- Email: [email protected]
Low energy (0–12 eV) electron driven reactions in linear and cyclic perfluorocompounds
International Journal of Mass Spectrometry 325–327 (2012) 95–99
Contents lists available at SciVerse ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Low energy (0–12 eV) electron driven reactions in linear and cyclic perfluorocompounds Janina Kopyra a,∗ , Iwona Szamrej a , Stefan Karolczak b , Eugen Illenberger c a b c
Department of Chemistry, Siedlce University, 3 Maja 54, 08-110 Siedlce, Poland Institute of Applied Radiation Chemistry, Technical University of Łód´z, Wróblewskiego 15, 93-590 Łód´z, Poland Institut für Chemie – Physikalische und Theoretische Chemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 3 April 2012 Received in revised form 6 June 2012 Accepted 6 June 2012 Available online 23 June 2012 Dedicated to Professor Eugen Nikolaev on the occasion of his 65th birthday. Keywords: Dissociative electron attachment Perfluorohexane Perfluoro(methylcyclohexane) Perfluorocyclohexanecarbonyl chloride Negative ion mass spectrometry
a b s t r a c t Dissociative electron attachment (DEA) to perfluorohexane (C6 F14 ), perfluoro(methylcyclo-hexane) (C6 F11 CF3 ) and perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) is studied in a crossed electronmolecular beam experiment with mass spectrometric detection of the fragment anions. We find that all molecules undergo effective decomposition in the subexcitation energy range (0–3 eV) yielding ions that arise from direct bond cleavages creating a fragment anion and its neutral counterpart but also from more complicated reactions due to substantial rearrangement in the transient anion formed upon resonant electron capture. All three compounds preferentially decompose into closed shell anions of the form Cn F2n+1 − . While in perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) DEA into Cl− represents the most intense reaction, formation of F− is in any case comparatively weak. The resonance positions for F− formation show specific features for the three compounds which may allow a correlation between the location of the DEA resonance and the site from which the F− ion originates. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is well known that perfluorocarbons are both thermally and chemically stable molecules. When they are exposed to low energy electrons even at no extra energy they may become unstable with respect to dissociative electron attachment (DEA). This is well documented in the literature where both the decomposition products and the kinetics of electron attachment to various fluorocompounds have been reported so far [1–4]. The interest to study perfluorinated compounds stems from their numerous applications, from purely technical to medical applications. They are widely used as, e.g., lubricants, heat exchange fluids, solvents with potential use for academic as well as industrial chemistry just to mention a few simple applications. More sophisticated applications include their use as polymers for (membrane) surface treatment, electrical insulators, chemical tracers or radiators for the detection of particle appearance (e.g., C6 F14 is used for the detection of the tau neutrino [5]). Another interesting usage concerns diagnostic pharmacology where C6 F14 is used as stabiliser of N2 microbubbles. This agent allows the detection of cardiovas-
∗ Corresponding author. Tel.: +48 256431136. E-mail address: [email protected] (J. Kopyra). 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2012.06.004
cular abnormalities and lesions of solid organs, such as tumours [6]. Here we study electron attachment to perfluorohexane, perfluoro(methylcyclohexane) and perfluorocyclohexanecarbonyl chloride in order to probe the susceptibility of these molecules by means of an electron-molecular crossed beam apparatus. The molecular structures of the investigated compounds are shown in Fig. 1. The aim of our experiments was to explore the influence of the molecular structure and the effect of the Cl atom in perfluorocyclohexanecarbonyl chloride on the corresponding DEA processes. We find that all compounds undergo effective DEA reactions at subexcitation energies (<3 eV) via direct bond cleavages and also more complex reactions involving the cleavage of different bonds and substantial rearrangement. While formation of the halogen ion F− is comparatively weak, Cl− is the strongest signal observed from the present compounds. 2. Experimental procedure The experiments were carried out with an electron/molecule crossed beams apparatus that has been described elsewhere [7]. It consists of an electron monochromator, an oven and a quadrupole mass analyzer that are housed in an UHV chamber. In brief, an incident electron beam of well-defined energy (FWHM ≈ 0.2 eV, electron current ≈ 10 nA) generated from a
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Fig. 1. Molecular structure of the perfluorohexane (A), perfluoro(methylcyclohexane) (B) and perfluorocyclohexanecarbonyl chloride (C).
trochoidal electron monochromator [8] perpendicularly intersects with an effusive molecular beam of gas phase perfluorohexane, perfluoro(methylcyclohexane) or perfluorocyclohexane-carbonyl chloride. The samples are liquid under normal conditions, however, the vapour pressures are sufficient to directly transfer the target molecules into the collision zone without extra heating. The pressure during the experiments was in a range of 10−6 mbar for all three investigated molecules, while the background pressure in the UHV system (no sample inside the system) was in a range of 10−9 mbar. Negative ions formed in electron–molecule collisions are extracted from the reaction zone towards a quadrupole mass analyzer and detected by a single pulse counting techniques. The intensity of negative ions is recorded as a function of the incident electron energy. The electron energy scale is calibrated by measuring the yield of the SF6 − ions, which exhibits a sharp peak near 0 eV. To avoid unwanted reaction between the target molecules and calibrant gas, the calibration is established before and after recording the ion yield of interest. Perfluorohexane (C6 F14 ) was obtained from Aldrich (99%). Perfluoro(methylcyclohexane) (C6 F11 CF3 ) and perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) were obtained from Apollo Scientific at a stated purity of 95% and >97%, respectively. All three samples have been used after degassing in order to remove any remaining gases in the container.
break-down of the Born–Oppenheimer approximation, the incoming electron may couple to vibrations of the target molecule thereby trapping the extra electron. A particular effective way to generate such vibrational Feshbach resonances is via dipole bound (DB) states (if the dipole moment of the target molecule is sufficiently high, that is above 2D). Such DB states can couple to valence state of the molecule thereby acting as doorways for DEA. Perfluorocompounds are of particular interest since they frequently form non-decomposed parent anions as reported previously for, e.g., C6 F14 from a single collision conditions experiment [9] and C6 F11 CF3 from a high pressure experiment [10]. Unfortunately, in our experiment the possible formation of the parent anions is outside the range covered by the mass spectrometer. Therefore in the following we have to focus on the formation of fragment negative ions up to 300 amu that arise from dissociative electron attachment to the perfluorocompounds under investigation.
3. Results and discussion In Figs. 2–5 we present the ion yield curves for negative ion formation due to low energy electron attachment to perfluorohexane (C6 F14 ), perfluoro(methylcyclohexane) (C6 F11 CF3 ) and perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) in the energy range 0–12 eV. All yields show pronounced resonance profiles in the low energy range 0–3 eV. The only exception is the formation of the F− anion at energies above 3 eV from all three investigated molecules and the O− anion in the energy range 6–7 eV from perfluorocyclohexanecarbonyl chloride. The resonance profiles of the anion yields clearly indicate that the corresponding fragments are formed via dissociative electron attachment (DEA). In this process the incoming electron is captured by the target molecule to form a transient negative ion (TNI)#− , which subsequently may undergo dissociation into a negatively charged fragment and one or more neutral fragments, viz., e− + RX → TNI#− → X− + neutral fragment(s) In the energy range below 3 eV the transient negative ions (TNIs) can be assigned as shape resonances, i.e., the excess electron accommodates the lowest normally unfilled orbital of the target molecule. While in the higher energy range the transient anions can be assigned as core excited resonances, i.e., the incoming electron excites the target molecule and becomes concomitantly captured by the molecule to form transitory ions with two electrons in normally unfilled orbitals. In that energy range, shape resonances of * character may also contribute. The assignment of resonances appearing near 0 eV is generally not straightforward. Due to the
Fig. 2. Ion yield curves of the fragment anions from perfluorohexane (C6 F14 ).
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Fig. 5. Ion yield curves of the fragment anions formed from exocyclic group of perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl).
3.1. Perfluorohexane (C6 F14 )
Fig. 3. Ion yield curves of the fragment anions from perfluoro(methylcyclohexane) (C6 F11 CF3 ).
Fig. 4. Ion yield curves of the fragment anions formed from the ring decomposition of perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl).
Fig. 2 shows the negative ions that are formed from DEA to gas phase perfluorohexane below 12 eV. We detect ions at m/e 269, 219, 169, 81 and 19 amu. From the stoichiometry, they can be assigned as C5 F11 − , C4 F9 − , C3 F7 − , C2 F3 − and F− , respectively. A closer inspection of the structure of the first three anions with a general formula Cn F2n+1 − clearly proves that we deal with a characteristic pattern of the decomposition of perfluorohexane due to the loss of particular neutral radicals. The heaviest anion C5 F11 − is formed by the loss of the neutral trifluoromethyl radical (CF3 ), while C4 F9 − and C3 F7 − anions generate the radicals C2 F5 and C3 F7 , respectively as neutral counterparts. All reactions are then due to direct C C bond cleavages. The most intensive fragment from DEA to perfluorohexane is the closed shell heptafluoro propyl anion C3 F7 − that is formed from the splitting of the central C C bond (Fig. 2) creating an anionic and a neutral fragment of equal size. Taking into account the typical C C bond dissociation enthalpy of about 3.9 eV [11] and the electron affinity of C3 F7 that is estimated to be >2.75 eV [9] the thermodynamic threshold is located near 1.1 eV and hence the appearance of this fragment ion agrees reasonably well with the thermodynamic threshold. Two further fragment anions (C4 F9 − and C2 F3 − ) appear within the same energy region as C3 F7 − which indicates a multichannel decomposition of the common transient negative ion at an energy around 1 eV. C4 F9 − is again produced via a single C C bond splitting creating the radical C2 F5 as neutral counterpart. With the electron affinity of C4 F9 of 3.2 eV [12] and the above quoted C C binding energy (3.9 eV) we arrive at a thermodynamic threshold for C4 F9 − formation of 0.7 eV. In the case of C2 F3 − the decomposition process is more complicated and requires a C C bond breaking and cleavage of two C F bonds. From energy considerations it is likely that in the course of the DEA reaction the two F atoms recombine to F2 and the C4 F8 fragment may reorganise to a stable linear or cyclic C4 F8 molecule. The strong signals of C2 F3 − have previously been observed from C3 F8 [9] and from CF3 C≡CH [13] with a peak maximum at 3.3 and 0.5 eV, respectively. It can be assigned as the trifluorovinyl anion with a structure of CF2 CF− or to the anion with a structure of CF3 C− . While the electron affinity of the CF3 C radical (2.8 eV [9]) is higher than that of CF2 CF (2.1 eV [9]), the heat of formation of two anions can be expected at approximately the
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same level. We can hence not conclude of the structure of the C2 F3 − anion from our results. The only fragment that appears already at the threshold energy (∼0 eV) is attributed to the C5 F11 − anion and corresponds to the loss of a neutral CF3 radical. This fragment was previously observed by Spyrou et al. [9], however at an energy of 2 eV. Due to the high value of the C5 F11 electron affinity (≥4.45 eV [9]) the formation of the C5 F11 − anion is indeed possible already near 0 eV. Finally we observe the formation of the F− anion at an energy around 3.5 eV and hence considerably higher than the fragments considered above. Although the electron affinity of the halogen atom is appreciably high (3.4 eV [14]) due to the high C F binding energy of 5.25 eV the formation of the F− is endothermic by about 1.9 eV, which is in agreement with our experimental observations.
3.2. Perfluoro(methylcyclohexane) (C6 F11 CF3 ) Perfluoro(methylcyclohexane) C6 F11 CF3 differs in stoichiometry from perfluoro-hexane by just one C atom. It has a six C-atom ring structure with a trifluoromethyl CF3 exocyclic group (for the molecular structure see Fig. 1). The yield curves for the five anions (C6 F11 − , C5 F11 − , C3 F7 − , C2 F3 − and F− ) are shown in Fig. 3. The anions can be divided into two groups, those which are produced via single bond cleavages (C6 F11 − and F− ) and those which are generated via a multiple bond breaking with intramolecular transfer of a fluorine atom (C5 F11 − , C3 F7 − and C2 F3 − ). As for perfluorohexane the fragment anions are generated at the restricted energy range below 3 eV with the only exception of F− that is visible via two energy domains at 3–4 eV and 8–12 eV. The relatively rich fragmentation of the perfluoro(methylcyclohexane) is in striking contrast to the observation of Alge et al. [15] who did not observe the formation of fragment anions in the range below 300 amu. As mentioned above the perfluorocyclohexyl anion (C6 F11 − ) is generated via the severing of the C C bond between the cyclic ring and the exocyclic group. From our experiment we are not able to predict whether the fragment anion preserves the cyclic structure or whether it changes into the linear one. On the other hand, from quantum chemical calculations by the group of Schaefer [16] it appears that cyclic perfluorocarbon radicals (Cn F2n−1 , n = 3–6) do strongly bind an extra electron. This is due to the stabilisation obtained from the delocalization of excess charge by the electronwithdrawing inductive and negative hyperconjugative effects of fluorine. Therefore it is very likely that the C6 F11 − anion from C6 F11 CF3 keeps the cyclic structure. The calculated (adiabatic) electron affinity for the c-C6 F11 radical is 3.2 eV. Taking into account the typical value of the C C binding energy we arrive at reaction enthalpy of around 0.4 eV for C6 F11 − formation. Further anions (C5 F11 − and C2 F3 − ) are observed at an energy around 0 eV with a very similar shape. Both species are generated due to the severing of two C C bonds and intramolecular fluorine transfer with the excess charge localised at either of the fragments, they are hence complementary with respect to the localisation of the excess electron. The C3 F7 − anion is also formed by the cleavage of the C C bonds and F atom transfer generating the linear closed shell CF3 CF2 CF2 − anion. This ion is observed at significantly higher energy (1.7 eV). Since there is no thermodynamic data available for the cyclic perfluorocompounds we shall not consider the energetic situation in any more details. We observe the formation of the F− anion in the two energy ranges around 3–4 eV and 8–12 eV. In the high energy range two overlapping resonances with peak maxima at 8.9 eV and 10.3 eV can clearly be distinguished. As will be considered in the next section by comparing the F− resonance profiles from the different compounds it is possible to specify the site of F− formation.
3.3. Perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) Figs. 4 and 5 show the ion efficiency curves for the fragments from perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl). The anions are formed either from the decomposition of the cyclic structure or from the exocyclic group of the target molecule. From the comparison of the positions and shapes of the resonances between C6 F11 C(O)Cl and C6 F11 CF3 we can easily conclude that the presence of the Cl atom and the carbonyl group changes the situation for DEA significantly and that most of the intensity is channelled into the Cl− anion arising from the cleavage of the C Cl bond. The fragment anions C5 F11 − , C4 F9 − and C2 F5 − (Fig. 4) are generated via a decomposition of the ring. All three ion yields are located close to 0 eV indicating a common intermediate negative ion. Among these three species the ion C5 F11 − has the highest intensity which is in line with the results of Spyrou et al. [9] that the electron affinity rises with the length of the carbon chain of Cn F2n+1 species. Further anions that arise from the exocyclic group are shown in Fig. 5. They have been detected at 63, 35 and 16 amu and are attributed to COCl− , Cl− and O− , respectively. While in the case of perfluoro(methylcyclohexane) the cleavage of the bond between the ring and the exocyclic group leads to the fragment with the extra charge localised at the C6 F11 unit for the perfluorocyclohexanecarbonyl chloride this is the exocyclic group that carries the charge. The formation of the COCl− anion may be attributed to elec∗ tron capture into a molecular orbital (MO) with predominant CO character. The same lowest unoccupied molecular orbital (LUMO) is likely to be involved in the formation of the halogen anion Cl− which is by far the most intense fragment. In this case the initial ∗ orbital is followed by a localisation of the extra charge at the CO conversion to the C∗ Cl MO and subsequent severing of the C Cl bond [17]. Formation of both anionic species has been recently observed from electron attachment to propionyl chloride, however in the case of COCl− at slightly higher energy near 0.5 eV [18]. O− is the only product from the side group that appears at higher energy with a peak maximum at around 6.5 eV. This core excited resonance may be associated to the excitation of the electron from the lone pair of the oxygen atom and concomitant electron capture by the electron–molecule potential of the excited state.
4. Conclusions In the present work the fragmentation of the gas phase perfluorohexane, perfluoro(methylcyclohexane) and perfluorocyclohexanecarbonyl chloride has been studied towards low energy electron interaction. We demonstrate that all three molecules undergo effective dissociation upon electron attachment which occurs at the restricted energy range below 3 eV with the exception of the appearance of the F− (from all three molecules) and O− (from perfluorocyclohexanecarbonyl chloride) anions at higher energy. In comparing F− formation from the three molecules under investigation we observe significant differences which may allow us to correlate the energy of the resonance with site from which the F− anion originates. While in linear C6 F14 we observe F− from one single resonance located at 3.5 eV, from C6 F11 CF3 it is formed at 3.5 eV and within two overlapping resonances in the range between 8 and 12 eV, from C6 F11 C(O)Cl it is exclusively formed within an overlapping features in the higher energy range. From that we may conclude that F− formation at low energy around 3.5 eV originates from either the linear structure or from the exocyclic CF3 group. F− formation at higher energies is then due to the DEA reactions involving the cyclic structure. It should be noted that quantum chemical calculations [19] predict that the extra electron
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accommodates * orbital dominated by the tertiary C F bond that followed by the stretch of the C F bond and its subsequent cleavage is an effective process. 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, in: L.G. Christophorou (Ed.), Electron Molecule Interactions and Their Applications, vol. 1, Academic Press, New York, 1984. [2] S.R. Hunter, L.G. Christophorou, J. Chem. Phys. 80 (1984) 6150. [3] S. Feil, T.D. Maerk, A. Mauracher, P. Scheier, C.A. Mayhew, Int. J. Mass Spectrom. 277 (2008) 41.
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[4] J. Kopyra, W. Barszczewska, J. Wnorowska, M. Fory´s, I. Szamrej, Acta Phys. Slov. 55 (2005) 447. [5] R. Forty, JHEP 12 (1999) 002. [6] C.P. Coyne, Comparative Diagnostic Pharmacology: Clinical and Research Applications in Living-System Models, Blackwell Publishing, 2006. [7] J. Kopyra, C. Koenig-Lehmann, I. Szamrej, E. Illenberger, Int. J. Mass Spectrom. 285 (2009) 131. [8] A. Stamatovic, G.J. Schultz, Rev. Sci. Instrum. 41 (1970) 423. [9] S.M. Spyrou, I. Sauers, L.G. Christophorou, J. Chem. Phys. 78 (1983) 7200. [10] B.H. Mahan, C.E. Young, J. Chem. Phys. 44 (1966) 2192. [11] S.J. Blanksby, G. Barney Ellison, Acc. Chem. Res. 36 (2003) 255. [12] P.W. Harland, J.C.J. Thynne, Int J. Mass Spectrom. Ion Physics 11 (1973) 445. [13] J. Kopyra, I. Dabkowska, E. Illenberger, Z. Phys. Chem. 225 (2011) 493. [14] The NIST Chemistry Webbook. http://webbook.nist.gov/chemistry. [15] E. Alge, N.G. Adams, D. Smith, J. Phys. B: Atom. Mol. Phys. 17 (1984) 3827. [16] P.P. Bera, L. Horny, H.F. Schaefer, J. Am. Chem. Soc. 126 (2004) 6692. [17] S.L. Chen, W.H. Fang, J. Phys. Chem. A 110 (2006) 944. [18] I. Bald, E. Illenberger, O. Ingolfsson, Chem. Phys. Lett. 442 (2007) 270. [19] A. Paul, C.S. Wannere, V. Kasalova, P.V.R. Schleyer, H.F. Schaefer, J. Am. Chem. Soc. 127 (2005) 15457.
Contents lists available at SciVerse ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Low energy (0–12 eV) electron driven reactions in linear and cyclic perfluorocompounds Janina Kopyra a,∗ , Iwona Szamrej a , Stefan Karolczak b , Eugen Illenberger c a b c
Department of Chemistry, Siedlce University, 3 Maja 54, 08-110 Siedlce, Poland Institute of Applied Radiation Chemistry, Technical University of Łód´z, Wróblewskiego 15, 93-590 Łód´z, Poland Institut für Chemie – Physikalische und Theoretische Chemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 3 April 2012 Received in revised form 6 June 2012 Accepted 6 June 2012 Available online 23 June 2012 Dedicated to Professor Eugen Nikolaev on the occasion of his 65th birthday. Keywords: Dissociative electron attachment Perfluorohexane Perfluoro(methylcyclohexane) Perfluorocyclohexanecarbonyl chloride Negative ion mass spectrometry
a b s t r a c t Dissociative electron attachment (DEA) to perfluorohexane (C6 F14 ), perfluoro(methylcyclo-hexane) (C6 F11 CF3 ) and perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) is studied in a crossed electronmolecular beam experiment with mass spectrometric detection of the fragment anions. We find that all molecules undergo effective decomposition in the subexcitation energy range (0–3 eV) yielding ions that arise from direct bond cleavages creating a fragment anion and its neutral counterpart but also from more complicated reactions due to substantial rearrangement in the transient anion formed upon resonant electron capture. All three compounds preferentially decompose into closed shell anions of the form Cn F2n+1 − . While in perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) DEA into Cl− represents the most intense reaction, formation of F− is in any case comparatively weak. The resonance positions for F− formation show specific features for the three compounds which may allow a correlation between the location of the DEA resonance and the site from which the F− ion originates. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is well known that perfluorocarbons are both thermally and chemically stable molecules. When they are exposed to low energy electrons even at no extra energy they may become unstable with respect to dissociative electron attachment (DEA). This is well documented in the literature where both the decomposition products and the kinetics of electron attachment to various fluorocompounds have been reported so far [1–4]. The interest to study perfluorinated compounds stems from their numerous applications, from purely technical to medical applications. They are widely used as, e.g., lubricants, heat exchange fluids, solvents with potential use for academic as well as industrial chemistry just to mention a few simple applications. More sophisticated applications include their use as polymers for (membrane) surface treatment, electrical insulators, chemical tracers or radiators for the detection of particle appearance (e.g., C6 F14 is used for the detection of the tau neutrino [5]). Another interesting usage concerns diagnostic pharmacology where C6 F14 is used as stabiliser of N2 microbubbles. This agent allows the detection of cardiovas-
∗ Corresponding author. Tel.: +48 256431136. E-mail address: [email protected] (J. Kopyra). 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2012.06.004
cular abnormalities and lesions of solid organs, such as tumours [6]. Here we study electron attachment to perfluorohexane, perfluoro(methylcyclohexane) and perfluorocyclohexanecarbonyl chloride in order to probe the susceptibility of these molecules by means of an electron-molecular crossed beam apparatus. The molecular structures of the investigated compounds are shown in Fig. 1. The aim of our experiments was to explore the influence of the molecular structure and the effect of the Cl atom in perfluorocyclohexanecarbonyl chloride on the corresponding DEA processes. We find that all compounds undergo effective DEA reactions at subexcitation energies (<3 eV) via direct bond cleavages and also more complex reactions involving the cleavage of different bonds and substantial rearrangement. While formation of the halogen ion F− is comparatively weak, Cl− is the strongest signal observed from the present compounds. 2. Experimental procedure The experiments were carried out with an electron/molecule crossed beams apparatus that has been described elsewhere [7]. It consists of an electron monochromator, an oven and a quadrupole mass analyzer that are housed in an UHV chamber. In brief, an incident electron beam of well-defined energy (FWHM ≈ 0.2 eV, electron current ≈ 10 nA) generated from a
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Fig. 1. Molecular structure of the perfluorohexane (A), perfluoro(methylcyclohexane) (B) and perfluorocyclohexanecarbonyl chloride (C).
trochoidal electron monochromator [8] perpendicularly intersects with an effusive molecular beam of gas phase perfluorohexane, perfluoro(methylcyclohexane) or perfluorocyclohexane-carbonyl chloride. The samples are liquid under normal conditions, however, the vapour pressures are sufficient to directly transfer the target molecules into the collision zone without extra heating. The pressure during the experiments was in a range of 10−6 mbar for all three investigated molecules, while the background pressure in the UHV system (no sample inside the system) was in a range of 10−9 mbar. Negative ions formed in electron–molecule collisions are extracted from the reaction zone towards a quadrupole mass analyzer and detected by a single pulse counting techniques. The intensity of negative ions is recorded as a function of the incident electron energy. The electron energy scale is calibrated by measuring the yield of the SF6 − ions, which exhibits a sharp peak near 0 eV. To avoid unwanted reaction between the target molecules and calibrant gas, the calibration is established before and after recording the ion yield of interest. Perfluorohexane (C6 F14 ) was obtained from Aldrich (99%). Perfluoro(methylcyclohexane) (C6 F11 CF3 ) and perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) were obtained from Apollo Scientific at a stated purity of 95% and >97%, respectively. All three samples have been used after degassing in order to remove any remaining gases in the container.
break-down of the Born–Oppenheimer approximation, the incoming electron may couple to vibrations of the target molecule thereby trapping the extra electron. A particular effective way to generate such vibrational Feshbach resonances is via dipole bound (DB) states (if the dipole moment of the target molecule is sufficiently high, that is above 2D). Such DB states can couple to valence state of the molecule thereby acting as doorways for DEA. Perfluorocompounds are of particular interest since they frequently form non-decomposed parent anions as reported previously for, e.g., C6 F14 from a single collision conditions experiment [9] and C6 F11 CF3 from a high pressure experiment [10]. Unfortunately, in our experiment the possible formation of the parent anions is outside the range covered by the mass spectrometer. Therefore in the following we have to focus on the formation of fragment negative ions up to 300 amu that arise from dissociative electron attachment to the perfluorocompounds under investigation.
3. Results and discussion In Figs. 2–5 we present the ion yield curves for negative ion formation due to low energy electron attachment to perfluorohexane (C6 F14 ), perfluoro(methylcyclohexane) (C6 F11 CF3 ) and perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) in the energy range 0–12 eV. All yields show pronounced resonance profiles in the low energy range 0–3 eV. The only exception is the formation of the F− anion at energies above 3 eV from all three investigated molecules and the O− anion in the energy range 6–7 eV from perfluorocyclohexanecarbonyl chloride. The resonance profiles of the anion yields clearly indicate that the corresponding fragments are formed via dissociative electron attachment (DEA). In this process the incoming electron is captured by the target molecule to form a transient negative ion (TNI)#− , which subsequently may undergo dissociation into a negatively charged fragment and one or more neutral fragments, viz., e− + RX → TNI#− → X− + neutral fragment(s) In the energy range below 3 eV the transient negative ions (TNIs) can be assigned as shape resonances, i.e., the excess electron accommodates the lowest normally unfilled orbital of the target molecule. While in the higher energy range the transient anions can be assigned as core excited resonances, i.e., the incoming electron excites the target molecule and becomes concomitantly captured by the molecule to form transitory ions with two electrons in normally unfilled orbitals. In that energy range, shape resonances of * character may also contribute. The assignment of resonances appearing near 0 eV is generally not straightforward. Due to the
Fig. 2. Ion yield curves of the fragment anions from perfluorohexane (C6 F14 ).
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Fig. 5. Ion yield curves of the fragment anions formed from exocyclic group of perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl).
3.1. Perfluorohexane (C6 F14 )
Fig. 3. Ion yield curves of the fragment anions from perfluoro(methylcyclohexane) (C6 F11 CF3 ).
Fig. 4. Ion yield curves of the fragment anions formed from the ring decomposition of perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl).
Fig. 2 shows the negative ions that are formed from DEA to gas phase perfluorohexane below 12 eV. We detect ions at m/e 269, 219, 169, 81 and 19 amu. From the stoichiometry, they can be assigned as C5 F11 − , C4 F9 − , C3 F7 − , C2 F3 − and F− , respectively. A closer inspection of the structure of the first three anions with a general formula Cn F2n+1 − clearly proves that we deal with a characteristic pattern of the decomposition of perfluorohexane due to the loss of particular neutral radicals. The heaviest anion C5 F11 − is formed by the loss of the neutral trifluoromethyl radical (CF3 ), while C4 F9 − and C3 F7 − anions generate the radicals C2 F5 and C3 F7 , respectively as neutral counterparts. All reactions are then due to direct C C bond cleavages. The most intensive fragment from DEA to perfluorohexane is the closed shell heptafluoro propyl anion C3 F7 − that is formed from the splitting of the central C C bond (Fig. 2) creating an anionic and a neutral fragment of equal size. Taking into account the typical C C bond dissociation enthalpy of about 3.9 eV [11] and the electron affinity of C3 F7 that is estimated to be >2.75 eV [9] the thermodynamic threshold is located near 1.1 eV and hence the appearance of this fragment ion agrees reasonably well with the thermodynamic threshold. Two further fragment anions (C4 F9 − and C2 F3 − ) appear within the same energy region as C3 F7 − which indicates a multichannel decomposition of the common transient negative ion at an energy around 1 eV. C4 F9 − is again produced via a single C C bond splitting creating the radical C2 F5 as neutral counterpart. With the electron affinity of C4 F9 of 3.2 eV [12] and the above quoted C C binding energy (3.9 eV) we arrive at a thermodynamic threshold for C4 F9 − formation of 0.7 eV. In the case of C2 F3 − the decomposition process is more complicated and requires a C C bond breaking and cleavage of two C F bonds. From energy considerations it is likely that in the course of the DEA reaction the two F atoms recombine to F2 and the C4 F8 fragment may reorganise to a stable linear or cyclic C4 F8 molecule. The strong signals of C2 F3 − have previously been observed from C3 F8 [9] and from CF3 C≡CH [13] with a peak maximum at 3.3 and 0.5 eV, respectively. It can be assigned as the trifluorovinyl anion with a structure of CF2 CF− or to the anion with a structure of CF3 C− . While the electron affinity of the CF3 C radical (2.8 eV [9]) is higher than that of CF2 CF (2.1 eV [9]), the heat of formation of two anions can be expected at approximately the
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same level. We can hence not conclude of the structure of the C2 F3 − anion from our results. The only fragment that appears already at the threshold energy (∼0 eV) is attributed to the C5 F11 − anion and corresponds to the loss of a neutral CF3 radical. This fragment was previously observed by Spyrou et al. [9], however at an energy of 2 eV. Due to the high value of the C5 F11 electron affinity (≥4.45 eV [9]) the formation of the C5 F11 − anion is indeed possible already near 0 eV. Finally we observe the formation of the F− anion at an energy around 3.5 eV and hence considerably higher than the fragments considered above. Although the electron affinity of the halogen atom is appreciably high (3.4 eV [14]) due to the high C F binding energy of 5.25 eV the formation of the F− is endothermic by about 1.9 eV, which is in agreement with our experimental observations.
3.2. Perfluoro(methylcyclohexane) (C6 F11 CF3 ) Perfluoro(methylcyclohexane) C6 F11 CF3 differs in stoichiometry from perfluoro-hexane by just one C atom. It has a six C-atom ring structure with a trifluoromethyl CF3 exocyclic group (for the molecular structure see Fig. 1). The yield curves for the five anions (C6 F11 − , C5 F11 − , C3 F7 − , C2 F3 − and F− ) are shown in Fig. 3. The anions can be divided into two groups, those which are produced via single bond cleavages (C6 F11 − and F− ) and those which are generated via a multiple bond breaking with intramolecular transfer of a fluorine atom (C5 F11 − , C3 F7 − and C2 F3 − ). As for perfluorohexane the fragment anions are generated at the restricted energy range below 3 eV with the only exception of F− that is visible via two energy domains at 3–4 eV and 8–12 eV. The relatively rich fragmentation of the perfluoro(methylcyclohexane) is in striking contrast to the observation of Alge et al. [15] who did not observe the formation of fragment anions in the range below 300 amu. As mentioned above the perfluorocyclohexyl anion (C6 F11 − ) is generated via the severing of the C C bond between the cyclic ring and the exocyclic group. From our experiment we are not able to predict whether the fragment anion preserves the cyclic structure or whether it changes into the linear one. On the other hand, from quantum chemical calculations by the group of Schaefer [16] it appears that cyclic perfluorocarbon radicals (Cn F2n−1 , n = 3–6) do strongly bind an extra electron. This is due to the stabilisation obtained from the delocalization of excess charge by the electronwithdrawing inductive and negative hyperconjugative effects of fluorine. Therefore it is very likely that the C6 F11 − anion from C6 F11 CF3 keeps the cyclic structure. The calculated (adiabatic) electron affinity for the c-C6 F11 radical is 3.2 eV. Taking into account the typical value of the C C binding energy we arrive at reaction enthalpy of around 0.4 eV for C6 F11 − formation. Further anions (C5 F11 − and C2 F3 − ) are observed at an energy around 0 eV with a very similar shape. Both species are generated due to the severing of two C C bonds and intramolecular fluorine transfer with the excess charge localised at either of the fragments, they are hence complementary with respect to the localisation of the excess electron. The C3 F7 − anion is also formed by the cleavage of the C C bonds and F atom transfer generating the linear closed shell CF3 CF2 CF2 − anion. This ion is observed at significantly higher energy (1.7 eV). Since there is no thermodynamic data available for the cyclic perfluorocompounds we shall not consider the energetic situation in any more details. We observe the formation of the F− anion in the two energy ranges around 3–4 eV and 8–12 eV. In the high energy range two overlapping resonances with peak maxima at 8.9 eV and 10.3 eV can clearly be distinguished. As will be considered in the next section by comparing the F− resonance profiles from the different compounds it is possible to specify the site of F− formation.
3.3. Perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl) Figs. 4 and 5 show the ion efficiency curves for the fragments from perfluorocyclohexanecarbonyl chloride (C6 F11 C(O)Cl). The anions are formed either from the decomposition of the cyclic structure or from the exocyclic group of the target molecule. From the comparison of the positions and shapes of the resonances between C6 F11 C(O)Cl and C6 F11 CF3 we can easily conclude that the presence of the Cl atom and the carbonyl group changes the situation for DEA significantly and that most of the intensity is channelled into the Cl− anion arising from the cleavage of the C Cl bond. The fragment anions C5 F11 − , C4 F9 − and C2 F5 − (Fig. 4) are generated via a decomposition of the ring. All three ion yields are located close to 0 eV indicating a common intermediate negative ion. Among these three species the ion C5 F11 − has the highest intensity which is in line with the results of Spyrou et al. [9] that the electron affinity rises with the length of the carbon chain of Cn F2n+1 species. Further anions that arise from the exocyclic group are shown in Fig. 5. They have been detected at 63, 35 and 16 amu and are attributed to COCl− , Cl− and O− , respectively. While in the case of perfluoro(methylcyclohexane) the cleavage of the bond between the ring and the exocyclic group leads to the fragment with the extra charge localised at the C6 F11 unit for the perfluorocyclohexanecarbonyl chloride this is the exocyclic group that carries the charge. The formation of the COCl− anion may be attributed to elec∗ tron capture into a molecular orbital (MO) with predominant CO character. The same lowest unoccupied molecular orbital (LUMO) is likely to be involved in the formation of the halogen anion Cl− which is by far the most intense fragment. In this case the initial ∗ orbital is followed by a localisation of the extra charge at the CO conversion to the C∗ Cl MO and subsequent severing of the C Cl bond [17]. Formation of both anionic species has been recently observed from electron attachment to propionyl chloride, however in the case of COCl− at slightly higher energy near 0.5 eV [18]. O− is the only product from the side group that appears at higher energy with a peak maximum at around 6.5 eV. This core excited resonance may be associated to the excitation of the electron from the lone pair of the oxygen atom and concomitant electron capture by the electron–molecule potential of the excited state.
4. Conclusions In the present work the fragmentation of the gas phase perfluorohexane, perfluoro(methylcyclohexane) and perfluorocyclohexanecarbonyl chloride has been studied towards low energy electron interaction. We demonstrate that all three molecules undergo effective dissociation upon electron attachment which occurs at the restricted energy range below 3 eV with the exception of the appearance of the F− (from all three molecules) and O− (from perfluorocyclohexanecarbonyl chloride) anions at higher energy. In comparing F− formation from the three molecules under investigation we observe significant differences which may allow us to correlate the energy of the resonance with site from which the F− anion originates. While in linear C6 F14 we observe F− from one single resonance located at 3.5 eV, from C6 F11 CF3 it is formed at 3.5 eV and within two overlapping resonances in the range between 8 and 12 eV, from C6 F11 C(O)Cl it is exclusively formed within an overlapping features in the higher energy range. From that we may conclude that F− formation at low energy around 3.5 eV originates from either the linear structure or from the exocyclic CF3 group. F− formation at higher energies is then due to the DEA reactions involving the cyclic structure. It should be noted that quantum chemical calculations [19] predict that the extra electron
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accommodates * orbital dominated by the tertiary C F bond that followed by the stretch of the C F bond and its subsequent cleavage is an effective process. 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, in: L.G. Christophorou (Ed.), Electron Molecule Interactions and Their Applications, vol. 1, Academic Press, New York, 1984. [2] S.R. Hunter, L.G. Christophorou, J. Chem. Phys. 80 (1984) 6150. [3] S. Feil, T.D. Maerk, A. Mauracher, P. Scheier, C.A. Mayhew, Int. J. Mass Spectrom. 277 (2008) 41.
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