Atmospheric Decomposition of Trifluoromethanol Catalyzed by Formic

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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

Atmospheric Decomposition of Trifluoromethanol Catalyzed by Formic Acid Parandaman Arathala, Josue E. Perez, and Amitabha Sinha J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09316 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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The Journal of Physical Chemistry

Atmospheric Decomposition of Trifluoromethanol Catalyzed by Formic Acid Arathala Parandaman, Josue E. Perez, and Amitabha Sinha*

Department of Chemistry and Biochemistry, University of California─San Diego, La Jolla, California 92093

Abstract Quantum chemistry calculations are used to investigate the energetics and kinetics of CF3OH decomposition catalyzed by a single formic acid (FA) molecule acting alone and in conjunction with a single water (H2O) molecule to form the products carbonyl fluoride (CF2O) and hydrofluoric acid (HF). While the uncatalyzed reaction has a barrier of ~44.7 kcal/mol, the presence of a FA molecule reduces the barrier to 6.4 kcal/mol, while the presence of both a FA and H2O molecule acting in unison decreases the barrier to -1.6 kcal/mol measured relative to the separated reactants. For comparison, we have also examined the decomposition of CF3OH catalyzed by HO2 and HO2 + H2O, which have been suggested in the literature to be an important atmospheric catalyst for CF3OH decomposition. In addition, we have also examined the loss of CF3OH via its bimolecular reaction with OH radicals. The rate constants for these various reactions were also calculated using canonical variational transition state theory coupled with small curvature tunneling corrections over the temperature range between 200-300K. Our results show that the rates for the CF3OH+FA and CF3OH+FA+H2O reactions are ~ 104 times faster compared respectively to the corresponding reactions involving CF3OH+HO2 and CF3OH+HO2+H2O at 300 K. Further, we find that although the CF3OH+FA reaction has a higher barrier compared to CF3OH+FA+H2O, measured relative to the separated reagents, its effective first order rate for CF3OH decomposition is significantly faster for temperatures above 240 K compared to that of CF3OH+FA+H2O. This trend arises from the higher unimolecular reaction barrier for the reactant complex associated with the CF3OH+H2O+FA reaction compared to that for CF3OH+FA, as well as the lower concentration of reactant dimer complexes for CF3OH+H2O+FA compared to the concentration of the monomer FA reactant in the CF3OH+FA reaction. Finally, our calculations show that the rate for CF3OH decomposition catalyzed by FA is ~ 104 times faster relative to the loss of CF3OH via its bimolecular reaction with OH radicals over the 200-300K temperature range. Thus, the present study suggests that among the various known loss mechanisms, unimolecular reaction catalyzed by FA is likely the dominant gas phase decomposition pathway for CF3OH in the troposphere.

* Address correspondence to: [email protected]

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I. Introduction: Trifluoromethanol (CF3OH) is produced in the atmosphere through the degradation of hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs).1-4 HFCs and HFEs are commonly used as refrigerants, fire suppressants, and in automotive air conditioning systems; they are viewed as a more environmental friendly alternative to chlorofluorocarbons (CFCs).1,5,6 With the increased usage of HFCS and HFEs, there has been considerable interest in understanding the atmospheric fate and decomposition mechanisms of these compounds.6 Through atmospheric oxidation, the various HFCs can generate CF3 radicals.7-10 These radicals react with atmospheric oxygen to form CF3O2, which then react with NO to form CF3O radicals.11,12 Once formed, the CF3O radicals can react with water vapor13,14 and hydrocarbons,15-20 to form CF3OH. Several studies have speculated that CF3O radicals can potentially participate in catalytic ozone destruction cycles in the stratosphere.21-23 Because the CF3OH and CF3O species are interconnected, understanding the fate of CF3OH is important for quantifying the fate of CF3O radicals in the atmosphere.24 The mechanisms associated with the atmospheric removal of CF3OH are not well understood. The lifetime of CF3OH with respect to photolysis is estimated to be about one million years at altitudes below 40 km, suggesting that direct photolysis of this molecule is not an important removal pathway.24 Further, the rate constant for the reaction of CF3OH with OH radicals, which results in the formation of CF3O, was found to be less than 2.0 х 10-17 cm3 molecule-1 s-1 at 298 K by DeMore et al.25 The slowness of this abstraction reaction has been also confirmed by computational studies which find that the rate coefficient for the CF3OH+OH reaction to form CF3O is 9.7 х 10-18 cm3 molecule-1 s-1 at 300 K.26 Given that atmospheric loss of


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CF3OH through photolysis and its reaction with OH radicals are both very slow, there has been interest in finding other possible removal mechanisms for this compound. Another

potential

route

for

CF3OH

decomposition

is

through

unimolecular

dissociation3,27 resulting in the formation of CF2O and HF as shown in Eq.1 CF3OH → CF2O + HF

(1)

The barrier height for this unimolecular reaction is estimated to be between 45-46 kcal/mol.1,5,2729

Most recently Long et al.30 used energies calculated at the CCSD(T)/6-311G(d,p)//B3LYP/6-

311G(d,p) level in conjunction with transition state theory to estimate the rate for the unimolecular reaction to be 1.9 х 10-27 s-1 at 298 K. Although the rate for the bare unimolecular reaction (shown in Eq.1) is very slow, the presence of a suitable catalyst can significantly reduce the reaction barrier. For example, using a single water molecule as catalyst, the unimolecular barrier height has been shown to be reduced to between 21.5 – 17.2 kcal/mol relative to the CF3OH + H2O separated reagents.1,5,29 An even more effective catalyst is the combination of HO2+H2O. Long et al.30 investigated the impact of this catalyst on the CF3OH unimolecular reaction and showed that at the CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p)) level, the reaction barrier is reduced to -0.1 kcal/mol, relative to the CF3OH+HO2+H2O separated reactants. Given that acids are known to reduce reaction barriers for many atmospheric reactions3134

and have significantly higher concentration compared to HO2, it is of interest to investigate the

potential impact of acid catalysis on the CF3OH unimolecular reaction. Formic acid for example is present in the troposphere at the parts per billion by volume (ppbv) levels.35,36 To the best of our knowledge there are no reports in the literature investigating the impact of formic acid, either acting alone or in conjunction with water, in catalyzing the decomposition of CF3OH. These



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reactions pathways can be represented schematically as shown in equations 2a and 2b where X represents the acid catalyst: CF3OH + X → CF2O + HF + X

(2a)

CF3OH + X + H2O → CF2O + HF + X + H2O

(2b)

In the present work, we have explored the effectiveness of formic acid, working alone and in conjunction with a single water molecule, in facilitating the decomposition of CF3OH by examining the energetics and kinetics associated with their respective reactions and comparing the results with those associated with the HO2 and HO2+H2O catalytic systems, all calculated at the same level of theory. In addition, we compare the effectiveness of FA as a catalyst relative to that of another atmospheric acid, nitric acid, which is also present in significant concentration.37 Our results suggest that FA lowers the CF3OH decomposition barrier more effectively compared to nitric acid and that at 300 K, the rates for the CF3OH+FA and CF3OH+FA+H2O reactions are ~ 104 times faster compared respectively to the corresponding reactions involving CF3OH+HO2 and CF3OH+HO2+H2O associated with HO2 catalysis.

II. Computational Methods: All electronic structure calculations were performed using the Gaussian-09 program.38 The stationary points involved in the decomposition of CF3OH reactions were optimized using the M06-2X hybrid density functional39 and Møller-Plesset perturbation theory (MP2).40 Both these computational methods were used in conjunction with the 6-311++G(3df,3pd) basis set. These two levels of theories have been shown to perform well for investigating unimolecular and bimolecular hydrogen atom transfer reactions.41-43 In the present study all transition states were optimized using the Gaussian-09 code; we also computed the harmonic vibrational frequencies 4 ACS Paragon Plus Environment

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associated with all stationary points at both levels of theory in order to determine if they correspond to a minimum or transition state along the reaction path. In addition, we carried out intrinsic reaction coordinate (IRC) calculations at the M06-2X level to confirm the connectivity between reactant complexes and product complexes with their corresponding transition states (TS). The stationary point energies determined at both the M06-2X and MP2 levels were further refined by performing single-point energy calculations at the CCSD(T) level44 also using the 6311++G(3df,3pd) basis. The computed total electronic energies (Etotal) and zero-point energy (ZPE) corrected electronic energies [Etotal(ZPE)] for the various stationary points are tabulated in Tables S1-S2 of the supporting information, while the computed barrier heights, vibrational frequencies, rotational constants, and geometries of all species are provided in Tables S3-S8.

III. Results and Discussion: A. Energies and Structure of Stationary Points Along Reaction Coordinate. We have investigated the unimolecular decomposition of CF3OH in the absence and presence of various catalysts. The potential energy surface for the unimolecular decomposition of CF3OH in the absence of any catalyst is shown in Figure 1. The barrier height is computed to be 44.7 kcal/mol at both the CCSD(T)//M06-2X and CCSD(T)//MP2 levels of theory. This result is consistent with the earlier value of 44.1 kcal/mol reported by Nguyen et al.28 at the CCSD(T)/CBS level as well as with other work.1,5,27,29,30 Hence, without a catalyst the barrier for CF3OH unimolecular reaction is too high and the reaction will be slow under typical atmospheric conditions. The presence of a suitable catalyst can change this situation and hence their impact needs to be explored.



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Due to the significant concentration of HO2 radicals in the atmosphere (~7×108 molecules/cm3)45 there has been interest in examining its potential in catalyzing the decomposition of CF3OH.30 Previous work by Long et al.30, performed at the CCSD(T)/6311G(d,p)//B3LYP/6-311G(d,p) level, found that HO2 catalysis reduced the unimolecular reaction barrier to 12 kcal/mol. We have re-examined the HO2 catalyzed decomposition of CF3OH at the CCSD(T)//MP2 and CCSD(T)//M06-2X levels to facilitate a direct comparison with FA catalysis. The optimized geometries of the stationary points for the HO2+CF3OH reaction and their relevant energies are shown in Figure 2. The present CCSD(T)//MP2 calculated barrier height is 10.1 kcal/mol relative to the separated reactants which is ~2 kcal/mol lower than the previously reported value obtained at the CCSD(T)/6-311G(d,p)//B3LYP/6311G(d,p) level.30 Previous study also suggests that the HO2 radical acting in conjunction with a H2O molecule can also be an effective catalyst for CF3OH decomposition.30 We have investigated the energetics of the CF3OH+HO2+H2O reaction at the CCSD(T)//M06-2X and CCSD(T)//MP2 levels of theory. In presence of three reagents (CF3OH, HO2, and H2O) the simultaneous collision of these species is unlikely as termolecular reactions are slow, therefore we have modelled the CF3OH+HO2+H2O reaction as occurring through three possible bimolecular encounters involving various dimer complexes as shown in Eqs. 3a-3c. HO2••H2O + CF3OH → CF2O + HF + HO2 + H2O

(3a)

CF3OH••H2O + HO2 → CF2O + HF + HO2 + H2O

(3b)

CF3OH••HO2 + H2O → CF2O + HF + HO2 + H2O

(3c)

The impact of each reaction shown in Eq.3a-3c is dictated by the atmospheric concentration of the corresponding dimer complex. These concentrations can be estimated from the dimer binding 6 ACS Paragon Plus Environment

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energy and corresponding equilibrium constant. The optimized structures of the relevant dimer complexes such as CF3OH••H2O (D2), HO2••H2O (D3), and CF3OH••HO2 (D4) are shown in Figure 3. The zero-point energy corrected binding energies of these three dimers are respectively 7.6, 6.6, and 7.3 kcal/mol at the CCSD(T)/M06-2X level. The calculated equilibrium constants for the CF3OH••H2O (D2), HO2••H2O (D3), and CF3OH••HO2 (D4) dimer complexes are given in Table S9 of the supporting information, and are respectively 1.1×10-20, 2.7×10-21, 8.4×10-22 cm3/molecule at 300 K. Using these equilibrium constant and dimer binding energies, the atmospheric concentration of the HO2••H2O, CF3OH••H2O, and CF3OH••HO2 dimers are estimated to be respectively 7.3×105, 4.1×108, 5.9×10-2 molecules/cm3 at 300K. In these calculations, we have used typical tropospheric concentrations of 3.9×1017 molecules/cm3 for H2O, 1011 molecules/cm3 for CF3OH, and 7×108 molecules/cm3 for HO2.30,42,45 These results suggest that the concentration of the CF3OH••HO2 dimer will be negligible in the atmosphere, and therefore reaction 3c was not considered any further in this study. Thus, there are basically two pathways, reactions 3a and 3b, associated with the CF3OH+HO2+H2O reaction. The energetics of the various stationary points for the CF3OH+HO2+H2O reaction are shown in Figure 4. As the figure illustrates, once formed the dimer complex can collide with the monomer reagent leading to the formation of the reactant complex (RC2) which then undergoes unimolecular reaction over the barrier associated with the transition state (TS2) to form the product complex (PC2) and finally the separated products. For example, the CF3OH••H2O dimer complex (D2) can collide with HO2 to form the reactant complex (RC2), which then undergoes unimolecular reaction over transition state (TS2) to form the product complex (PC2) and finally the HF and CF2O separated products. In an analogous manner the HO2••H2O dimer (D3) can collide with CF3OH to also form RC2, which then undergoes unimolecular reaction over TS2 to



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form the products (see Fig. 4). Our calculations suggest that the barrier height for the CF3OH+HO2+H2O reaction is 1.1 kcal/mol relative to the separated reactants; for comparison Long et al30 reported a barrier height of -0.1 kcal/mole for this reaction at the CCSD(T)/6311G(d,p)//B3LYP/6-311G(d,p) level. The PES for the decomposition of CF3OH catalyzed by FA is shown in Figure 5. The transition state for the reaction (TS3) is 6.4 kcal/mol higher in energy relative to the separated reactants. We investigated the impact of adding a single water molecule along with FA by looking at the CF3OH+FA+H2O reaction system. As in the case of CF3OH+HO2+H2O, the CF3OH+FA+H2O reaction involves three reagents and thus is modeled as occurring via three distinct bimolecular encounters involving three possible dimers reacting with a monomer reagent as represented in Eqs. 4a-4c. FA••H2O + CF3OH → CF2O + HF + FA + H2O

(4a)

CF3OH••H2O + FA → CF2O + HF + FA + H2O

(4b)

CF3OH••FA + H2O → CF2O + HF + FA + H2O

(4c)

The most stable structures of the FA••H2O (D6), CF3OH••H2O (D2), and CF3OH••FA (D7) dimer complexes are shown in Figure 3. The binding energy of the FA••H2O dimer is found to be 8.0 kcal/mol and agrees well with the value reported in prior studies.32,46,47 The CF3OH••FA dimer is even more stable with a binding energy of 10.2 kcal/mol. These dimer complexes can react by colliding with the monomer third reactant as represented in Eq.4a-4c and shown in Fig. 6. As shown in the figure, these reaction paths lead to the formation of a reactant complex (RC4) which then goes through a ten-membered ring transition state (TS4) to form the product complex (PC4) and finally the separated products. Interestingly, the calculated barrier height of this reaction is -1.6 kcal/mol relative to the CF3OH+FA+H2O separated reactants, and 20.5 kcal/mol 8 ACS Paragon Plus Environment

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higher than the reactant complex (RC4). This value also suggests that the transition state (TS4) is lower in energy compared to that for the CF3OH+HO2+H2O reaction. We note that there is a second transition state for the CF3OH+FA+H2O reaction corresponding to an alternate arrangement of the catalysts with the position of the water and FA molecules interchanged relative to that shown in Fig.6. This alternate TS configuration is shown in Figure S1 of the supporting information. Because this alternate TS is 2.1 kcal/mol higher in energy compared to TS4, it was not considered any further for kinetic analysis. Given the favorable impact of FA and FA+H2O in reducing the barrier for CF3OH decomposition, we were curious about the impact of another important atmospheric acid, nitric acid, in catalyzing the reaction. The atmospheric concentration of FA and HNO3 are comparable, with the concentration of FA being slightly higher in the troposphere.36,48 The PES for the CF3OH+HNO3 reaction is shown in Figure 7. The transition state for this reaction, TS5, lies 13.6 kcal/mol above the CF3OH+HNO3 separated reactants. The lowest energy path for the corresponding reaction with one water molecule, CF3OH+HNO3+H2O, is shown in Figure 8. The CF3OH+HNO3+H2O reaction also has a second higher energy TS where the position of HNO3 and water molecules are interchanged; this higher energy reaction path is shown in Fig. S2 of the supporting information. As Fig.7 and Fig. 8 show, the CF3OH+HNO3, and CF3OH+HNO3+H2O reaction, with transition states TS5 and TS6, have barrier heights respectively of 13.6, and 3.4 kcal/mol relative to the separated reactants. These barriers are higher than those associated with formic acid and the formic acid water systems. Thus, based on the computed barriers of the TS and the relative binding energy/equilibrium constant for the corresponding dimer complexes (see Figs. 5,6,7, and 8) we conclude that the HNO3 and HNO3+H2O catalytic systems will not be as effective as FA and FA+H2O in facilitating the unimolecular dissociation of CF3OH.



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B. Theoretical Kinetic Analysis. Using the ab-initio calculated energies we have carried out rate calculations to determine the relative impact of the HO2 and HO2+H2O catalytic systems, considered to be an important catalyst in the literature,30 with that of the FA and FA+H2O systems at the same level of theory. Given that the calculations in the previous section show that HNO3 and HNO3+H2O are not as effective in lowering the barrier for CF3OH decomposition compared to that achievable by FA and FA+H2O, we did not investigate the kinetics of the CF3OH+HNO3, and CF3OH+HNO3+H2O reaction systems. For the kinetic analysis, we have followed an approach similar to that described in our earlier work and below we provide an outline of the procedure.42 The rate analysis consists of first calculating the rate constants associated with the reactions of interest. The reactions represented by Eqs. 3a-3b and 4a-4c both start with the formation of a barrierless reactant complex (RC) generated from the separated reactants, which then undergoes unimolecular reaction through a transition state to form the products CF2O and a trimer complex (Y) as represented by Eq. 5.

In Eq. 5, R1, R2, RC, and Y represent respectively reactant-1, reactant-2, the reactant complex, and the trimer product complex. For this situation the overall rate constant is given by Eq. 6 𝑘𝑘1 𝑅𝑅𝑅𝑅 𝑘𝑘 = � � 𝑘𝑘 = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘2 𝑘𝑘−1 2

(6)


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𝑅𝑅𝑅𝑅 In Eq.6 𝐾𝐾𝑒𝑒𝑒𝑒 is the equilibrium constant associated with the formation of the reactant complex

(RC) from the two starting reactants (R1 and R2) and was calculated using the partition functions

associated with R1, R2 and RC. The vibrational frequencies and rotational constants required for the partition function calculation are computed at the M06-2X/6-311++G(3df,3pd) level as outlined in the previous section and the partition functions themselves are calculated using standard formulas from statistical mechanics.49 Further, the unimolecular rate constant (k2) was computed using canonical variational transition state theory50,51 (CVT) with small curvature tunneling52 (SCT) as implemented in the POLYRATE (2016) code,53 and given by Eq. 7. ∗ 𝑘𝑘B T QGT (𝑠𝑠 ∗ ) −𝑉𝑉(𝑆𝑆𝑇𝑇 ) 𝑘𝑘 𝑘𝑘2 (CVT⁄SCT) = 𝜅𝜅 𝑒𝑒 𝐵𝐵 QRC ℎ

(7)

In Eq. 7, 𝜅𝜅 is the tunneling parameter, computed using the small curvature approach, kB is the Boltzmann’s constant, h is Planck’s constant, 𝑠𝑠 ∗ is the value of the reaction coordinate at the

energy maximum, QGT (𝑠𝑠 ∗ ) and QRC are the partition function of the transition state and reactant

complex, respectively, V(s∗ ) is the potential energy at the barrier maximum, and T the

temperature in Kelvin.

As discussed in the previous section, we assume that the CF3OH+FA+H2O and CF3OH+HO2+H2O reactions occur via bimolecular encounters where a dimer complex collides with the remaining monomer reactant. For the CF3OH+FA+H2O reaction system, for example, this leads to three possible pathways involving FA••H2O+CF3OH, CF3OH••FA+H2O, and CF3OH••H2O+FA as given in Eqs. 4a-4c. These three bimolecular reaction paths lead to the formation of the same reactant complex (RC), which is assumed to be in equilibrium with the reactants. Eventually this RC undergoes unimolecular reaction through a transition state to form CF2O + Y products (here Y is the HF••FA••H2O trimer complex) as shown in Eq. 5. The 11 ACS Paragon Plus Environment


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magnitudes of the unimolecular rate constant (k2) calculated using the CVT/SCT method are 𝑅𝑅𝑅𝑅 ) given in Table S10. The equilibrium constant for the formation of the reactant complex (𝐾𝐾𝑒𝑒𝑒𝑒

were calculated using Eq. 8 and their values are tabulated in Table S11 of the supporting information. 𝑅𝑅𝑅𝑅 𝐾𝐾𝑒𝑒𝑒𝑒 = 𝑄𝑄

𝑄𝑄𝑅𝑅𝑅𝑅

𝑅𝑅1 𝑄𝑄𝑅𝑅2

exp �−

𝐸𝐸𝑅𝑅𝑅𝑅 −𝐸𝐸𝑅𝑅 𝑘𝑘𝐵𝐵 𝑇𝑇

�

(8)

In Eq.8, QR1, QR2, and QRC represent the partition functions of reactant-1 (the dimer complex), reactant-2 (the monomer) and the corresponding reactant complex (RC), while ER and ERC are respectively the zero-point corrected total energies of the reactants and reactant complex. The bimolecular rate constants for each of the FA••H2O+CF3OH, CF3OH••H2O+FA, and 1 CF3OH••FA+H2O reaction paths are calculated using respectively the expressions: 𝑘𝑘4𝑎𝑎 = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘2 ,

2 3 𝑘𝑘4𝑏𝑏 = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘2, and 𝑘𝑘4𝑐𝑐 = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘2 , and are given in Table S10. Where, the equilibrium constants 1 2 3 𝐾𝐾𝑒𝑒𝑒𝑒 , 𝐾𝐾𝑒𝑒𝑒𝑒 , and 𝐾𝐾𝑒𝑒𝑒𝑒 are associated with the formation of the reactant complex (RC) from their

respective dimer and monomer combination. Finally, the total rate constants (cm3 molecule-1 s-1)

for the CF3OH+FA+H2O reaction is obtained by adding the contribution from each of the three FA+H2 O

pathways giving 𝑘𝑘total

= 𝑘𝑘4𝑎𝑎 + 𝑘𝑘4𝑏𝑏 + 𝑘𝑘4𝑐𝑐 ; this total rate constant is also given in Table S10.

For comparison, the rate constant for the CF3OH+HO2+H2O reaction was also calculated

using the CVT/SCT method. Since the atmospheric concentration of the HO2••CF3OH dimer is negligible, there are basically two main reaction pathways for this reaction involving the HO2••H2O+CF3OH and CF3OH••H2O+HO2 bimolecular encounters. The corresponding 4 bimolecular rate constants (k3a and k3b) were calculated using the expression: 𝑘𝑘3a = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘2 , and

5 4 5 𝑘𝑘3b = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘2 , where the equilibrium constants 𝐾𝐾𝑒𝑒𝑒𝑒 , and 𝐾𝐾𝑒𝑒𝑒𝑒 , are associated with the formation of

the reactant complex (RC) for this reaction from the corresponding dimer and a monomer 12 ACS Paragon Plus Environment

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combinations. As in the case for CF3OH+FA+H2O, the equilibrium constant for the reactant 𝑅𝑅𝑅𝑅 ) associated with the CF3OH+HO2+H2O reaction was calculated using Eqn. 8 and complex (𝐾𝐾𝑒𝑒𝑒𝑒

their values are given in Table S11 of the supporting information. The total rate coefficient was HO +H2 O

then calculated by adding the contributions from the two paths via 𝑘𝑘total2

= 𝑘𝑘3a + 𝑘𝑘3b and

their values over the temperature range from 200-300 K are given in Table S12. A comparison of the present CVT/SCT rate results with those of Long et al.30 shows that our calculated rate coefficients are ~2 orders of magnitude smaller than theirs. This disagreement likely arises from the following important differences between the two studies: (1) The present CCSD(T)/6311++G(3df,3pd)/M06-2X/6-311++G(3df,3pd) calculated reaction barrier is 8 kcal/mol measured relative to the CF3OH••H2O+HO2 separated reactants while that reported by Long et al.30, calculated at the CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) level, is 2 kcal/mol lower, (2) the present rate constants are calculated using canonical variational transition state theory (CVT) while those in the Long et al.30 work were calculated using transition state theory, which gives an upper bound for the reaction rate constant, finally, (3) Long et al.30 used an Eckart barrier to make tunneling corrections, while in the present work these corrections are implemented using the small curvature tunneling (SCT) method.52 In order understand the role played by the water molecule, we have also performed rate constant calculations for the HO2+CF3OH and FA+CF3OH reactions where water is absent and HO2 and FA are respectively the sole catalysts. The calculated unimolecular (k2) and bimolecular rate constants for these reactions are given in Table S13 of the supporting information. As the reaction with OH radicals can potentially also be an important removal mechanism for atmospheric CF3OH molecules, we have also investigated the rate of the OH+CF3OH reaction. There are no reports in the literature on the kinetics of this reaction at 13 ACS Paragon Plus Environment


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atmospherically relevant temperatures below 298 K. The work of DeMore et al.25 suggest that the rate constant for this reaction is less than 2.0 х 10-17 cm3 molecule-1 s-1 at 298 K. Brudnik et al.26 investigated this reaction over the temperature range between 300 and 1000 K using the CTST/Wigner method and reported a value of 9.7 х 10-18 cm3 molecule-1 s-1 for the rate constant at 300 K. In the present work, we have carried out rate constants calculations using the CVT/SCT method over the 200-300 K temperature range. For these calculations stationary points on the OH+CF3OH potential energy surface were determined at the CCSD(T)/6311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory. Hydrogen atom abstraction by OH radical from CF3OH was found to have a barrier of 9.2 kcal/mol measured as the energy difference between the TS and the separated reactants. The calculated rate coefficients are given in Table. S13 of the supporting information. Our data suggests that the rate constant for the CF3OH+OH abstraction reaction at 300 K is 1.75 х 10-18 cm3 molecule-1 s-1; this is approximately five times smaller than the value reported by Brudnik et al.26 using the TST/Wigner method. In order to assess the atmospheric impact of the various reactions discussed above on the decomposition of CF3OH, it is necessary to compare the rates of these reactions on an equal footing. This is most easily done by determining the effective first order rate constant for CF3OH loss associated with each reaction. As an example, the rate, ν4a, for the FA••H2O+CF3OH bimolecular reaction path (Eqn. 4a) can be written in terms of the corresponding reactant concentrations and the bimolecular rate constant as follows: ν4𝑎𝑎 = k4a [FA••H2O] [CF3OH]

(9)


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Further, the concentration of the FA••H2O dimer in Eq.9 can be expressed in terms of the corresponding monomer reactant concentrations and the equilibrium constant for dimer formation, thus giving the following expression: ν4a = k4a ĸ4𝑎𝑎 𝑒𝑒𝑒𝑒 [FA][H2O][CF3OH]

(10)

Combining the first four terms on the right side of Eq. 10, we can express the rate in terms of an ′ effective first-order rate constant (𝑘𝑘4𝑎𝑎 ) for CF3OH decomposition as: ′ ν4a = k4a ĸ4𝑎𝑎 𝑒𝑒𝑒𝑒 [FA][H2O][CF3OH] = 𝑘𝑘4𝑎𝑎 [CF3OH]

(11)

′ The constant 𝑘𝑘4𝑎𝑎 = k4a ĸ4𝑎𝑎 𝑒𝑒𝑒𝑒 [FA][H2O] is thus the effective first-order rate constant for CF3OH

loss via this path. By following a similar procedure the effective first order rate constants for the CF3OH••H2O+FA and CF3OH••FA+H2O reaction paths can also be determined and are given as

′ ′ 4𝑐𝑐 4𝑏𝑏 4𝑐𝑐 = k4b ĸ4𝑏𝑏 𝑘𝑘4𝑏𝑏 𝑒𝑒𝑒𝑒 [FA][H2O] and 𝑘𝑘4𝑐𝑐 = k4c ĸ𝑒𝑒𝑒𝑒 [FA][H2O]. Here ĸ𝑒𝑒𝑒𝑒 and ĸ𝑒𝑒𝑒𝑒 are the equilibrium

constants for forming respectively the CF3OH••H2O and CF3OH••FA dimers. Using an analogous

approach,

the

effective

first

order

rate

constants

associated

with

the

CF3OH+HO2+H2O, CF3OH+FA, CF3OH+HO2, and CF3OH+OH reactions can also be obtained, and these are given as follows: ′ ν3a = k 3a [HO2••H2O][CF3OH] = k3a ĸ3𝑎𝑎 𝑒𝑒𝑒𝑒 [HO2][H2O][CF3OH] = 𝑘𝑘3a [CF3OH]

′ ν3b = k 3b [CF3OH••H2O][HO2] = k3b ĸ3𝑏𝑏 𝑒𝑒𝑒𝑒 [HO2][H2O][CF3OH] = 𝑘𝑘3b [CF3OH] ′ νFA = k FA [FA][CF3OH] = 𝑘𝑘FA [CF3OH]

′ νHO2 = k HO2 [HO2][CF3OH] = 𝑘𝑘HO [CF3OH] 2

(12) (13) (14) (15)

′ νOH = kOH [OH][CF3OH] = 𝑘𝑘𝑂𝑂𝑂𝑂 [CF3OH]

(16) 15

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The required equilibrium constants for the formation of the various dimers are given in Table S9 of the supporting information and the concentrations of the various reagents were taken from the literature and set to the following values: [FA]=4.4×1011 molecules/cm3, [HO2]=7×108 molecules/cm3, [H2O]=3.9×1017 molecules/cm3, [OH]=1×106 molecules/cm3.36,42,45 Finally, for reactions having multiple branches, the total effective first-order rate coefficient for the overall reaction is obtained by adding the individual effective first-order rate constants for each branch. This is illustrated below for the HO2+H2O+CF3OH and FA+H2O+CF3OH reactions through Eq. (17) and (18): ′ ′ ′ (HO2 + H2 O + CF3 OH) = 𝑘𝑘3a 𝑘𝑘𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 + 𝑘𝑘3b

′ ′ ′ ′ (FA + H2 O + CF3 OH) = 𝑘𝑘4𝑎𝑎 𝑘𝑘𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 + 𝑘𝑘4𝑏𝑏 + 𝑘𝑘4𝑐𝑐

(17) (18)

In this way the total effective first order rate coefficients (in s-1) for the HO2+CF3OH, FA+CF3OH, OH+CF3OH, HO2+H2O+CF3OH, and FA+H2O+CF3OH reactions are obtained and summarized in Table 1. The corresponding tunneling factors are given in Table S14 of the supporting information. Results from Table S14 suggest that tunneling plays an important role in the CF3OH+OH reaction as it increases the rate by 1-3 orders of magnitude over the temperatures between 200-300 K; in the case of the other reactions it shows very little effect below 250 K. Figure 9 shows how the total effective first order rate constants for the HO2+CF3OH, FA+CF3OH, OH+CF3OH, HO2+H2O+CF3OH, and FA+H2O+CF3OH reactions vary with temperature over the range between 200-300 K. It is clear from figure that the total effective first order rate constants (keff) for the HO2+CF3OH, FA+CF3OH, and OH+CF3OH reactions increase with temperature while those for FA+H2O+CF3OH and HO2+H2O+CF3OH respectively decrease and are independent of temperature. These differences arise mainly due to the fact that the HO2+CF3OH, FA+CF3OH, and OH+CF3OH reactions exhibit a significant


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positive energy barrier while the FA+H2O+CF3OH reaction has a negative barrier and the barrier for the HO2+H2O+CF3OH reaction is only slightly positive. The data from Table 1 and Figure 9 also suggests that the effective first-order rate constant for FA+H2O catalysis is ~105 - 104 times larger compared to those for HO2+H2O catalysis over the temperature range considered. Thus, in the atmosphere decomposition of CF3OH through FA+H2O catalysis is far more effective compared to that by HO2+H2O catalysis. Further compared to that for the FA+H2O+CF3OH reaction, we find that the effective first order rate constant for the FA+CF3OH reaction is larger over most of the temperature range of atmospheric interest; this occurs even though the energy of the TS associated with the FA+CF3OH reaction is higher compared to that for the FA+H2O+CF3OH reaction. Only for temperatures below ∼200 K is the FA+H2O+CF3OH reaction faster compared to the FA+CF3OH reaction. This trend arises due to the difference in

the unimolecular barrier heights for the two reactions as well as the difference in the concentration of the FA••H2O dimer complex versus the FA reactant concentration involved in the two reactions; both factors favor the reaction without water. For example, looking at Fig. 5 and 6, we see that the calculated unimolecular barrier heights from RC to TS for the CF3OH+FA and CF3OH+FA+H2O reactions are respectively 16.5 and 17.5 kcal/mol. Using these barrier heights, the unimolecular rate constants (k2) are calculated to be respectively 1.84 s-1 and 1.18×10-1 s-1 at 300 K. The concentration of reactants also appears in the keff calculations and based on the computed equilibrium constant, we find that the concentration of the dimer to be [FA••H2O] = 1.5×109 molecule/cm3 at 300 K while the concentration of formic acid is [FA]= 4.4×1011.36 Several other studies have also reported observing similar behavior for the impact of a single water molecule on other reactions.54-56 Finally, comparing the effective first-order rate constant for the CF3OH+FA reaction with that for CF3OH+OH, we find that the effective first-



order rate constant for CF3OH+FA is ~104 larger compared to that for the CF3OH+OH reaction over the 200-300 K temperature range. Thus, among the various gas phase atmospheric CF3OH decomposition pathways considered, catalysis by FA (i.e. CF3OH+FA) is the most effective.

IV. Conclusions: The impact of several atmospheric chemical species including HO2, HO2+H2O, FA, FA+H2O, HNO3, HNO3+H2O, and OH on the gas phase decomposition of CF3OH has been explored at the CCSD(T)//MP2 and CCSD(T)//M06-2X levels of theory. Further, the CVT/SCT method has been used to calculate and compare the effective first order rate constants for CF3OH decomposition catalyzed by HO2, HO2+H2O, FA, and FA+H2O as well as that for the OH+CF3OH reaction over the temperature range between 200-300K. Our calculations show that the FA catalyzed decomposition of CF3OH (i.e. FA+CF3OH) is significantly faster compared to those involving HO2+CF3OH, HO2+H2O+CF3OH, FA+H2O+CF3OH, and OH+CF3OH over the atmospheric relevant temperature range. Only for temperatures below ∼200 K is the

FA+H2O+CF3OH reaction found to be faster than the FA+CF3OH reaction. The present study also shows that formic acid is a significantly more effective atmospheric catalyst for CF3OH decomposition compared to HO2; this is primarily due to the larger atmospheric concentration of FA relative to HO2. The decomposition of CF3OH catalyzed by formic acid (FA) is also found to be more effective compared to nitric acid catalysis. Thus, among the various gas phase decomposition pathways examined, the unimolecular reaction of CF3OH catalyzed by formic acid is the fastest. Other organic acids such as acetic acid is expected to behave similarly, and


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thus decomposition of CF3OH catalyzed by organic acids is expected to be the dominant gas phase loss mechanism for CF3OH molecules in the troposphere.

Supporting Information: Tables S1-S8, containing optimized geometries of all the stationary points in terms of their Zmatrices, their vibrational frequencies and rotational constants, reaction barrier heights, their calculated total electronic energies including zero-point energy corrections and imaginary frequencies of various TS as discussed in the text at different levels of theories. Table S10-S14, containing equilibrium constants for dimers, reactant complex formation from their corresponding dimers + reagent, rate constants and tunneling factors for the FA+H2O, HO2+H2O, FA, HO2, and OH catalyzed decomposition of CF3OH. Figure S1-S2, showing second configuration of the reactants involving FA+H2O+CF3OH and HNO3+H2O+CF3OH calculated at CCSD(T)//M06-2X level.

Acknowledgement: A.S. thanks the National Science Foundation for support of this work under the Grant CHE-1566272. AS also thanks the W. M. Keck Foundation, through computing resources at the W. M. Keck Laboratory for Integrated Biology, for allowing use of their computers.



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N2O5 from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). Atmos. Chem. Phys. 2008, 8, 3529−3562. (49) McQuarrie, D. A. Statistical Mechanics; University Science Books: Sausalito, CA, 2000. (50) Truhlar, D. G.; Garrett, B. C. Variational Transition-State Theory. Acc. Chem. Res. 1980, 13, 440−448. (51) Garrett, B. C.; Truhlar, D. G. Criterion of Minimum State Density in the Transition State Theory of Bimolecular Reactions. J. Chem. Phys. 1979, 70, 1593-1598. (52) Liu, Y. P.; Lynch, G. C.; Truong, T. N.; Lu, D. H.; Truhlar, D. G.; Garrett, B. C. Molecular Modeling of the Kinetic Isotope Effect for the [1,5]-Sigmatropic Rearrangement of cis-1,3Pentadiene. J. Am. Chem. Soc. 1993, 115, 2408-2415. (53) Zheng, J.; Zhang, S.; Lynch, B. J.; Corchado, J. C.; Chuang, Y.- Y.; Fast, P. L.; Hu, W.-P.; Liu, Y.-P.; Lynch, G. C.; Nguyen, K. A. et al. POLYRATE, Version 2016; University of Minnesota: Minneapolis, MN, 2016. (54) Iuga, C.; Alvarez-Idaboy, J. R. On the Possible Catalytic Role of a Single Water Molecule in the Acetone + OH Gas Phase Reaction: A Theoretical Pseudo-Second-Order Kinetics Study. Theor. Chem. Acc. 2011, 129, 209–217. (55) Iuga, C.; Alvarez-Idaboy, J. R.; Reyes, L.; Vivier-Bunge, A. Can a Single Water Molecule Really Catalyze the Acetaldehyde + OH Reaction in Tropospheric Conditions? J. Phys. Chem. Lett. 2010, 1, 3112-3115. (56) Ali, M. A.; Balaganesh, M.; Lin, K. C. Catalytic Effect of a Single Water Molecule on the OH+ CH2NH Reaction. Phys. Chem. Chem. Phys. 2018, 20, 4297-4307.



Figure Captions Figure 1. Potential energy profile for the uncatalyzed gas-phase decomposition of CF3OH calculated at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level. The symbols correspond to: TS (transition state) and PC (product complex) respectively. Figure 2. Potential energy profile for the HO2 catalyzed decomposition of CF3OH at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The symbols correspond to: RC1 (reactant complex), TS1 (transition state), PC1 (product complex), and D1 (HF••H2O dimer). Figure 3. Optimized structures of the various dimer and trimer complexes computed at MP2/6311++G(3df,3pd) level of theory. The symbols correspond to: D1 (HF••H2O dimer) D2 (CF3OH••H2O dimer), D3 (HO2••H2O dimer), D4 (CF3OH••HO2 dimer), D5 (HF••FA dimer), D6 (FA••H2O dimer), D7 (CF3OH••FA dimer), D8 (HF••HNO3 dimer ) D9 (CF3OH••HNO3 dimer), D10 (HNO3••H2O dimer), T1 (HF••HO2••H2O trimer), T2 (HF••FA••H2O trimer), and T3 (HF••HNO3••H2O trimer). Figure 4. Potential energy profile for the HO2+H2O catalyzed decomposition of CF3OH calculated at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level. The symbols correspond to: D2 (CF3OH••H2O dimer), D3 (HO2••H2O


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dimer), D4 (CF3OH••HO2 dimer), RC2 (reactant complex), TS2 (transition state), and PC2 (product complex). Figure 5. Potential energy profile for the formic acid catalyzed decomposition of CF3OH calculated at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level. The symbols correspond to: RC3 (reactant complex), TS3 (transition state), and PC3 (product complex). Figure 6. Potential energy profile for the FA+H2O catalyzed decomposition of CF3OH calculated at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level. The symbols correspond to: D2 (CF3OH••H2O dimer), D6 (FA••H2O dimer), D7 (CF3OH••FA dimer), RC4 (reactant complex), TS4 (transition state), and PC4 (product complex). Figure 7. Potential energy profile for the nitric acid catalyzed decomposition of CF3OH calculated at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level. The symbols correspond to: RC5 (reactant complex), TS5 (transition state), and PC5 (product complex). Figure 8. Potential energy profile for the HNO3+H2O catalyzed decomposition of CF3OH calculated at the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory. The energies shown in parentheses are computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level. The symbols correspond to: D2 (CF3OH••H2O dimer), D9 27 ACS Paragon Plus Environment


(CF3OH••HNO3 dimer), D10 (HNO3••H2O dimer), RC6 (reactant complex), TS6 (transition state), and PC6 (product complex). Figure 9. Comparison of the total effective first-order rate constants for the FA+H2O catalyzed decomposition of CF3OH with that for the CF3OH+HO2+H2O, CF3OH+HO2, CF3OH+FA, and CF3OH+OH reactions over the temperatures between 200 and 300 K.


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Fig. 1



Fig. 2


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Fig. 3



Fig. 4


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Fig. 5



Fig. 6


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Fig. 7



Fig. 8


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Fig. 9


The Journal of Physical Chemistry 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

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Table 1. Total effective first order rate constants (𝑘𝑘 ′ in s-1) for the gas phase decomposition of CF3OH using various catalysts over the temperatures between 200 and 300 K. T (K) 200 210 220 230 240 250 260 270 280 290 300

′ (FA + H2 O + CF3 OH) 𝑘𝑘𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡

8.55 х 10-10 6.32 х 10-10 4.84 х 10-10 3.80 х 10-10 3.08 х 10-10 2.55 х 10-10 2.15 х 10-10 1.85 х 10-10 1.61 х 10-10 1.42 х 10-10 1.27 х 10-10

′ (HO2 + H2 O + CF3 OH) 𝑘𝑘𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡

6.69 х 10-15 6.38 х 10-15 6.14 х 10-15 6.02 х 10-15 5.92 х 10-15 5.86 х 10-15 5.82 х 10-15 5.83 х 10-15 5.85 х 10-15 5.91 х 10-15 5.95 х 10-15

′ 𝑘𝑘HO (HO2 + CF3 OH) 2

2.83 х 10-16 7.05 х 10-16 1.67 х 10-15 3.75 х 10-15 8.07 х 10-15 1.66 х 10-14 3.26 х 10-14 6.15 х 10-14 1.12 х 10-13 1.97 х 10-13 3.36 х 10-13


′ 𝑘𝑘FA (FA + CF3 OH)

3.59 х 10-10 6.19 х 10-10 1.04 х 10-9 1.72 х 10-9 2.75 х 10-9 4.28 х 10-9 6.54 х 10-9 9.75 х 10-9 1.42 х 10-8 2.04 х 10-8 2.87 х 10-8

′ 𝑘𝑘𝑂𝑂𝑂𝑂 (OH + CF3 OH)

5.20 х 10-14 7.85 х 10-14 1.16 х 10-13 1.70 х 10-13 2.47 х 10-13 3.53 х 10-13 4.98 х 10-13 6.91 х 10-13 9.53 х 10-13 1.30 х 10-12 1.75 х 10-12

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TOC Graphic


Atmospheric Decomposition of Trifluoromethanol Catalyzed by Formic

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