Thermal Decomposition of 2,3,3,3- and trans-1,3,3,3

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Thermal Decomposition of 2,3,3,3- and trans-1,3,3,3-Tetrafluoropropenes Akira Matsugi, and Kazuo Takahashi J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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The Journal of Physical Chemistry

Thermal Decomposition of 2,3,3,3- and trans-1,3,3,3-Tetrafluoropropenes Akira Matsugi*a and Kazuo Takahashib

a

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa,

Tsukuba, Ibaraki 305-8569, Japan. b

Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku,

Tokyo 102-8554, Japan *Corresponding Author. E-mail: [email protected]

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ABSTRACT The thermal decomposition reactions of 2,3,3,3- and trans-1,3,3,3-tetrafluoropropenes (TFPs) have been studied both experimentally and computationally to elucidate their kinetics and mechanism. The experiments were performed by observing the temporal profiles of HF produced in the decomposition of the tetrafluoropropenes behind shock waves at temperatures of 1540–1952 K (for 2,3,3,3-TFP) or 1525–1823 K (for trans-1,3,3,3-TFP) and pressure of 100–200 kPa in Ar bath. The reaction pathways responsible for the profiles were explored based on quantum chemical calculations. The decomposition of 2,3,3,3-TFP was predicted to proceed predominantly via direct 1,2-HF elimination to yield CHCCF3, while trans-1,3,3,3-TFP was found to decompose to HF and a variety of isomeric C3HF3 products including CHCCF3, CF2CCHF, CCHCF3, and CF2CHCF. The C3HF3 isomers can subsequently decompose to either CF2 + CHCF or CF2CC + HF products. Multichannel RRKM/master-equation calculations were performed for the identified decomposition channels. The observed formation rates and apparent yields of HF are shown to be consistent with the computational predictions.


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1. INTRODUCTION Hydrofluoroolefins (HFOs), such as tetrafluoropropenes (TFPs), have been proposed as replacements for conventional hydrofluorocarbon (HFC) refrigerants. In particular, 2,3,3,3-TFP (HFO-1234yf) and

trans-1,3,3,3-TFP (HFO-1234ze(E)) have

received

considerable attention as promising alternatives owing to their short atmospheric lifetimes and hence low global warming potentials.1 HFOs generally have shorter atmospheric lifetimes than saturated HFCs mainly owing to their enhanced reactivity arising from the presence of a carbon–carbon double bond.2–4 Evaluating their environmental impact is crucial for the development of alternative refrigerants, and a number of studies have been reported on the atmospheric chemistry of TFPs.1–6 Another important aspect for risk assessment of HFOs use is that the high reactivity of HFOs can also reduce their thermal stability that potentially increases their decomposition reactivity and flammability.7–10 Therefore, a detailed understanding of their fundamental combustion and thermolysis behaviors is important to ensure safe use. Nevertheless, kinetic data for high-temperature reactions related to combustion and pyrolysis of HFOs are very sparse.11 In the present study, the thermal decomposition of 2,3,3,3- and trans-1,3,3,3-TFPs 2,3,3,3-TFP → products

(R1)

trans-1,3,3,3-TFP → products

(R2)

has been studied by shock tube/laser absorption experiments and quantum chemical and RRKM/master-equation calculations. Experimentally, the temporal absorption profiles of hydrogen fluoride (HF) produced from the decomposition reactions were recorded behind reflected shock waves. The profiles indicated the existence of secondary HF formation reactions as well as the direct formation from R1 and R2. They were analyzed on the basis of computationally suggested reaction mechanism to reveal their kinetics and reaction pathways.



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2. METHODS 2.1. HF Measurement behind Shock Waves. The experiments were performed using a double-piston-actuated diaphragm-less shock tube. The apparatus and method have been described elsewhere,12,13 and only specific details are presented here. HF produced from the thermal decomposition of TFPs behind the reflected shock waves was detected by its IR absorption near 2476 nm. The HF concentration was calculated from Beer's law using the absorption cross-section parameterized in the previous study, based on the Rausian– Sobel’man line profile,14 with the estimated uncertainty of 5%.12 The sample gases used were mixtures of 20 or 50 ppm of 2,3,3,3-TFP (>99% purity; Honeywell International Inc.) or trans-1,3,3,3-TFP (>99% purity; Honeywell International Inc.) diluted in Ar (>99.9999% purity; Taiyo Nippon Sanso). The mixtures were prepared from pressure measurements using capacitance manometers in a glass vacuum line, stored in glass vessels, and allowed to homogenize for at least 12 hours prior to the experiments. The measurements were performed for the temperature ranges of 1540–1952 K (2,3,3,3-TFP) and 1525–1823 K (trans-1,3,3,3-TFP) at pressures of ~100 and ~200 kPa. Experimental conditions and the observed HF profiles are summarized in Figs. S1 and S2 in the Supporting Information. 2.2. Quantum Chemical Calculations. Quantum chemical calculations were performed to unravel possible reaction pathways in the decomposition reactions of 2,3,3,3and 1,3,3,3-TFPs and related secondary reactions. Geometries of stationary points were optimized using density functional theory with the ωB97X-D hybrid functional15,16 and the split-valence 6-311++G(d,p) basis set implemented in the Gaussian 09 program.17 Numerical integration of the exchange-correlation potential was performed with an ultrafine grid with 99 radial shells and 590 angular points per shell. Harmonic vibrational frequencies were also calculated at the same level of theory. The optimized geometries and harmonic frequencies are listed in Table S1 of the Supporting Information. Single-point energies at the optimized 4 ACS Paragon Plus Environment

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structures were refined by an explicitly correlated coupled cluster method, CCSD(T)-F12b,18– 20

with correlation-consistent polarized valence triple-ζ basis sets optimized for explicitly

correlated methods, cc-pVTZ-F12.21 The studied energy surface involved a transition state for internal rotation around carbon-carbon double bond that exhibited open-singlet character and cannot properly be described by single-determinant basing methods. For this transition state, geometry optimization and vibrational analysis were performed using the spin-unrestricted ωB97X-D method and the potential energy was evaluated as the CCSD(T)-F12b energy of the corresponding triplet state subtracted by the energy gap between the singlet and triplet states. The energy gap was calculated by the explicitly correlated multireference configuration interaction method (MRCI-F12)22–24 with Davidson’s quadruples correction (+Q) using cc-pVTZ-F12 basis sets. The reference wave functions for the MRCI-F12 calculation were obtained from complete active space self-consistent field (CASSCF) calculation with a two-electron/two-orbital active space. Molpro 2012.1 program25 was employed for the coupled-cluster and multireference configuration interaction calculations. 2.3. Rate Calculations. The rate constants for major reaction pathways were evaluated by transition state theory and RRKM/master-equation calculations. The pressure-dependent rate constants for decomposition reactions were calculated as the steady-state solutions to the master equation26,27 using the SSUMES program.28 The decomposition reactions handled in this study were essentially multiple-well systems, and the steady-state decomposition process from a specific reactant was treated as described in ref. 27. This treatment obscures detailed kinetics for interconversion among isomeric wells, yet is considered to be appropriate for the present study because only the overall decomposition processes are of concern. Density of states and microscopic rate constants were calculated using the modified version of UNIMOL program,29 based on the computed rovibrational properties and energies. The frequency and 5 ACS Paragon Plus Environment


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zero-point energy (ZPE) scaling factors of 0.950 and 0.975, respectively, were adopted.30,31 Rigid-rotor and harmonic oscillator approximations were adopted except for internal rotations of the CF3-, CHF2-, and CH2F-rotors, which were treated as hindered rotors by using the Pitzer−Gwinn approximations.32,33 One-dimensional semiclassical tunneling corrections were included by assuming an asymmetric Eckart potential determined using the imaginary frequencies of transition state.34 The collision frequencies were calculated using Lennard-Jones collision parameters estimated by the combining rules (arithmetic mean for the diameters, σ, and geometric mean for the well depths, ε) from the pure gas values. The parameters for the buffer gas, Ar, were taken from literature35 (σ = 3.54 Å and ε/kB = 93 K), while those for the C3H2F4 and C3HF3 isomers were represented by the parameters for 2,3,3,3-TFP (σ = 5.01 Å and ε/kB = 271 K) and trifluoroallene (CF2CCHF; σ = 4.71 Å and ε/kB = 307 K), respectively. The values for the latter two molecules are estimated from the empirical relations36 using the critical temperature and volume estimated by using the group-interaction contribution method.37 The collisional energy transfer probability was estimated by the exponential down model38 with an average downward energy transfer, 〈∆Edown〉, of 1200 cm−1. This value is based on the recent evaluation of 〈∆Edown〉 ≈ 800–1000 cm−1 for the thermal decomposition of 1,1,1-trifluoroethane and 1,1-difluoroethylene13,39 at similar temperature range as the present experiment and its expected molecular size dependence.40 The effects of the value of 〈∆Edown〉 on the resultant rate constants are shown in Fig. S3 in the Supporting Information.


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3. RESULTS AND DISCUSSION 3.1. HF Time Profile. Examples of the HF time profiles observed following the decomposition of the TFPs are shown in Fig. 1. Time zero in the profiles indicates the passage of reflected shock waves where a schlieren spike due to density deflection is visible. HF formation from the decomposition of 2,3,3,3- and trans-1,3,3,3-TFPs exhibited a similar behavior: the HF concentration increased following the passage of the reflected shock waves, and reached an asymptote value as seen in the profiles at high temperatures, which indicates the apparent yield of HF to be about 1.5–1.8 per 2,3,3,3- or trans-1,3,3,3-TFP decomposed. The plateau was not completely flat but a slow and gradual rise of the HF concentration can be seen. These observations suggest that there are at least two secondary reactions contributing to the HF profiles. The unimolecular decompositions of 2,3,3,3- and trans-1,3,3,3-TFPs are anticipated to directly produce HF exclusively through HF elimination pathways. The observed profiles suggest that the remaining C3HF3 isomers decompose with rate constants comparable to, or higher than, those for the decomposition of the TFPs and with the HF yield of about 0.5–0.8. The late gradual rise of HF is possibly caused by decomposition of different C3HF3 isomers at a slower rate or from some bimolecular reactions between the decomposition products. Also, a closer look at the early profile at high temperatures indicated that the formation of HF was accompanied by a short incubation period. A similar behavior has also been observed in the decomposition of fluoroethane, 1,1,1-trifluoroethane, and 1,1-difluoroethylene.12,13 The most plausible explanation for this is that the unimolecular decomposition of the reactants firstly produced vibrationally excited HF, and its relaxation41 is observed as the incubation period.12,13,39 Since the HF profiles are considered to have a complex dependence on kinetics and pathways of the primary and secondary reactions, they were analyzed with the aid of computational investigations on the reaction mechanisms.



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Figure 1. Example of HF mole fraction profiles observed in the decomposition of 2,3,3,3and trans-1,3,3,3-TFPs behind the reflected shock waves. The white lines represent the fitted profiles.

3.2. Reaction Pathways. Figure 2 shows the calculated energy diagrams for the decomposition of 2,3,3,3- and trans-1,3,3,3-TFPs. For both molecules, the predominant reaction pathways were three- or four-center HF elimination channels. Direct bond fission reactions were too endothermic to be feasible and are not depicted here. The weakest bonds of 2,3,3,3- and trans-1,3,3,3-TFPs were both found to be the C-C single bond with the bond energies of 447 and 464 kJ mol−1, respectively, while the C-H and C-F bond energies were larger than 470 kJ mol−1. These values are significantly larger than the barrier heights for the HF elimination channels, and contribution of the bond dissociation channels is expected to be minor as will be described later. The direct dissociation of 2,3,3,3-TFP is possible only through the 1,2-HF elimination 8 ACS Paragon Plus Environment

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pathway to produce 3,3,3-trifluoropropyne (CHCCF3) and HF (channel R1a) with the barrier height of 326 kJ mol–1. There are other reaction pathways in which 2,3,3,3-TFP firstly isomerizes to 1,1,2,3-TFP (CF2CFCH2F) by a 3,1-fluorine shift reaction followed by HF elimination via a three- or four-center transition state to yield 1H-trifluorovinylmethylene (CF2CFCH) + HF (R1b) or trifluoroallene (CF2CCHF) + HF (R1c), respectively. However, these channels have only minor contributions to the decomposition of 2,3,3,3-TFP because of the small density of states for the five-membered ring transition state for the isomerization and the high energy barriers for the elimination transition states compared with the transition state of the direct 1,2-HF elimination from 2,3,3,3-TFP. The decomposition of trans-1,3,3,3-TFP has several reaction pathways. 1,3,3,3-TFP has two stereoisomers depending on the configuration around the double bond. The trans (E) configuration is more stable than the cis (Z) isomer by 10 kJ mol−1. They can isomerize to each other by the internal rotation around the double bond or by the sequential 1,2- and 2,1-hydrogen shift reactions through the 1,3,3,3-tetrafluoropropylidene (CFCH2CF3) intermediate. As mentioned earlier, the potential energy of the transition state for the torsional isomerization was calculated from its triplet-state energy and the energy gap between the singlet and triplet states. This evaluation would not cause a serious error in the present case since the energy gap was computed to be only –2.9 kJmol−1. Both trans and cis configurations of 1,3,3,3-TFP can also isomerize to 1,1,3,3-TFP (CF2CHCHF2) by 3,1-fluorine shift reactions. The barrier heights for the trans–cis and 3,1-fluorine shift isomerization reactions were low enough for the isomerizations to be competitive with the HF elimination reactions. There are several pathways for HF elimination from each of the isomers: 1,2-, 2,3-, and 1,1-elimination from trans-1,3,3,3-TFP, 2,3- and 1,1-elimination from cis-1,3,3,3-TFP, and 1,2-, 2,3-, and 3,3-elimination from 1,1,3,3-TFP. Their transition states have similar energies ranging from 319 to 353 kJ mol−1 relative to trans-1,3,3,3-TFP except



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for the 1,2-elimination from 1,1,3,3-TFP, which has the energy of 408 kJ mol−1. Therefore, four

product

channels,

CHCCF3

+

HF

(R2a),

CF2CCHF

+

HF

(R2b),

2H-trifluoromethylvinylidene (CCHCF3) + HF (R2c), and 2H-trifluorovinylmethylene (CF2CHCF) + HF (R2d), are expected to have comparable branching fractions in the decomposition of trans-1,3,3,3-TFP.

Figure 2. Energy diagrams for the decomposition of (a) 2,3,3,3-TFP (CH2CFCF3) and (b) trans-1,3,3,3-TFP (CHFCHCF3(E)). ZPE-corrected energies relative to the reactants are shown. R1a-c and R2a-e indicate the decomposition channels.


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The experimental HF profiles suggested that the isomeric C3HF3 products in the decomposition of the TFPs can subsequently decompose to produce further amounts of HF. The energy diagram for the decomposition of the C3HF3 isomers is shown in Fig. 3. The global minimum is CHCCF3, and the energies shown are relative to that of CHCCF3. All of the five proposed C3HF3 products are interconnected through isomerization pathways. CHCCF3, the main product in the decomposition of 2,3,3,3-TFP, can directly isomerize to CCHCF3, CF2CCHF, and CF2CFCH by 1,2-hydrogen, 3,1-fluorine, and 3,2-fluorine shift reactions, respectively. CF2CHCF can be reached from these isomers through the following pathways: 3,1-fluorine shift from CCHCF3, 3,2-hydrogen shift from CF2CCHF, and interconversion

of

CF2CFCH

through

the

formation

of

the

cyclic

isomer,

1H-trifluorocyclopropene (c-CHCFCF2). HF can be produced from CF2CCHF by the three-center HF elimination reaction, which leaves difluoropropadienylidene (CF2CC). There exists

another

fragmentation

channel

that

gives

fluoroacetylene

(CHCF)

and

difluoromethylene (CF2) via C=C bond fission of the trifluorovinylmethylenes, CF2CHCF and CF2CFCH. The transition states for the HF elimination, the C=C bond fission, and some of the isomerization reactions have energies comparable with each other; therefore, the two fragmentation channels are expected to be competitive in the decomposition of the C3HF3 isomers. This would explain the apparent HF yields observed in the decomposition of the TFPs.



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Figure 3. Energy diagrams for the decomposition of C3HF3 isomers. ZPE-corrected energies relative to CHCCF3 are shown.

The C3HF3 energy diagram also suggests that the interconversions between the C3HF3 isomers are feasible during their decompositions, indicating that the thermal decomposition rates do not significantly differ among the C3HF3 isomers depicted in Fig. 3. Therefore, the late gradual rise observed in the measured HF profiles is unlikely to be caused by the decomposition of C3HF3 isomers. Instead, bimolecular reactions between the decomposition products may have generated the additional HF. One candidate for such a reaction is the reaction between CHCF and CF2 that proceeds on the same energy surface as C3HF3 decomposition. The addition of CF2 to either carbon site of CHCF has barrier heights of 33– 41 kJ mol−1, forming the chemically activated adducts that can undergo dissociation back to the reactants, isomerization, or subsequent decomposition to CF2CC + HF. In the same manner, CF2CC can add to CHCF to form 1H- and 2H-trifluorobutatrienylmethylene (CF2CCCFCH and CF2CCCHCF) with the computed barrier heights of 8.8 and 10.6 kJ mol–1, respectively. The energy diagram for these chemically activated reactions is shown in Fig. 4. CF2CCCFCH

can

easily

isomerize

to

CF2CCCHCF

through


the

formation

of

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1H-trifluorovinylidenecyclopropene, with the same mechanism as in the isomerization between the trifluorovinylmethylenes. Analogously to the reactions on the C3HF3 surface, the hydrogen shift reaction of CF2CCCHCF forms trifluoropentatetraene (CF2CCCCHF), which then dissociates to difluoropentatetraenylidene (CF2CCCC) and HF. Marked contrast between the CF2 + CHCF and CF2CC + CHCF reactions is that the latter one is much more effective at generating HF because of the lower entrance barrier heights and the lower energies required for the subsequent isomerization and decomposition.

Figure 4. Energy diagrams for the reaction between CF2CC and CHCF. ZPE-corrected energies relative to the reactants are shown.

3.3. Predicted Kinetics. The explored energy surfaces revealed sequential decomposition pathways of the TFPs. They firstly decompose to the isomeric C3HF3 products, which then subsequently decompose to CF2 + CHCF or CF2CC + HF. There also exist succeeding bimolecular reactions that potentially contribute to the HF formation profiles. Here, the predicted rate constants are presented for each of the reaction steps.



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There are three and five decomposition channels from 2,3,3,3- and trans-1,3,3,3-TFPs, respectively: 2,3,3,3-TFP → CHCCF3 + HF

(R1a)

2,3,3,3-TFP → CF2CFCH + HF

(R1b)

2,3,3,3-TFP → CF2CCHF + HF

(R1c)

trans-1,3,3,3-TFP → CHCCF3 + HF

(R2a)

trans-1,3,3,3-TFP → CF2CCHF + HF

(R2b)

trans-1,3,3,3-TFP → CCHCF3 + HF

(R2c)

trans-1,3,3,3-TFP → CF2CHCF + HF

(R2d)

trans-1,3,3,3-TFP → CFCCHF2 + HF

(R2e).

The RRKM/master-equation calculations were performed considering all of the wells and dissociation channels shown in Fig. 2. As noted before, contribution of the direct bond dissociation channels are expected to be minor because of the high energies required for them. Here, the branching fraction for the bond cleavage channel was preliminarily evaluated for the channel having the weakest bond energy: 2,3,3,3-TFP → CH2CF + CF3

(R1d).

The microscopic rate constant for this barrierless channel was estimated as an inverse Laplace transform of a standard Arrhenius expression of the temperature-dependent rate constant.26 For the Arrhenius expression, the pre-exponential factor was presumed to be 1016 s−1 and the activation energy was assumed to be the same as the bond dissociation energy, 447 kJ mol−1. The master-equation calculation suggested that this channel has the branching fraction less than 0.5% even at the highest temperature studied (2000 K). Therefore, the bond cleavage channels are considered to have negligible contribution to the overall decomposition of the TFPs. Of the HF-elimination channels, the CF2CCHF + HF channel (R1c) in R1 and the


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CFCCHF2 + HF channel (R2e) in R2 were found to have negligible contributions to the decomposition processes; their branching fractions were calculated to be less than 0.1%, and hence, they will not be further discussed. The calculated total rate constants for the decomposition of 2,3,3,3-TFP (k1) and trans-1,3,3,3-TFP (k2) are shown in Fig. 5. The rate constants are in the falloff region under the studied temperature and pressure ranges. The symbols shown are the rate constants estimated from the measured HF profiles, as will be discussed later. The decomposition of trans-1,3,3,3-TFP is a factor of 2–4 faster than that of 2,3,3,3-TFP owing to the number of reaction pathways it can access. Fig. 6 shows the calculated branching fraction for R1a in R1. The decomposition of 2,3,3,3-TFP predominantly produces the CHCCF3 + HF products with a branching fraction larger than 0.95 in the studied temperature and pressure ranges. Its temperature dependence reflects the gap in the threshold energy between R1a and R1b. The branching fraction also depends on pressure because of perturbation in the internal population at high-energy region caused by slow energy transfer rate at low pressures. The branching fractions in the decomposition of trans-1,3,3,3-TFP at a pressure of 100 kPa are shown in Fig. 7. The four channels, R2a–d, are competing, and its order at low temperatures is in accordance with the lowest threshold energies for each channel: 319, 324, 337, and 337 kJ mol−1 for R2d, R2a, R2c, and R2b, respectively. The difference decreases as the temperature increases, and the averaged branching fractions under the experimental conditions are approximately 0.25:0.2:0.2:0.35 for R2a–d, sequentially. The channel-specific rate constants for R1 and R2 at various pressures were parameterized in the modified Arrhenius expression and are given in Table 1.



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Figure 5. Arrhenius plots of the calculated and experimental total rate constants for the thermal decomposition of (a) 2,3,3,3-TFP and (b) trans-1,3,3,3-TFP.

Figure 6. Predicted branching fractions for the CHCCF3 + HF channel (R1a) in the thermal decomposition of 2,3,3,3-TFP. 16 ACS Paragon Plus Environment

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Figure 7. Predicted branching fractions in the thermal decomposition of trans-1,3,3,3-TFP at a pressure of 100 kPa.

Table 1. Modified Arrhenius Expression of the Computed Rate Constants for the Thermal Decomposition Reactions of 2,3,3,3- and trans-1,3,3,3-TFPs in the Temperature Range of 1000–2000 K at Various Pressures.a reaction

A (s−1)

pressure

n

Ea/R (K)

(kPa) 2,3,3,3-TFP → CHCCF3 + HF (R1a)

10

4.08×1061

−13.634

54944

100

1.10×10

44

−8.492

49971

2.27×10

38

−6.852

48201

2.06×10

26

−3.401

44288

10000

2.64×1015

−0.300

40594

10

5.04×10

76

−18.085

64648

4.59×10

61

−13.544

61272

7.07×10

55

−11.843

59627

1.63×10

42

−7.901

55430

10000

1.40×1028

−3.881

50775

10

5.83×10

67

−15.549

55804

1.19×10

52

−10.897

51763

3.04×10

46

−9.271

50112

1000

3.10×1033

−5.526

46042

10000

1.33×1019

−1.423

41265

10

71

−16.554

58146

200 1000 2,3,3,3-TFP → CF2CFCH + HF (R1b)

100 200 1000 trans-1,3,3,3-TFP → CHCCF3 + HF (R2a)

100 200

trans-1,3,3,3-TFP → CF2CCHF + HF (R2b)

2.69×10



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

100

2.83×1056

−12.062

54604

200

6.57×10

50

−10.413

53009

1.41×10

37

−6.458

48831

10000

1.32×1021

−1.872

43562

10

5.24×10

72

−16.886

58849

1.02×10

58

−12.461

55456

2.44×10

52

−10.814

53880

4.37×10

38

−6.834

49697

10000

2.47×1022

−2.184

44363

10

9.66×10

64

−14.799

53684

3.44×10

49

−10.266

49379

2.89×10

44

−8.798

47821

1000

2.08×1033

−5.599

44240

10000

22

−2.435

40504

1000 trans-1,3,3,3-TFP → CCHCF3 + HF (R2c)

100 200 1000 trans-1,3,3,3-TFP → CF2CHCF + HF (R2d)

100 200

a

Page 18 of 42

1.66×10 n

Parameters for the modified Arrhenius expression, k = A (T/K) exp(–Ea/RT), are given.

For the thermal decomposition on the C3HF3 energy surface, the rate constants were calculated for all of the six isomers depicted in Fig. 3. It was found that the predicted rate constant for the CCHCF3 decomposition was nearly identical to that for the CHCCF3 decomposition due to their rapid partial equilibration during the course of the thermal decomposition. For the same reason, the rate constants for the decomposition of c-CHCFCF2, CF2CHCF, and CF2CFCH isomers were also nearly identical. Hereafter, no distinction was made for these kinetically indistinguishable species, and they are represented in lumped form as CHCCF3/CCHCF3 and c-CHCFCF2/CF2CHCF/CF2CFCH. Since each C3HF3 isomer has two product channels, the rate parameters are reported for the following reactions: CHCCF3/CCHCF3 → CF2CC + HF

(R3a)

CHCCF3/CCHCF3 → CF2 + CHCF

(R3b)

CF2CCHF → CF2CC + HF

(R4a)

CF2CCHF → CF2 + CHCF

(R4b)

c-CHCFCF2/CF2CHCF/CF2CFCH → CF2CC + HF

(R5a)


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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


c-CHCFCF2/CF2CHCF/CF2CFCH → CF2 + CHCF

(R5b)

Figures 8 and 9 show, respectively, the total rate constants (k3, k4, and k5) and the branching fractions for the HF formation channels in the thermal decomposition of the C3HF3 isomers at a pressure of 100 kPa. The modified Arrhenius expressions of the channel-specific rate constants are listed in Table 2. The rate constant was largest for the decomposition of c-CHCFCF2/CF2CHCF/CF2CFCH because of the existence of the direct dissociation channels to CF2 + CHCF. The predicted branching fraction also indicated a preference for these channels. The rate constant for the decomposition of CF2CCHF was slightly smaller than those of c-CHCFCF2/CF2CHCF/CF2CFCH; although CF2CCHF has access to the direct HF elimination channel, its relative barrier height is slightly higher than those for the dissociation from c-CHCFCF2/CF2CHCF/CF2CFCH. The thermal decomposition of CHCCF3 was the slowest among the studied isomers; its rate constant was a factor of 2–3 smaller than those for the decomposition of the other isomers at high temperatures. The branching fractions suggest that the experimentally observed secondary HF can be generated from the decomposition of any C3HF3 isomer considered, but its amount depends on the structure of the isomers.



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

Figure 8. Predicted rate constants for thermal decomposition of the C3HF3 isomers at a pressure of 100 kPa.

Figure 9. Predicted branching fractions in the thermal decomposition of the C3HF3 isomers at a pressure of 100 kPa.


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Table 2. Modified Arrhenius Expression of the Computed Rate Constants for the Thermal Decomposition Reactions of the C3HF3 Isomers in the Temperature Range of 1000–2000 K at Various Pressures.a reaction

A (s−1)

pressure

n

Ea/R (K)

(kPa) CHCCF3/CCHCF3 → CF2CC + HF (R3a)

10

9.54×1063

−14.417

56006

100

9.62×10

58

−12.767

55644

6.19×10

55

−11.804

54910

1000

6.97×1047

−9.453

52789

10000

1.09×1038

−6.610

49823

10

2.91×10

65

−14.886

57696

100

1.14×10

61

−13.324

57945

200

3.68×1057

−12.251

57254

1000

2.49×10

47

−9.225

54668

6.71×10

31

−4.714

49927

1.14×10

33

−6.108

38795

100

2.71×1016

−1.209

33868

200

11

0.296

32353

−1

200

CHCCF3/CCHCF3 → CF2 + CHCF (R3b)

10000 CF2CCHF → CF2CC + HF (R4a)

10

1000

3.616

28996

−16

8.081

24201

10

1.68×1040

−8.089

43922

100

1.17×10

29

−4.603

42366

9.82×10

24

−3.361

41635

1.49×10

14

−0.145

39105

10000

1.18×10−2

4.514

34293

10

1.14×1029

−5.022

36623

100

5.28×10

15

−0.962

34820

1.55×10

13

−0.166

34730

1000

2.37×10

8

1.343

34455

10000

1.79×101

3.460

32887

10

25

−4.439

31612

−5

10000 CF2CCHF → CF2 + CHCF (R4b)

200 1000 c-CHCFCF2/CF2CHCF/CF2CFCH → CF2CC + HF (R5a)

200

c-CHCFCF2/CF2CHCF/CF2CFCH → CF2 + CHCF (R5b)

100

8.67×10 3.11×10

9.00×10

4.232

21864

−13

6.567

19154

1000

2.03×10−29

11.266

13718

10000

−45

15.924

8445

200

a

2.00×10

7.10×10 5.04×10 1.27×10 n

Parameters for the modified Arrhenius expression, k = A (T/K) exp(–Ea/RT), are given.



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

Page 22 of 42

The rate constants for the subsequent bimolecular reactions, CF2 + CHCF → products

(R6)

CF2CC + CHCF → products

(R7)

have been calculated for high-pressure limiting conditions. The results can be represented as k6 = 6.07 × 10−21 T

2.661

exp(−3749 K / T) cm3 molecule−1 s−1 and k7 = 2.21 × 10−20 T

2.550

exp(−775 K / T) cm3 molecule−1 s−1, respectively, in the temperature range of 1000–2000 K. They give the rate constants of k6 = 3.5 × 10−13 cm3 molecule−1 s−1 and k7 = 2.9 × 10−12 cm3 molecule−1 s−1 at 1800 K. The former one should have a negligible contribution to the measured HF profiles given that the expected concentrations for these products are ≈1014 molecules cm−3 under the present experimental conditions. Furthermore, the chemically activated adducts formed in R6 must overcome the isomerization barriers, which have comparable heights with the entrance harriers, to produce additional HF. Therefore, R6 was excluded in the kinetic analysis of the HF profiles presented below. On the other hand, the latter reaction has no pronounced isomerization barriers and is considered to generate HF with the yield close to unity. Its rate constant is predicted to be large enough to exhibit a small but observable contribution to the experimental profiles.

3.4. Analysis of the HF Profile. On the basis of the computational investigations, the reactions R1–R5 and R7 are considered to be responsible for the formation of HF from the decomposition of 2,3,3,3- and trans-1,3,3,3-TFPs. However, it is difficult to obtain the experimental rate constants directly from the profile. In particular, because the decomposition reactions of the TFPs and the C3HF3 isomers have similar rate constants as shown in Figs. 4 and 7, contributions from the primary and secondary HF formations cannot be clearly separated. The situation is further complicated in the case of trans-1,3,3,3-TFP, for which multiple decomposition pathways are accessible. Accordingly, no attempts were made to 22 ACS Paragon Plus Environment

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extract rate constants purely from the profile. Instead, the experimental rate constants were retrieved only for the primary decomposition reactions, R1 and R2, assuming that the rest of the kinetic parameters were identical to the computationally predicted values. In the kinetic analysis, HF profiles were simulated using the reaction mechanism comprised of R1–R5 and R7 with the rate constants and branching fractions fixed to those derived from the RRKM/master-equation calculations except for the total rate constants for the decomposition of the reactants, which were determined by nonlinear least-squares fits of the experimental HF profiles to the simulated ones. Here, the reaction R7 was assumed to exclusively produce CF2CCCC + HF with the rate constant equal to that at the high-pressure limit. Furthermore, the CHCCF3 + HF and CF2CCHF + HF channels in R1 and R2 have large excess energies that can be distributed to the vibrational energy of HF. As practiced in the previous studies,12,13 HF formed in these channels was presumed to be produced in its vibrationally excited state, HF*, and its relaxation reaction HF* + Ar → HF + Ar

(R8)

was also induced in the mechanism. The rate constant for R8 was taken from literature.41 This model, ignoring vibrational distribution of the nascent HF, is seemingly crude yet considered to be reasonable for the purpose of deriving the rate constant.39 The simulated profiles are shown in Figs. 1 and S1 and S2 in the Supporting Information. They successfully captured the general behavior of the profiles. The apparent HF yields can be accurately explained by the computed branching fractions. The obtained rate constants for R1 and R2 are listed in Tables 3 and 4, respectively. They are in accord with the RRKM/master-equation predictions as shown in Fig. 4. There were no discernible difference between the rate constants measured at pressures of 100 and 200 kPa beyond the scatter of the data. This result is also consistent with the calculation predicting only weak pressure-dependence of the rate constants between these pressures. These agreements corroborate the suggested decomposition pathways and



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

their kinetics. To examine the effect of changing the rate parameters for the secondary decomposition reactions (R3–5) on the derived rate constants for the primary reactions (R1 and R2), sensitivities for the rate constants were evaluated. The sensitivity, Sk, can be defined as the ratio of the fractional change in the rate constant (obtained from the least-squares fit) for R1 or R2 to the fractional change in presumed values of the rate constants for the secondary reactions. In the calculations, the ratio of the rate constants for the secondary reactions (namely, k3:k4:k5) as well as the branching fractions in the reactions R1–R5 were fixed to the computationally predicted values. The resultant sensitivities are listed in Tables 3 and 4. The sensitivity ranged from −0.08 to −0.67 depending on the temperature. Under the low temperature conditions (< 1650 K), the derived rate constants are only moderately sensitive to the rate parameters for the secondary reactions assumed. The absolute values of the sensitivity increase as the temperature rises, but are still much less than unity. If one assumes that the secondary reactions have the logarithmic uncertainties (∆log10 k) of 0.2 in the predicted rate constants, they can propagate into the logarithmic uncertainty of, on average, ~0.1 in the obtained k1 and k2. This value is smaller than the root-mean-square (RMS) deviations between the experimental and predicted log10 k1 and log10 k2, both of which were 0.17. The root-sum-square of the propagated uncertainty and the RMS deviation is 0.2, which would represent overall uncertainties in the predicted k1 and k2. The uncertainties in the predicted rate constants for R1–R5 are expected to be similar to each other; therefore, it is considered reasonable to assign ∆log10 k = ±0.2 as the uncertainties in the all of the decomposition rate constants computed in the present study. In some of the high-temperature profiles, a small discrepancy was observed in the formation rate of HF at the late-stage. However, the formation rate is too small to quantify the rate constant for the responsible reaction, R7. Indeed, the deviation is visible in some of the 24 ACS Paragon Plus Environment

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profiles but not discernible in others. Therefore, no justification could be presently made for this reaction. Further examination of the succeeding reactions is desirable since bimolecular reactions, such as R7, potentially play important roles for elucidating thermolysis behavior and thermal stability of dense TFPs. Moreover, the HF formation pathways in R6 and R7 suggest existence of further succeeding reactions between CF2(CC)n (n ≥ 2) and CHCF that produce CF2(CC)n+1 + HF. Such reaction is not efficient in the present experimental condition due to the slow rate of R7. Nevertheless, this type of reactions, or reactions between CF2/CF2(CC)n species and unsaturated HFCs in general, may contribute to carbon growth chemistry in pyrolysis of HFOs.

Table 3. Summary of Experimental Conditions and Results of Kinetic Analysis for Reaction R1. k1 (s−1)

temperature

pressure

XTFPa

(K)

(kPa)

(ppm)

1540

106

50

1.5×103

−0.09

50

3.8×10

3

−0.12

3.3×10

3

−0.11

3

−0.16

1598 1599

106 113

50

Skb

1623

105

50

6.5×10

1718

107

50

1.4×104

−0.55

50

2.0×10

4

−0.65

2.4×10

4

−0.67

3.5×10

4

−0.62

4

−0.50

1740 1772 1833

109 105 106

50 50

1889

107

50

4.2×10

1893

103

50

7.2×104

−0.63

50

8.4×10

4

−0.59

3.9×10

3

−0.22

4

−0.57

1952 1615

108 108

20

1759

108

20

1.8×10

1817

105

20

4.2×104

−0.61

20

6.1×10

4

−0.57

4.8×10

3

−0.32

3

−0.42

1889 1654

107 202

50

1720

207

50

8.4×10

1760

203

50

1.9×104

−0.52

50

4

−0.48

1843

206

3.6×10



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

1695 1748

203 205

20

7.0×103

−0.42

20

1.2×10

4

−0.47

4

−0.43

1804

207

20

2.3×10

1819

206

20

2.9×104

−0.45

20

4.1×10

4

−0.45

6.6×10

4

−0.52

1857 1886

208 205

20

a

Initial mole fraction of 2,3,3,3-TFP.

b

Sensitivity of k1 to the rate constants for the secondary reactions.

Table 4. Summary of Experimental Conditions and Results of Kinetic Analysis for Reaction R2. temperature

pressure

XTFPa

(K)

(kPa)

(ppm)

1525

111

50

2.2×103

−0.10

50

3.4×10

3

−0.08

4.9×10

3

−0.10

7.0×10

3

−0.10

4

−0.17

1540 1549 1573

109 107 107

50 50

k2 (s−1)

Skb

1612

107

50

1.1×10

1667

110

50

1.7×104

−0.46

50

2.5×10

4

−0.48

2.8×10

4

−0.50

4

−0.52

1688 1730

108 109

50

1804

108

50

5.8×10

1823

106

50

7.7×104

−0.55

20

5.7×10

3

−0.12

1.2×10

4

−0.22

4

−0.43

1565 1608

109 107

20

1694

109

20

2.5×10

1725

108

20

3.1×104

−0.49

20

6.5×10

4

−0.50

1.6×10

4

−0.46

4

−0.46

1784 1668

106 205

50

1702

204

50

2.1×10

1711

206

50

2.3×104

−0.48

50

5.0×10

4

−0.48

1.3×10

4

−0.40

4

−0.43

1762 1652

204 205

20

1708

209

20

2.6×10

1732

206

20

3.1×104

−0.41

20

4

−0.44

1768

205

5.7×10

a

Initial mole fraction of trans-1,3,3,3-TFP.

b

Sensitivity of k2 to the rate constants for the secondary reactions.


Page 26 of 42

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4. CONCLUSION The combined experimental and computational modeling study revealed the main reaction pathways and rate constants for the thermal decomposition of 2,3,3,3- and trans-1,3,3,3-TFPs. The primary decomposition reactions of the TFPs were found to proceed through HF elimination pathways that leave a variety of isomeric C3HF3 products. 2,3,3,3-TFP predominantly decomposes to CHCCF3 + HF with the 1,2-HF elimination mechanism, while the decomposition of trans-1,3,3,3-TFP was predicted to produce CHCCF3, CF2CCHF, CCHCF3, and CF2CHCF with the branching fractions comparable to each other. The C3HF3 isomers subsequently decompose to either CF2 + CHCF or CF2CC + HF with the rate constants close to those for the decomposition of the TFPs. The branching fractions of these secondary reactions explain the apparent HF yields of 1.5–1.8 seen in the experiment. The predicted rate constants are shown to be consistent with the observed formation rate of HF in the thermal decomposition of both 2,3,3,3- and trans-1,3,3,3-TFPs.

SUPPORTING INFORMATION Experimental conditions and observed HF profiles (Figs. S1 and S2), effects of 〈∆Edown〉 on calculated rate constants (Figs. S3), and optimized geometries and harmonic frequencies for stationary points (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported in part by JSPS KAKENHI Grant Numbers 15K17991 and 27 ACS Paragon Plus Environment


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15K01231.

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(22) Werner, H.-J.; Knowles, P. J. An Efficient Internally Contracted Multiconfiguration-Reference Configuration Interaction Method. J. Chem. Phys. 1988, 89, 5803–5814. (23) Knowles, P. J.; Werner, H.-J. An Efficient Method for the Evaluation of Coupling Coefficients in Configuration Interaction Calculations. Chem. Phys. Lett. 1988, 145, 514– 522. (24) Shiozaki, T.; Knizia, G.; Werner, H.-J. Explicitly Correlated Multireference Configuration Interaction: MRCI-F12. J. Chem. Phys. 2011, 134, 034113. (25) Werner, H-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schutz, M. et al. MOLPRO version 2012.1, a package of ab initio programs. University College Cardiff Consultants Limited: Cardiff, U.K., 2012. (26) Forst, W. Unimolecular Reactions: A Concise Introduction: Cambridge University Press: Cambridge, 2003. (27) Yamauchi, N.; Miyoshi, A.; Kosaka, K.; Koshi, M.; Matsui, H. Thermal Decomposition and Isomerization Processes of Alkyl Radicals. J. Phys. Chem. A 1999, 103, 2723–2733. (28) Miyoshi, A. SSUMES software, rev. 2014.05.20m1, The University of Tokyo: Tokyo, Japan, 2014. (29) Gilbert, R. G.; Smith S. C.; Jordan, M. J. T. UNIMOL Program Suite, School of Chemistry, Sydney University: Sydney, Australia, 1993. (30) Alecu, I. M.; Zheng, J.; Zhao, Y.; Truhlar, D. G. Computational Thermochemistry: Scale Factor Databases and Scale Factors for Vibrational Frequencies Obtained from Electronic Model Chemistries. J. Chem. Theory Comput. 2010, 6, 2872–2887. (31) Matsugi, A.; Shiina, H. Kinetics of Hydrogen Abstraction Reactions from Fluoromethanes and Fluoroethanes. Bull. Chem. Soc. Jpn. 2014, 87, 890–901. (32) Pitzer, K. S.; Gwinn, W. D. Energy Levels and Thermodynamic Functions for Molecules with Internal Rotation. I. Rigid Frame with Attached Tops. J. Chem. Phys. 1942, 10, 428– 440. (33) Knyazev, V. D. Density of States of One-Dimensional Hindered Internal Rotors and Separability of Rotational Degrees of Freedom. J. Phys. Chem. A 1998, 102, 3916–3922. (34) Garrett, B. C.; Truhlar, D. G. Semiclassical Tunneling Calculations. J. Phys. Chem. 1979, 83, 2921–2926.


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TOC Graphic


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Figure 1. Example of HF mole fraction profiles observed in the decomposition of 2,3,3,3- and trans-1,3,3,3TFPs behind the reflected shock waves. The white lines represent the fitted profiles. 88x109mm (600 x 600 DPI)

ACS Paragon Plus Environment


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Figure 2. Energy diagrams for the decomposition of (a) 2,3,3,3-TFP (CH2CFCF3) and (b) trans-1,3,3,3-TFP (CHFCHCF3(E)). ZPE-corrected energies relative to the reactants are shown. R1a-c and R2a-e indicate the decomposition channels. 130x123mm (600 x 600 DPI)


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Figure 3. Energy diagrams for the decomposition of C3HF3 isomers. ZPE-corrected energies relative to CHCCF3 are shown. 64x31mm (600 x 600 DPI)



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Figure 4. Energy diagrams for the reaction between CF2CC and CHCF. ZPE-corrected energies relative to the reactants are shown. 55x26mm (600 x 600 DPI)


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Figure 5. Arrhenius plots of the calculated and experimental total rate constants for the thermal decomposition of (a) 2,3,3,3-TFP and (b) trans-1,3,3,3-TFP. 113x171mm (600 x 600 DPI)



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Figure 6. Predicted branching fractions for the CHCCF3 + HF channel (R1a) in the thermal decomposition of 2,3,3,3-TFP. 65x56mm (600 x 600 DPI)


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Figure 7. Predicted branching fractions in the thermal decomposition of trans-1,3,3,3-TFP at a pressure of 100 kPa. 65x56mm (600 x 600 DPI)



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Figure 8. Predicted rate constants for thermal decomposition of the C3HF3 isomers at a pressure of 100 kPa. 69x64mm (600 x 600 DPI)


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Figure 9. Predicted branching fractions in the thermal decomposition of the C3HF3 isomers at a pressure of 100 kPa. 65x56mm (600 x 600 DPI)



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Table of Contents Image 32x13mm (600 x 600 DPI)


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Thermal Decomposition of 2,3,3,3- and trans-1,3,3,3

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