; published online xxxx 2017)

Running title: Computational kinetics of CH2OO + HNO3

a)

Corresponding authors. Yuan-Pern Lee, E-mail: [email protected]. Tel:

+886-3-5131459. Fax: +886-3-5713491. M. C. Lin, E-mail: [email protected]. Tel: +886-3-5131696.

ACS Paragon Plus Environment


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ABSTRACT The kinetics and mechanisms for the reaction of the Criegee intermediate CH2OO with HNO3 and the unimolecular decomposition of its reaction product CH2(O)NO3 are important in atmospheric chemistry. The potential-energy profile of the reactions predicted with the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method shows that the initial association yields a pre-reaction complex that isomerizes by H migration to yield excited intermediate nitrooxymethyl hydroperoxide NO3CH2OOH* with internal energy ~44 kcal mol−1. A fragmentation of this excited intermediate produces CH2(O)NO3 + OH with its transition state located 5.0 kcal mol−1 below that of the reactants. Further decomposition of CH2(O)NO3 produces HCO + HNO3, forming a catalytic cycle for destruction of CH2OO by HNO3. The rate coefficients and product-branching ratios were calculated in temperature range 250−700 K at pressure 20−760 Torr (N2) using the variational-transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate coefficient for reaction CH2OO + HNO3 at 295 K, 5.1×10−10 cm3 molecule−1 s−1, agrees satisfactorily with the experimental value, (5.4 ± 1.0)×10−10 cm3 molecule−1 s−1. The predicted branching ratios at 295 K are 0.21 for the formation of NO3CH2OOH and 0.79 for CH2(O)NO3 + OH at pressure of 40 Torr (N2), and 0.79 for the formation of NO3CH2OOH and 0.21 for CH2(O)NO3 + OH at 760 Torr (N2). This new catalytic conversion of CH2OO to HCO + OH by HNO3 might have significant impact on atmospheric chemistry.

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1.

INTRODUCTION Criegee intermediates, carbonyl oxides proposed to be produced in ozonolysis

reactions, are important in organic chemistry and atmospheric chemistry.1−5 The reactions of ozone with alkenes are important in the atmosphere because they are responsible for the non-photolytic production of OH, formation of H2SO4, organic acids, and aerosols in the atmosphere.6−9 These intermediates were detected directly in the gaseous phase only recently,10−13 when a new reaction scheme using photolysis of CH2I2 in O2 to generate the simplest Criegee intermediate, formaldehyde oxide (CH2OO), was employed. This novel method of production facilitated direct spectral detection; consequently research on Criegee intermediates has become highly active. Our understanding of related important reactions involving Criegee intermediates in the atmosphere is becoming clarified.3−5 The rate coefficient for the reaction CH2OO + H2O was found to be small,10,

14−16

but that for CH2OO + (H2O)2 is much greater and with a negative

temperature dependence;15−18 the reaction of CH2OO with water consequently becomes the most important channel for the loss of CH2OO after considering the concentrations of water dimer and other trace reactants. The reactions of CH2OO with SO2 are reported also to be important; rate coefficients of CH2OO + SO2 reported from various laboratories are in the range (3.4−4.1)×10−11 cm3 molecule−1 s−1.10, 14, 19−21

The reactions of CH2OO with organic acids are also important; rate coefficients

for the reactions of CH2OO with HCOOH and CH3COOH were reported to be (1.1±0.1)×10‒10 and (1.3±0.1)×10−10 cm3 molecule−1 s−1, respectively.22 Foreman et al.23 investigated the kinetics of the reaction of CH2OO with inorganic acids HCl and HNO3 and reported rate coefficients 4.6×10‒11 and 5.4×10‒10 2



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cm3 molecule−1 s−1, respectively. These results seem to indicate that the reactions of Criegee intermediates with HNO3 are likely to be competitive with the reaction with water vapor in polluted urban areas under conditions of modest relative humidity. Even though the reaction of CH2OO + HNO3 might play an important role in atmospheric chemistry, the reaction mechanism has been little investigated with quantum-chemical calculations. Foreman et al.23 employed the CCSD/cc-pVDZ method to optimize geometries of reactants, entry-channel complexes, transition states and products, followed by calculations of energy with the CCSD(T)/aug-cc-pVTZ method to derive a potential-energy scheme. These authors reported that CH2OO reacts with HNO3 to form nitrooxymethyl hydroperoxide (NMHP; NO3CH2OOH) via formation of the C−O bond with the nitro group followed by transfer of hydrogen from HNO3 to the terminal oxygen of CH2OO; the process is barrierless and has exothermicity ~44 kcal mol−1. Because of this large exothermicity, further decomposition of NMHP likely occurs. However, neither detailed mechanism nor prediction of related rate coefficients was reported. The reaction mechanism and the nature of the decomposition products of NMHP are expected to play important roles in determining the impacts, in addition to the destruction of CH2OO, of reaction CH2OO + HNO3 in atmospheric chemistry. Here we report an investigation of possible reaction paths and their corresponding rate coefficients to assess the importance of the title reaction in the atmosphere. We found a new catalytic cycle for decomposition of CH2OO to HCO and OH by HNO3.

2.

COMPUTATIONAL METHODS The geometries of the reactants, intermediates, transition states and products of

the title reaction were optimized with the B3LYP method;24, 3


25

for improved energies,

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single-point calculations at the CCSD(T) level were used.26 The aug-cc-pVTZ basis set27 was used throughout this work. Calculations of vibrational wavenumbers were performed at the B3LYP/aug-cc-pVTZ level based on the optimized geometries; these wavenumbers are unscaled. The intrinsic reaction coordinate (IRC) analyses28 were performed to connect each transition state with designated reactants and products. All calculations were made with program Gaussian 09.29 Calculations of the rate coefficients for the key product channels were made with program Variflex30 based on the microcanonical Rice-Ramsperger-Kassel-Marcus (RRKM) theory and variational transition-state theory VTST.31

-32

The component rates were evaluated at the E/J- resolved level; the

pressure dependence was predicted with one-dimensional master-equation calculations using the Boltzmann probability of the intermediate (NO3CH2OOH*) for the J-distribution. For a barrierless association or decomposition, the variational transition state was approximated with a Morse function, V(R) = De {1 − exp[−β (R−Re)]}2, in conjunction with an anisotropy potential function to represent the minimum potential-energy path (MEP), which was used in calculations of the rate coefficients. In the above equation, R is the reaction coordinate (i.e., the distance between two bonding atoms), De is the bond energy without including the zero-point energy, and Re is the equilibrium value of R. We approximated the potential in this region of the surface with the method described by Miller and Klippenstein.33 The potential for the conserved degrees of freedom corresponds to the normal mode vibrations in the separated fragments and is assumed to be the same as fragments. Also, in an anisotropic potential, it is assumed that the stretching potential is used in conjunction with a potential form of Vanisotropic = V0 [1 − cos2(θ1−θ1e) × cos2(θ2−θ2e)], in which V0 is the stretching potential which is represented by a Morse potential, θ1 4



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(or θ2) is the rotational angle between the fragment 1 (or 2) and the reference axis, and θ1e, (or θ2e) is the equilibrium bond angle of fragment 1 (or 2). The exponent of the angular dependence function is set to 2. For the reaction NO3CH2OOH → CH2(O)NO3 + OH, θ1 and θ2 values are 110.4º and 101.9º, respectively. The variational dissociation curve for NO3CH2OOH → CH2(O)NO3 + OH was calculated at the B3LYP/aug-cc-pVTZ level to cover the separation of two oxygen atoms in the OOH moiety from 1.45 to 6.45 Å at step size 0.1 Å, while other geometric parameters were fully optimized. The computed potential energies were fitted satisfactorily to the Morse potential function with parameter β = 1.56 Å−1. The barrierless association of CH2OO + HNO3 was calculated similarly with β = 1.23 Å−1.

3.

RESULTS AND DISCUSSION

1.

Potential-energy surface and mechanism of reaction Figure 1 presents a scheme of the potential energy of the reaction of CH2OO

with HNO3 according to the energies obtained with the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method. The optimized geometric parameters of the reactants, intermediates, and transition states computed with the B3LYP/aug-cc-pVTZ method and their structures are presented in Figure 2. Cartesian coordinates of main species in the reaction channels of CH2OO + NO3 calculated at the B3LYP/aug-cc-pVTZ level are presented in Table S1, available in the Supporting Information. In Figure 1, the initial association of CH2OO and HNO3 proceeds via a pre-reaction complex, LM1, with binding energy 14.7 kcal mol−1, in which the CH2O−O bond is lengthened to 1.391 Å, as compared with 1.351 Å in CH2OO. LM1 can isomerize readily by transfer of H from the O–H bond in HNO3 and the formation of the C−O bond via TS1 with a negligible barrier (−0.1 kcal mol−1) to form an 5


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intermediate complex NMHP, NO3CH2OOH. The process is exothermic by 44.2 kcal mol−1 at level CCSD(T)//B3LYP/aug-cc-pVTZ, in agreement with the previously reported value predicted with the CCSD(T)/aug-cc-pVTZ//CCSD/cc-pVDZ method.23 We confirmed TS1 by verifying its connectivity with LM1 and NO3CH2OOH with IRC calculations. The IRC analysis was carried out at the B3LYP/aug-cc-pVTZ level, as shown in Figure S1, Supporting Information. We elucidated further a detailed mechanism following the fragmentation of the excited intermediate, NO3CH2OOH. As shown in Figure 1, the NO3CH2OOH intermediate can dissociate through direct cleavage of the O−OH bond without an intrinsic barrier. This channel, producing CH2(O)NO3 + OH, has the least energy that is 5.0 kcal mol−1 below that of the initial reactants CH2OO + HNO3. In the second mechanism, excited intermediate NO3CH2OOH* can undergo migration of one H atom in CH2 to one O atom in the ONO group through TS2 (energy −5.4 kcal mol−1), producing HC(O)OOH and trans-HONO with a barrier of 38.8 kcal mol−1 and exothermicity of 34.3 kcal mol−1. At TS2, the lengths of the breaking C···H and O···N bonds were predicted with the B3LYP/aug-cc-pVTZ method to be 1.274 Å and 2.084 Å respectively, and the forming H···ONO bond to be 1.384 Å (Figure 2). Other reaction channels proceed via transition states with energy greater than that of the reactants; they are expected to be negligible. The lowest-energy reaction channel occurs through a direct H-abstraction by O atoms in the OH group from one H atom in the neighboring CH2 group of NO3CH2OOH via TS3 to produce HC(O)O + NO2 + H2O with exothermicity 74.2 kcal mol−1; TS3 has a barrier of 3.3 kcal mol−1. The fourth channel occurs via abstraction of H by the NO2 group in NO3CH2OOH (TS4) with barrier of 11.4 kcal mol−1, leading to the formation of CH2O + HONO + 1O2 with exothermicity of 11.6 kcal mol−1. Intermediate NO3CH2OOH can decompose also via migration of one H 6



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atom in CH2 to the neighboring O (TS5) to yield HC(O)OH + NO2 + OH with exothermicity 69.3 kcal mol−1; the barrier height at TS5 is 14.2 kcal mol−1. The direct cleavage of the N−O bond and the O−O bond eventually producing CH2O(O) + NO2 + OH is predicted to be endothermic by 20.3 kcal mol−1 without an intrinsic barrier. 2.

Decomposition of CH2(O)NO3 To evaluate the role of the decomposition of CH2(O)NO3 radical, we investigated

its fragmentation at level CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ; the related geometries and PES are shown in Figures 2 and 3, respectively. The profile of potential energy shown in Figure 3 indicates that the fragmentation of the CH2(O)NO3 radical might produce HCO + HNO3 with exothermicity 5.1 kcal mol−1 via migration of an H atom from the CH2 group to the O atom in the NO2 moiety via a five-centered TS6 with barrier 16.1 kcal mol−1. Another path yields products CH2O + NO3 through the cleavage of the C−N bond in CH2(O)NO3 via TS7 with barrier 25.1 kcal mol−1; this process is endothermic by 9.2 kcal mol−1 at level CCSD(T)//B3LYP. At TS7, the length of the breaking C−N bond is predicted to be 1.899 Å, significantly greater than the corresponding bond length of 1.445 Å in CH2(O)NO3 (see Figure 2). The path with a large barrier of 30.5 kcal mol−1 at TS8 proceeds through migration of H to the neighboring O atom, followed by fragmentation into HC(O)OH + NO2; this reaction is predicted to be exothermic by 64.3 kcal mol−1. There are no experimental data for the decomposition reaction for comparison with the predicted energy barriers. However, we can compare the predicted heat of reaction for CH2O + NO3 → HNO3 + HCO with available experimental value. The heat of reaction predicted at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level is −14.3 kcal mol−1 which is close to the experimental value, −13.7 kcal mol−1.34, 7


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The experimental values utilized in the calculation are based on the enthalpies of formation at 0 K for CH2O = −25.1 ± 0.2 kcal mol−1; NO3 = 18.9 kcal mol−1; HNO3 = −29.8 ± 0.1 kcal mol−1; HCO = 9.89 kcal mol−1. Based on this comparison, the barriers and product energies calculated at the CCSD(T)/aug-cc-pVTZ// B3LYP/aug-cc-pVTZ level may be accurate to within ±1.0 kcal mol−1. 3.

Calculations of rate coefficients According to the predicted reaction paths and vibrational energies, rate

coefficients and branching ratios for individual product channels of the bimolecular reaction CH2OO + HNO3 have been calculated for temperature range 250−700 K with program Variflex30 based on the variational transition-state microcanonical RRKM theory. In these calculations, we used the energies predicted at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level. The rotational parameters and vibrational wavenumbers employed are listed in Table S2, available in the Supporting Information. The energy transfer was computed on the basis of an exponential down model with down = 300 cm−1 for N2. For the tight transition states involving H-migration, corrections for Eckart tunneling were made. As shown in the potential-energy profile for CH2OO + HNO3, the reaction occurs via a loose variational transition state giving pre-reaction complex LM1, which is readily transformed by migration of H via TS1 to yield excited intermediate NO3CH2OOH*. The rate coefficient for the formation of the excited intermediate was treated with the unified statistical formulation of Miller36 including multiple reflection corrections.37,38 The Lennard-Jones parameters employed in the title reaction are approximated as follows: for NO3CH2OOH, ε/k = 467.9 K and σ = 4.29 Å, estimated on the basis of CH2OO, ε/k = 520 K and σ = 3.79 Å and HNO3, ε/k = 421 K and σ = 4.80 Å, and for buffer gas N2, ε/k = 106 K and σ = 3.74 Å. The rate coefficients for 8



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the following low-energy product channels of low energy are considered in this work:

CH2OO + HNO3 → NO3CH2OOH* → NO3CH2OOH

(1)

→ CH2(O)NO3 + OH

(2)

→ HC(O)OOH + HONO

(3)

→ HC(O)O + NO2 + H2O

(4)

Calculated rate coefficients at 295 K and pressure range of 20−100 and 760 Torr are also listed in Table 1. The rate coefficients computed in temperature range of 250−700 K and pressure range of 20−100 and 760 Torr are fitted to a three-parameter Arrhenius equation k(T) = AT n exp (−Ea/RT) ;

(5)

the derived parameters are also presented in Table 1; rate coefficients at each temperature and pressure are listed in Table S3, available in the Supporting Information. Figure 4 summarizes the total and individual product rate coefficients of reaction CH2OO + HNO3 at pressure 40 Torr N2 for comparison with experimental data. The results reveal that the dominant products in the low-temperature range are NO3CH2OOH (k1) and CH2(O)NO3 + OH (k2) with respective rate coefficients k1 = 1. 1×10−10 cm3 molecule−1 s−1 and k2 = 4.0×0−10 cm3 molecule−1 s−1 at 295 K, giving the total rate coefficient, 5.1×10−10 cm3 molecule−1 s−1, which agrees with the experimental result, (5.4±1)×10−10 cm3 molecule−1 s−1.23 The fact that the predicted total rate coefficient is in agreement with the experimental value suggests that our VTS curve reliably predicts the enthalpic and entropic changes (i.e. Gibbs free energy changes) during the association process. The calculated pressure-dependent rate coefficients for the formation of 9


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competing products at 295 K covering pressure range 20−760 Torr are displayed in Figure 5. The results show clearly that the formation of product CH2(O)NO3 + OH (k2) is dominant over the adduct product channel NO3CH2OOH (k1) at P < 150 Torr, with a branching ratio 89 % at 20 Torr decreasing to 60 % at 100 Torr. At P > 150 Torr, the formation of NO3CH2OOH (k1) becomes dominant (79 % at 760 Torr) with a concomitant decrease in branching ratio for formation of CH2(O)NO3 + OH (k2) (21 % at 760 Torr). Figure 6 shows the branching ratios of the individual product channels at 40 Torr in temperature range 250−700 K. The results show that, at 295 K, the formation of CH2(O)NO3 + OH (k2) is dominant with 79 % yield; the remaining 21 % gives NO3CH2OOH (k1). As the temperature increases, the deactivation of the excited product becomes less competitive; CH2(O)NO3 + OH becomes the sole product. As shown in Figure 6, channels 3 and 4 producing HC(O)OOH + HONO (k3), HC(O)O + NO2 + H2O (k4), respectively, are negligible throughout the entire temperature range. 4.

Thermal decomposition of CH2(O)NO3 The thermal decomposition of CH2(O)NO3 has three possible channels, CH2(O)NO3 → HCO + HNO3

(6)

→ CH2O + NO3

(7)

→ HC(O)OH + NO2

(8)

Rate coefficients at 295 K and pressure range of 20−100 and 760 Torr are predicted with corrections for Eckart tunneling using the Chemrate code,39 as listed in Table 2; the three parameters in the Arrhenius equation, Eq. (5), derived from fitting calculated rate coefficients in temperature range of 250−700 K and pressure range of 20−100 and 760 Torr are also presented in Table 2. Rate coefficients at each temperature and pressure are listed in Table S4, available in the Supporting Information. For reaction 10



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(8), Eq. (5) reproduces the calculated results to only about a factor of two at 250 K, as indicated in Fig. S2, available in the Supporting Information. Under all conditions, the decomposition of CH2(O)NO3 produces predominantly HCO + HNO3 reaction (6). The rate coefficients have a positive pressure dependence, reflecting the nature of collisional activation as shown in Table 2 and Figure S2. The rate coefficient for generation of HCO + HNO3 (k6) via migration of H to the neighboring O atom of the NO3 group is predicted to be >106 greater than that for the formation of CH2O + NO3 via breaking of the C−O bond at 295 K. As the temperature increases, the bond breaking becomes more competitive. The predicted rate coefficient k8 for the formation of HC(O)OH + NO2 is smaller than k7, attributable to its greater barrier at TS8, 30.5 kcal mol−1. The results cited above and summarized in Tables 1 and 2 clearly indicate that the CH2OO + HNO3 reaction at room temperature under atmospheric pressure condition is dominated by collisional stabilization of NO3CH2OOH* by reaction (1) and the production of CH2(O)NO3 + OH by reaction (2), to be followed by the fragmentation of CH2(O)NO3 to generate HCO + HNO3 in reaction (6). HNO3 is thus functioning as a catalyst converting CH2OO into OH and CHO. Although the production of HCOOOH + HONO via TS2 appears to have a similar barrier with that of the CH2(O)NO3 + OH reaction as aforementioned, the tighter structure of TS2, comparing with the loose variational TS for the latter process renders the former reaction non-competitive as shown by the line labeled for HONO formation. 5.

Implications for atmospheric chemistry As Foreman et al. discussed, reaction CH2OO + HNO3 becomes increasingly

competitive with water dimers as the relative humidity decreases, and dominates under dry conditions with RH < 30 %.23 These authors expected a negative 11


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dependence of this reaction on temperature and proposed that the importance of this reaction would be even more significant at low temperature. Our calculations indicate that the total rate coefficient remains nearly the same as temperature decreases. However, the effective competition between HNO3 and water is clearly predicted. More importantly, the previous authors considered the title reaction only as a sink reaction for CH2OO. We investigated the decomposition channels of the adduct and showed that OH + CH2(O)NO3 is the major channel at low pressure and still has ~20 % yield at 760 Torr of N2. Furthermore, the calculations show that CH2(O)NO3 further decomposes to form HCO and HNO3; because HNO3 was regenerated, HNO3 catalytically decomposes CH2OO into OH and HCO via reactions (2) and (6). It is expected that the same reaction paths hold for larger Criegee intermediates; reaction (2) for the larger species still generates OH and reaction (6) might still regenerate HNO3. Hence, the importance of the reactions of HNO3 with Criegee intermediates is not only the significant decay of Criegee intermediates by HNO3 but also the catalytic production of OH and HCO. These reactions should perhaps be included in the model for production of OH under conditions of dim light in the polluted and dry regions. Finally, because a significant amount of the adduct NO3CH2OOH is produced at high pressure, its photochemistry deserves further investigation so that we understand the products and their net effects.

4.

CONCLUSION The reaction of CH2OO with HNO3 and the decomposition of CH2(O)NO3 have

been studied in detail at the CCSD(T)/aug-cc-pVTZ level of calculations based on B3LYP/aug-cc-pVTZ optimized geometries in conjunction with statistical theory calculations. The reaction occurs readily through the formation of a pre-reaction 12



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complex with binding energy 14.7 kcal mol−1; the migration of H in HNO3 to CH2OO yields excited intermediate NO3CH2OOH* with internal energy 44.2 kcal mol−1. The excited intermediate readily dissociates into products CH2(O)NO3 + OH through breaking the O−O bond with no intrinsic barrier. Other product channels via the excited intermediate are negligible. The major decomposition product, CH2(O)NO3 radical, can further decompose to give HCO + HNO3 through migration of H via TS6 with an activation barrier of 16.1 kcal mol−1. The rate coefficients for all product channels and their branching ratios have been computed with the microcanonical VTST/RRKM theory in temperature range 295−700 K and pressure range 20−760 Torr (N2); the predicted rate coefficients depend on temperature and pressure. Our predicted total rate coefficient at 295 K, 5.1×10−10 cm3 molecule−1 s−1, agrees with experimental value (5.4±1)×10−0 cm3 molecule−1 s−1.23 Among the product channels, the formation of CH2(O)NO3 + OH (k2) is dominant in temperature range 295−700 K and pressure below 150 Torr. The predicted branching ratios at 295 K for NO3CH2OOH (k1) and CH2(O)NO3 + OH (k2) account for 0.21 and 0.79, respectively, at 40 Torr and 0.79 (k1) and 0.21 (k2) at 760 Torr . This reaction becomes increasingly competitive with water dimers as the relative humidity decreases and dominates under dry conditions (RH < 30 %). The importance of this reaction is the catalytic generation of OH and HCO from CH2OO, in addition to the effective competition between HNO3 and water for CH2OO in the atmosphere under dry conditions.

ASSOCIATED CONTENT

Supporting Information. Cartesian coordinates of major species in the reaction (Table S1). Vibrational wavenumbers and rotational parameters for reactants, 13


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transition states, and products optimized at the B3LYP/ aug-cc-pVTZ level of theory (Table S2). Rate coefficients k for various channels of the reaction CH2OO + HNO3 at various pressures and temperatures (Table S3). Rate coefficients k at 295 K and Arrhenius parameters for the unimolecular decomposition of CH2(O)NO3 into various products at various pressures and temperatures (Table S4). The IRC analysis of the connectivity of TS1 with LM1 and NO3CH2OOH (Figure S1). Plot of rate coefficients at pressure 40 Torr for the decomposition of CH2(O)NO3 reaction to form HCO + HNO3 (k6), CH2O + NO3 (k7), and HC(O)OH + NO2 (k8) (Figure S2).

AUTHOR INFORMATION

Corresponding Author *Yurn-Pern Lee. E-mail: [email protected]. Tel: +886-3-5131459. FAX: +886-3-5713491. *M. C. Lin, E-mail: [email protected]. Tel: +886-3-5131696. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Ministry of Science and Technology, Taiwan (Grant No.

MOST105-2745-M-009-001-ASP) and Ministry of Education, Taiwan ("Aim for the Top University Plan" of National Chiao Tung University) supported this work. The National Center for High-Performance Computation provided computer time. MCL would also like to acknowledge the support from the Ministry of Science and Technology for his distinguished visiting professorship at NCTU. TABLE 1. Rate coefficients at 295 K and Arrhenius parametersa for various channels 14



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of the reaction CH2OO + HNO3 at various pressures and temperatures. P A Ea/R k (295 K) n b /Torr /cm3 molecule−1 s−1 /K −17.92 4471 5.7×10−11 NO3CH2OOH (k1) 20 3.70×1040 40 −17.30 4425 1.1×10−10 1.87×1039 38 60 −16.96 4413 1.5×10−10 3.45×10 37 −16.66 4383 1.8×10−10 80 6.96×10 100 −16.43 4363 2.1×10−10 2.01×1037 30 760 −13.96 3991 4.2×10−10 8.88×10 CH2(O)NO3 + OH (k2) 20 −5.83 1640 4.5×10−10 2.91×107 8 40 −6.10 1816 4.0×10-10 2.14×10 −6.27 1940 3.6×10-10 60 7.77×108 9 80 −6.39 2037 3.3×10−10 2.00×10 100 −6.48 2112 3.1×10−10 3.89×109 11 760 −6.90 2817 1.1×10−10 1.61×10 20 HC(O)OOH + HONO (k3) −5.65 1516 2.0×10−13 3.01×103 4 40 −5.91 1690 1.8×10−13 2.10×10 60 −6.10 1828 1.6×10−13 9.41×104 5 80 −6.22 1923 1.5×10−13 2.26×10 100 −6.32 2009 1.4×10−13 5.08×105 7 760 −6.64 2682 4.6×10−14 1.02×10 HC(O)O + NO2 + H2O (k4) 20 1.60 23 1.3×10−17 1.55×10−21 −21 40 1.36 161 1.2×10−17 8.96×10 60 1.16 275 1.1×10−17 3.80×10−20 −19 80 0.97 386 1.1×10−17 1.58×10 −19 100 0.85 462 1.0×10−17 3.95×10 760 −0.86 1571 5.0×10−18 1.39×10−13 6 ktotal (= k1 + k2 + k3 + k4) 20 −5.55 1457 5.0×10−10 3.48×10 40 −5.60 1478 5.1×10−10 5.11×106 6 60 −5.64 1492 5.1×10−10 6.60×10 80 −5.66 1501 5.1×10−10 7.81×106 6 100 −5.68 1507 5.1×10−10 8.75×10 760 −5.88 1583 5.2×10−10 3.57×107 a k(T) = AT n exp (−Ea/RT) predicted for various temperatures 295−700 K in units cm3 molecule−1 s−1 for k. Products

b

in unit /cm3 molecule−1 s−1.

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TABLE 2. Rate coefficients (s−1) at 295 K and Arrhenius parametersa for the unimolecular decomposition of CH2(O)NO3 into various products at various pressures and temperatures. P A Ea/R k (295 K) n 3 −1 −1 /Torr /cm molecule s /K /s−1 HCO + HNO3 (k6) 20 −10.05 10760 0.82 3.80×1040 39 40 10813 0.98 −9.74 9.29×10 60 −9.50 10824 1.07 2.69×1039 38 80 −9.29 10820 1.12 8.91×10 38 100 10812 1.16 −9.13 3.44×10 32 760 −6.86 10424 1.39 2.87×10 38 CH2O + NO3 (k7) 20 15541 6.8×10−7 −8.85 3.92×10 36 40 −8.11 15412 7.2×10−7 3.95×10 −7.63 60 15314 7.4×10−7 1.88×1035 34 80 −7.27 15235 7.5×10−7 1.89×10 100 −6.98 15169 7.5×10-7 2.99×1033 25 760 −4.17 14420 7.8×10−7 2.72×10 HC(O)OH + NO2 (k8) 20 12.92 6289 4.7×10−8 1.85×10-30 -33 40 13.99 6009 4.7×10−8 1.68×10 60 14.62 5837 4.7×10−8 2.57×10-35 -36 80 15.07 5713 4.7×10−8 1.35×10 15.41 100 5615 4.7×10−8 1.34×10-37 -46 760 18.38 4735 4.7×10−8 3.12×10 a k(T) = AT n exp (−Ea/RT) predicted for various temperatures 295−700 K in units cm3 molecule−1 s−1 for k; these questions reproduce most data to within 20 % except k7. Products

16



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Captions of Figures` Figure 1. Schematic potential-energy profiles for reaction CH2OO + HNO3. Relative energies (ZPE-corrected, kcal mol−1 ) were calculated at level CCSD(T)/aug-cc-pVTZ //B3LYP/aug-cc-pVTZ. Figure 2. Optimized geometries of reactants, transition states (TS), intermediates, and products of reaction CH2OO + HNO3 calculated at level B3LYP/aug-cc-pVTZ (bond length in Å and bond angle in degree). Figure 3. Schematic potential-energy profiles for the decomposition of CH2(O)NO3. Relative

energies

(ZPE-corrected,

kcal

mol−1)

were

calculated

at

level

CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ. Figure 4. Predicted rate coefficients at pressure 40 Torr for reaction CH2OO + HNO3 forming NO3CH2OOH (k1), CH2(O)NO3 + OH (k2), HC(O)OOH + HONO (k3), HC(O)O + NO2 + H2O (k4), and ktotal = k1 + k2 + k3 + k4 (dash line) based on the CCSD(T)//B3LYP/aug-cc-pVTZ results. The experimental value of Foreman et al.23 is shown as a blue star. Figure 5. Pressure dependence of rate coefficients for all product channels for the reaction CH2OO + HNO3 at T = 295 K. The high-pressure limit is shown in a green line. Figure 6. Temperature dependence of the branching ratios of products of reaction CH2OO + HNO3 at pressure 40 Torr.

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Figure 1. J. Phys. Chem./Lee et al. (double column)

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Figure 2. J. Phys. Chem./Lee et al. (single column)

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Computational Chemical Kinetics for the Reaction of Criegee

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