IR spectroscopy of gaseous fluorocarbon ions: The perfluoroethyl anion

Chemical Physics 398 (2012) 118–123

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IR spectroscopy of gaseous fluorocarbon ions: The perfluoroethyl anion Maria Elisa Crestoni a, Barbara Chiavarino a, Joel Lemaire b, Philippe Maitre b, Simonetta Fornarini a,⇑ a b

Dipartimento di Chimica e Tecnologie del Farmaco, Università di Roma ‘‘La Sapienza’’, P. le A. Moro 5, I-00185 Roma, Italy Université Paris Sud, Laboratoire de Chimie Physique – UMR8000 CNRS, Faculté des Sciences – Bâtiment 350, 91405 Orsay Cedex, France

a r t i c l e

i n f o

Article history: Available online 12 March 2011 Keywords: IRMPD spectroscopy FT–ICR mass spectrometry Negative hyperconjugation Gas phase ion chemistry

a b s t r a c t The first IR spectrum of a perfluorinated carbanion has been obtained in the gas phase by IRMPD spectroscopy. Quantum chemical calculations at the MP2/cc-pVTZ level were performed yielding the optimized geometries and IR spectra for a covalently bound C2F5 species and for conceivable loosely bound F(C2F4) complexes. Both the computational results and the IR characterization point to a covalent structure for the assayed species in agreement with the reactivity pattern displayed with selected neutrals. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Fluorocarbon plasmas are widely used for the etching of silicon and silicon compounds in semiconductor device manufacturing industry. A notable amount of research has addressed the study of plasma conditions and components that contribute to the competing processes of deposition and etching in fluorocarbon plasmas, ultimately directed to optimize the features of microelectronic circuits [1]. Both gas and plasma-surface interactions affect the SiO2 etch process and the mechanisms of selective SiO2 to Si and Si3N4 etching have been extensively studied [2]. In addition, fully dense fluorocarbon films deposited from plasmas have been considered for use as dielectric interlayers that can have dielectric constants of less than 2.0 [3]. A rich chemistry appears to be involved in the overall film growth and etching processes. Both charged and radical species are active in these etching and deposition processes [4] and mass spectrometric methods have identified the presence of various fluorocarbon ions [5]. In the present study, that we wish to dedicate to Mario Capitelli in recognition of his paramount contribution to plasma science, the focus is on perfluoroethyl anion, C2F5, a prototypical perfluorinated anion, which is characterized by its reactivity and IR spectroscopic features. The study moves in particular from two specific questions. The first one arises from a very recent investigation of the gas phase ion–molecule reactions in C2F4 where C2F5 ions formed by the addition of F to C2F4 display a peculiar behaviour suggesting the existence of distinct isomeric species [6]. The Van’t Hoff plots for the clustering reaction F + C2F4 ? F(C2F4) show an increase of the equilibrium constant above room temperature, followed by a decrease at higher temperatures, which has been ⇑ Corresponding author. Tel.: +39 (0) 6 49913510. E-mail address: [email protected] (S. Fornarini). 0301-0104/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2011.03.005

interpreted as due to the existence of both a loosely bound and a tightly bound complex. The second question attached to C2F5 regards the operation of negative hyperconjugation which may affect its structure and bonding features. For some time the importance of negative hyperconjugation has been under discussion. First introduced in 1950 by Roberts to explain certain electronic influences of the CF3 group [7], it is now a well accepted concept particularly in cases where fluorine is involved in bonding [8]. Landmark features ascribed to this effect are the bond length and bond angle situation in CF3O. Negative hyperconjugation is also held responsible for the comparatively high acidity of CF3OH with respect to that of CF3SH [9]. Other related examples are the structures of OC2F5, OCF(CF3)2, and OCFCF2O anions investigated by single crystal X-ray cristallography, all indicative of a negative hyperconjugation effect [8]. Negative hyperconjugation has been shown to be a general phenomenon dictating conformational stabilities and controlling chemical reactivity. In negative hyperconjugation a p electron pair (a lone pair in the present case) interacts with an acceptor r⁄ orbital and their interaction is maximized when an antiperiplanar orientation is adopted and a small gap exists between the interacting orbitals [10,11]. The direction of ‘‘electron flow’’ is of p ? r⁄ type, such as the pC ! rCF interaction in FCH2CH2. It has been often labeled ‘‘anionic hyperconjugation’’ but it is not at all restricted to anions and has been more generally termed ‘‘negative hyperconjugation’’ [12]. In this way p-bond character is built into bonds that nominally have only r character at the expense of weakening adjoining C–F r bonds through the population of the corresponding r⁄ orbital. Ab initio calculations including correlation corrections have been performed on three fluorine-substituted ethyl carbanions, namely CF2HCH2, CF3CH2, and CF3CHF [13]. Three criteria were considered indicative of negative hyperconjugation based on (i) the

M.E. Crestoni et al. / Chemical Physics 398 (2012) 118–123

length of the C–F bond anti with respect to the lone pair (C–Fanti) in the carbanion as compared with the bond length in the conjugate acid, (ii) the charge on the F atom anti to the lone pair in the anion with respect to the conjugate acid and (iii) the 18F/19F equilibrium isotope effect for the F atom anti to the lone pair in the deprotonation of the conjugate acid. All criteria concurred in pointing to a weakening of the C–Fanti bond on ionization. Hyperconjugation appeared more pronounced when less inductive stabilization of the carbanion was expected. In the series of ethyl carbanions C2F5 presents exaustive fluorine substitution gaining utmost inductive stabilization. Thus the stabilizing interaction between r⁄(C–Fb) and the lone pair electrons on Ca is expected to be smaller in CF3CF2 than, for example, in CF3CH2, as already pointed out [11]. In terms of resonance, negative hyperconjugation may be represented by the contribution of a no-bond resonance structure, e. g. A M B. -

-

CF3-CF2 A

F CF2=CF2 B

A thorough understanding of the structure and bonding features of simple charged species is considerably aided by IR spectroscopy of the naked ions in the gas phase. The sensitive technique of IRMPD (Infra Red Multiple Photon Dissociation) spectroscopy of mass selected ions using the beamline of the free electron laser at CLIO (Centre Laser InfraRouge d’Orsay) is used here to obtain the IR spectrum of CF3CF2 in the 600–1400 cm1 region [14]. The spectroscopic data are integrated with the results of quantum chemical calculations and with the observed reactivity pattern towards few exemplary neutral molecules. 2. Materials and methods 2.1. IRMPD measurements The experiments have been performed on the instrumental platform combining a movable FT–ICR mass spectrometer with the laser optics devices allowing the IR radiation of the CLIO free electron laser (FEL) to shine on the ion cloud trapped in the FT– ICR cell [15]. The cell is placed within a 1.25 T permanent magnet. Ions were formed by electron ionization of perfluoropropane (Fluorochem Ltd.), admitted by a pulsed valve. The ions were selected, allowed to relax for about 1.5 s and then irradiated for a time lapse of 1 s, set by a fast electromechanical shutter synchronized with the FEL. The FEL is based on a 10–50 MeV electron accelerator and the radiation output is delivered in 10 ls long macropulses fired at a repetition rate of 25 Hz. Each macropulse comprises about 600 micropulses, each a few picoseconds long. In the present experiments, the mean IR power was about 800 mW and the electron energy was set at 38 MeV to provide high power IR radiation over the 600–1400 cm1 range. For each wavelength, the mass spectrum is recorded averaging from an ion signal accumulated over four sequences. The IRMPD spectrum is obtained plotting the photofragmentation yield R, expressed as a function of the intensities of the parent and fragments ions as R = ln{Iparent /(Iparent + RIfragment)}, as a function of the radiation frequency. 2.2. Quantum chemical methods Quantum chemical calculations have been carried out to characterize the CF3CF2 anion and related species. In order to determine the structures, energies and vibrational properties, calculations have been performed using the second order Möller– Plesset perturbation theory (MP2) with the cc-pVTZ basis set. All calculations were performed using the Spartan’08 program pack-

119

age [16]. The identification of the stationary points as minima or transition states is ensured by the analysis of the vibrational frequencies. Relative energies are reported at 0 K, including correction by unscaled zero point vibrational energies. Calculated IR spectra are obtained by scaling the harmonic frequencies by a factor of 0.934, unless stated otherwise. This factor is found to optimize the agreement between calculated and measured frequencies in the IRMPD spectrum of CF3CF2 anion (vide infra). In order to facilitate the comparison, calculated IR bands are convoluted assuming a Lorentzian profile with a 20 cm1 full-width at half-maximum (fwhm). The charge distribution is obtained by Mulliken analysis. 2.3. Gas phase ion chemistry experiments The experiments were performed using a commercial Bruker Spectrospin Apex TM 47e mass spectrometer equipped with a cylindrical ‘‘infinity’’ cell within a 4.7 T superconducting magnet and with an external ion source. In this source, the ionization of perfluoropropane admitted through a gas inlet was effected by a 50 eV electron beam. The ions formed in the external source are led into the FT–ICR cell by an ion guide. After a delay time the C2F3 ions of interest were mass selected by ejecting unwanted ions using rf sweeps and single shots. The selected ions were then let to react with a neutral reagent leaked in the cell at room temperature by a needle valve at a constant pressure, typically in the range of 108–107 mbar. The pressure readings were calibrated according to standard procedures already described [17]. The kinetics of the ion–molecule reactions were monitored by recording 10–20 averaged scans for each mass spectrum in series of runs corresponding to increasing reaction time. In these conditions the kinetics respond to pseudo first order and the rate constants (kobs) were obtained from the slope of the semilogarithmic decrease of reactant ion abundance versus reaction time. The pseudo first order rate constants divided by the substrate concentration yield the bimolecular rate constants (kexp) which are normalized by the collisional frequency to give the reaction efficiencies (U, namely the % ratio of kexp relative to the collision rate constant, kc, calculated by the parameterized trajectory theory [18]). 3. Results and discussion An outline is given first on the reactivity features of gaseous C2F5 ions and on the results of quantum chemical calculations before reaching a more comprehensive description of the properties of C2F5 as they emerge from the IRMPD spectral characteristics. C2F5 ions may be readily obtained by F addition to perfluoroethene [6,19] or by dissociative electron capture in perfluoropropane [20,21] The latter method was used in the present work. 3.1. Reactivity features of C2F5 ions C2F5 reacts readily with perfluoropropene yielding an adduct ion C5F11, whereas it is unreactive with perfluoroethene in a selected ion flow tube apparatus [19]. Interestingly, the same reactivity is observed when C2F5 is formed either by F addition to C2F4 or by dissociative electron capture from C3F8 suggesting a common covalent structure, viewed more likely than an alternative [F C2F4] complex [19,22]. The major reaction channel for the C2F5 reaction with hexafluoropropene oxide leads to C4F9 and CF2O as stable neutral fragment [23]. The reactivity of C2F5 has been examined also towards ozone leading to F and CF3 with 80% reaction efficiency, whereas O2 is unreactive [24]. No ionic products have been observed from the fast reaction of C2F5 with atomic hydrogen suggesting that an electron detachment process is occurring [25].

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The gas phase reactions of C2F5 ions with few exemplary neutrals examined in this study are summarized in Table 1. The reaction with SiF4 is a fast process yielding SiF5 as the major product with 49% reaction efficiency. In this reaction C2F5 displays F releasing ability, which is conceivable given the high fluoride affinity of SiF4 (D(F4Si–F) determined equal to 259 ± 17 kJ mol1) [26]. A representative kinetic plot showing the time dependence of relative ion abundances is shown in Fig. 1. The C2F5 ion decay is clearly exponential, suggesting that the reactant ion population conforms to a single structure. The presence of two quite different isomers should rather produce a kinetic plot reflecting the combination of two exponential decays with distinct time constants. Minor amounts of an adduct ion [C2F5SiF4] start to appear after few seconds, though never exceeding 10% of the total ion abundance (Fig. 1). The electron affinity (EA) of pentafluoronitrobenzene is fairly high, equal to 1.50 ± 0.25 eV [27]. However, the EA of the C 2 F 5 radical is considerably higher (2.2 ± 0.3 eV) [27] and the reaction of pentafluoronitrobenzene with C2F5 yields NO2 as the only product ion with a high efficiency (U = 53%). This reaction is reminiscent of the reaction between fluoride ion and nitrobenzene which proceeds at a rate close to the collisional frequency [28]. By analogy, it may be inferred that an efficient nucleophilic aromatic substitution reaction is performed, consistent with a covalently bonded C2F5 species. A similar reactivity is observed with OP(OMe)3 where now the aliphatic carbon atom of a methyl group is attacked by C2F5 and (MeO)2PO2 is released as the leaving group. The two compounds COS and SO2 display a similarly low

Table 1 Ion–molecule reactions of C2F5- ions with selected neutrals (X). Neutral (X)



SiF4 C6F5NO2 OP(OMe)3 COS SO2 (CH3CO)2CH2 a b c

Product ion(s)a [X + F] (95) [X + C2F5] (5) [NO2] [(MeO)2PO2] [X + C2F5] [X + C2F5] [X–H]

kexpb

Uc

3.7

49

8.6 1.8 0.067 0.068 12.2

53 10.6 0.67 0.56 97

Branching ratios observed at initial reaction time are in parentheses. Units of 1010 cm3 s1. U = (kexp/kcoll)  100.

100

reactivity (U = 0.6%–0.7%) yielding adduct ions. Interestingly, the CF  2 carbene anion is found to react with COS by a formal Sabstraction process releasing neutral CO [29]. Both COS and SO2 have still fairly high fluoride ion affinities of 133 and 183 kJ mol1, respectively, [26] so that the lack of F transfer reactivity is not compatible with a weakly bound [F C2F4] complex, rather speaking in favour of a covalently bound species. The fast proton transfer reaction with (CH3CO)2CH2 (PA((CH3CO)2CH = 1438 kJ mol1) and the absence of reactivity with CH3C(O)OCH3 (PA(CH3OC(O)CH2 = 1573 kJ mol1) is in line with the bracketing of the proton affinity of C2F5 (PA = 1567 kJ mol1) obtained from the gas phase reaction of a series of basic anions with C2F5H [27]. 3.2. Calculated structural features of C2F5 ions The lowest energy optimized geometry of C2F5 is shown in Fig. 2, namely the Cs structure 1 of the 1A0 ground state of C2F5. The molecular structure is quite similar to the one reported by King, Pettigrew and Schaefer in their thorough study of C2Fn molecules and C2Fn anions using several density functional theory methods [30]. The CF bond of the CF3 group oriented anti to the lone pair on the adjacent carbon is 1.368 Å long, somewhat longer than the other two CF bonds (1.355 Å), in agreement with the qualitative orbital description of the B resonance structure. Indeed, it has been noted previously that the CF bond nominally anti (Cb–Fan ti) to the unshared pair on Ca is longer than the other C–F bonds on the same carbon (Cb–Fgauche) [11,13]. The molecular geometries of C2F5 and of the conjugate acid, C2F5H, are reported in Fig. 1S (Appendix A. Supplementary material). The length of the Cb–Fanti bond in the C2F5 carbanion (1.368 Å) with respect to the length of same bond (1.327 Å) in C2F5H shows an increase of 0.041 Å, to be compared with a 0.021 Å difference for the Cb–Fgauche bond between the two species. However, the bond lengthening effect is smaller than in ethyl carbanions having less fluorine atoms [13]. This result has been explained as due to a comparatively decreased importance of anionic hyperconjugation in C2F5 because of the inductive effect of fluorine lowering the energy of the lone pair orbital. In a similar way, any contribution of a no bond resonance structure B should appear in a shorter C–C bond and indeed it is found that the C–C bond length in C2F5 is 0.012 Å shorter than in C2F5H (supplementary Fig. 1S). It may be further reported that the charge on Fanti is more negative than the charge on Fgauche in C2F5, amounting to a difference of 0.090. Evaluating the difference between the negative charge on the F atom anti to hydrogen in CF3CF2H and the charge on

I (%)

75

50

1 (0)

2 (141)

3 (194)

25

5

10

15

+

20

Time (s) Fig. 1. Time dependence of the relative ion abundances when selected [C2F5] ions (}, at m/z 119) are allowed to react with SF4 at 1.8  108 mbar forming SiF5 (j, at m/z 123) and [C2F5SiF4] (N, at m/z 223).

4 (195)

F

-

(224)

Fig. 2. Geometries and relative energies (in parentheses, kJ mol1) of isomeric structures for C2F5 and of C2F4 + F fragments at MP2/cc-pVTZ level.

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3.3. Vibrational features of C2F5 ions When IR radiation of proper wavenumber is shined on the C2F5 ion cloud trapped in the cell of the FT–ICR mass spectrometer a photofragmentation process is activated involving loss of fluoride ion. It is also possible that electron detachment is taking place as well, as suggested by a decrease of the parent ion signal that is not quite balanced by the increase in the abundance of the fragment ion at m/z 19. The photofragmentation process is ensured by the absorption of multiple photons in resonance with an active vibrational mode of the sampled species and can be monitored by recording the relative ion intensities of the parent and fragment ion (Iparent and Ifragment) and plotting the photofragmentation yield R as a function of the radiation wavenumber. The derived IRMPD spectrum of C2F5 is shown in Fig. 3 (bottom panel: exp). The spectrum is dominated by two overlapping bands with maxima at 1053 and 1119 cm1. Other weaker bands appear at 1208, 908 and 819 cm1, characterized by a bandwidth (fwhm) of ca. 40 cm1, in line with the fwhm typically observed under FT–ICR MS conditions. It should be noted that a better resolution (fwhm  15– 20 cm1) can be achieved when molecular ions are thermalized through multiple collisions with He in a 3D ion-trap [31], or with Ar in a linear ion trap prior to their transfer into the ICR cell of a hybrid FT–ICR instrument [14a]. In the present case, because electron ionisation is performed inside the ICR cell, it is likely that the heated tungsten filament leads to an effective temperature of the molecular ions significantly higher than room temperature, thereby leading to a broadening of the absorption bands. The wavenumber and relative intensity of the IRMPD features are listed in Table 2 where they are faced with the calculated harmonic frequencies of 1 obtained in this work and in a previous report [30]. Detailed descriptions and assignments are based on inspection of animations of the normal modes. The stable, covalent structure 1 does in fact account for all the major vibrational features of the sampled C2F5 ions. As shown in Fig. 3, there is good agreement between the recorded IRMPD spectrum and the IR spectrum calculated for 1. On the contrary, species 2–4 present very

4

3

2

1

2

exp

R 1

800

1000

1200

1400

-1

Wavenumber (cm

(

Fgauche yields a less negative value of 0.065. Once again, development of negative charge on Fanti is consistent with the contribution of negative hyperconjugation as depicted in B. A partial double bond character of the CC bond as shown in B should impair the rotation around the C–C bond of C2F5. The calculated energy profile as a function of the FCaCbF dihedral angle shows a relatively high rotational barrier of 27 kJ mol1 as illustrated in supplementary Fig. 2S (Appendix A. Supplementary material) . The barrier for the rotation around the C–C bond is smaller in the conjugate acid C2F5H, as shown in supplementary Fig. 3S (Appendix A. Supplementary material) where the eclipsed conformation is a maximum at 16.2 kJ mol1 relative to the minimum energy staggered conformation. The rotational barrier for the radical is even lower, 11.6 kJ mol1, as shown in supplementary Fig. 4S (Appendix A. Supplementary material). Fig. 2 shows also the relative energy of the C2F4 + F dissociation products, at 224 kJ mol1 relative to 1, properly accounting for the observed F transfer reaction to SiF4. The possible existence of isomers other than 1 lying in local minima on the [C2F5] potential energy surface was carefully explored, also in view of the report suggesting two possible isomers, tightly bound and loosely bound with respect to the C2F4 + F dissociation products [6]. However, a wide variety of different initial geometries consistently converged into structure 1, unless symmetry restrictions were imposed. In the latter case species 2–4 were obtained, each characterized, though, by one imaginary frequency. The harmonic frequency analysis for 1–4 provides also the calculated IR spectra presented in the following paragraph.

Fig. 3. IRMPD spectrum of [C2F5] ions (bottom panel) and calculated IR spectra for isomeric species 1–4 (upper panels).

different IR spectra (top panels in Fig. 3) and any contribution to the sampled ion population is rather disproven. It may be useful to view the IR spectrum of C2F4. In this spectrum the most active modes are due to b2u and b1u CF stretching vibrations at 1337 and 1186 cm1, respectively [27,30]. In isomer 2, if one considers the perturbed C2F4 unit, the related modes appear at 1265 and 1122 cm1. The two overlapping bands with maxima at 1053 and 1119 cm1 can thus be considered as IR diagnostic of the strongly bound nature of the C2F5 ion. These two bands can be assigned to the two CF3 asymmetric stretching modes. While these two vibrational modes would be degenerate in an ideal C3v symmetry CF3 group, they are no longer degenerate in C2F5. If one considers the Cs-symmetry structure 1 of C2F5, the CF3 asymmetric stretching of a00 symmetry corresponds to the asymmetric combination of the C–Fgauche bonds. On the contrary, the a0 symmetry normal mode mainly involves the C–Fanti bond. Since the C–Fanti bond is weakened through negative hyperconjugation, one can thus expect that the band corresponding to the CF3 asymmetric stretching of a0 symmetry is red-shifted with respect to the a00 symmetry mode, and that the resulting frequency splitting provides a quantification of the negative hyperconjugation effect. In this context, it is interesting to compare the presently reported IRMPD spectrum of C2F5 with the one of the C 2 F 5 radical which has been obtained by matrix isolation methods [32–34]. Table S1 in the supporting information

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Table 2 Infrared spectrum of C2F5 (Cs symmetry).a Label A B C D E a b c d

Observed frequencyb 1208 (0.34) 1119 (1.0) 1053 (0.92) 908 (0.32) 819 (0.50)

Calculated harmonic frequency c

1217 (176) 1096 (312)c 1045 (338)c 914 (91)c 829 (124)c

Symm. d

1232 1122d 1039d 908d 837d

0

a a00 a0 a0 a00

Mode description CC str – CF2 sym str – CF3 sym str CF3 asym str CF3 asym str CF2 sym str CF2 asym str

Frequencies are in cm1. Relative intensities are given in parentheses. Values obtained by calculations at MP2/cc-pVTZ level, intensities (km mol1) are given in parentheses. Values reported by calculations at B3LYP/DZP++ level (no scaling factor applied) [30].

lists the experimental IR frequencies of C 2 F 5 and the harmonic frequencies calculated at the MP2/cc-pVTZ level. For both the radical and anionic species, the CF3 asymmetric stretching of a0 symmetry is predicted at a lower frequency relative to the CF3 asymmetric stretching of a00 symmetry. Relative to the radical, the a00 mode is red shifted in the anion by 108 cm1 while the a0 vibration is red shifted by 131 cm1, comparing the corresponding frequencies in the IR spectrum of the radical and in the IRMPD spectrum of the anion. This finding may suggest that the Cb–Fanti bond is somewhat weaker in the anion, providing circumstantial indication of a negative hyperconjugation effect. It may be interesting to compare the IR spectral features of gaseous C2F5 ions with another gaseous fluorocarbon ion, namely (CD3)2CF+ [35]. This species has been studied in detail by IRMPD spectroscopy using the experimental platform at the Felix center [36]. The multiple bond character of the CF bond has been ascertained, based on the CF stretching frequency observed at 1425 cm1, likely close to the high energy end for this vibrational mode. Although the molecular environment is clearly very different, also in this previous report the electronic and structural features of a fluorocarbon naked ion could be assessed, establishing landmark features for the unperturbed ionic species. Parenthetically, IRMPD spectroscopy of fluorotrimethylborate anion has been recently reported, pointing out a role for both electronegativity and hyperconjugation in affecting the vibrational modes and related force constants [37]. As a final comment, it should be stressed that deriving the IR spectrum of isolated gas phase molecular ions can be challenging in the cases of small and strongly bound systems as in the present study. In these cases, it can be difficult to drive the non-coherent absorption of multiple photons which strongly relies on the efficiency of intramolecular vibrational energy redistribution, and more sensitive spectroscopic methods than IRMPD have to be utilized. For example, in the case of CH5+, IR absorption features were revealed by monitoring the yield of an IR induced proton transfer reaction [38]. Alternatively, as in the case of the prototypical 2Ccarbocation, namely C2H5+, the argon tagging method has been adopted [39,40]. Interestingly, while the dissociation energy of C2H5+, releasing C2H3+ and H2, is ca. 200 kJ mol1, a value similar to the dissociation energy of C2F5 ions (see Fig. 2), efforts to obtain photodissociation of C2H5+ failed even using the high intensity FEL light.

4. Conclusions A prototypical perfluoroalkyl anion, C2F5, has been studied in the gas phase by complementary techniques, aiming to obtain a thorough description of this species, in particular with regard to the possible existence of isomeric structures and to the operation of negative hyperconjugation effects. The reactivity pattern displayed towards exemplary neutrals conforms to the behaviour expected for a covalently bound species. Nucleophilic substitution

reactions are fast with both an aliphatic and an aromatic substrate and F transfer reactivity is observed in correspondence to an estimated exothermic process. Quantum chemical calculations at the MP2/cc-pVTZ level show the covalent pentafluoroethyl anion 1 to be the absolute minimum on the [C2F5]– potential energy surface and yield an IR spectrum matching nicely the experimental IRMPD spectrum recorded at the CLIO European facility. The strongest vibrational features at 1053 and 1119 cm1 are assigned to the CF asymmetric stretching of the CF3 group. In this group the CF bond nominally anti (Cb–Fanti) to the unshared pair on Ca appears to exert a negative hyperconjugation effect that may be illustrated by a contribution of the B resonance structure. The effect is weak, certainly weaker than in ethyl anions with less extensive fluorine substitution [11,13]. Nevertheless, the observed splitting (66 cm1) of the two CF asymmetric stretching of the CF3 group is the signature of the negative hyperconjugation effects in C2F5–. In conclusion, C2F5 ions formed by dissociative electron capture in perfluoropropane and investigated by their reactivity towards neutrals and by IRMPD spectroscopy conform to a uniform ion population, best described as a covalently bound pentafluoroethyl anion 1. Acknowledgements This work was supported by the CNRS (PICS program), by the Italian Ministero dell’Istruzione, dell’Università e della Ricerca and by the European Community’s Seventh Framework Programme (FP7/2007-2013, under grant agreement n 226716) which provided also travel funding to M. E. C. and B. C. for access to the European multi-user facility CLIO. B. C., M. E. C. and S. F. congratulate Mario Capitelli to his 70th birthday, wish him many more healthful and productive years and thank him for an invaluable friendly relationship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemphys.2011.03.005. References [1] (a) A. Milella, F. Palumbo, R. d’Agostino, in: R. d’Agostino, P. Favia, Y. Kawai, H. Ikegami, N. Sato, F. Arefi-Khonsari (Eds.), Advanced Plasma Technology, WileyVCH Verlag GmbH & Co, KGaA, Weinheim, 2008, p. 175; (b) G.K. Vinogradov, P.I. Nevzorov, L.S. Polak, D.I. Slovetskii, Vacuum 32 (1982) 529. [2] M. Schaepkens, G.S. Oehrlein, J. Electrochem. Soc. 148 (2001) C211. [3] I. Reid, G. Hughes, Semiconductor Sci. Technol. 21 (2006) 1354. [4] E.R. Fuoco, L. Hanley, J. App. Phys. 92 (2002). [5] (a) A.N. Goyette, Y. Wang, M. Misakian, J.K. Olthoff, J.K.J. Vacuum, Sci. Technoi., A 18 (2000) 2785; (b) W. Schwarzenbach, G. Cunge, J.P. Booth, J. Appl. Phys. 85 (1999) 7562. [6] K. Hiraoka, N. Mochizuki, A. Wada, H. Okada, T. Ichikawa, D. Asakawa, I. Yazawa, Int. J. Mass Spectrom. 272 (2008) 22. [7] J.D. Roberts, R.L. Webb, E.A. McElhill, J. Am. Chem. Soc. 72 (1950) 408.

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