J Mol Model (2015) 21: 69 DOI 10.1007/s00894-015-2629-x

ORIGINAL PAPER

Theoretical insight on atmospheric chemistry of HFE-365mcf3: reactions with OH radicals, atmospheric lifetime, and fate of alkoxy radicals (CF3CF2CH(O•)OCH3/CF3CF2CH2OCH2O•) Debajyoti Bhattacharjee & Bhupesh Kumar Mishra & Ramesh Chandra Deka

Received: 7 December 2014 / Accepted: 15 February 2015 / Published online: 7 March 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract In the present work, theoretical study on the mechanism and kinetics of the gas-phase reactions of CF3CF2CH2OCH3 (HFE-365mcf3) with the OH radicals have been performed using meta-hybrid modern density functional M06-2X in conjunction with 6-31+G(d,p) basis set. Reaction profiles for OH-initiated hydrogen abstraction are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels. Our calculations reveal that hydrogen abstraction from the –CH2 group is thermodynamically more facile than that from the –CH3 group. This is further ascertained by the calculated C-H bond dissociation energy of CF3CF2CH2OCH3 molecule. The rate constants of the titled reactions are computed over the temperature range of 250–450 K. The calculated rate constant value at 298 K is found to be in reasonable agreement with the experimental results. The atmospheric life time of HFE-365mcf3 is estimated to be 42 days. The atmospheric fate of the alkoxy radicals, CF3CF2CH(O•)OCH3 and CF3CF2CH2OCH2O• are also investigated for the first time using the same level of theory. Out of three plausible decomposition channels, our results clearly point out that reaction with O2 is the dominant atmospheric sink for the decomposition of CF3CF2CH(O•)OCH3 radical in the atmosphere.

Electronic supplementary material The online version of this article (doi:10.1007/s00894-015-2629-x) contains supplementary material, which is available to authorized users. D. Bhattacharjee : R. C. Deka (*) Department of Chemical Sciences, Tezpur University, TezpurNapaam, Assam 784028, India e-mail: [email protected] B. K. Mishra (*) Department of Chemistry, D. N. Government College, Itanagar, Arunachal Pradesh 791113, India e-mail: [email protected]

Keywords Alkoxy radical . Bond dissociation energy . Hydrofluoroether . Isodesmic reactions . Rate constant

Introduction In order to reduce the adverse effect of chlorofluorocarbons (CFCs) toward Earth’s stratospheric ozone layer their commercial production is banned as per the Montreal protocol and an international effort has gone into finding environmentally more acceptable alternatives to replace chlorofluorocarbons [1]. In this context, a number of CFCs replaceable compound such as hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) have been developed for short term use because of their non-reliability toward protection of ozone layer. Volatile organic compounds like hydrofluoroethers (HFEs) are designed and widely recommended as a third generation replacement for CFCs, HFCs, and HCFCs in applications such as cleaning of electronic equipments, heat transfer fluid in refrigerators, lubricant deposition, and foam blowing agents [2, 3]. Hydrofluoroethers do not contain chlorine and bromine atoms that cause the ozone depletion [4]. In addition the rate constants for the reaction of HFEs with OH radical suggest that their atmospheric lifetime should be relatively short and thus HFEs appear to have less impact for global warming [5]. The tropospheric oxidation of HFEs is known to produce corresponding hydrofluorinated esters (FESs) [6–8] which on hydrolysis produce environmentally burdened product like trifluoroacetic acid (TFA) and COF2. In the present work, we have studied the mechanism and kinetics of H-abstraction reaction from CF3CF2CH2OCH3 with OH radicals using DFT methods. Our calculation suggests that one reaction channel each from –CH2 group and two

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transition states from –CH 3 group are feasible for the CF3CF2CH2OCH3 +OH reactions as given below.

Two experimental studies were reported for reactions (R1R2) by Oyaro et al. [9] and Thomsen et al. [10]. Oyaro et al. [9] first studied the hydrogen abstraction channel of these reactions using a relative rate method and the experimental rate constants were reported as (6.42 ± 0.33) × 10−13 cm3 molecule−1 s−1 at 298 K. Subsequently, Thomsen et al. [10] performed another experimental study by using FTIR smog chamber technique and they also reported a rate constant value as k(OH + CF 3 CF 2 CH 2 OCH 3 ) = (5.78 ± 1.02) × 10 −13 cm 3 molecule−1 s−1 at 296±1 K. However, both these experimentally measured values are in reasonable agreement with that was estimated empirically by Urata et al. [11] (3.93 × 10−13 cm3 molecule−1 s−1) on the basis of bond dissociation enthalpies (BDEs) of the C–H bond by using the artificial neural network (ANN) technique. Moreover, experimental studies provided only the total rate constant and it is difficult to predict the detailed mechanism and thermo chemistry. Thus, for better understanding of mechanistic pathways, kinetics, and thermochemistry we must rely on quantum chemical methods. To the best of our knowledge, this is the first detailed theoretical study on gas-phase reactions on OHinitiated hydrogen abstraction reactions of HFE-365mcf3. The tropospheric degradation of CF3CF2CH2OCH3 is initiated by attack of OH radicals which leads to the formation of alkyl radical CF3CF2C•HOCH3. The latter reacts with atmospheric O 2 to produce peroxy radical, CF 3 CF 2 CH(OO • )OCH 3 . In a polluted atmosphere the peroxy radical thus formed may further reacts with other oxidizing species such as NO2 and NO that ultimately leads to the formation of alkoxy radical CF3CF2CH(O•)OCH3. On the other hand, alkoxy radical, CF3CF2CH2OCH2O• may also be generated through hydrogen abstraction from the -CH3 group. The chemistry of alkoxy radicals, thus generated has been a subject of extensive experimental and theoretical investigations as these species are interesting intermediates in the atmospheric oxidation of volatile organic compounds. During the recent past, substantial theoretical studies have been performed to understand the reactivity of the atmospheric fate of similar alkoxy radicals [12–18]. Oyaro et al. [9] studied the product distribution of CF3CF2CH2OCH3 with OH radicals and Cl atoms at 298 K and reported that C-C bond scission leading to CF3C•F2 and methyl formate (CH 3 OCHO) is not an important

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atmospheric sink for the consumption of CF 3CF2CH(O• )OCH 3 radical. Recently, Thomsen et al. [10] experimentally investigated the thermal decomposition of alkoxy radical CF3CF2CH(O•)OCH3 produced by OH-initiated oxidation of CF 3CF 2CH2OCH 3 in a FTIR smog chamber. They concluded that the atmospheric fate of CF3CF2CH(O•)OCH3 radical is C-C bond scission. Due to this discrepancy, there is a need to perform quantum mechanical calculations to determine the energetics involved during the decomposition of CF3CF2CH(O•)OCH3 radical. No theoretical study has been performed to elucidate the dissociative pathways of alkoxy radicals. This motivated us to investigate the decomposition and reactivity mechanism of this radical on a sound theoretical basis. There are two potential pathways for decomposition of alkoxy radicals produced from CF3CF2CH2OCH3 that involve bond scissions and oxidation processes. These are represented as follows:

Bond dissociation energies (BDEs) of the breaking C-H bonds are known to be strongly correlated with the observed reactivity trend for the hydrogen abstraction reaction, and the ether linkage (−O-) is important for the reactivity of the hydrofluoroethers [19, 20]. Thus, we present BDE of the two types of C-H bonds in CF3CF2CH2OCH3. In addition, the knowledge of accurate enthalpy of formation (ΔfH°298) for CF3CF2CH2OCH3, CF3CF2C•HOCH3 and CF3CF2CH2OC•H2 radicals is of vital importance for determining the thermodynamic properties and atmospheric modeling. However, no theoretical or experimental study on standard enthalpy of formation has been reported so far for these species. Here, we predict the enthalpies of formation using isodesmic reactions at M06-2X/6-31+G(d,p) level.

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Computational methods Geometry optimization of the species were made at the M062X level [21] of theory using 6-31+G(d,p) basis set. The 631+G(d,p) basis set was used because the same basis set was used for developing the model functional. Previous works have shown that the M06-2X hybrid density functional provides reliable results for thermo chemistry and kinetics [22–26]. The M06-2X is a hybrid meta-DFT method with a high percentage of HF exchange, and it has broadest applicability with average mean absolute errors of about 1.3, 1.2, and 0.5 kcal mol−1 for thermochemical, barrier height, and noncovalent interaction calculations, respectively [27]. Since the formation of pre- and post-reaction complex modifies the shape of potential energy surface for the hydrogen abstraction reactions by OH radicals and hence affects the tunneling factor [28, 29]. As a result the rate constant for hydrogen abstraction also changes. Thus we also validated pre- and post reactive complexes along the entry and exit of the reaction path. In order to determine the nature of different stationary points on the potential energy surface, vibrational frequencies calculations were performed using the same level of theory at which the optimization was made. Further, to ascertain that the identified transition states connect reactant and products smoothly, intrinsic reaction coordinate (IRC) calculations [30] were also performed at the M06-2X/6-31+G(d,p) level. All calculations were performed with the Gaussian 09 suite of program [31].

Results and discussion The calculated enthalpy of reactions (ΔrH°298) and reaction free energies (Δ r G° 298 ) at 298 K for the reactions of CF3CF2CH2OCH3 with OH radicals are recorded in Table 1. The enthalpy of reaction (ΔrH298°) values given in Table 1 for R1 and R2 show that both the reactions are significantly exothermic and exergonic in nature and thus thermodynamically feasible. There are two potential hydrogen abstraction sites of CF3CF2CH2OCH3, namely the –CH2 and –CH3 group. It can be seen from the geometrical parameters and stereographical orientation the hydrogen atoms in the –CH3 groups are not

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equivalent. Two transition states (TS2a and TS2b) are therefore located for CF3CF2CH2OCH3 +OH reactions from the – CH3 group. Optimized geometries of reactants and transition states obtained at the M06-2X/6-31+G(d,p) level are shown in Fig. 1; whereas for the reactant complexes, product complexes and products are depicted in Fig. S1 of Supporting information. In the entrance channel for reactions of hydrogen abstraction by OH radicals (R1 and R2), pre-reactive complexes (RC1 and RC2a) have been located in the present work. However, for reaction channels R1 and R2b, we have obtained the same pre-reactive complexes (RC1/RC2b). In the exit channels, there are also product complexes occurring before the release of the final products, which are labeled as PC1, PC2a, and PC2b. In pre-reactive complex RC1, four hydrogen bonds are formed between the O1…H6, O2…H1, O2…H3, and F2…H6 bonds with the bond distances of 2.374, 2.671, 2.588, and 2.221 Å, respectively. However, in the complex RC2a, the two hydrogen bonds are formed between the O1…H6 and O2…H3 bonds with the bond distances of 1.943 and 2.831 Å, respectively while the other bond lengths are very close to those in equilibrium structures of CF3CF2CH2OCH3 molecule. At the same time, the postreaction hydrogen bonded complexes (PC1, PC2a, and PC2b) with energy less than the corresponding products are located at the exits of the reaction channels R1 and R2 for reactions with OH radicals which can be identified with relatively strong C–H…O and O-H…F bonds, as shown in Fig. S1. So it is clear that the reaction channels R1 and R2 may proceed via indirect mechanisms. During the formation of transition states, the important structural parameters have to be observed are one of the C-H bond of the leaving hydrogen and the newly formed bond between H and O atoms in the OH radical. Visualization of the optimized structure of TS1, TS2a, and TS2b further reveals that the breaking C-H bond is found to be longer in a range of 6.01–10.10 % than the observed C-H bond length in isolated CF3CF2CH2OCH3; whereas the forming O…H bond length is longer by 37.35-52.44 % than the O-H bond length in H2O. The fact that the elongation of forming bond is larger than that of the breaking bond indicates that the barrier of the reactions (R1-R2) is near the

Table 1 Reaction enthalpies, free energies of reactions (R1-R2), and bond dissociation energy (D0298) calculated at M06-2X/6-31+G(d,p) level of theory along with literature values. All values are in kcal mol−1 Reaction channels

ΔrH°298

ΔrG°298

Reaction R1 Reaction R2 Bond dissociation type

−22.09 −18.52 M06-2X/ 6-31+G(d,p) 96.78 100.35

−23.56 −19.15 Literature valuea 94.59 97.15

a

Urata et al. [11]

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Fig. 1 Optimized geometries of reactants and transition states at M06-2X/6-31+G(d,p) level. Bond lengths are in Angstroms

CF3CF2CH2OCH3

TS1

TS2a

TS2b

CF3CF2CH(O•)OCH3

TS3

TS4

TS5

•

CF3CF2CH2OCH2O

TS6

TS7

corresponding reactants. This means the reaction will proceed via early transition state structure which is in consonance with Hammond’s postulate [32] applied to an exothermic hydrogen abstraction reaction.

Results obtained by frequency calculations for species involved in reactions (R1-R2) are recorded in Supporting information (Table S1). These results show that the reactants, reactant complexes, product complexes, and products have stable

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minima on their potential energy surface characterized by the occurrence of only real positive vibrational frequencies. On the other hand, transition states are characterized by the occurrence of only one imaginary frequency obtained at 944i, 1334i, and 868i cm−1 for TS1, TS2a, and TS2b, respectively. Visualization of the normal-mode corresponding to the calculated imaginary frequencies shows a well defined transition state geometry connecting reactants and products during transition. The existence of transition state on the potential energy surface is further ascertained by intrinsic reaction coordinate (IRC) calculation [30] performed at the same level of theory. The relative energies (including ZPE) for all the species involved in reaction channels (R1-R2) obtained from M06-2X/ 6-31+G(d,p) results are recorded in Table S2. These results reveal that energy barriers for H atom abstraction by OH radicals from the –CH2 and –CH3 groups are −0.21, 2.68, and 0.14 kcal mol−1 for TS1, TS2a, and TS2b, respectively at M06-2X/6-31+G(d,p) level of theory. A potential energy diagram of the title reaction is constructed and shown in Fig. 2. In the construction of energy diagram, zero-point energy corrected total energy data as recorded in Table S2 are utilized. These energies are plotted with respect to the ground state energy of CF3CF2CH2OCH3 +OH including ZPE arbitrarily taken as zero. Moreover, spin contamination is not important for the CF3CF2CH2OCH3 + OH reactions becauseis found to be 0.76 at M06-2X/6-31+G(d,p) before annihilation that is only slightly larger than the expected value of= 0.75 for doublets. The barrier height values suggest that hydrogen abstraction by OH radical from the –CH2 group of CF3CF2CH2OCH3 is more facile than that from the –CH3 group. However, this finding contradicts with the observation of Thomsen et al. [10] that hydrogen abstraction by OH radicals proceeds 44±5 % from the –CH2 group and 56±5 %

from the –CH3 group. This is further confirmed by C-H bond dissociation energies calculations. The calculated bonddissociation energies, BDE (D0298) of the C-H bonds of CF3CF2CH2OCH3 molecule along with literature values are recorded in Table 1. The D0298 value obtained from the M062X/6-31+G(d,p) results for C–H bonds in the –CH2 and –CH3 sites of CF 3 CF 2 CH 2 OCH 3 amount to be 96.78 and 100.35 kcal mol−1, respectively. Moreover, our calculated BDE values for the C–H bonds are found to be in reasonable agreement with the theoretical values of 94.59 and 97.15 kcal mol−1, respectively for –CH2 and –CH3 sites of CF3CF2CH2OCH3 reported by Urata et al. [11] using ANN technique. Recently, it has been established that this functional (M06-2X) was better in computing the bond dissociation enthalpies [33, 34]. The standard enthalpy of formation (Δ f H° 298 ) for CF3CF2CH2OCH3 and the radicals CF3CF2C•HOCH3 (P1) and CF3CF2CH2OC•H2 (P2) generated from hydrogen abstraction, can be valuable information for understanding the mechanism and thermochemical properties of their reactions and most importantly for atmospheric modeling, but these values are not yet reported. The group-balanced isodesmic reactions, in which the number and types of bonds are conserved, are used as working chemical reactions herein to calculate the ΔfH°298. Here, three isodesmic reactions are used to estimate the enthalpies of formation of CF3CF2CH2OCH3. The used isodesmic reactions are as follows:

TS2a

-1

Relative Energy + ZPE (kcal mol )

2.68

TS2b 0.14

0.00

TS1

R + OH

-0.21 RC2a -4.78 RC1/RC2b

P2 + H2O

-5.72

-18.72 P1 + H2O

R = CF3CF2CH2OCH3

PC2b

P1 = CF3CF2CHOCH3

-22.91

P2 = CF3CF2CH2OCH2

-22.52

PC2a -23.27

PC1 -27.85

Fig. 2 Potential energy profile for the CF3CF2CH2OCH3 +OH reactions. Relative energies (in kcal mol−1) at M06-2X/6-31+G(d,p) level

All geometrical parameters of the species involved in the isodesmic reactions (R8-R10) were first optimized at the M06-2X/6-31+G(d,p) level. The ΔfH°298 values for CH3F: −55.62 kcal mol−1, CH2F2: − 107.77 kcal mol−1 , CHF3: −166.08 kcal mol −1 , CH 3 : 34.82 kcal mol −1 and CF 3 : −111.75 kcal mol−1 were taken from ref. [35], where as for C H 3 O C H 3 : − 44 .0 k ca l mo l − 1 , C F 3 C H 2 O C H F 2 : 319.38 kcal mol−1, CF3CH2CF3: −336.5 kcal mol−1, CH3CF3: −178.94 kcal mol−1, CH 3CH 3: −20.24 kcal mol −1, and CHF2OCHF2: −259.1 kcal mol−1 were taken from ref. [36] to evaluate the required enthalpies of formation. The calculated values of enthalpies of formation are listed in Table 2. As can be seen from Table 2, the values of ΔfH°298 for the species obtained by the three working chemical reactions are

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Table 2 Enthalpies of formation (ΔfH0298) for the species at 298 K from the isodesmic reactions along with literature values. All values are in kcal mol−1 Species

Reaction schemes

M06-2X/ 6-31+ G(d,p)

Literature valuea

CF3CF2CH2OCH3

R5 R6 R7 Average

−307.20 −307.99 −308.49 −307.89 −263.25

−309.27

CF3CF2C•HOCH3 (P1) CF3CF2CH2OC•H2 (P2) a

−259.68

Kondo et al. [36]

consistent with each other. The average Δ f H° 298 for CF3CF2CH2OCH3 calculated from M06-2X/6-31+G(d,p) result is found to be −307.89 kcal mol−1. It is seen that the calculated ΔfH°298 for CF3CF2CH2OCH3 at M06-2X/6-31+ G(d,p) method is found to be in good agreement with a theoretical value of −309.27 kcal mol−1 reported by Kondo et al. [36] using both bond additivity corrected MP2 method (BACMP2/6-31G**) and atom additivity corrected MP2 method (AAC-MP2/6-31G**). The ΔfH°298 values for CF3CF2C•HOCH3 (P1) and CF3CF2CH2OC•H2 (P2) radicals can also be easily calculated from the reported ΔrH°298 values for reactions R1 and R2 in Table 1, the calculated ΔfH°298 value for CF3CF2CH2OCH3 and the experimental ΔfH°298 values for H2O (−57.8 kcal mol−1) and OH (8.93 kcal mol−1) radical. The ΔfH°298 values for P1 and P2 radicals calculated from the M06-2X/6-31+G(d,p) results are −263.25 and −259.68 kcal mol−1, respectively. The rate constant for title reactions are calculated by using canonical transition state theory [37] given by the following expression: kB T QTS −ΔE ; exp h QA : QB RT #

k ¼ σΓðTÞ

ð1Þ

where, σ is the reaction path degeneracy, Γ(T) is the tunneling correction factor at temperature T. QTS, QA, and QB are the total partition functions (per unit volume) for the transition states and reactants, respectively. ΔE# is the barrier height including zero point energy correction, kB is the Boltzmann constant, h is the Planck’s constant, and R represents the universal gas constant. The tunneling correction was estimated by using the Eckart’s unsymmetrical barrier method [38, 39]. All vibrational modes, except for the lowest vibrational mode, were treated quantum mechanically as separable harmonic oscillators, whereas for the lowest-frequency mode, the partition function was evaluated by the hindered-rotor approximation as proposed by Chuang and Truhlar [40]. The total

partition function is approximated by the product of translational, rotational, vibrational, and electronic partition functions. In the calculation of reactant electronic partition function, the excited state of the OH radicals is included, with a 140 cm−1 splitting due to spin-orbit coupling. The procedure proposed by Singleton and Cvetanovoic [41] are incorporated for taking into consideration the effect of pre- and post-reactive complex on reaction kinetics. The contribution from each of the channels is taken into account while calculating the total rate coefficient (kOH) for the CF3CF2CH2OCH3 +OH reactions. The computed rate constant values in the temperature range of 250–450 K are recorded in Table 3. At 298 K, our calculated kOH value using M06-2X/6-31+G(d,p) barrier heights is 3.59 × 10−13 cm3 molecule−1 s−1 which is in reasonable agreement with the experimental value of (5.78 ± 1.02) × 10−13 cm3 molecule−1 s−1 reported by Thomsen et al.11 and (6.42 ± 0.33) × 10−13 cm 3 molecule−1 s−1 by Oyaro et al. [9]. However, our calculated rate constant (3.59× 10−13 cm3 molecule−1 s−1) is in very good agreement with the value of 3.93 × 10−13 cm3 molecule−1 s−1 estimated empirically by Urata et al. [11] using the artificial neural network (ANN) technique. Although, rate constants for H-abstraction from the –CH3 group is found to be slightly higher than that from the –CH2 group for CF 3 CF 2 CH 2 OCH 3 + OH reactions. Of course the Habstraction from the –CH2 group is thermodynamically more favorable because of its more exothermic nature.

Atmospheric consequences Atmospheric lifetime The atmospheric sink of VOCs depends on various processes like photolysis, wet and dry deposition and reaction with atmospheric oxidants such as OH radicals, Cl atoms, O3, and NO3 radicals. Previous reports [42, 43],

Table 3 Rate constant values (in cm3 molecule−1 s−1) for hydrogen abstraction reactions of CF 3 CF 2 CH 2 OCH 3 with OH radicals as calculated from M06-2X/6-31+G(d,p) results T (K)

kTS1

kTS2a

kTS2b

kOH

250 298 300 350 400 450

1.59×10−13 1.31×10−13 1.30×10−13 1.14×10−13 1.06×10−13 1.01×10−13

2.95×10−14 2.85×10−14 3.91×10−14 5.08×10−14 6.48×10−14 8.08×10−14

1.49×10−13 2.00×10−13 1.33×10−13 1.24×10−13 1.22×10−13 1.20×10−13

3.37×10−13 3.59×10−13 3.01×10−13 2.89×10−13 2.92×10−13 3.02×10−13

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reveal that photolysis and reactions with NO3 and O3 radicals are unimportant. The hydrophobic nature and volatility of ethers will render wet deposition and dry deposition an unlikely removal process. Thus, atmospheric lifetime (τeff) of HFEs is mainly controlled by OH-initiated hydrogen abstraction. In order to minimize the errors resulting from neglecting the specific temperature dependences, when calculating OH-based lifetimes, the use of 272 K as an average tropospheric temperature and methyl chloroform (CH3CCl3), as a chemical of well known source and sink, has been suggested [44]. Thus, lifetime estimations for HFEs are generally calculated on the basis of gas-phase removal by OH only and with methyl chloroform (MCF) as reference: FE τH OH ¼

k M C F ð272K Þ M C F τ ; k H FE ð272K Þ OH

where τHFE OH is the lifetime for HFE-365mcf3, kHFE and kMCF are the rate constants for the reactions of OH radicals with HFE-365mcf3 and methyl chloroform (MCF), respectively at T=272 K and τMCF OH = 5.99 years [45]. Taking the values of rate constant for kMCF =6.14×10−15 from ref. [45] and calculated rate constant of kHFE =3.17×10−13 cm3 molecule−1 s−1 at 272 K, the estimated lifetime is found to be 42 days which is in reasonable agreement with the reported value of 20 days by Thomsen et al. [10]. Atmospheric fate of alkoxy radical Theoretical calculations at the M06-2X/6-31+G(d,p) level of theory were carried out in order to explore the nature of the reaction mechanism for the unimolecular decomposition of CF 3CF2 CH(O • )OCH3 and CF3 CF 2CH 2OCH 2 O• radicals in the gas phase which are envisaged to occur via reactions (R3-R7). Optimized geometries of radicals and transition states involved in these reactions obtained at the M06-2X/6-31+G(d,p) level are shown in Fig. 1. Transition states obtained on the potential energy surfaces of reactions (R3-R7) are characterized as TS3, TS4, TS5, TS6, and TS7, respectively. The search was made along the minimum energy path on a relaxed potential energy surface. Harmonic vibrational frequencies of the stationary points were calculated and are given in Table S3 in Supporting information. These results show that the reactant and products have stable minima on their potential energy surface characterized by the occurrence of only real and positive vibrational frequencies. On the other hand, transition vectors for all the transition states TS3, TS4, TS5, TS6, and TS7 were obtained at 450i, 1029i, 1284i, 1047i, and 1539i cm−1, respectively as recorded in Table S3. The existence of these transition states on the potential energy surface are further ascertained by

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intrinsic IRC calculation [30] performed at the same level of theory. The thermochemical data calculated at M06-2X/ 6-31+G(d,p) level are summarized in Table S4. Free energy values show that two reaction channels (R5 and R7) are significantly exergonic (ΔG