Atmospheric Oxidation Mechanism and Kinetics of Hydrofluoroethers

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Atmospheric Oxidation Mechanism and Kinetics of Hydrofluoroethers, CH3OCF3, CH3OCHF2 and CHF2OCH2CF3 by OH radical - A Theoretical Study Subramani Ponnusamy, Sandhiya Lakshmanan, and Kittusamy Senthilkumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01890 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry

Atmospheric Oxidation Mechanism and Kinetics of Hydrofluoroethers, CH3OCF3, CH3OCHF2 and CHF2OCH2CF3 by OH radical - A Theoretical Study S. Ponnusamya, L. Sandhiyab and K. Senthilkumar*, a a b

Department of Physics, Bharathiar University, Coimbatore, India-641 046.

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, USA *Corresponding author: Fax No: +91-422-2422387, E-Mail: [email protected]

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Abstract In the present work the reaction mechanism of two segregated hydrofluoroethers (HFEs), CH3OCF3 (HFE-143a) and CH3OCHF2 (HFE-152a) and a non-segregated HFEs, CHF2OCH2CF3 (HFE-245fa2) with OH radical is studied using electronic structure calculations. The initial reaction between HFE and OH radical is studied by considering two (three for CHF2OCH2CF3) pathways, H-atom abstraction and C-O bond breaking, OH addition reaction and C-C bond breaking, OH addition reaction, which leads to the formation of alkyl radical intermediate. The dominant atmospheric fate of initially formed alkyl radical intermediate is its reaction with O2. The peroxy radicals thus formed exit through the reaction with HO2 radical and NO radical resulting in the formation of products, carbonyl fluoride

(COF2),

trifluoromethylformate,

trifluoro(hydroperoxymethoxy)methane,

difluoro(hydroperoxy methoxy)methane, difluoromethylformate, 2-(difluoromethoxy)-1,1,1trifluoro-2-hydroperoxy ethane and difluoromethyl ester. The rate constant is calculated for the initial H-atom abstraction reaction using canonical variational transition state theory with small curvature tunnelling corrections over the temperature range of 272-350 K. The atmospheric lifetime and global warming potential of HFEs are obtained from the calculated reaction potential energy surface and rate constant. The results are discussed with respect to the atmospheric implications of CH3OCF3 (HFE-143a), CH3OCHF2 (HFE-152a) and CHF2OCH2CF3 (HFE-245fa2).


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Introduction: Hydrofluoroethers

(HFEs)

are

being

considered

as

an

alternative

to

chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) for wide variety of commercial applications, such as cleaning of electronic equipments, heat transfer fluid in refrigerators, lubricant deposition and foam blowing agents. HFEs do not contain chlorine and bromine atoms that are responsible for ozone depletion.1-10 Fluorinated compounds may exhibit strong absorption in the earths infrared atmospheric transparency window, between 8 and 13µm, thus acting as greenhouse gases.11 To find the global warming potential (GWP) and lifetime of a compound in the atmosphere, the information about infrared absorption spectrum and rate of the reaction are required. The atmospheric lifetime ofhydrogen-containing trace gases are primarily controlled by their reactions with hydroxyl radical in the troposphere. Similar to most of the volatile organic compounds, HFEs contain C-H bonds and are removed from the troposphere through their reaction with OH radical. Also, the presence of -O- linkage in HFEs enhance their reactivity with the atmospheric reactive species, resulting in shorter atmospheric lifetime which limits their accumulation in the atmosphere and decreases their potential impact as green house gas. The reaction between HFEs and OH radical will proceed by H-atom abstraction and C-O bond breaking, OH radical addition reactions, leading to the formation of alkyl radicals.12 Further, the alkyl radicals will further react with other atmospheric species, resulting in the formation of end products of HFEs.1 A number of experimental and theoretical studies on the reaction of HFEs with OH radical are available in the literature.12-19 Chen et al.14 experimentally studied the mechanism and kinetics of the reaction of HFEs, CF3OCH3 and CF3OC(O)H with OH radical and reported the rate constant for H-atom abstraction reaction at 298 K as 1.19x10-14 and 1.64x10-14 cm3molecule-1s-1. Orkin et al.18 experimentally studied the reaction of HFEs, 3 ACS Paragon Plus Environment


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CH3OCF3(HFE-143a), CH3OCHF2 (HFE-152a) and CHF2OCH2CF3 (HFE-245fa2) with OH radical and reported the rate constant for above HFEs as 1.3x10-14, 3.5x10-14 and 1.14x10-14 cm3molecule-1s-1, respectively. Wu et al.12 theoretically studied the H-atom abstraction and C-O bond breaking reactions of CF3OCHF2 with OH radical and Cl atom and reported

the

rate

constant

for

H-atom

abstraction

reaction

of

CF3OCHF2 as

3.3x10-14 cm3molecule-1s-1. The rate constant calculated for the reaction of HFEs with OH radical clearly show their relatively short atmospheric lifetime and thus HFEs appear to have less impact on global warming.20 In all these studies, the mechanism for the secondary reactions from the radical initiated reactions are not explored. Predicting the end products formed from these reactions is crucial in order to determine the fate of HFEs in the atmosphere. Further, the above mentioned HFEs can be categorized as segregated and nonsegregated HFEs. The segregated HFEs are CH3OCF3 and CH3OCHF2, where the perfluorinated part of the molecule is separated from the hydrogenated part by an ether linkage, i.e. RHORF. Whereas, in the non-segregated HFEs, there is a hydrogen/fluorine atom on both sides of the ether linkage. Earlier studies21-23 show that fluorinated esters are the major oxidation products from segregated HFEs and depending on the dominant H-atom abstraction reactions, the oxidation products from non-segregated HFEs differs. Hence, it will be of interest to compare the reactivity and oxidation products formed from these HFEs. With the above exposures, in the present study the reaction mechanism and kinetics of the HFEs, CH3OCF3(HFE-143a), CH3OCHF2 (HFE-152a) and CHF2OCH2CF3 (HFE-245fa2) with OH radicalare studied by using high level electronic structure calculations and canonical variational transition state theory. To find the end products of HFEs in the atmosphere, the subsequent reactions of initially formed alkyl radicals with other atmospheric species, such as O2, HO2 and NO radicals are also studied. The rate constant calculated for the favorable initial reactions are used to determine the lifetime and the global warming potential of HFEs.


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Computational Details: The geometry of the reactants, intermediates, transition states and products was optimized using M06-2X24 functional with 6-311++G(d,p) basis set. The meta-hybrid exchange and correlation functional, M06-2X is the most widely used method for studying the chemical reactions involving radicals.25-27 Recent studies show that DFT calculations with M06-2X functional perform well for thermochemical and reaction mechanism studies.28-31 All minima were confirmed with positive frequencies and the transition state structure had single imaginary frequency. Each transition state was verified and connected to the designated reactants and products by performing intrinsic reaction coordinate (IRC) calculations. In IRC calculations, the minimum energy path (MEP) was constructed from respective saddle point geometry and the calculations were performed in mass-weighted Cartesian coordinates. Further, we have performed the single point energy calculations at CCSD(T)/6-311++G(d,p), CCSD(T)/aug-cc-pVTZ and QCISD(T)/6-311++G(d,p) level of theory on the geometries optimized at M06-2X/6-311+G(d,p) level of theory. The enthalpy and Gibbs free energy were calculated by including the thermodynamic corrections to the energy at 298.15 K and at 1 atmospheric pressure. All the electronic structure calculations were performed using Gaussian 09 program package.32 The computed zero point vibrational energy (ZPVE) corrected potential energy surface and associated thermochemical parameters were directly utilized to calculate the rate constant as a function of temperature. The rate constants for the favaorable H-atom abstraction reactions reactions are calculated by using canonical variational transition state theory (CVT), in which the temperature dependent rate constant is expressed as,

k CVT (T ) = min s k c

GT

(T , s )


(1)


where, the generalized transition state (GTS) rate constant kc

GT

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(T , s ) at the dividing surface s

is,

kc

GT

σk BT Qc GT (T , s) −βV (T , s) = e βh φc R (T )

( 2)

MEP ( s )

where, σ is the symmetry factor (The overall symmetry factor is given by the ratio of rotational symmetry numbers corresponding to reactant and transition state. For the reaction of OH radical with CH3OCF3 and CH3OCHF2 the overall symmetry factor is 3 and for CHF2OCH2CF3 the symmetry factor is 2), kB is the Boltzmann constant and h is the Planck’s constant, φc (T ) is the reactants classical partition function per unit volume, R

V MEP ( s )

is the

GT

classical potential energy at the point s on the minimum energy path, Qc (T , s) is the classical partition function for the generalized transition state at the dividing surface, s. The quantity ( β h )-1 is called the universal transition state frequency factor. To account for the dynamical quantum effects of reaction coordinate tunnelling, a multiplicative transmission coefficient k (T ) is included in equation 1 as,

kc

CVT

(T ) = k (T )k CVT (T )

(3)

The transmission coefficient k (T ) corresponding to tunnelling is evaluated through small curvature approximation to the vibrational adiabatic potential energy surface. The rate constant calculations were performed by using Gaussrate 2010 program33 which is an interface program between Gaussian 09 and Polyrate 2010A programs.34 Results and Discussion: As shown in Schemes 1-3, the initial reaction between HFE and OH radical is studied by considering two (three for CHF2OCH2CF3) pathways, H-atom abstraction reaction and C-O bond breaking, OH addition reaction and C-C bond breaking, OH addition reaction. The structure and energetical parameters of the reactive species were calculated at 6 ACS Paragon Plus Environment

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M06-2X/6-311++G(d,p) level of theory. In addition, the single point energy calculations were

performed

at

QCISD(T)/6-311++G(d,p)

CCSD(T)/6-311++G(d,p), level

of

theories

with

CCSD(T)/aug-cc-pVTZ the

geometry

optimized

and at

M06-2X/6-311++G(d,p) level of theory. The calculated energitical parameters of the the reactive species are summarized in Tables S1-S3. As given in Tables S1-S3, the relative energy (∆E) calculated at different methods are comparable with each other. Hence, in the following sections, the structure and energetics calculated from M06-2X/6-311++G(d,p) level of theory are used to discuss and analyze the different reaction pathways studied. CH3OCF3 + •OH reaction: The Scheme for the initial and secondary reactions of CH3OCF3 + OH radical is shown in Scheme 1. The relative energy, enthalpy and Gibbs free energy of the corresponding reactive species are summarized in Table S1. The optimized structure of the transition state, reactant, reactant complex, intermediate complex, intermediate and products are shown in Figures 1 and S1.



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Scheme 1: The Scheme for the reaction of CH3OCF3 with OH radical and subsequent reactions

of

initially

formed

alkyl

radical.

The

energy

barrier

calculated

at

M06-2X/6-311++G(d,p) level of theory is given in kcal/mol. As shown in Scheme 1, the H-atom abstraction from the methyl group leads to the formation of alkyl radical intermediate, •CH2OCF3 (I1) (I1) with H2O through the reactant complex, RC1, transition state, TS1 and intermediate complex, IC1. Due to the high electronegativity of the F and O-atoms, the reactant and intermediate complexes are stabilized by the attractive interactions between O and H atoms, and between F and H atoms. The energy barrier for this H-atom abstraction reaction is 8.2 kcal/mol. As shown in Figure 1, in TS1, the C2-H3 bond is elongated by 0.1 Å with respect to the reactant complex, RC1 and the distance between cleaved H-atom of CH3OCF3 and O atom of OH radical is 1.358 Å which is 0.4 Å longer than the O-H bond length of H2O in IC1.The reaction corresponding to the formation of I1+H2O is an exothermic and exoergic with an enthalpy (∆H) and Gibbs free energy (∆G) of -16 and -17.8 kcal/mol.The second pathway is C-O bond breaking, OH addition reaction which leads to the formation of alkyl radical intermediates, CH3O• (I2) and CF3O• (I3) along with CF3OH and CH3OH through the transition states, TS2 and TS3. In TS2


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and TS3, the bond length of cleaved (C-O) bond is increased by 0.5 Å with respect to respective reactant complex, RC1. The energy barrier for the transition states,TS2 and TS3 is 26.1 and 28.3 kcal/mol, respectively. The reaction enthalpy and Gibbs free energy for the formation of I2+CF3OH is -8.1 and -10.5 kcal/mol and for I3+CH3OH, ∆H and ∆G are -5.1 and -8.3 kcal/mol, respectively. By comparing the energy barrier for the H-atom abstraction and C-O bond breaking, OH-addition reactions, it is observed that the H-atom abstraction reaction is more favourable than the C-O bond breaking, OH addition reaction. Relatively high energy barrier associated with the transion states, TS2 and TS3 is due to the large bond dissociation energy (~105 kcal/mol) involved in C-O bond breaking step.35 The alkyl radical intermediate, •CH2OCF3 (I1) formed from the H-atom abstraction reaction further undergoes unimolecular dissociation and subsequent reactions with other atmospheric species. The alkyl radical intermediate, I1 dissociates into carbonyl fluoride (COF2) and •CH2F (I4) through the transition state, TS4 with an energy barrier of 13.6 kcal/mol. In TS4, the C2-O1 bond is elongated by 0.57 Å with respect to the I1, and F1 atom of CF3 group is shifted to the C2 position.The relative enthalpy and Gibbs free energy of I4 + COF2 complex is -4.2 and -5.5 kcal/mol. Note that the COF2 is a toxic gas and it is removed/transferred through hydrolyse reaction (CO2 + HF) and secondary aerosol formationin the atmosphere.1 As shown in Scheme 1, the reaction of alkyl radical intermediate, I1 with O2 leads to the formation of peroxy radical intermediate, I5 in a barrierless reaction. The reaction associated with the formation of I5 is exothermic with an enthalpy of -42.3 kcal/mol. In the next propagation step, the peroxy radical, I5 reacts with HO2 radical to form the product, trifluoromethylformate (P1) along with O2 through the reactant complex (RC2), transition state (TS5) and product complex (PC1). The energy barrier for this reaction is 11.4 kcal/mol. In TS5, the distance between H3 and O3 atoms is 1.419 Å which is shorter by 0.11 Å than



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that of the RC2. The reaction corresponding to the formation of a product, P1 is exothermic and exoergic with an enthalpy and Gibbs free energy of -27.5 and -29.8 kcal/mol. Under the atmospheric condition,the peroxy radical intermediate, I5 reacts with NO radical, in which the addition of NO radical at the terminal O-atom of peroxy radical intermediate, I5 leads to the formation of an intermediate, I6 in a barrierless reaction with an enthalpy of -39.4 kcal/mol. Further, I6 dissociates into I7 and NO2 through the transition state, TS6 with an energy barrier of 8.3 kcal/mol. The exit pathway for I7 is its reaction with O2, which leads to the formation of a product, trifluoro(hydroperoxymethoxy) methane (P2) and HO2 radical. The energy barrier for this reaction is 11.7 kcal/mol. As given in Table S1, the reactions associated with the formation of intermediates, I5 and I6 and the products, P1and P2 are exothermic.

TS1

TS2

TS4

TS3


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TS6

TS5

TS7 Figure 1: The optimized structure of the transition states for the initial and subsequent reactions of CH3OCF3. CH3OCHF2 + •OH reaction: The Scheme for the reaction of CH3OCHF2 + OH radical and its subsequent reactions is shown in Scheme 2. The relative energy, enthalpy and Gibbs free energy of the corresponding reactive species are summarized in Table S2. The optimized structure of the transition state, reactant, reactant complex, intermediate complex, intermediate and products is shown in Figures 2 and S2.



Scheme 2: The Scheme for the reaction of CH3OCHF2 with OH radical and subsequent reactions of initially formed alkyl radical. The energy barrier calculated at M06-2X/6311++G(d,p) level of theory is given in kcal/mol. As shown in scheme 2, the H-atom abstraction from CH3 and CHF2 groups of CH3OCHF2 leads to the formation of alkyl radical intermediates, I8 and I9 with H2O through the corresponding reactant complexes, RC4 and RC5, transition states, TS8 and TS9 and intermediate complexes, IC5 and IC6. The energy barrier for TS8 and TS9 is 7.6 and 8.2 kcal/mol, and are comparable with the energy barrier of 8 kcal/mol reported for the H-atom abstraction reaction of CF3OCHF2 by OH radical.12 The presence of F atom in CHF2 group slightly increase the energy barrier corresponding to H-atom abstraction from CHF2 group. In TS8 and TS9, the C-H bond is elongated by 0.1 Å with respect to the reactant complex (RC4 and RC5) and the distance between cleaved H-atom and O atom of OH radical is 1.375 and 1.389 Å which is 0.3 and 0.4 Å longer than the O-H bond length of H2O in IC5 and IC6.The enthalpy (∆H) and Gibbs free energy (∆G) for the formation of I8 + H2O is -19.3 and -21.2 kcal/mol and for the formation of I9 + H2O ∆H and ∆G are -17.8 and


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-19.3 kcal/mol, respectively. The next pathway for the reaction of CH3OCHF2 with OH radical is C-O bond breaking, OH addition reaction. In this two step process, CHF2O• and CH3O• are formed along with the CH3OH and CHF2OH through the transition states, TS10 and TS11, with an energy barrier of 29.8 and 26.1 kcal/mol, respectively. The relative enthalpy corresponding to the formation of I10+CH3OH and I2+CHF2OH is -5.1 and -8.7 kcal/mol, and are comparable with the enthalpy of -5.4 and -8.9 kcal/mol reported for the C-O bond breaking reaction of CF3OCHF2 with OH radical.12 As shown in Scheme 2, the alkyl radical intermediate, I8 dissociates into COF2 and CH3 radical (I11) through the transition state, TS12, and intermediate complex, IC9. The energy barrier associated with the transition state,TS12 is 17.2 kcal/mol and the reaction is slightly exothermic and exoergic with ∆H and ∆G of -6.6 and -8.4 kcal/mol, respectively. As shown in Scheme 2, the alkyl radical intermediate, I8 can react with O2 and the peroxy radical intermediate, I12 is formed in a barrierless reaction with an enthalpy of -46.5 kcal/mol. Further the peroxy radical intermediate, I12 reacts with HO2 and NO radicals. The reaction of I12 with HO2 radical leads to the formation of stable product, difluoro(hydroperoxy)methane, P3 through the reactant complex, RC6, transition state,TS13 and product complex, PC3. The energy barrier for TS13 is 8.7 kcal/mol. Here, H4-atom of HO2 radical binds with the oxygen atom (O3) of the I12 to form the product, P3. In TS13, the bond length between O3 and H4 atoms (See Figure 2 for labelling of atoms) is 1.583 Å which is 0.4 Å shorter than that of

the RC6. As shown in Scheme 2, the peroxy radical

intermediate, I12 is oxidized by NO radical in which addition of NO radical at terminal O-atom of peroxy radical intermediate, I12 leads to the formation of intermediate, I13 in a barrierless reaction with an enthalpy of -42.5 kcal/mol. In the next step, I13 dissociates into an intermediate, I14 and NO2 radical through the transition state, TS14 with an energy barrier of 6.2 kcal/mol. The exit pathway for I14 is its reaction with O2 which leads to the formation



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of a product, difluoromethylformate, (P4) and HO2 radical.The energy barrier for the above reaction is 7.8 kcal/mol.The product, P4 is formed by abstracting the H3-atom of I14 by O2 through the reactant complex, RC7, transition state, TS15 and product complex, PC4. In TS14, the distance between C2 and H3 atoms is 1.281Å which is 0.18 Å longer than that of RC7 and the distance between H3 and O2 atoms is 1.429Å which is 0.44 Å longer than that of HO2 in PC4. A pulse radiolysis experimental study on the reaction between HFE radicals and O2 showed that there is an increase in the yield of ester compound along with HO2 at high concentrations of O221 and the calculated energy barrier reveals the importance of product channel P4 + HO2 (i.e. ester like compound P4) in accordance with the experimental observation. The calculated relative enthalpy (∆H) and Gibbs free energy (∆G) shows that the intermediates and products formed from the initial and subsequent reactions are exothermic and exoergic.

TS9

TS8

TS11

TS10


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TS12

TS13

TS15

TS14

Figure 2: The optimized structure of the transition states for the initial and subsequent reactions of CH3OCHF2. CHF2OCH2CF3 + •OH reaction: The Scheme for the reaction of CHF2OCH2CF3 with OH radical is shown in Scheme 3. The relative energy, enthalpy and Gibbs free energy of the corresponding reactive species are summarized in Table S3.The optimized structure of the transition state, reactant, reactant complex, intermediate complex, intermediates and products are shown in Figures 3 and S3.



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Scheme 3: The Scheme for the reaction of CHF2OCH2CF3 with OH radical and subsequent reactions

of

initially

formed

alkyl

radical.

The

energy

barrier

calculated

at

M06-2X/6-311++G(d,p) level of theory is given in kcal/mol. As shown in Scheme 3, the H-atom abstraction from C1 and C2 atom of CHF2OCH2CF3 leads to the formation of alkyl radical intermediates, I15 and I16 along with H2O molecule through the corresponding reactant complex, RC8 and RC9, transition state, TS16 and TS17 and intermediate complex, IC10 and IC11. The calculated energy barrier for TS16 and TS17 is 9.7 and 9 kcal/mol, respectively. The calculated energy barrier for TS17 is comparable with the energy barrier of 8.4 kcal/mol reported for H-atom abstraction from CH2 group of CH3CH2OCF3 by OH radical.36 In TS16 and TS17, the C-H bond is elongated by 0.11 Å with respect to the reactant complexes, RC8 and RC9, and the distance between cleaved H-atom of CHF2OCH2CF3 and O atom of OH radical is 1.628 and 1.345 Å which is 0.32 and 0.6 Å longer than the O-H bond length of H2O in IC10 and IC11.The enthalpy (∆H) and Gibbs free 16 ACS Paragon Plus Environment

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energy (∆G) for the formation of I15+H2O is -9.3 and -11.5 kcal/mol and for I16+H2O ∆H and ∆G is -12.4 and -14.8 kcal/mol, respectively.The next pathway for the reaction of CHF2OCH2CF3 with OH radical is, C-O bond breaking, OH addition reaction which leads to the formation of CHF2O• (I10) + CH2OHCF3 and CH2CF3O• (I17)+CHF2OH through the transition states, TS18 and TS19 and intermediate complexes, IC12 and IC13. The energy barrier associated with TS18 and TS19 is 32.1 and 30.7 kcal/mol. As shown in Scheme 3, the C-C bond breaking, OH addition reaction leads to the formation of an intermediate CHF2OC•H2 (I18) and CF3OH through the reactant complex, RC9, transition state, TS20 and intermediate complex, IC14. The energy barrier corresponding to the transition state,TS20 is 27 kcal/mol. By comparing the energy barrier for the H-atom abstraction, C-O/C-C bond breaking, OH-addition reactions, it is observed that H-atom abstraction reaction is more favourable. The energy barrier associated with the transition state, TS21 coresponding to the formation of COF2 and •CH2CF3 (I19) through the dissociation of I16 is 13.6 kcal/mol. As shown in Scheme 3, the reaction of I16 with O2 leads to the formation of peroxy radical intermediate, I20 in a barrierless reaction with an enthalpy and Gibbs free energy of -36.3 and -37.9 kcal/mol, respectively. In the next propagation step, the peroxy radical, I20 reacts with HO2 radical, forming the product, 2-(difluoromethoxy)-1,1,1-trifluoro-2-hydroperoxy ethane (P5) and O2 through the reactant complex, RC10, transition state, TS22 and product complex, PC5. The energy barrier associated with the transition state, TS22 is 17.6 kcal/mol. In TS22, the distance between O3 and H4 atoms is 1.538 Å, which is 1 Å shorter than that of RC10. The alternative propagation step for the peroxy radical, I20 is its reaction with NO radical. As shown in Scheme 3, the intermediate, I21 is formed in a barrierless reaction. The reaction corresponding to the formation of I21 is exothermic with an enthalpy of -30.1 kcal/mol. Then the intermediate, I21 dissociates into another radical intermediate, I22 there by eliminating



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NO2 radical through the transition state, TS23. The energy barrier associated with the transition state, TS23 is 14.3 kcal/mol. This reaction is exothermic with an enthalpy of -24.5 kcal/mol. Further, the intermediate, I22 dissociates via C-C bond dissociation into a product,difluoromethyl ester (P6) and CF3 radical through the transition state, TS24 with an energy barrier of 10.8 kcal/mol and the reaction is exothermic and exoergic with ∆H and ∆G of -28.9 and -30.5 kcal/mol. This reaction is an analogous to the unimolecular dissociation of C4F9OCH2CH2O•, where its atmospheric fate is determined via C-C bond scission reaction.37 The products, trifluoromethylformate, trifluoro(hydroperoxymethoxy) methane, difluoro(hydroperoxymethoxy)methane, difluoromethylformate, 2(difluoromethoxy)-1,1,1trifluoro-2-hydroperoxy ethane and difluoromethyl esterformed from the initial and subsequent reactions of studied HFEs with OH radical and other atmospheric reactive species are less toxic than the parent HFEs and the above products are easily degradable in the atmosphere through atmospheric reactions. That is, the esters are less reactive towards gas-phase atmospheric oxidants38 and dissolve in aerosols, where oxidation in the liquid phase dominates. As discussed in the introduction part, esters are the major oxidation products from the segregated HFEs (CH3OCF3 and CH3OCHF2) and alkyl fluoroformates are the major oxidation products from non-segregated HFEs (CHF2OCH2CF3).

TS16 TS17


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TS18

TS19

TS21

TS20

TS23

TS22



TS24 Figure 3: The optimized structure of the transition states associated with the initial and subsequent reactions of CHF2OCH2CF3. Kinetics: The rate constant calculated for the favourable initial reactions using canonical variational transition state theory (CVT) with small curvature tunnelling (SCT) corrections over the temperature range of 272 - 350 K and at 1 atmospheric pressure is summarized in Tables S4-S6. As observed from Tables S4-S6, the ratio between CVT and TST rate constant is ~ 1 over the studied temperature range of 272-350 K. That is, the variational effect is negligible on the rate constant of studied reactions, whereas, for the studied reactions the tunnelling effect is appreciable and is in the order of 101.


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Figure 4: The Arrhenius plot for the rate constant of initial H-atom abstraction reaction of HFEs. The Arrhenius plot for initial H-atom abstraction reactions is shown in Figure 4. The plot shows that the rate constant is increasing as the temperature increase. At 298 K, the calculated rate constant for the H-atom abstraction reaction of CH3OCF3 (I1+H2O) is 6.9x10-14 cm3molecule-1s-1 and the calculated rate constant for the H-atom abstraction reaction from CH3 (I8+H2O) and CHF2 (I8+H2O) groups of CH3OCHF2 is 8.3x10-14 and 7.6x10-14 cm3molecule-1s-1, respectively. For H-atom abstraction reaction from CHF2 (I15+H2O) and CH2 (I16+H2O) groups of CHF2OCH2CF3 the rate constant are 2.9x10-14 and 3.4x10-14 cm3molecule-1s-1, respectively. The calculated rate constants are comparable with the previous experimental rate constant values.12,14,18 As given in Scheme 2 and 3, Tables S2, S3, S5 and S6, the calculated energy values, rate constants and branching ratio show that for CH3OCHF2 and CHF2OCH2CF3 both the hydrogen atom abstraction reactions are equally contributing to the transformation of HFEs in



the atmosphere. Hence, we have calculated the total rate constant and is used to calculate the lifetime and Global warming potential. The atmospheric lifetime39 of HFEs is calculated by using the formula,

τ=

1 k OH [OH ]

where, k OH is the rate constant of HFE at temperature 298 K and the average concentration of OH radical in the atmosphere is 1.0x106 molecule. cm-3.40,41 The calculated lifetime of studied HFEs is around 4 to 5 years. The calculated lifetime of HFEs is comparable with experimental value of 5.6 years. Global-warming potential (GWP) is an important atmospheric parameter which provides a relative measure of heat trapped by the greenhouse gas in the atmosphere and it is calculated by using the equation, t

GWP =

−t

ai ∫ e τ dτ 0

AGWPCFC −11

where a i is the total instantaneous infrared radiative forcing (W m-2 ppbv-1) and t is the time horizon. The total instantaneous radiative forcing ( a i ) is calculated by using the equation, a i = ∑ A k F(υ k ) k

where, Ak is the absorption cross-section in cm2 molecule-1 and F(υk ) is the radiative forcing function per unit cross section per wavenumber (W m-2 (cm-1))-1 (cm2 molecule-1)-1).39,42 While calculateing the radiative forcing we have used the cros-section values of 25.3, 13.88 and 14.6 cm2 molecule-1 for CH3OCF3, CH3OCHF2 and CHF2OCH2CF3 provided by Orkin et al.18 for the corresponding normal modes, 1289, 1217 and 1058 cm-1. The GWP of HFE is calculated relative to the absolute global warming potential (AGWP) of CFC-11, which is determined from the radiative transfer model of the atmosphere. The AGWP of CFC11 is 6730, 4750 and 1620 relative to CO2 for 20, 100 and 500 years time horizon.43 The 22 ACS Paragon Plus Environment

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calculated lifetime and GWP of the studied HFEs at 20, 100 and 500 years time horizon are summarized in Table 1. Table 1: The calculated lifetime and global warming potential of CH3OCF3, CH3OCHF2 and CHF2OCH2CF3

Molecule CH3OCF3 CH3OCHF2 CHF2OCH2CF3 CFC-11

Lifetime (Years) 4.5

20 2130

GWP 100 500 660 180

Ref.

5.15

2550

739

225

(18)

4

876

243

62

This work

2.7

1025

291

89

(18)

5

2691

854

236

This work

5.6

3042

889

270

(18)

45

6730

4750

1620

(43)

This work

The calculated lifetime and GWP of studied HFEs are comparable with the previous experimental results.18 While comparing the lifetime and GWP of studied HFEs with that of CFC-11 it is observed that studied HFEs are the best alternative for CFC, particularly CH3OCHF2 (HFE-152a) is the best one with relatively less GWP. Conclusions The atmospheric degradation mechanism and kinetics of hydrofluoroethers, CH3OCF3(HFE-143a), CH3OCHF2 (HFE-152a) and CHF2OCH2CF3 (HFE-245fa2) by reaction with OH radical is studied using electronic structure calculations and canonical variational transition state theory. The initial reaction between HFE and OH radical is studied by considering two (three for CHF2OCH2CF3) pathways, H-atom abstraction and C-O bond breaking, OH addition reaction and C-C bond breaking, OH addition reaction, which leads to the formation of alkyl radical intermediate. The energy barrier for H-atom abstraction reaction is around 9 kcal/mol and for C-O (as well as C-C) bond breaking, OH addition



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reaction the energy barrier is around 30 kcal/mol. The dissociation and subsequent reactions are studied for the alkyl radical intermediate formed from the initial H-atom abstraction reaction which leads to the formation of products, carbonyl fluoride (COF2) and trifluoromethylformate, methoxy)methane,

trifluoro(hydroperoxymethoxy)methane, difluoromethylformate,

difluoro(hydroperoxy

2-(difluoromethoxy)-1,1,1-trifluoro-2-

hydroperoxy ethane and difluoromethyl ester. The calculated thermochemical parameters show that the reactions associated with the above products are exothermic and exoergic. At 298 K, the calculated rate constant for the H-atom abstraction reaction of CH3OCF3 (I1+H2O) is

6.9x10-14 cm3molecule-1s-1 and the calculated rate constant for the H-atom abstraction

reaction from CH3 (I8+H2O) and CHF2 (I8+H2O) groups of CH3OCHF2 is 8.3x10-14 and 7.6x10-14 cm3molecule-1s-1, respectively. For H-atom abstraction reaction from CHF2 (I15+H2O) and CH2 (I16+H2O) groups of CHF2OCH2CF3 the rate constant are 2.9x10-14 and 3.4x10-14 cm3molecule-1s-1, respectively. Based on the rate constant, the lifetime and GWP of HFEs are calculated, and the results are comparable with the previous experimental values. The fluorinated species formed in the present reactions are same as those obtained during degradation of hydrofluorocarbons,44 and these products will not affect the stratospheric ozone. The results obtained from the present investigation confirmed that the studied HFEs, CH3OCF3, CH3OCHF2 and CHF2OCH2CF3 are the better alternatives to chloroflurocarbons. Hence, the use of HFEs in industrial applications as alternatives to CFCs may be expected to have less impact on the atmosphere and climate.


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Acknowledgements: Authors thank the University Grants Commission (UGC), Govt. of India, for granting the Major Research Project. The authors are thankful to UGC and Department of Science and Technology (DST), India for funding to establish the high performance computing facility under the SAP and PURSE programs. Supporting Information: The calculatedrelative energy (∆E), enthalpy (∆H) and Gibbs free energy (∆G) of the reactive species involved in the initial and subsequent reactions of HFEs with OH radical are summarized in Tables S1-S3.The calculated TST, TST (SCT), CVT and CVT(SCT) rate constants of favourable H-atom abstraction reaction is given in Tables S4-S6. The optimized structure of the reactants, reactive complex, intermediate complex, intermediate, product complex and products formed from the initial and subsequent reactions of HFEs with OH radical are shown in Figures S1-S3. References: (1)

(2)

(3)

(4) (5) (6) (7) (8) (9)

Bai, F. Y.; Ma, Y.; Lv, S.; Pan, X.-M.; Jia, X.-J. Theoretical insight into OHand Cl- initiated oxidation of CF3OCF2CF2H and fate of CF3OC(X)(CF3)2 and CF3OCF2CF2X radicals (X=O, O2). Scientific Reports. 2017, 7, 40264. Mishra, B. K.; Lily, M.; Deka, R. C. A theoretical insight into atmospheric chemistry of HFE-7100 and perfluoro-butyl formate: reactions with OH radicals and Cl atoms and the fate of alkoxy radicals. New J. Chem. 2017, 40, 6148-6155. Ravishankara, A. R.; Turnipseed, A. A.; Jensen, N. R.; Barone, S.; Mills, M.; Howark, C. J.; Solomon, S. Do hydrofluorocarbons destroy stratospheric ozone? Science. 1994, 263, 71-75. Hanmmitt, J. K. Future Emission Scenarios for chemicals that May deplete stratospheric Ozone. Nature. 1987, 330, 711-716. Beara, P. P.; Francisco, J. S.; Lee, T. J. Identifying the molecular origin of Glopal warming. J. Phys. Chem. A. 2009, 113, 12694-12699. A. McCulloch. CFC and Halon replacements in the environment. J. Fluorine Chem. . 1999, 100, 163-173. Powell, R. L. CFC phase-out: have we met the challenge? J. Fluorine Chem. 2002, 114, 237-250. Sekiya, A.; Misaki, S. The potential of hydrofluoroethers to replace CFCs, HCFCs and PFCs. J. Fluorine Chem. 2000, 101, 215. Tsai, W. T. Environmental risk assessment of hydrofluoroethers (HFEs). J. Hazard. Mater. 2005, 119, 69-78.



(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

Imasu, R.; Suga, A.; Matsuno, T. Radiative Effects and Halocarbon Global Warming Potentials pf Reflacement compounds for Chlorofluorocarbons J. Meteorol. Soc. Jpn. 1995, 73, 1123-1136. Bera, P. P.; Francisco, J. S.; Lee, T. J. Design and Strategies to minimize the radiative efficiency of global warming molecules. PNAS. 2010, 107, 90499054. Wu, J.-Y.; Liu, J.-Y.; Li, Z.-S.; Sun, C.-C. Theoretical Study of the Reactions of CF3OCHF2 with the Hydroxyl Radical and the Chlorine Atom. ChemPhysChem. 2004, 5, 1336-1344. Andersen, M. P. S.; Nielsen, O. J.; Wallington, T. J.; Hurley, M. D.; DeMore, W. B. Atmospheric Chemistry of CF3OCF2CF2H and CF3OC(CF3)2H: Reaction with Cl Atoms and OH Radicals, Degradation Mechanism, Global Warming Potentials, and Empirical Relationship between k(OH) and k(Cl) for Organic Compounds. J. Phys. Chem. A. 2005, 109, 3926-3934. Chen, L.; Kutsuna, S.; Nohara, K.; Takeuchi, K.; Ibusuki, T. Kinetics and Mechanisms for the Reactions of CF3OCH3 and CF3OC(O)H with OH Radicals Using an Environmental Reaction Chamber. J. Phys. Chem. A. 2001, 105, 10854-10859. Chen, L.; Kutsuna, S.; Tokuhashi, K.; Sekiya, A. Kinetics and Mechanisms of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 Reactions with OH Radicals. J. Phys. Chem. A. 2006, 110, 12845-12851. Tokuhashi, K.; Takahashi, A.; Kaise, M.; Kondo, S.; Sekiya, A.; Yamashita, S.; Ito, H. Rate Constants for the Reactions of OH Radicals with CH3OCF2CHF2,CHF2OCH2CF2CHF2, CHF2OCH2CF2CF3, and CF3CH22OCF2CHF2 over the TemperatureRange 250-430 K. J. Phys. Chem. A. 2000, 104, 1165-1170. Ninomiya, Y.; Kawasaki, M.; Guschin, A.; Molina, L. T.; Molina, M. J.; Wallington, T. J. Atmospheric Chemistry of n-C3F7OCH3: Reaction with OH Radicals and Cl Atoms and Atmospheric Fate of n-C3F7OCH2O(•) Radicals. Environ. Sci. Technol. 2000, 34, 2973-2978. Orkin, V. L.; Khamaganov, V. G.; Guschin, A. G. Photochemical Properties of Hydrofluoroethers CH3OCHF2, CH3OCF3 and CHF2OCH2CF3: Reactivity toward OH, IR Absorption Cross Sections, Atmospheric Lifetimes, and Global Warming Potentials. J. Phys. Chem. A. 2014, 118, 10770-10777. Fontana, G.; causa, M.; Gianotti, V.; Marchionni, G. Computational studies of the reaction of the Hydroxyl radical with Hydrofluorocarbons (HFCs) and Hydrofluoroethers (HFEs). J. Fluorine Chem. 2001, 109, 113-121. Blowers, P.; Moline, D. M.; Tetrault, K. F.; Wheeler, R. R.; Tuchawena, S. L. Global warming potentials of hydrofluoroethers. Environ. Sci. Technol. 2008, 42, 1301-1307. Christensen, L. K.; Sehested, J.; Nielsen, O. J.; Bilde, M.; Wallington, T. J.; Guschin, A.; Molina, L. T.; Molina, M. J. Atmospheric Chemistry of HFE7200 (C4F9OC2H5): Reaction with OH Radicals and Fate of C4F9OCH2CH2O(•) and C4F9OCHO(•)CH3 Radicals. J. Phys. Chem. A. 1998, 102, 4839-4845. Wallington, T. J.; Hurley, M. D.; Fedotov, V.; Morrell, C.; Hancock, G. Atmospheric Chemistry of CF3CH2OCHF2 and CF3CHClOCHF2: Kinetics and Mechanisms of Reaction with Cl Atoms and OH Radicals and Atmospheric Fate of CF3C(O•)HOCHF2 and CF3C(O•)ClOCHF2 Radicals. J. Phys. Chem. A. 2002, 106, 8391-8398. 26 ACS Paragon Plus Environment

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Page 27 of 29 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


(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

(32)

(33) (34)

(35)

(36)

Wallington, T. J.; Schneider, W. F.; Sehested, J.; Bilde, M.; Platz, J.; Nielsen, O. J.; Christensen, L. K.; Molina, M. J.; Molina, L. T.; Wooldridge, P. W. Atmospheric Chemistry of HFE-7100 (C4F9OCH3): Reaction with OH Radicals, UV Spectra and Kinetic Data for C4F9OCH2· and C4F9OCH2O2· Radicals, and the Atmospheric Fate of C4F9OCH2O· Radicals. J. Phys. Chem. A. 1997, 101, 8264-8274. Zhao, Y.; Truhlar, D. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215-241. Luo, S.; Zhao, Y.; Truhlar, D. G. Validation of electronic structure methods for isomerization reactions of large organic molecules. Phys. Chem. Chem. Phys. 2011, 13, 13683-13689. Xu, X.; Alecu, I. M.; Truhlar, D. G. How Well Can Modern Density Functionals Predict Internuclear Distances at Transition States? J. Chem. Theory. Comput. 2011, 7, 1667-1676. Zheng, J.; Zhao, Y.; Truhlar, D. G. The DBH24/08 Database and Its Use to Assess Electronic Structure Model Chemistries for Chemical Reaction Barrier Heights. J. Chem. Theory. Comput. 2009, 5, 808-821. Karton, A.; arnopolsky, A.; Lamere, J. F.; Schatz, G. C.; Martin, J. M. L. Highly Accurate First-Principles Benchmark Data Sets for the Parametrization and Validation of Density Functional and Other Approximate Methods. Derivation of a Robust, Generally Applicable, Double-Hybrid Functional for Thermochemistry and Thermochemical Kinetics. J. Phys. Chem. A. 2008, 112, 12868-12886. Zheng, G.; Zhao, Y.; Truhlar, D. G. Thermochemical Kinetics of HydrogenAtom Transfers between Methyl, Methane, Ethynyl, Ethyne, and Hydrogen. J. Phys. Chem. A. 2007, 111, 4632-4642. Ponnusamy, S.; Sandhiya, L.; Senthilkumar, K. The atmospheric oxidation mechanism and kinetics of 1,3,5-trimethylbenzene initiated by OH radicals – a theoretical study. New. J. Chem. 2017, 41, 10259-10271. Ponnusamy, S.; Sandhiya, L.; Senthilkumar, K. Mechanism and Kinetics of the Reaction of Nitrosamines with OH Radical: A Theoretical Study. Int. J. Chem. Kinet. 2017, 49, 339-353. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Rob, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. Zheng, J.; Zhang, S.; Corchado, J. C.; Chuang, Y. Y.; Coitino, E. L.; Ellingson, B. A.; Truhlar, D. G. GAUSSRATE version, 2009. Zheng, J.; Zhang, S.; Lynch, B. J.; Corchado, J. C.; Chaung, Y. Y.; Fast, P. L.; Hu, W. P.; Liu, Y. P.; Lynch, B. J.; Nguyen, K. A.et al. POLYRATE version, 2010. Oyaro, N.; Sellevåg, S. R.; Nielsen, C. J. Atmospheric Chemistry of Hydrofluoroethers: Reaction of a Series of Hydrofluoroethers with OH Radicals and Cl Atoms, Atmospheric Lifetimes, and Global Warming Potentials. J. Phys. Chem. A. 2005, 109, 337-346. Mishra, B. K.; Lily, M.; Chakrabartty, A. K.; Deka, R. C.; Chandra, A. K. A DFT study on kinetics of the gas phase reactions of CH3CH2OCF3 with OH radicals and Cl atoms. J. Fluorine Chem. 2014, 159, 57-64. 27 ACS Paragon Plus Environment


(37)

(38)

(39)

(40) (41)

(42)

(43)

(44)

Japar, S. M.; Wallington, T. J.; Rudy, S. J.; Chang, T. Y. Ozone-forming potential of a series of oxygenated organic compounds. Environ. Sci. Technol. 1991, 25, 416-420. Oyaro, N.; Sellevåg, S. R.; Nielsen, C. J. Study of the OH and Cl-Initiated Oxidation, IR Absorption Cross-Section, Radiative Forcing, and Global Warming Potential of Four C4-Hydrofluoroethers. Environ. Sci. Technol. 2004, 38, 5567-5576. Srinivasulu, G.; Rajakumar, B. Theoretical Investigations on the Kinetics of H Abstraction Reactions from CF3CH(OH)CF3 by OH Radicals. J. Phys. Chem. A. 2013, 117, 4534-4544. Kurylo, M. J.; Orkin, V. L. Determination of Atmospheric Lifetimes via the measurement of OH Radical Kinetics. Chem. Rev. 2003, 103, 5049-5076. Wingenter, O. W.; Blake, D. R.; Blake, N. J.; Sive, B. C.; Rowland, F. S.; Atlas, E.; Flocke, F. Tropospheric hydroxyl and atomic chlorine concentrations, and mixing timescales determined from hydrocarbon and halocarbon measurements made over the Southern Ocean J. Geophys. Res. 1999, 104, 21819-21828. Pinnock, S.; Hurley, M. D.; Shine, K. P.; Wallington, T. J.; Smith, T. J. Radiative forcing of climate by hydrochlorofluorocarbons and hydrofluorocorbons. J. Geophys. Res. Atmos. 1995, 100, 23227-23238. World Meteorological Organization Scientific Assessment of Ozone Depletion: 2010; Global Ozone Research and Monitoring Project Report No. 52; Geneva, S. 2011, 516. Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. Hydrofluorocarbons and stratospheric ozone Farad. Discuss. 1995, 100, 5564.


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TOC Graphic


Atmospheric Oxidation Mechanism and Kinetics of Hydrofluoroethers

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