On the kinetics of thermal electron attachment to perfluoroethers

Chemical Physics Letters 519–520 (2012) 25–28

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On the kinetics of thermal electron attachment to perfluoroethers J. Kopyra a,⇑, J. Wnorowska a, W. Barszczewska a, S. Karolczak b, I. Szamrej a a b

Department of Chemistry, Siedlce University, 3 Maja 54, 08-110 Siedlce, Poland Institute of Applied Radiation Chemistry, Technical University of Łódz´, Wróblewskiego 15, 93-590 Łódz´, Poland

a r t i c l e

i n f o

Article history: Received 8 August 2011 In final form 4 November 2011 Available online 11 November 2011

a b s t r a c t In this Letter we report the results of the measurements of the rate coefficients for thermal attachment to several perfluoroethers namely perfluorodiglyme (C6F14O3), perfluorotriglyme (C8F18O4), perfluoropolyether (CF3–(OCF(CF3)CF2)n–(OCF2)m–OCF3) and perfluorocrownether ((C2F4O)5). Rate coefficients were obtained under thermal conditions in the temperature range 298–378 K. The increase of the rates with temperature follows the Arrhenius law and the activation energies have been obtained from the slope of the ln(k) vs. 1/T. The respective values of the rate coefficients (at 298 K) and activation energies are as follows: 7.7 ± 1.2  1011 cm3 s1 (0.18 ± 0.005 eV), 6.7 ± 2.1  1011 cm3 s1 (0.25 ± 0.004 eV), 2.1 ± 0.2  1010 cm3 s1 (0.16 ± 0.010 eV), 3.1  1011 cm3 s1 (0.27 ± 0.003 eV) for C6F14O3, C8F18O4, CF3–(OCF(CF3)CF2)n–(OCF2)m–OCF3 and (C2F4O)5. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Over the last decades there have been many investigations of low energy electron attachment to halocontaining aliphatic hydrocarbon molecules [1–4]. This is partly due to the use of halosubstituted molecules in a number of technological applications like plasma etching industry, gas discharge lasers, gaseous dielectrics, electronic industry. Chlorofluorocarbons (CFCs) have been designed to possess particular physical properties as high volatility, low-toxicity, non-flammability, thermal stability and chemical inertness. However, the lack of reactivity is also the major drawback of such compounds since, in the 1970s, it became clear that they are accumulating in the stratosphere and effectively deplete the earth’s ozone shield [5]. At present production and application of CFCs and most of the halons is prohibited in the developed countries, although they are still used in selected installations. Fluorosubstituted ethers appear to be promising replacements for chlorofluorocarbons. They are considered as ‘environmentally friendly’ molecules because they have zero ozone depletion potential (fluorine is inert in the stratosphere). The atmospheric lifetimes of fluorocontaining ethers which have at least one hydrogen atom within the molecular structure are much shorter [6] in comparison to those of CFCs. They can be removed from the troposphere by oxidation with highly reactive radical species, e.g., OH radicals [7]; thereby reducing their global warming potential. However, to the authors’ knowledge there are no data on the atmospheric lifetimes of perfluorinated ethers. Besides the less harmful effects on the earth atmosphere ethers properties fulfill

⇑ Corresponding author. E-mail address: [email protected] (J. Kopyra). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.11.018

the highly demanding applications such as cleaning agents, foaming additives or as heat exchange fluids. Here we report for the first time the swarm results of electron attachment to perfluoroethers C6F14O3, C8F18O4, CF3– (OCF(CF3)CF2)n–(OCF2)m–OCF3 and (C2F4O)5 in carbon dioxide buffer gas. The experiments have been carried out to obtain rate coefficients (k) of the thermal attachment processes and to probe their temperature dependence. It appears that the molecules attach electrons with relatively low rate coefficients that are accompanied with relatively high activation energies. These kinetic data may be of great importance if the strong temperature dependence would be of interest for the practical applications. Unlike perfluoroalkanes (with comparable length of an alkyl chain) the presently studied perfluoroethers attach electrons purely dissociatively as appears from the beam studies [8]. The only exception is perfluorocrownether where both dissociative and non-dissociative processes have been reported. The observation of both mechanisms for the latter system in principle could influence the temperature dependence of the rate coefficient. It is known from the earlier studies [9] that for molecules that undergo dissociative electron attachment the increase of the temperature results in increase of the k value, while in the case of pure non-dissociative attachment in decrease of the k value. This will be discussed in detail later in the letter. 2. Experiment The experimental procedure of the time-resolved electron swarm technique, known also as a Pulsed-Townsend (PT) technique, has been described in detail previously [10]. The main part of the experimental set-up consists of a cylindrical chamber with two parallel stainless-steel electrodes. The chamber can be heated

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J. Kopyra et al. / Chemical Physics Letters 519–520 (2012) 25–28

using heating jacket produced by the Watlow company. An electronic controller enables stabilization of the temperature within 0.5 °C. An electron swarm is photoelectrically produced using a 5 ns Nd:YAG laser operating on fourth harmonics (266 nm, 10 Hz). The laser beam after passing through the optics is focused by converging lens. It then enters the chamber through a quartz window and strikes the center of the photocathode. In the presence of a carbon dioxide used as the buffer gas the generated photoelectrons very quickly reach steady-state equilibrium of energy distribution. The concentration of the electron acceptor was chosen to obtain the rate of electron disappearance in a range of 105 molecule cm3 s1. In the PT technique electrons traversing in the homogeneous electric field E, induce a change of the anode potential. In the pure buffer gas this potential grows linearly as electrons move towards the collecting electrode. In the mixture consisting of scavenger and buffer gas the electrons are captured by the scavenging gaseous additives, therefore the increase of the potential is no longer linear. The output signal is amplified by preamplifier, registered on the oscilloscope and stored in the computer memory. The thermal electron attachment rate coefficient is determined from the shape of the pulse of the temporal variation of the collector potential. The scavenger–buffer gas mixtures were prepared inside the chamber by injecting the investigated gas and then introducing the buffer gas to obtain total pressure of the gas mixtures in the range 300–610 Torr (1 Torr = 3.24  1016 molecule cm3 at T = 298 K). The experiments were carried out at a few different initial concentrations of electron scavenger in carbon dioxide. Commonly 50 pulses were registered for a given density reduced electric field value (E/N), and then averaged and analyzed. The procedure was repeated for several E/N values in the range of (1.5 – 3)  1017 V cm2 molecule1, at which electrons in carbon dioxide are in thermal equilibrium with gas molecules. The investigated compounds C6F14O3 perfluoro(diethylene glycol dimethyl ether); perfluorodiglyme, C8F18O4 perfluoro(triethylene glycol dimethyl ether); perfluorotriglyme and (C2F4O)5 eicosafluoro-[15]-crown-5 ether; perfluorocrownether were provided by Exfluor Research Corp. with a stated purity of 98%, 98% and 99%, respectively. While CF3–(OCF(CF3)CF2)n–(OCF2)m–OCF3 Galden HT135; perfluoropolyether, with a purity of 98%, was purchased from Solvay Solexis S.p.A. These chemicals were used after degassing by freeze–pump–thaw cycles with liquid nitrogen. The CO2 gas from Fluka with quoted purity of 99.998% was used without further purification.

3. Results and discussion In this work we have investigated an electron attachment to a series of fluoro-substituted ethers namely perfluorodiglyme, perfluorotriglyme, perfluoropolyether and perfluorocrownether (for the molecular structure see Figure 1). The results of the fundamental quantities such as rate coefficients (k) and activation energies (Ea) that have been obtained within our studies are summarized in Table 1. To the best of our knowledge there are no available kinetic data on electron attachment to these molecules. The only paper that reports the total absolute electron attachment rate coefficients for fluoroethers comes from Oak Ridge National Laboratory [11]. The data were, however, obtained for much smaller molecules such as fluorodimethylethers: CF3OCF3, CF3OCF2H, CF2HOCF2H, CF3OCH3. In that case the experiments were performed in N2 and Ar as the buffer gases, which are known as weak thermalizing agents. This means that the rate coefficients (approximately 1012 cm3 s1 for the lowest mean electron energy applied and above thermal energy) were obtained under non-thermal conditions. Thus any direct comparison with presently obtained

A B

O

F3C

F3C

O

CF2CF2

CF2CF2

C

O

O

CF2CF2

CF2CF2

O

O

CF3

CF2CF2

O

CF3

CF3-(OCFCF2)n-(OCF2)m-OCF3 CF3

D

F2

F2 F2

O

F2

F2

O

O

F2 O F2

O

F2 F2

F2

Figure 1. Molecular structures of perfluoroethers under investigation: (A) – perfluorodiglyme (C6F14O3); (B) – perfluorotriglyme (C8F18O4); (C) – perfluoropolyether (CF3–(OCF(CF3)CF2)n–(OCF2)m–OCF3) and (D) – perfluorocrownether ((C2F4O)5).

Table 1 Thermal electron attachment rate coefficients k(298 K) and activation energies Ea derived from the kinetic data for some perfluoroethers.

a

Molecule

k(298 K) (cm3 s1)

Ea (eV)

Trange (K)

C6F14O3 C8F18O4 CF3–(OCF(CF3)CF2)n–(OCF2)m–OCF3 (C2F4O)5

7.7 ± 1.2  1011 6.7 ± 2.1  1011 2.1 ± 0.2  1010 a 3.1  1011

0.18 ± 0.005 0.25 ± 0.004 0.16 ± 0.010 0.27 ± 0.003

298–368 298–378 298–378 318–368

Present data extrapolated to 298 K.

(under thermal conditions) k values for the perfluoroethers is not justified. Nevertheless it is worth noting that in the case of perfluorodimethyl ether the rate coefficient was slightly smaller than for partially fluorosubstituted ethers. A similar trend was observed for the fluorocarbons, where the thermal electron attachment rate coefficients for the fluoro-containing ethanes, e.g., CH3CHF2 (7.6  1013 cm3 s1), CH3CF3 (1.0  1012 cm3 s1), CH2FCF3 (3.7  1012 cm3 s1) [12] were one to two orders of magnitude higher than for perfluoroethane C2F6 (<1  1013 cm3 s1) [13]. It seems that it is more rationale to compare our data for perfluoroethers with these for perfluoroalkanes [n-CNF2N+2 (N = 1–6)] that have been studied in a high pressure swarm apparatus. Hunter and Christophorou [13] reported that for CF4 and C2F6 the electron attachment is purely dissociative process and the rate coefficients were below the detection limit of the system; they concluded that k values for both molecules are <1013 cm3 s1. For C3F8, n-C4F10, n-C5F12 and n-C6F14 the thermal attachment rates increase with the size of the molecule. The respective rate coefficients are equal to 1.8  1012, 4.0  1011, <1.5  1010 and 2.3  1010 cm3 s1. For all four molecules the formation of the parent negative ion was reported as the predominant process at the gas pressures used in the experiments. This was confirmed by single collision experiments where the formation of long-lived parent anion for n-C4F10, n-C5F12 and n-C6F14 was observed [14,15]. Further examples of perfluorocompounds which form the non-decomposed molecular anion are SF6 and C6F6 both showing pronounced resonance peaks close to zero eV [16,17]. Based on these observations, similar behavior was in fact expected for perfluoroethers under investigation. Recent electron attachment studies on perfluorodiglyme, perfluorotriglyme and perfluorocrownether [8] by means of electron-molecule beam

J. Kopyra et al. / Chemical Physics Letters 519–520 (2012) 25–28

technique revealed however that only in the case of perfluorocrownether the molecular anion is formed and at energy distinctly above zero eV; with a peak maximum at around 2 eV. Otherwise, the fluorocompounds undergo efficient fragmentation and the most intensive dissociative channel for these three molecules results in the formation of anionic fragment due to the cleavage of the O–C bond. In these systems fragmentary anions appear exclusively at low energy domain 0–4 eV. The only exception is represented by F, which in the case of perfluorodiglyme and perfluorocrownether is observable via two broad and clearly separated resonances in the energy range 3–6 eV and 12–16 eV while from perfluorodiglyme F is visible as neighboring (dual) signals in the energy range 3–7 eV. In our PT experiment the disappearance of the electrons from the swarm is monitored. The obtained rate coefficients correspond therefore to (an overall) attachment processes including all the reaction channels. The results for the investigated systems in terms of k vs. T and ln(k) vs. 1/T are shown in Figure 2. These data represent the average of a few measurements (three to five series) carried out at different initial pressures of perfluoroethers in the range 0.110–0.134 Torr, 0.079–0.099 Torr, 0.033–0.051 Torr and 0.070– 0.110 Torr for C6F14O3, C8F18O4, CF3–(OCF(CF3)CF2)n–(OCF2)m– OCF3 and (C2F4O)5, respectively. As one can see the rate coefficients obtained at 298 K (Table 1) for the set of fluoro-substituted ethers are within the same order of magnitude. A few comments can be made after an inspection of the Table 1 and Figure 2: A. There is a strong increase in the rate coefficients with rising the temperature. B. The rate coefficients for the perfluoroethers, even in the relatively narrow temperature range (298–378 K), increase by two orders of magnitude. C. The elongation of the chain of the molecules does not have a strong influence on the rate coefficient values. In fact the values of rate coefficients for investigated systems change in a different manner than for perfluoroalkanes. We observe a slight decrease of rates with increase of the number of the carbon atoms (hence also the number of fluorine atoms) by going from perfluorodiglyme to perfluorotriglyme and to perfluorocrownether. The only exception is perfluoropolyether for which the rate coefficient is by order of magnitude higher than those obtained for the other molecules. For all systems, as mentioned above, we observed strong Arrhenius-type rise of the thermal attachment rate coefficients for the temperature range from 298 K to 378 K (T = Te = TG) (Figure 2).

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The validity of the Arrhenius equation has been recently theoretically investigated by Fabrikant and Hotop [18] by means of R-matrix calculations. They have demonstrated that in both exo- and endothermic reactions the equation holds over a finite intermediate temperature range. Since the present experiments have been conducted in the temperature range where, according to above mentioned calculations, the Arrhenius law holds and because we observe clear linear dependence of ln(k) vs. 1/T the activation energies can indeed be obtained from the slope of the curves in Figure 2 (right panel). The values of activation energies Ea are listed in Table 1. Ea is quite frequently regarded as the energy of the crossing point between the potential curve of neutral molecule and the respective anion curve. However, due to the tunneling effect that can take place from vibrational level below the barrier it is likely that the activation energy of the process is slightly lower than the energy of the crossing point. The activation energy thus defines effective barrier energy for the electron attachment process. Up to date there are several approaches that aim to correlate the values of rate coefficients with the molecular and structural parameters. Christophorou [1] has demonstrated that the rate coefficients for a group of haloalkanes depend on EA and/or VAE (EA – electron affinity, VAE – vertical attachment energy). Later on similar dependence of kbeam vs. VAE (kbeam – rate coefficient derived from beam experiments) has been proposed by Burrow et al. [19]. Hotop et al. [20] have shown the relation of the k values with the polarizability of the molecule. This relation is, however, valid only for highly halogenated halocarbons. Another relation has been established by Szamrej et al. [2,3]. They have demonstrated that the rate coefficients depend on the polarizability of the attaching center of the molecule, a part of molecule that is responsible for the capture of the electron. Recently we proposed [10,21] the exponential relationship between the dissociative electron attachment (DEA) rate coefficient determined at room temperature k(298 K) for a set of halo-containing alkanes and the activation energy. This relation can be expressed by the Arrhenius equation

kð298 KÞ ¼ A exp

  Ea kB T

ð1Þ

where preexponential factor A can be regarded as rate coefficient when the activation energy for the electron attachment process is equal zero (like in the case of CCl4, where k value is equal to 3.79  10–7 cm3 s1), which means that every collision is effective and leads to the products. Factor A can also be estimated with an analytical approximation of Troe and co-workers [22] that gives the rate coefficient for capture processes in the range

Figure 2. Dependence of k vs. T and ln(k) vs. 1/T for perfluoroethers: (s) – perfluorodiglyme (C6F14O3); (h) – perfluorotriglyme (C8F18O4); (D) – perfluoropolyether (CF3– (OCF(CF3)CF2)n–(OCF2)m–OCF3) and (e) – perfluorocrownether ((C2F4O)5).

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(2.8–3.8)  10–7 cm3 s1 (at T = 300 K) for polar and polarizable molecules. In our previous paper [21] it has been assumed that value of A is equal to 5  10–7 cm3 s1 as determined taking into account the maximum s-wave scattering cross section p(k/2p)2, where k is the de Broglie wavelength of the electron. While kBT in Eq. (1) is a constant value at given and constant T (at T = 298 K, kBT = 25.68 meV). This exponential relation between k and Ea has been theoretically confirmed by R-matrix calculations for temperature-dependent rate coefficients of exothermic dissociative attachment reactions with barrier towards dissociation [23]. The question can be raised whether the other groups of molecules would follow the proposed exponential relation between k(298 K) and Ea. There are already known some molecules, e.g., sulfur containing compounds (SF5C6H5, SF5C2H3, S2F10, SF5Br, SF5Cl) [24,25] for which the DEA reactions do not obey the Arrhenius relation. In these cases the relatively low rates are accompanied with low activation energy. In some cases as for SF5Cl there is no temperature dependence of k. Here we provide an evidence that also in the case of perfluoroethers namely perfluorodiglyme, perfluorotriglyme and perfluoropolyether the increase of the rate coefficients with growing temperature follows the Arrhenius behavior. Not unexpected is that the respective rate coefficient and activation energy data pairs (Table 1), to a good approximation, are consistent with the recently proposed relation between k and Ea [10,21]. Even for the system like perfluorocrownether that forms long-lived parent anion (as it is obvious from the crossed beam experiment mentioned above) the relation is valid; although, as was suggested by Hotop et al. [23], in such cases the behavior might be different due to the temperature dependent autodetachment time of the transient anion and also the necessity of vibrational redistribution or collisional removal of the excess energy of the complex anion. Similar temperature dependence of the rate coefficient as for perfluorocrownether was observed for cyclic compound perfluoromethylcyclohexane. In this case Alge et al. [26] reported the formation of the parent negative ion via a three-body process with a helium bath gas as a stabilizing agent. Since in our experiment we do not observe any pressure dependence of k value in a covered pressure range (419–606 Torr) the stabilization of the molecular anion most likely proceeds via intramolecular vibrational redistribution. In such cases its likely that a kind of deviation from the Arrhenius behavior might appear at high temperatures due to a stronger competition of the DEA reactions and formation of the metastable parent ion and/or because the formation of the parent anion may not be any more favorable at high temperatures. Unfortunately, due to the limitation of working conditions in our apparatus we are only able

to study the rate coefficients over a relatively narrow temperature range. In order to explore the behavior of the systems that form non-decomposed molecular anions additional measurements in a broader temperature range would be necessary. Acknowledgments This work has been supported by the Polish Ministry of Science and Higher Education (scientific funds for the years 2008–2011) and the Grant No. 204 057 31/1485. References [1] L.G. Christophorou, Z. Phys. Chem. 195 (1996) 195. [2] W. Barszczewska, J. Kopyra, J. Wnorowska, I. Szamrej, J. Phys. Chem. A 107 (2003) 11427. [3] W. Barszczewska, J. Kopyra, J. Wnorowska, I. Szamrej, Int. J. Mass Spectrom. 233 (2004) 199. [4] D. Smith, P. Spanel, Adv. At. Mol. Opt. Phy. 32 (1994) 307. [5] M.J. Molina, F.S. Rowland, Nature 249 (1974) 810. [6] S.A. Montzka, et al., Controlled substances and other source gases, Chapter 1 in World Meteorological Organization: Scientific assessment of ozone depletion: 2002, Global Ozone Research and Monitoring Project; Report No. 47, Geneva, 2003. [7] G. Fontana, M. Causa, V. Gianotti, G. Marchionni, J. Flu. Chem. 109 (2001) 113. [8] C. Mitterdorfer et al., Int. J. Mass Spectrom. 306 (2011) 63. [9] L.G. Christophorou, D.L. McCorkle, A.A. Christodoulides, in: L.G. Christophorou (Ed.), Electron Molecule Interactions and their Applications, vol. 1, Academic Press, New York, 1984. [10] J. Kopyra, J. Wnorowska, M. Forys´, I. Szamrej, Int. J. Mass Spectrom. 268 (2007) 60. [11] S.M. Spyrou, S.R. Hunter, L.G. Christophorou, J. Chem. Phys. 81 (1984) 4481. [12] J. Kopyra, W. Barszczewska, J. Wnorowska, M. Forys´, I. Szamrej, Acta Phys. Slov. 50 (2005) 447. [13] S.R. Hunter, L.G. Christophorou, J. Chem. Phys. 80 (1984) 6150. [14] S.M. Spyrou, I. Sauers, L.G. Christophorou, J. Chem. Phys. 78 (1983) 7200. [15] P.W. Harland, J.C.J. Thynne, Int. J. Mass Spectrom. Ion Phys. 11 (1973) 445. [16] L.G. Christophorou, in: L.G. Christophorou (Ed.), Electron Molecule Interactions and Their Applications, vol. 1, Academic Press, New York, 1984. [17] L. Suess, R. Parthasarathy, F.B. Dunning, J. Chem. Phys. 117 (2002) 11222. [18] I.I. Fabrikant, H. Hotop, J. Chem. Phys. 128 (2008) 124308. [19] G.A. Gallup, K. Aflatooni, P.D. Burow, J. Chem. Phys. 118 (2003) 2562. [20] D. Klar, M.-W. Ruf, I.I. Fabrikant, H. Hotop, J. Phys. B 34 (2001) 3855. [21] J. Kopyra, J. Wnorowska, M. Forys´, I. Szamrej, Int. J. Mass Spectrom. 291 (2010) 13. [22] E.I. Dashevskays, I. Litvin, E.E. Nikitin, J. Troe, Phys. Chem. Chem. Phys. 10 (2008) 1270. [23] H. Hotop, M.-W. Ruf, J. Kopyra, T.M. Miller, I.I. Fabrikant, J. Chem. Phys. 134 (2011) 064303. [24] T.M. Miller, A.A. Viggiano, W.R. Dolbier, T.A. Sergeeva, J.F. Friedman, J. Phys. Chem. A 111 (2007) 1024. [25] J.M. Van Dorren, T.M. Miller, A.A. Viggiano, P. Spanel, D. Smith, J.C. Bopp, J. Troe, J. Chem. Phys. 128 (2008) 094309. [26] E. Alge, N.G. Adams, D. Smith, J. Phys. B: At. Mol. Phys. 17 (1984) 3827.