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Kinetics and Mechanistic Investigations of Atmospheric Oxidation of HFO-1345fz by OH Radical: Insights from Theory Pradeep Kumar Rao and Shridhar P. Gejji* Department of Chemistry, Savitribai Phule Pune University, Pune 411007, India S Supporting Information *

ABSTRACT: HFO-1345fz (CF3CF2CHCH2 or 3,3,4,4,4-pentafluoro-1-butene) belongs to a class of hydrofluoro-olefins and represents a new generation of potential foam expansion agents. Its atmospheric impact and environmental acceptability can be estimated from the studies of kinetics and mechanism of its oxidative degradation. The molecular insights accompanying the reaction pathways in terms of the characterization of intermediates or products and radiative properties should prove useful for large-scale industrial applications. Systematic mechanistic gas-phase kinetics investigations on the reactivity of HFO-1345fz with the •OH facilitating a variety of degradation routes have been carried out employing the M06-2x-based density functional theory. Structure and energetics of different reaction pathways such as hydrogen abstraction, •OH addition, isomerization−dissociation, or interaction with atmospheric O2 have been analyzed. The formation of gaseous products from the interaction of HFO-1345fz with •OH in the absence and presence of NOx atmospheric conditions has been reported. Calculated branching ratios have shown that the addition channel dominates such oxidative degradation, whereas the abstraction channel contributes negligibly to the global rate constant and addition of •OH to the terminal carbon is favored over the nonterminal one. The rate constants for all reaction channels were computed by conventional transition state theory (TST) and canonical variation transition state theory (CVT) including small curvature tunneling (SCT) over the temperature range of 200−1000 K at atmospheric pressure. The CVT calculated rate constant for the reaction at 298 K was shown to be 1.17 × 10−12 cm3 molecule−1 s−1, which compares well with the 1.24 × 10−12 cm3 molecule−1 s−1 as obtained from TST and is in excellent agreement with the experiments reported earlier. The atmospheric lifetime, radiative efficiency, and global warming potential (GWP) have also been obtained.

1. INTRODUCTION The adverse environmental impact of chlorinated hydrocarbons such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) on the depletion of stratospheric ozone has been of serious concern1 to life and vegetation on earth. Significant attention has been directed toward finding alternatives to CFCs by environmentally friendly substitutes such as hydrofluorocarbons (HFCs) or HCFCs in the past few decades. Degradation of HFCs or HCFCs, however, showed serious after effects in the atmosphere, which restricted their use to certain applications. To this direction hydrofluoro-olefins (HFOs), particularly CxF2x+1CHCH2, a class of unsaturated HFCs that are more reactive and exhibit low global warming potentials (GWPs), have been proposed as an alternative to the saturated HFCs in many applications. Among other HFOs, HFO-1345fz (CF3CF2CHCH2 or 3,3,4,4,4-pentafluoro-1butene) has emerged as a new-generation foam expansion agent.2 Long-chain perfluorinated carboxylic acids (PFCAs, CxF2x+1C(O)OH), potentially toxic acids) have been observed in fish and mammals in remote areas.3−5 The PFCAs with no known natural sources are directly emitted to the environment primarily via industrial processes. Atmospheric degradation of CxF2x+1CHCH2 possibly serves as one of the sources of PFCAs (CxF2x+1C(O)OH) in remote areas. The atmospheric © XXXX American Chemical Society

persistence of CF3CF2CHCH2 (HFO-1345fz) and the indirect potential environmental impact of its atmospheric degradation thus should be crucial. With this perspective, the present endeavor focuses on understanding the degradation of HFO-1345fz by •OH leading to the formation of PFCAs. A reaction profile, energy barriers, and thermodynamic parameters for each step accompanying the various reaction pathways have been derived. The work further provide the contributions from the •OH addition as well as the hydrogen abstraction channels under atmospheric conditions. In particular, we examine the oxidation initiation channels followed by the reactions with the alkyl, alkoxy, and alkylperoxy intermediates active in subsequent oxidation steps. It has been realized that the CxF2x+1CHCH2 compounds do not undergo photolysis6 and are not expected to be removed effectively by either wet or dry deposition because the concentration of chlorine atoms in the atmosphere is not sufficient to have any impact on the lifetimes of CF3CF2CHCH2. Accordingly, the reactions with • OH are expected to be the major loss process for HFOs. Moreover, abundance of the •OH radicals7 and their reactivity Received: November 10, 2016 Revised: December 26, 2016 Published: December 27, 2016 A

DOI: 10.1021/acs.jpca.6b11312 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. continued

B


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Figure 1. continued

C


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Figure 1. Optimized structures of all species involved in the addition reaction channel from M06-2x/6-311++G(d,p) theory.

Figure 2. Energy profile for the initiation step of the oxidation of HFO-1345fz by •OH.

chamber have shown that the rate constant for reaction R1 (at 296 K) was (1.36 ± 0.25) × 10−12 cm3 molecule−1 s−1 at 700 Torr of N2 or N2/O2 diluents. Interestingly, the recent pulsed laser photolysis laser-induced fluorescence experiments by Jiménez et al.9 revealed that the rate constant was determined to be (1.21 ± 0.04) × 10−12 cm3 molecule−1 s−1 at 298 K, consistent with the FTIR results. The rate constants further were found to be nearly independent of the total pressure.

make them a major sink for removal of HFO-1345fz in the troposphere. Detailed analysis of the reaction mechanism, kinetics, and thermochemistry of the following reaction CF3CF2CHCH 2 + OH → Products

(R1)

has been given in this account. On the experimental front, earlier experiments by Sulbaek Andersen et al.8 using the long path length FTIR smog D


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The Journal of Physical Chemistry A Scheme 1. Schematic Representation of the Sequence of Steps for the •OH Addition Channel

These authors used H2O2 and HNO3 as precursors for the •OH radicals. The experiments provide the global rate constants; the contributions from individual reaction channels to the global rate constant are not accessed directly. On the other hand, theoretical methods provide detailed information on the electronic structure and spectral characteristics of the reactants, intermediates (IMs), transition states (TSs), and products and the site-specific rate constants. The specificity of the overall reaction can thus be explored. To the best of the authors’ knowledge, the rate constant for the oxidation of HFO-1345fz by •OH has been reported only at the room temperature. Detailed kinetics or a degradation mechanism has not yet been determined. The present work precisely focuses on understanding the products accompanying the degradation of HFO1345fz. The calculated rate constants over the temperature range of 200−1000 K should prove useful for modeling of atmospheric reactions and combustion phenomena. Furthermore, molecular insights on such atmospheric degradation mechanisms and degradation products allows one to examine the potential atmospheric impact from the emissions of HFO1345fz.

theory11,12 in conjunction with Pople’s 6-311++G(d,p) basis set.13 The stationary point structures along the potential energy surfaces, including reactants, prereactive (RCs) and postreactive (PCs) complexes, IMs, and products, were located as the local minima and the TSs accompanying each channel. The stationary point structures obtained were confirmed to be local minima from the normal vibrational frequencies, all of which turn out to be real, whereas the TS was characterized through an imaginary frequency for one of its normal vibrations. The normal vibrations were assigned by visualizing the atomic displacements about their mean (equilibrium) positions employing the GausView (version 5)14 program. SCF energies incorporating zero-point corrections (scaled by 0.9489)15,16 were obtained. It was demonstrated earlier that the kinetics of reactions can be modeled accurately by the use of the M06-2x theory.17−20 A smooth transformation from the reactant to product via the TS along the minimum energy paths was tracked with the use of intrinsic reaction coordinate (IRC) calculations. BSSE (basis set superposition error)21 corrected binding energies of reaction complexes and TSs were calculated from ΔE = ΔEcomplex/TS − EReactant‑1 − EReactant‑2, where the Ecomplex/TS refers to energies of the hydrogen-bonded RC and PC or TS. EReactant‑1 and EReactant‑2 refer to the zero-pointcorrected SCF energies of CF3CF2CHCH2 and •OH, respectively.

2. COMPUTATIONAL METHOD The electronic structures and SCF energies for the species involved in various reaction pathways were obtained using the Gaussian 09 suite of programs.10 The optimizations were carried out employing M06-2x-based density functional E


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3. RESULTS AND DISCUSSION 3.1. Energetics and Mechanism. Oxidation of the title molecule by •OH can proceed by either hydrogen abstraction or •OH addition channels with the radical interacting with a terminal CH2 or middle −CH site. The reaction was initiated with the hydrogen-bonded RCs located at the entrance of the reaction, whereas the PCs are found at the exit of the abstraction channel of the reaction. Optimized structures of reactants, RCs, PCs, and IMs accompanying the reaction reveal the presence of C−H···O and O−H···F interactions, as depicted in Figure 1. Selected bond distances and bond angle parameters are given also. Calculated vibrational frequencies of these species are summarized in Tables S1 of the Supporting Information. The energy profiles for the initiation step of the addition as well as abstraction channels are depicted in Figure 2. The products of preliminary reactions subsequently follow a sequence of reactions that include isomerization, dissociation, and reaction with atmospheric O2. From the energy profile of the initiation, it is evident that the addition channel prevails over the abstraction. The decomposition of IM3 and IM4 for preliminary reactions accompanying the addition channel are discussed in the following. Zero-point-corrected energies relative to those of reactants for each channel are shown in Figure 2. 3.1.1. H-Abstraction Channel. The title molecule with hydrogen atoms bound to terminal and central carbon suggests that the hydrogen abstraction is facilitated via either

Path-1 CF3CF2CHCH 2 + OH → CF3CF2CH•−CH 2(OH) (R3)

Path-2 CF3CF2CHCH 2 + OH → CF3CF2CH(OH)−CH 2• (R4)

As shown in Scheme 1, the electrophilic addition yields CF3CF2CH•−CH2(OH) and CF3CF2CH(OH)−CH2•, IMs and follows decomposition paths Path-1a, Path-1b, Path-2a, Path-2b, and Path-2c. The hydrogen-bonded prereactive complex RC3, the local minimum, turns out to be 1.95 kcal mol−1 lower in energy than the reactant. Further, the addition to terminal carbon reaction R3 can be traced via the transition state TS3, which again shows a marginal barrier of −0.02 kcal mol−1. An IM CF3CF2CH•−CH2(OH) or IM3 was also identified. On the other hand, the addition to the middle carbon as in reaction R4 (Path-2) is shown to be accompanied by TS4 (with a barrier of 0.11 kcal mol−1) and the IM CF3CF2CH(OH)−CH2• (IM4). Thermodynamic parameters such as the enthalpy of reaction, free energy of reaction, and free energy of activation for the addition channel are shown in Table 1. Subsequent decomposition of IM3 and IM4 can occur Table 1. Energy Barrier and Thermodynamic Properties (in kcal mol−1) for the Addition Channel of the Initiation Step at 298 K

CF3CF2CHCH 2 + OH → CF3CF2CHCH• + H 2O (R1a and R1b)

or CF3CF2CHCH 2 + OH → CF3CF2CH•CH 2 + H 2O

s.n.

reaction

ΔE

ΔrH

ΔrG

ΔG#

1 2

R3 R4

−0.02 0.11

−30.13 −31.01

−20.77 −20.58

7.79 9.08

via unimolecular isomerization−dissociation or bimolecular reaction with atmospheric O 2 . A large abundance of atmospheric O2 and a high NOx atmospheric condition suggest the possibilities of Path-1b and Path-2c. Stabilization of RCs and TSs were examined by BSSE-corrected binding energies, which are given in Table S3. As shown, TS4 with a binding energy of 1.26 kcal mol−1 is predicted to be more stable compared to that of TS3 (0.92 kcal mol−1). It may therefore be conjectured that TS3 can be transformed more easily to products implying the •OH addition to terminal carbon is favored. The separations corresponding to simultaneous formation and cleavage of the bond in TS3 are predicted to be 2.070 and 1.349 Å, respectively, which turn out to be 46 and 2% longer than the corresponding equilibrium bond lengths in isolated IM3 and CF3CF2CHCH2 molecules, respectively. Similarly, in TS4, the separations corresponding to the breaking and formation of bonds are 1.348 and 2.072 Å, respectively, nearly 2 and 47% larger than the equilibrium bond lengths of the reactant molecule and IM4. The large separation corresponding to bond formation compared to cleavage indicates that both TSs are reactant-like, and it may therefore be inferred that the reaction is exothermic and proceeds via the early TS. These results on the “early” or “late” TSs are consistent with the inference drawn earlier in the literature.22−24 The initiation of HFO-1345fz oxidation is followed by the formation of intermediates IM3 and IM4, which dissociate by a series of steps for unimolecular isomerization, dissociation, and reaction with atmospheric O2, yielding the different products. These pathways are discussed briefly in the following, and the

(R2)

Thus, reactions R1a, R1b, and R2 imply H-abstraction from terminal carbon and that from the nonterminal carbon. Moreover, reaction R1 proceeds via the complex mechanism with the RCs and PCs located along the path, which subsequently transform to products. The hydrogen-bonded RC1 and RC2 both located at the entrance are 2.97 and 3.04 kcal mol−1, respectively, lower in energy relative to the reactants, whereas the PC1 and PC2 turn out to be 8.49 and 8.03 kcal mol−1 lower in energy, respectively, and were identified at the exit channel. Both reactions R1a and R1b transform to the product CF3CF2CHCH• + H2O shown as P1 via the transition states TS1a and TS1b. On the other hand, reaction R2 transforms to CF3CF2CH•CH2 + H2O (P2) via TS2, the corresponding barrier being 5.45 kcal mol−1. An energy profile accompanying the hydrogen abstraction is displayed in Figure 2. It is discernible that reaction R2 is energetically favored over reaction R1a or reaction R1b. The enthalpy of reaction (ΔrH), Gibb’s free energy of reaction (ΔrG), and Gibb’s free energy of activation ΔG# are summarized in Table S2 of the Supporting Information. As may be inferred, reaction R1a is largely exothermic and exergonic (with both ΔrH and ΔrG less than zero). Hydrogen abstraction from nonterminal carbon is thus preferred over abstraction from terminal carbon. The optimized geometries of the species located along the abstraction channel are shown in Figure S1 of the Supporting Information. 3.1.2. Addition Channel. The channels for •OH addition to terminal or nonterminal carbons are represented by F


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The Journal of Physical Chemistry A accompanying energy barrier (ΔE) and reaction enthalpies (ΔrH) are given in Table 2.

CF3CF2CH(O•)−CH 2(OH)

Table 2. Reaction Enthalpies and Energy Barriers (in kcal mol−1) of the Individual Reaction of the Addition Channel

CF3CF2CH(O•)−CH 2(OH) + O2

s.n.

reaction

ΔE

ΔrH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

R3-1 R3-2 R3-1a R3-2a-1 R3-2a-2 R3-2a-3 R3-2a-4 R3-2a-1a R4-1 R4-2 R4-3 R4-1a R4-1b R4-3a-1 R4-3a-2 R4-3a-3 R4-3a-1a

40.00 −36.36 21.29 8.55 16.10 −26.82 28.70 −55.90 33.42 29.49 −35.81 13.99 13.40 14.15 −24.61 26.03 −23.79

2.55 −53.61 16.12 13.68 11.79 −64.74 18.31 −53.78 9.40 28.20 −68.06 14.11 9.30 11.13 −63.55 19.50 −4.67

→ CH(O)CH 2(OH) + CF3CF2•

→ CF3CF2C(O)CH 2(OH) + HO2

→ CF3CF2C(O)CH 2(OH) + H

CH 2•OH + O2 → CH 2O + HO2

(R3-1)

CF3CF2CH 2−CH 2(O ) → CF3CF2CH 2 + CH 2O (R3-1a)

The reaction, therefore, progresses via the intermediate IM3 followed by isomerization (reaction R3-1) facilitated via fourmembered transition state TS3-1 (10.83 kcal mol−1) formed as a result of H atom transfer from the radical to the middle carbon. The IM CF3CF2CH2−CH2(O•) (IM3-1) with a relative stabilization energy of −29.36 kcal mol−1 was noticed. Subsequent dissociation brings about C−C bond scission, leading to CF3CF2CH2• and CH2(O) products with a corresponding energy of −11.32 kcal mol−1 relative to the reactants (CF3CF2CHCH2 + •OH). Alternatively, the oxidation may follow Path-1b

CF3CF2CH(OH)−CH 2• → CF3CF2CH(O•)−CH3 •

CF3CF2CH(O )−CH3 → CF3CF2CH(O) + •

CH3•

(R4-1) (R4-1a)

•

CF3CF2CH(O )−CH3 → CF3CF2 + CH(O)−CH3 (R4-1b)

CF3CF2CH(OH)−CH 2 → CH(OH)CH 2 + CF3CF2• •

(R4-2)

CF3CF2CH•−CH 2(OH) + O2

Thus, reaction R4-1 refers to isomerization of IM4 via the transfer of a H atom from the OH to terminal carbon. A fourmembered TS4-1 was identified, which leads to the intermediate IM4-1 [CF3CF2CH(O•)−CH3]. Subsequent dissociation via terminal C8−C10 (reaction R4-1a) and C5−C8 (reaction R4-1b) bond scission engenders stable products CF3CF2CH(O) and CH(O)CH3,, respectively, is facilitated by transition states TS4-1a (−6.57 kcal mol−1) and TS4-1b (−7.16 kcal mol−1). On the other hand, Path-2b reveals dissociation of IM4 (reaction R4-2), leading to CH(OH)CH2 as the stable product via TS4-2. As shown in Figure 4a, the energy profiles for Path-2a and Path-2b suggest that reaction R4-2 favors Path2b. It is also evident that Path-2c is facilitated via a series of reactions similar to those predicted Path-1b, and thus, interaction of IM4 with atmospheric O2 engenders alkoxy radical IM4-2a CF3CF2CH(OH)−CH2(O•). Three different pathways are suggested for the decomposition of the alkoxy

(R3-2)

CF3CF2CH(OO•)−CH 2(OH) + NO (R3-2a)

In atmospheric conditions with rich NOx, IM3 first reacts with the atmospheric O2, forming the peroxy radical IM3-2, which subsequently reacts with NO, transforming to NO2 via the alkoxy radical intermediate, IM3-2a. The decomposition of IM3-2a leads to a variety of different pathways including (i) C8−C10 bond scission, (ii) C5−C8 bond scission, (iii) reaction with O2 (H-abstraction), and (iv) hydrogen atom elimination, as shown below CF3CF2CH(O•)−CH 2(OH) → CF3CF2CH(O) + CH 2•(OH)

(R3-2a-1a)

As evidenced from Figure 3b, among reactions R3-2a-1, R3-2a2, R3-2a-3, and R3-2a-4, reaction R3-2a-3, representing hydrogen atom abstraction from the radical site by O2, is dominant, which proceeds via transition state TS3-2a-3 with an energy barrier of −26.86 kcal mol−1 being the lowest. Stable products CF3CF2C(O)CH2(OH) and HO2 are obtained. Alternatively, the same product can be obtained by reaction R3-2a-4 via H-elimination facilitated by transition state TS3-2a4, which reveals an energy barrier of 28.70 kcal mol−1. On the other hand, reaction R3-2a-1 (terminal C−C bond scission) and R3-2a-2 (middle C−C bond scission) proceed through transition states TS3-2a-1 and TS3-2a-2 with the corresponding energy barriers being 16.10 and 8.55 kcal mol−1, respectively. As shown in Scheme 1, Path-2a and Path-2b are facilitated through the formation of intermediate IM4, which decomposes through the isomerization−dissociation pathways represented in the following reactions

•

→ CF3CF2CH(O•)−CH 2(OH) + NO2

(R3-2a-4)

The energy profiles for Path-1 along with stabilization energies of all of the species relative to IM3-2a are depicted in Figure 3a,b. An intermediate IM3-2a-1a [CH2•(OH)] subsequently reacts with O2, forming the stable product formaldehyde. The transition state TS3-2a-1a, revealing a barrier of −55.90 kcal mol−1, was identified.

CF3CF2CH•−CH 2(OH) → CF3CF2CH 2−CH 2(O•)

→ CF3CF2CH(OO•)−CH 2(OH)

(R3-2a-3)

CF3CF2CH(O•)−CH 2(OH)

As shown in Scheme 1, the addition of •OH to the terminal carbon of the title molecule can follow Path-1a represented by

•

(R3-2a-2)

(R3-2a-1) G


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Figure 3. Potential energy (kcal mol−1) diagram for the step involving the •OH to the terminal carbon of HFO-1345fz: (a) Path-1a and (b) Path-1b. See the text for details. H


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Figure 4. Potential energy (kcal mol−1) diagram for the steps involving the •OH addition to the middle carbon of HF-1345fz: (a) Path-2a and Path2b; (b) Path-2c. See the text for details. I


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numerical integration program of Brown28 based on the Eckart29 unsymmetrical potential barrier. The initiation step for the oxidation of HFO-1345fz by •OH is the rate-determining step. We, therefore, estimate the corresponding rate constant. It was pointed out earlier that the rate constant for the abstraction channel was nearly negligible. The rate constants for the reactions of •OH with the title molecule for the abstraction and addition channels are given in Table 3 along with the tunnelling factor. The overall

radical, which include (i) C−C bond scission, (ii) reaction with O2, and (iii) H atom elimination, as shown below CF3CF2CH(OH)−CH 2(O•) → CH 2O + CF3CF2CH•(OH)

(R4-3a-1)

CF3CF2CH(OH)−CH 2(O•) + O2 → CF3CF2CH(OH)−CH(O) + HO2

(R4-3a-2)

Table 3. Tunnelling Γ(T) and Rate Constants (in cm3 molecule−1 s−1) for the Title Reaction

CF3CF2CH(OH)−CH 2(O•) → CF3CF2CH(OH)−CH(O) + H

(R4-3a-3)

s.n.

•

CF3CF2CH (OH) + O2 → CF3CF2CHO + HO2

1 2 2

(R4-3a-1a)

The energy profile for Path-2c displayed in Figure 4b indicates that the reaction with O2 prevails over C−C bond scission and the H-elimination channel. The energy barrier for reaction R43a-2 was predicted to be −24.61 kcal mol−1, which is lower than those of reactions R4-3a-1 and R4-3a-3. 3.2. Kinetics and Branching Ratio. The rate constants for the initiation step of oxidation of HFO-1345fz, reaction with the •OH, are evaluated from transition state theory (TST)25 ‡ ⎛ ΔE ⎞ kBT QTS ⎟ k = Γ(T ) · exp⎜ − ⎝ RT ⎠ h QR

4 5

(2)

with the symbols having usual meanings. To calculate the partition function of reactant and TSs, lower vibrational frequencies associated with the torsional mode were treated as a hindered rotor, and the Chaung and Truhlar27 approximation was used to derive the hindered rotor partition function. The vibrational partition function was evaluated from corr Q vib

=

v=i

rate constant 2.92 × 10−17 1.90 × 10−17 8.42 × 10−17 1.23 × 10−12 5.26 × 10−15 1.24 × 10−12 1.21 × 10−12 1.36 × 10−12

kCVT(T , s) = min kGT(T , s)

(4)

s

kGT(T , s) =

Q vibQ HR ∏Q

tunneling

Abstraction Channel R1a 23.89 R1b 11.91 R2 20.18 Addition Channel R3 1.24 R4 1.21 Overall Rate Constant this work Jiménez et al.9 Sulbaek Andersen et al.8

rate constant at 298 K for the oxidation from M06-2x/6-311+ +G(d,p) theory turns out to be 1.24 × 10−12 cm3 molecule−1 s−1, which shows excellent agreement with the 1.23 ± 0.04 × 10−12 cm3 molecule−1 s−1 observed from the experiments by Jiménez et al.9 and those with 1.36 ± 0.25 × 10−12 cm3 molecule−1 s−1 by Sulbaek Andersen et al.8 The overall rate constant is the sum of rate constants of the abstraction and addition channels. The individual rate constants for reaction R3 (•OH addition to terminal carbon) and R4 (•OH addition to middle carbon) turn out to be 1.23 × 10−12 and 5.26 × 10−15 cm3 molecule−1 s−1, respectively. Along with conventional TST, we analyzed the variational effect on submerged TSs accompanying the reactions R3 and R4. The rate constant derived from variational transition state theory (VTST)30,31 over the temperature range of 200−1000 K were obtained by employing the Polyrate 2010-A32 program. Canonical variational transition state theory (CVT) is based on the idea of varying the TS dividing surface along the reaction coordinate that minimizes the rate constant with the use of the following equations

(1)

where Γ(T) is the tunnelling correction factor, Q‡TS and QR are the partition functions for the TSs and reactants, respectively, and ΔE denotes the energy barrier for the reaction calculated from the zero-point-corrected energy difference between the TS and reactant (ETS − ER). The total partition function used in the rate constant calculation was taken as the product of vibrational, translational, rotational, and electronic partition functions. The electronic partition function of •OH was corrected by considering the splitting of the ground-state level between the 2Π ground state and the excited state, which reveals a separation of 139.7 cm−1.26 The corrected electronic partition function of •OH is calculated from Q elec(OH) = 2 + 2 exp[(139.7 cm−1)hc /kT ]

reaction

(3)

⎛ − V (s ) ⎞ kBT QGT(T , s) · ·exp⎜ MEP ⎟ R h Φ (T ) ⎝ kBT ⎠ GT

(5)

CVT

In the above equations, k and k denote rate constants obtained from generalized TST and CVT, respectively, σ is reaction path degeneracy, QGT and ΦR are the partition functions of a generalized TS and reactants, respectively, and VMEP(s) is the potential energy of the generalized TS at reaction coordinate s. T is the temperature in Kelvin, kB and h have their usual meanings. The rate constants for the addition channel calculated by TST and CVT methods including small curvature tunnelling (SCT)33 are given in Table 4. In the case of a negative barrier or a barrierless addition channel, the tunnelling effect is insignificant and becomes effective only at the lower

where Qvib is the vibrational partition function that considers all vibrations to be a harmonic oscillator, QHR is the hindered rotor partition function for the lower vibrational frequencies, and Qv=i corresponds to the partition function for normal-mode vibration for the hindered rotor. The tunnelling correction Γ(T) was defined through the ratio of quantum mechanical to classical mechanics barrier crossing rates. Γ(T) depends on the forward and reverse potential barriers and the numerical value of imaginary frequencies of the corresponding TS. The tunnelling was calculated using the modified version of the J


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The Journal of Physical Chemistry A Table 4. Calculated Rate Constants (in cm3 molecule−1 s−1) for the Addition Channel (R3 and R4) over a Temperature Range of 200−1000 K Using TST and CVT Methods TST

The present work demonstrates that the calculated branching ratio for the title reaction proceeds via the addition reaction channel and the contribution from abstraction being negligible. It is discernible here that the % contribution of the individual reaction rate to the overall rate constant accompanying the addition channel in the temperature range between 200 and 1000 K displayed in Figure 6 suggests that the •OH attack on the terminal carbon is favored over that for the nonterminal one.

CVT

T (K)

kR3 × 1012

kR4 × 1015

kTotal × 1012

kR3 × 1012

kR4 × 1015

kTotal × 1012

200 298 400 500 600 700 800 900 1000

1.86 1.23 1.17 1.24 1.37 1.52 1.72 1.93 2.17

6.38 5.26 5.60 5.97 6.35 6.83 7.39 8.03 8.74

1.87 1.24 1.18 1.25 1.38 1.53 1.73 1.94 2.18

1.74 1.16 1.10 1.18 1.28 1.38 1.50 1.67 1.83

5.46 4.57 4.75 4.99 5.17 5.41 5.70 6.00 6.33

1.75 1.17 1.11 1.19 1.29 1.39 1.51 1.68 1.84

temperatures. The ratios of TST and CVT in the studied temperature range reported in the Table S4 of the Supporting Information turn out to be greater than unity, which suggests that the variational effect is significant. A plot of the rate constants calculated using TST and CVT vs 103/T within the temperature range of 200−1000 K for the addition channel is displayed in Figure 5. It may readily be noticed that the

Figure 6. Branching ratios vs 103/T within the temperature range of 200−1000 K for the addition channel (reactions R3 and R4).

3.3. Atmospheric Implication. 3.3.1. Atmospheric Lifetimes. The atmospheric lifetimes of HFO-1345fz due to its reaction with tropospheric •OH is estimated from 1 τ= k OH[OH] (7) where kOH denotes the rate constant for the reaction of the title molecule with the OH radical and [OH] is the average concentration of the OH radical in the atmosphere. The atmospheric lifetime of HFO-1345fz was estimated to be 9.3 days considering the average OH radical concentration34 as 1× 106 molecules cm−3, consistent with the atmospheric lifetime of 9 days reported by Jiménez et al.9 and 8.5 days reported by Sulbaek Andersen et al.8 3.3.2. Global Warming Potential (GWP). The GWP35 provides a relative measure of how much heat is trapped by a greenhouse gas. The GWP for HFO-1345fz relative to CO2 was calculated on the basis of harmonic vibrational frequencies (vk), IR intensities (Ak) of corresponding frequencies, and the atmospheric lifetime (τ) of the HFO-1345fz reaction with •OH obtained within the framework of M06-2x/6-311++G(d,p) theory using the equation36

Figure 5. Rate constant for reactions R3 (k × 1012) and R4 (k × 1015) pathways derived from TST and CVT as a function of 103/T between 200 and 1000 K.

addition channel reveals nonlinear behavior over the temperature range studied. Moreover, the variational effect is significant for reaction R4 compared to that for reaction R3. To the best of our knowledge, experimental or theoretical studies comparing the rate constants for reactions R3 and R4 for different temperatures have not been reported as of yet. The rate constant calculated from the CVT at 298 K turns out to be 0.998 times lower than that determined using the CTST method. The branching ratio that provides contribution from the individual channel to overall rate constant was determined by the equation Branching Ratio =

k individual % koverall

TH

GWP( i H) =

∫0 RF( i t ) dt TH

∫0 RFCO2(t ) dt

=

AGWP(TH) i AGWPCO2 (TH)

(8)

If Ak is the radiative forcing efficiency (RF), τ is the atmospheric lifetime for HFO-1345fz, and TH is the time horizon. Assuming that removal of HFO-1345fz from the

(6) K


Article


Table 5. Atmospheric Lifetimes, τ (in days), Total Radiative Forcing, Ai (in (W m−2 ppbv−1), and GWP at 20, 100, and 500 year Time Horizons (THs) τ

Ai

9.3 9.0 7.6

0.026 0.022 0.176

GWP TH = 20 years

TH = 100 years

TH = 500 years

1.12 0.89 6.00

0.36

0.11

2.00

1.00

■

atmosphere follows exponential decay, the AGWPi, the absolute GWP,37 can be calculated from ⎛ ⎛ TH ⎞⎞ ⎟⎟ AGWPi = A i τ ⎜1 − exp⎜ − ⎝ τ ⎠⎠ ⎝

∑ Ak F(υk̅ ) k

this work Jiménez et al.9 Andersen et al.39

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11312. Vibrational frequencies, energy barrier and thermodynamic properties for the abstraction channel, BSSEcorrected binding energies, ratios of the rate constants, and optimized structures of the species along the hydrogen abstraction channel (PDF)

(9)

where Ai denotes the total instantaneous IR radiative forcing efficiency in W m−2 ppbv−1. The radiative forcing efficiency38 was further calculated from RFi =

ref

■

(10)

The frequencies below 3000 cm−1 were used for the determination of instantaneous radiative forcing (Ai). In the above F(υk) is the radiative forcing function per unit cross section per wavenumber in W m−2 (cm−1) −1 (cm2 molecule−1) −1 . In the present work, AGWP for CO2 (reference compound) was taken to be 0.192, 0.676, and 2.223 W m−2 ppm−1 for 20, 100, and 500 years time horizon.39 Calculated atmospheric lifetimes and GWPs are compared with the experiments in Table 5. The lower GWP parameters further suggest that the unsaturated HFO-1345fz should find applications such as a better foam expansion agent over currently used HFCs or HCFCs.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-20 2560122. Fax: +91-20-225691728. ORCID

Shridhar P. Gejji: 0000-0003-3305-2475 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS P.K.R. is thankful to the University Grant Commission (UGC) for providing financial support in the form of a D. S. Kothari postdoctoral fellowship. S.P.G. acknowledges support from the Research Project (37(2)/14/11/2015-BRNS) from the Board of Research in Nuclear Sciences (BRNS), India. Authors thank the Center for Development of Advanced Computing (CDAC), Pune for providing the National Param Supercomputing Facility.

4. CONCLUSIONS Detailed systematic analysis of atmospheric degradation routes and degradation products accompanying the gas-phase reaction of HFO-1345fz by OH radical has been carried out. The temperature dependence of the absolute rate constant over a wide temperature range of 200−1000 K has been given. Different channels (hydrogen abstraction and •OH addition) for oxidative degradation of HFO-1345fz by •OH involving isomerization−decomposition pathways in the absence of NOx and reaction with atmospheric O2 in the presence of high NOx atmospheric conditions have been studied. The calculated branching ratio suggests that the addition channel contributes more than 99% to the overall reaction rate and electrophilic addition of •OH occurs predominantly via the terminal carbon of the double bond. The estimated lifetime for the HFO-1345fz was about 9.3 days. In other words, they should survive longrange transport in the troposphere and the influence of secondary products. The present calculations suggest that CF3CF2C(O)CH2(OH) is the major degradation product in the presence of high NOx atmospheric conditions, whereas CH2(O) is favored in the absence of NOx with the CF3CF2CH(OH)CH(O) and CH2CH(OH) being formed to the lesser extent. Formation of CxF2x+1C(O)OH (PFCAs) was not observed, and therefore, atmospheric degradation of CF3CF2CHCH2(HFO-1345fz) by •OH is not expected to be a source of bioaccumulative PFCAs. Finally, it has been concluded that HFO-1345fz seems to be a better foam expansion agent over the saturated HFCs or HCFCs owing to its short lifetime and low GWP.

■

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