Gas-phase fluorination of acetylene by XeF+: Formation, structure and reactivity of C2H2F+ isomeric ions

4 May 2001

Chemical Physics Letters 339 (2001) 71±76

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Gas-phase ¯uorination of acetylene by XeF‡: Formation, structure and reactivity of C2H2F‡ isomeric ions Fulvio Cacace a, Marina Attin a b, Antonella Cartoni b, Federico Pepi a,* a b

Universit a degli studi di Roma La Sapienza, Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Piazzale Aldo Moro 5, 00185 Roma, Italy Universit a di Roma, Tor Vergata, Dipartimento di Scienze e Tecnologie Chimiche, Via della Ricerca Scienti®ca 1, 00133 Roma, Italy Received 5 January 2001; in ®nal form 21 February 2001

Abstract The gas-phase reactivity of XeF‡ towards acetylene was investigated by triple quadrupole mass spectrometry. XeF‡ promotes both F‡ and Xe‡ transfer to acetylene, yielding C2 H2 F‡ and C2 H2 Xe‡ , respectively. The C2 H2 F‡ ions formed were probed by low-energy collisionally activated dissociation mass spectrometry and characterized as CH2 @CF‡ , namely the isomer identi®ed as the most stable by previous theoretical studies. The 1-¯uorovinyl cation reacts in the gas-phase with typical nucleophiles …CH3 COCH3 ; CH3 CN; CH3 OH; C2 H4 †, as a Brùnsted acid and/or as a ¯uorinating agent, depending on the thermochemistry of the processes involved. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction The e€ect of F substitution on the thermodynamic stability of vinyl cation has been the subject of many theoretical studies in the past three decades [1±8]. In particular, the structure and stability of C2 H2 F‡ isomeric ions, formally the conjugate acids of ¯uoroacetylene, have been investigated with a variety of semiempirical and ab initio computational methods, from the early study at the SCF-MO level of theory (1975) to a very recent, comprehensive study performed at the B3LYP/6-311G(d,p) and the MP2/6-311G(d,p) levels of theory (2000). Among the three structures investigated, ion I is theoretically identi®ed as

*

Coresponding author. Fax: +390-649-91-36-02. E-mail address: [email protected] (F. Pepi).

the minimum on the C2 H2 F‡ potential energy surface.

In contrast with the theoretical interest witnessed by the above calculations, relevant experimental data are relatively scant [9±12]. To the best of our knowledge, no structural characterization of isolated C2 H2 F‡ ions has been reported to date, nor is any information available on ion±molecule reactions leading to the formation of gaseous ¯uorovinylium cations. This state of a€airs is even more surprising in view of the intrinsic interest of ¯uorinated species in atmospheric and environmental chemistry and of the importance of vinyl cations as reactive

0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 2 4 8 - 2

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F. Cacace et al. / Chemical Physics Letters 339 (2001) 71±76

intermediate in thermal, as well as in photochemical reactions [13]. In this Letter we report the results of a massspectrometric study on the formation, the structure and the reactivity of gaseous ¯uorovinylium cations, C2 H2 F‡ , obtained from the reaction of XeF‡ ions with acetylene. For comparison purposes we discuss as well the results of related studies on the ¯uorination of ethylene and acetonitrile [14,15], prototypal examples of the gasphase electrophilic ¯uorination of model p systems promoted by XeF‡ ions, a process strictly related to the selective ¯uorination of alkenes by XeF‡ salts in solution [16]. 2. Experimental All chemicals were research-grade products from Aldrich Chemical and were used without further puri®cation. The gases were obtained from Matheson Gas Products and from Fluorochem with a stated purity in excess of 99.95 mol% and used as such. The experiments were performed with a Model Quattro triple quadrupole (TQ) instrument from VG Micromass. The ions generated in the chemical ionization (CI) or electron impact (EI) source and mass selected with the ®rst quadrupole (Q1) were driven into the collision cell, actually a RF-only hexapole (Q2), containing the neutral reagent whose pressure, measured by a ionization gauge, was adjusted from 4  10 6 to 2  10 5 Torr. The ion±molecule reactions were performed at collision energies from 0 to 6 eV (laboratory frame). The center of mass collision energy Ecm for the system is given by Ecm ˆ Elab ‰m=…M ‡ m†Š, where Elab is the nominal laboratory energy and M and m represent the masses of the reactant ion and neutral target, respectively. Total cross-section …rtot † was calculated by use of rtot ˆ Ip =INl, where Ip and I are the measured intensities for the product and reactant ion beams, respectively, N is the number density of the neutral target, and l is the e€ective path length of the collision cell. The collisionally activated dissociation (CAD) spectra were recorded utilizing Ar as the target gas at pressures up to 1  10 5 Torr at collision energies from 10 to 100

eV (laboratory frame). The charged products were analyzed with the third quadrupole (Q3), scanned at a frequency of 150 amu s 1 , accumulating about 150 scans for each run. 3. Experimental results 3.1. Formation of C2 H2 F ‡ ions The C2 H2 F‡ ions were obtained from the reaction XeF‡ ‡ C2 H2 ! C2 H2 F‡ ‡ Xe

…1†

that provides a further example of the ¯uorinating ability of XeF‡ [17]. The latter is the most abundant ion in the CI spectrum of undiluted XeF2 vapor, recorded at a source pressure of 1  10 4 Torr, and is accompanied by Xe‡ and by a small amount of the XeF‡ 2 molecular ion. The occurrence of reaction (1) was demonstated in the triple quadrupole mass-spectrometer experiments by mass selecting one of the XeF‡ isotopomers in the ®rst quadrupole and allowing it to react with acetylene in the RF-only gas cell, at pressures up to 1:5  10 5 Torr (Fig. 1). At such low pressures, no adducts between XeF‡ and acetylene were observed. Fluorination is accompanied by formation of C2 H2 Xe‡ ions, a reaction channel peculiar of the XeF‡ ions reactivity towards molecules containing multiple bonds, like ethylene and acetonitrile [14,15]. Reaction (1) was studied at ion energies from 0 to 6 eV (laboratory frame) by changing the voltage applied to the ion source block and to the third quadrupole and measuring the products ion

Fig. 1. Formation of C2 H2 F‡ and C2 H2 Xe‡ ions from the reaction of mass-selected 132 XeF‡ ions (m=z 151) with C2 H2 .

F. Cacace et al. / Chemical Physics Letters 339 (2001) 71±76

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3.2. Structural characterization of C2 H2 F ‡ ions

Fig. 2. Energy resolved mass-spectrum of the ionic products obtained from reaction (1).

intensities as a function of the translational energy of the reactant ion (Fig. 2). Moreover, C2 H2 F‡ ions of known structure, generated from the EI of 1,1-di¯uoroethylene and 1-chloro-2-¯uoroethylene according to processes (2) and (3), were used as models to characterize the structure and the reactivity of the ionic population from reaction (1). EI

‡=

CH2 @CF2 ! CH2 @CF2

! CH2 @CF‡ ‡ F I

…2† EI

ClCH@CHF ! ClCH@CHF‡= ! CH@CHF‡ ‡ Cl II

…3†

Process (2) and (3) are expected to generate a population of ions I of CH2 @CF‡ connectivity and a population of ions II of CH@CHF‡ connectivity, respectively.

The structure of C2 H2 F‡ ions generated from the ¯uorination of acetylene was probed by collisionally activated dissociation (CAD) mass spectrometry, comparing the spectra of the ions from reaction (1) with those of the model ions. The lowenergy CAD spectra of the ions obtained from processes (1)±(3), recorded at a collision energy of 60 eV (laboratory frame), are closely similar, displaying the C2 H‡ 2 fragment at m=z 26, arising from F loss, as the major peak (Table 1). The dissociation channel yielding the CF‡ fragment ion at m=z 31 is structurally informative, suggesting the existence of a CH2 group in all the ionic population assayed (vide infra). The above structural evidence on the connectivity of the C2 H2 F‡ ions was con®rmed by the survey of their reactions with selected nucleophiles (`reactive probing'). The C2 H2 F‡ ion reacts predominantly as a Brùnsted acid or a ¯uorinating agent, depending on the proton anity (PA) of the nucleophile used. The higher PA of …CH3 †2 CO, CH3 CN, CH3 OH than that of ¯uoroacetylene makes their protonation by C2 H2 F‡ considerably exothermic and hence the only reaction channel observed. As the PA of the neutral reagent is lowered, ¯uorination becomes energetically favored, e.g. in the case of ethylene. All the C2 H2 F‡ ionic populations sampled show the same reactivity, as illustrated in Table 2. 4. Discussion 4.1. Fluorination of acetylene The gas-phase reaction of the XeF‡ cation with acetylene results in F‡ and Xe‡ transfer, as previously noted with molecules, such as ethylene and

Table 1 CAD Spectra of C2 H2 F‡ ions from various source taken at a collision energy (laboratory frame) of 60 eV Fragment (m=z)

Relative intensities (%) Reaction (1)

Model ion I

Model ion II

C2 H‡ 2 (26) C2 H‡ (25) CF‡ (31)

66.8 13.3 19.9

67.9 10.7 21.4

68.6 11.3 20.1

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F. Cacace et al. / Chemical Physics Letters 339 (2001) 71±76

Table 2 Reactivity of C2 H2 F‡ ions toward selected nucleophiles Nucleophile ‡

C2 H2 F ions

…CH3 †2 CO

CH3 CN

CH3 OH

C2 H4

CO

Reaction (1) Model ion I Model ion II

H‡ transfer H‡ transfer H‡ transfer

H‡ transfer H‡ transfer H‡ transfer

H‡ transfer H‡ transfer H‡ transfer

H‡ =F‡ transfer H‡ =F‡ transfer H‡ =F‡ transfer

No reaction No reaction No reaction

acetonitrile, containing multiple bonds. In particular, acetylene displays the same reactivity showed by the triple C±N bond of acetonitrile, whereas ¯uorination of ethylene was not observed. The discrepancy is only apparent because at the low pressures of the reaction cell, the low eciency of collisional deactivation causes the ¯uoroethylene ion, excited by the large exotermicity of its formation process, 440:1 kJ mol 1 , to undergo HF loss, endothermic by only 156:0 kJ mol 1 . On the other hand, the scarce tendency of ¯uorinated acetylene to undergo HF loss re¯ects the lower exotermicity of its formation process, 311:7 kJ mol 1 , and the higher endothermicity, as large as 452:7 kJ mol 1 , of its dissociation into HF and C2 H‡ due to the considerable instability of the ionic fragment [18]. From a mechanistic point of view, the electrophilic association of XeF‡ to the p electrons system of acetylene is expected to yield two distinct adducts, electrostatically bound through the xenon and the ¯uorine atoms, respectively, as previously demonstrated in the case of acetonitrile [15].

In the low-pressure range of the reaction cell, the gaseous ion±molecule complexes IV and V initially formed undergo ¯uorine and xenon atom loss yielding the ¯uorovinylium cation and C2 H2 Xe‡ , respectively. The occurrence of these dissociation processes was con®rmed by the detection of the C2 H2 F‡ and C2 H2 Xe‡ fragments in the CAD spectra of the adduct between XeF‡ and

acetylene obtained in the high pressure range of the CI source. It is worth to note the formation and the unexpected stability of C2 H2 Xe‡ ions. Considering the ionization potential of C2 H2 and Xe, 11.4 and 12.1 eV, respectively, and the existence of low-lying excited state for the C2 H‡ 2 ion, the particular stability of C2 H2 Xe‡ ions could be attributed to the formation of two resonant ‰C2 H‡ 2 ±XeŠ and ‰C2 H2 ±Xe‡ Š complexes stabilized by charge exchange coupling. As apparent from Fig. 2, ¯uorination is characterized by an energy barrier, whereas no barrier is associated with Xe‡ transfer whose eciency remains constant over the energy range investigated. A total cross-section, rtot , of 7:6  10 14 cm2 …20%† at a center of mass collision energy of 0.6 eV was estimated for the reaction (1), with a branching ratio of 8:1 in favor of ¯uorination. 4.2. Structure and reactivity of C2 H2 F ‡ ions The structural evidence obtained from the CAD spectra points to the formation of a single ionic population from the three di€erent processes used to generate the C2 H2 F‡ ions. In particular, the decomposition that leads to the CF‡ fragment upon loss of a CH2 group is consistent with the presence of ions characterized by the CH2 @CF‡ connectivity. Formation of separated CH and H fragments, rather than of CH2 , as the neutral dissociation products can be excluded on energetic grounds. It is interesting to note that the CF‡ fragment is present as well in the CAD spectrum of the model ion II, showing that also this species, excited by the large exothermicity of its formation process, can isomerize into the more stable ion I via a 1,2 hydrogen shift. Considering that the three

F. Cacace et al. / Chemical Physics Letters 339 (2001) 71±76

possible isomeric structure di€er only for the position of one H atom, we cannot exclude the presence of II and III isomers in the ionic populations assayed. However any such ions initially formed, in the high-pressure CI source are expected to undergo fast isomerization into the more stable isomer I. The C2 H2 F‡ ions reactivity towards selected bases/nucleophiles in the TQ gas cell re¯ects the thermochemistry of the processes involved. The large exothermicity of the proton transfer process to bases, such as CH3 OH …PA ˆ 754:3 kJ mol 1 †, CH3 CN …PA ˆ 779:2 kJ mol 1 † and …CH3 †2 CO …PA ˆ 812:0 kJ mol 1 †, of PA higher than that of ¯uoroacetylene …PA ˆ 686:0 kJ mol 1 † causes protonation to be the only reaction channel detectable. Conversely, ¯uorination is the main reaction pathway in the case of ethylene …PA ˆ 680:5 kJ mole 1 †, whose slightly endothermic protonation is observed to occur only to a low extent, probably allowed by the traslational energy excess of the ionic reagent. No reactions were observed with CO, whose protonation and ¯uorination by C2 H2 F‡ are highly endothermic. The results of reactive probing experiments con®rm the structural identity of the ionic population generated from the ¯uorination of acetylene with those of the model ions from the EI of 1,1di¯uoroethylene and 1-chloro-2-¯uoroethylene, but fail to demonstrate the existence of a pure population of ion I. Indeed, the ability of the C2 H2 F‡ ions to undergo H‡ and/or F‡ transfer to gaseous nucleophiles is consistent with all three isomeric structures I, II and III. 5. Conclusion The present results provide a further signi®cant example of the ability of the XeF‡ cations to promote F‡ transfer to molecules containing C±C multiple bonds. Since we have been unable to exploit formation processes of the C2 H2 F‡ ions less exothermic than reaction (1), the reactivity pattern is a€ected by the large internal energy excess of the primary ionic products. Unavoidably, the ionic population largely evolves into ions of the CH2 @CF‡ structure, that characterizes the most

75

‡

stable ‰C2 ; H2 ; FŠ isomers, according to the available theoretical evidence. The results from this study support the view that the reaction of XeF‡ ions with simple unsaturated molecules such as ethylene, acetylene and acetonitrile proceeds in the gas-phase by a mechanism strictly related with that of the selective ¯uorination promoted by the XeF‡ salts in solution [16]. In absence of counterions and solvating molecules the primary acetylene¯uoroxenonium ions undergo competitive dissociation into ¯uorinated acetylene or C2 H2 Xe‡ , a charged product peculiar of, and stable only in, the gas-phase. Acknowledgements Work carried out with the ®nancial support of the Universities of Rome `La Sapienza' and `Tor Vergata' and of Consiglio Nazionale delle Ricerche (CNR). The authors express their gratitude to Mr. Giuseppe D'Arcangelo who performed the TQ measurements. References [1] C.U. Pittman Jr., L.D. Kispert, T.B. Patterson Jr., J. Phys. Chem. 77 (1973) 494. [2] I.G. Csizmadia, V. Lucchini, G. Modena, Theoret. Chim. Acta 39 (1975) 51. [3] Y. Apeloig, Pv.R. Schleyer, J.A. Pople, J. Am. Chem. Soc. 42 (1977) 3004. [4] P. Kollmann, S. Nelson, S. Rothenberg, J. Phys. Chem. 82 (1978) 1403. [5] P.S. Martin, K. Yates, I.G. Csizmadia, Can. J. Chem. 67 (1988) 2178. [6] D.A. Stams, T.D. Thomas, D.C. McLaren, D. Ji, T.H. Morton, J. Am. Chem. Soc. 122 (1990) 1427. [7] Y. Apeloig, R. Biton, H. Zuilhof, G. Lodder, Tetrahedron Lett. 35 (1994) 265. [8] K.V. Alem, G. Lodder, H. Zuilhof, J. Phys. Chem. A 104 (2000) 2780. [9] R.M. O'Malley, K.R. Jennings, M.T. Bowers, V.G. Anicich, Int. J. Mass Spectrom. Ion Phys. 11 (1973) 89. [10] H.S. Tan, M.J.K. Pabst, J.L. Franklin, Int. J. Mass Spectrom. Ion Phys. 21 (1976) 297. [11] J. Dannacher, A. Shmelzer, J. Stadelmann, J. Vogt, Int. J. Mass Spectrom. Ion Phys. 31 (1979) 175. [12] M. Vincenti, R.G. Cooks, Org. Mass Spectrom. 23 (1988) 317. [13] P.J. Stang, Z. Rappoport, M. Hanack, L.R. Subramanian, Vinyl Cations, Academic Press, New York, 1979.

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[14] M. Attin a, F. Bernardi, F. Cacace, I. Rossi, Chem. Eur. J. 5 (1999) 1186. [15] M. Attin a, F. Cacace, A. Cartoni, M. Rosi, J. Phys. Chem. A 104 (2000) 7574. [16] N.S. Ze®rov, A.G. Gakh, V.V. Zhdankin, P.J. Stang, J. Org. Chem. 56 (1991) 1416.

[17] F. Cacace, G. de Petris, F. Pepi, M. Rosi, A. Troiani, J. Phys. Chem. A 103 (1999) 2128. [18] Data taken from NIST Chemistry WebBook; NIST standard Reference Database No. 69 ± February 2000 release; data collection of the National Institute of Standards and Technology (http://webbook.nist.gov).