Chemical Physics Letters 461 (2008) 21–27

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Chemical Physics Letters

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Gas-phase ion chemistry of BF3/CH4 mixtures: Activation of methane by BFþ2 ions

Federico Pepi a,*, Alessandra Tata a, Stefania Garzoli a, Marzio Rosi b

a Università degli studi di Roma ‘La Sapienza’, Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Piazzale Aldo Moro 5, 00185 Roma, Italyb Università di Perugia, Dipartimento di Ingegneria Civile e Ambientale e Istituto di Scienze e Tecnologie Molecolari del CNR, Via Elce di Sotto 8, 06123 Perugia, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 May 2008In final form 26 June 2008Available online 2 July 2008

0009-2614/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.06.083

* Corresponding author. Fax: +39 0649913602.E-mail address: Federico.Pepi@uniroma1.it (F. Pep

The gas-phase ion chemistry of BF3/CH4 mixtures was investigated by the joint application of mass-spec-trometric and ab initio theoretical methods. The addition of BFþ2 to CH4 led to the observation of BF2CHþ4and BFCHþ3 ions in the gas phase. Collisionally activated dissociation (CAD) mass-spectrometric resultsand theoretical calculations performed at the B3LYP and CCSD(T) levels identified the F2B—CHþ4 andF—B—CHþ3 ions as the most stable isomers on the [BFnCHn+2]+ (n = 1, 2) potential energy surfaces. It isworth noting that C–N bond formation was observed when BFCHþ3 ions were allowed to react with NH3.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Methane is a plentiful feedstock in the chemical industry, butnotoriously difficult to functionalize in a selective manner. Meth-ane activation by transition metal catalysts is crucial for manyindustrial processes such as the Degussa reaction were CH4 is cou-pled to NH3 for the production of hydrogen cyanide. Nevertheless,an understanding of the mechanisms of many important catalyticreactions in the condensed phase is still limited.

In the last three decades a large number of gas-phase mass-spectrometric studies have been conducted on the reactivity ofmethane–transition metal cluster in order to obtain a simplifiedmodel that could enhance the mechanistic knowledge of this pro-cess and contribute to the development of new catalysts.

Gas-phase C–H and C–C bond activation by transition-metalions was initially studied by using guided ion-beam and ion cyclo-tron resonance mass-spectrometric techniques [1–4]. Many transi-tion-metal ions are capable of dehydrogenating methane yieldingthe reactive methylidene ion, MCHþ2 . Both Beauchamp and Arment-rout have demonstrated the greater reactivity of the third-rowtransition-metal ions with respect to their first and second rowcongeners explained in terms of orbital size and promotion and ex-change energies leading to a more efficient spin–orbit coupling [5–7].

Dihydrogen elimination of methane by gaseous cationic and an-ionic platinum clusters with up to 24 Pt atoms was also recentlyreported by Uggerud [8] and Bondybey [9].

The addition of methane to ammonia, mediated by transition-metal ions, was studied both experimentally and computationallyby Schwarz and coworkers [10–14]. These studies demonstratedthat C–N bond formation is efficiently catalyzed by Pt+, Ptþ2 , PtAu+

and PtClþ2 gaseous ions.

ll rights reserved.

i).

Moreover, non-metallic ionic species were able to activatemethane r bonds in the gas phase. A great efficiency in the meth-ane activation by BHþ2 ions has been demonstrated by DePuy et al.[15]. In particular, methane reacts rapidly with BHþ2 by additionfollowed by H2 elimination leading to the HBCHþ3 ion.

Many efficient ion/molecule reactions involving non-metallicions were also able to functionalize C–H inert bonds of aromaticcompounds such as benzene and toluene [16–18].

Recently we reported the first experimental observation of gas-eous ions obtained from the gas-phase ionization of BF3/HN3 andBF3/NH3 mixtures [19,20].

In this Letter, we report the gas-phase ion chemistry of BF2CHþ4and BFCHþ3 ions synthesized by the reaction between BFþ2 and CH4

under typical mass-spectrometric chemical ionization (CI) condi-tions. These species are relatively unknown, only the appearanceenergy of the BFCHþ3 ion and a theoretical study of the structureand energetics of [B,C,F,H3]+ isomers have been reported [21,22].

In addition, the reactivity of BFCHþ3 towards ammonia was ex-plored, finding relevant analogies with that of bare CHþ3 ions [23–26] and ionic species such as PtCHþ3 and HBCHþ3 .

2. Experimental and theoretical methods

Boron trifluoride was purchased from Sigma–Aldrich Co. with astated purity of 99.9 mol%.

2.1. Mass-spectrometric experiments

Triple quadrupole mass-spectrometric experiments were per-formed with a TSQ700 instrument from Thermofinnigan Ltd. Theions generated in the chemical ionization (CI) source were driveninto the collision cell, actually a RF-only hexapole, containing theneutral reagent. The collisionally activated dissociation (CAD)spectra were recorded using Ar as the target gas at pressures of

22 F. Pepi et al. / Chemical Physics Letters 461 (2008) 21–27

about 0.1 mTorr and at collision energies ranging from 0 to 40 eV(laboratory frame). An upper limit of 2–3 eV for the kinetic energyof the reactant ion at nominal collision energy of 0 eV (laboratoryframe) and an ion-beam energy spread of about 1 eV can be esti-mated by using cut-off potentials. Laboratory ion energies (lab)are converted to center-of-mass (CM) energies by using the for-mula ECM = ELAB m/(m + M), where m is the mass of neutral reactantand M is the mass of the ionic reagent. Experimental cross sections,rtot, were determined by the relation IR/Itot = exp(�rnl) where IR isthe intensities of the transmitted ion beam, Itot is the total ionintensities, n is the number density of the neutral gas and l is theeffective gas cell length. Individual product cross sections, rp, werecalculated by rp = r(Ip/Iptot) where Ip represents the intensity ofthe product ion and Iptot the total product ion intensities.

Ion/molecule reactions inside the collision cell were recorded atneutral reactant gas pressures ranging from 0.1 to 1.2 mTorr and ata nominal collision energy of 1 eV (laboratory frame). The pres-sures values reported do not represent the actual reactant gas pres-sure inside the cell but correspond to the readings of a pirani gaugeon the gas introduction line. The charged fragments and/or prod-ucts were analyzed with the third quadrupole, scanned at a fre-quency of 150 amu s�1.

2.2. Computational methods

Density functional theory, using the hybrid [27] B3LYP func-tional [28], was used to optimize the geometry of relevant speciesand evaluate their vibrational frequencies. Although it is wellknown that density functional methods using non-hybrid func-tionals sometimes tend to overestimate bond lengths [29], hybridfunctionals as B3LYP usually provide geometrical parameters inexcellent agreement with experiment [30]. Single point energy cal-culations at the optimized geometries were performed using thecoupled-cluster single and double excitation method [31] with aperturbational estimate of the triple excitation [CCSD(T)] approach[32], in order to include extensively correlated contributions[33,34]. Transition states were located using the synchronous tran-sit-guided quasi-Newton method due to Schlegel and coworkers[35]. The 6-311+G(2d) basis set was used [36–39]. Zero-point en-ergy corrections evaluated at the B3LYP/6-311+G(2d) level wereadded to CCSD(T) energies. All calculations were performed usingGAUSSIAN 03 [40].

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0 0,1 0,2 0,3 0,4 0,5 0,6

Pressure

Rel

ativ

e In

tens

itie

s

0.1 0.60.2 0.3 0.4 0.500.0

0.1

0.1

0.2

0.3

Fig. 1. Profiles of the ionic intensities of the products obtained from the

3. Results

3.1. Formation of BF2CHþ4 and BFCHþ3 ions

The high pressure triple quadrupole (TQ) BF3/Ar (1:3) CI massspectrum is dominated by the BF3/CI peaks at m/z 48/49 and m/z115, 116 and 117 characterized by the boron isotopic patternand corresponding to the BFþ2 and B2Fþ5 ions, respectively. The addi-tion of a small amount of methane to the CI plasma leads to theappearance of ionic species at m/z 64/65 and m/z 44/45, formallycorresponding to the BF2CHþ4 and BFCHþ3 ions, respectively.

To investigate the process leading to the formation of these twoionic species the 11BFþ2 and 11B2Fþ5 ions were mass selected with thefirst quadrupole and left reacting with CH4 in the collision cell at anominal collision energy of 1 eV (laboratory frame) and at pres-sures ranging from 0.1 to 1.2 mTorr. The 11BFþ2 ions form the11BFCHþ3 ion at m/z 45 whereas the reaction involving the 11B2Fþ5species (Fig. 1) gives rise to the 11BFCHþ3 ion as well as the11BF2CHþ4 addition product at m/z 65 and the dissociation intothe 11BFþ2 fragment.

3.2. Characterization of BF2CHþ4 and BFCHþ3 ions

Collisionally activated dissociation (CAD) mass spectrometryhas been used to gain structural information on the BF2CHþ4 andBFCHþ3 ions.

The addition of BFþ2 to methane should reasonably lead to iso-mer 1. An additional isomer, 2, could be generated from 1 througha proton shift on a fluorine atom of the BF2 moiety.

The cross sections for the CAD of the 11BF2CHþ4 ions recorded at cen-ter-of-mass collision energies ranging from 0 to 8 eV is reported inFig. 2a. The spectrum is dominated by the 11BFþ2 fragment which ac-counts for 80% of the whole ionic fragmentation pattern at a colli-sion energy of 8 eV. The 11BFþ2 ion appears at a collision energycomparable to that characteristic of the lower fragmentation chan-

0,7 0,8 0,9 1 1,1 1,2 1,3

(mTorr)

11BF2+

11BF2CH4+

11BFCH3+

0.7 0.8 0.9 1.11.0 1.2 1.3

reaction of 11B2Fþ5 with CH4 at different collision cell gas pressures.

0

1E-15

2E-15

3E-15

4E-15

5E-15

6E-15

7E-15

8E-15

9E-15

0 1 2 3 4 5 6 7 8

collision energy (eV, CM)

cros

s se

ctio

n (c

m2 )

11BF2+

11BF,CH3+

0

1E-16

2E-16

3E-16

4E-16

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

collision energy (eV, CM)

cros

s se

ctio

n (c

m2 )

CH3+

11BF+

11BCH3+

Fig. 2. (a) Cross sections for the CAD of the [11BF2,CH4]+ ions as a function of the center-of-mass collision energy. (b) Cross sections for the CAD of the [11BF,CH3]+ ions as afunction of the center-of-mass collision energy.

F. Pepi et al. / Chemical Physics Letters 461 (2008) 21–27 23

nel leading to the formation of the 11BFCHþ3 ion, through the loss ofa HF molecule.

Taking into account that the BFCHþ3 ions are generated by theloss of the HF molecule from the BF2CHþ4 adduct, it is reasonableto assume the formation of a species characterized by F—B—CHþ3connectivity. The cross sections for the CAD of the 11BFCHþ3 ions re-corded at center-of-mass collision energies ranging from 0 to 24 eVis reported in Fig. 2b.

The CHþ3 ion represents the main fragmentation channelwhereas only at very high center-of-mass collision energies ap-pears the CH3

11B+ and 11BF+ fragment ions.

3.3. Reactivity of the BF2CHþ4 and BFCHþ3 ions with ammonia

The reactivity of the BF2CHþ4 and BFCHþ3 ions towards NH3

was studied with the triple quadrupole mass spectrometer by

24 F. Pepi et al. / Chemical Physics Letters 461 (2008) 21–27

mass selecting the reactant ions with the first quadrupole andletting them to react in the collision cell with the neutralsubstrate introduced at pressures ranging from 0.1 to 1.2mTorr.

The BF2CHþ4 ion reacts with NH3 by ligand exchange leading tothe formation of the BF2NHþ3 adduct and to BFNHþ2 ion throughoutthe subsequent loss of an HF molecule. High intensities of NHþ4were also observed due to a fast proton transfer from BF2CHþ4 tothe highly basic ammonia.

A similar reactivity was observed when the BFCHþ3 ions were al-lowed to react with ammonia. The formation of the [BFCH3,NH3]+

addition product is followed by the loss of a HF molecule, leadingto the [BCH5N]+ ion. In this case it is interesting to note an addi-tional dissociation of the [BFCH3,NH3]+ adduct that through theloss of an H2 molecule generates the [BFCH4N]+ ion. The formationof the latter species may be indicative of the formation of a carbon–nitrogen bond. Very low amount of NHþ4 were also observed,although this reactivity cannot be attributed unequivocally to theprotonating ability of this ion since may also derive from self-pro-tonation reaction involving the ammonia radical cation formed bycharge transfer. Reactant gas pressures of about 1 mTorr give riseto multiple collision conditions and consecutive reactions takeplace as evidenced by the subsequent addition of NH3 to the[BCH5N]+ product ion.

To clarify the mechanism of the H2 elimination and demon-strate whether the H atoms originated from the ammonia and/orthe methyl group, the reaction was studied by using deuterium la-beled species. The use of CD4 to generate BFCDþ3 ions at m/z 47 andm/z 48, respectively, was precluded by the overlapping of thesespecies with 11BFOH+ and 10BFþ2 ions, always present in the CI massspectrum of BF3. However, the reactivity of the BFCHþ3 ions withND3 was studied. Unfortunately, no useful information about H2

elimination was obtained from this experiment since a fast andcomplete H/D exchange was observed in the [BFCH3,ND3]+ com-plex initially formed in the collision cell.

B

F

F

1

113.3

1.275

1.839

H

H

B CF

3

1.229

1.480 109.3

H

H

H1.097

109.7

B C

F

F

2a

1.655

109.5

1.511

0.948

122.3

102.6

H

H

H

1.260

H

1.095

1.091

107.5

142.8

Fig. 3. Optimized geometries of the [BF2,CH4]+ and [BF,CH3]+ ions

3.4. Theoretical calculations

In order to rationalize the mass-spectrometric results, theoreti-cal calculations were performed by using an approach based on thedensity functional theory using the hybrid B3LYP functional. Addi-tional calculations were carried out using the coupled-cluster sin-gle and double excitation method with a perturbational estimate ofthe triple excitations CCSD(T) approach. The optimized structuresof the BF2CHþ4 and BFCHþ3 ions are shown in Fig. 3. The relativeenergies of these species, computed at 0 K both at the B3LYP andCCSD(T) level of theory are reported in Table 1 together with thebarrier heights for their isomerization processes and the energyof selected dissociation reactions.

The addition of BFþ2 to CH4 gives rise to isomers 1 and 2. Isomer2 may exist in two forms, 2a and 2b, that show almost the samestability but differ in the H–F–B–C dihedral angle. Isomer 1 is com-puted to be the most stable species, being more stable than 2a by7.5 (6.5) kcal mol�1 at the B3LYP (CCSD(T)) level of theory.

The formation of 1 via the addition of BFþ2 to CH4 is exothermicby 26.1 (27.3) kcal mol�1 whereas the same species is obtainedfrom the reaction of B2Fþ5 and CH4 in a process that is almost ther-moneutral. The interconversion between ions 1 and 2a requiresovercoming an activation barrier, computed to be 29.6(32.2) kcal mol�1 at the B3LYP (CCSD(T)) level of theory. Isomer 1can lose an HF molecule through this isomerization process gener-ating the BFCHþ3 ion, 3.

The same species can be obtained directly from isomer 2 by theloss of an HF molecule in a process endothermic by 14.1(14.8) kcal mol�1 at the B3LYP (CCSD(T)) level of theory.

The BFCHþ3 ion is very stable. The dissociation of the carbon–boron bond leading to CHþ3 and BF, that represents its easiest dis-sociation channel, requires 116.0 (108.4) kcal mol�1 at the B3LYP(CCSD(T)) level of theory. Ion 3 can isomerize to species 4 withHFBCHþ2 connectivity, which is less stable than 3 by 107.0(105.3) kcal mol�1 at B3LYP (CCSD(T)) level of theory. Moreover,

C

1.088

116.6

104.1

H

1.123

H

B C

F

F

2b

1.645

110.3

1.506

0.951

120.4

105.6

H

H

H

1.268

H

1.095

1.090

106.7

143.5

B CF

4

1.420 1.351 115.7

HH

H1.088

122.10.971 120.5

. Bond lengths are in angstroms and bond angles in degrees.

Table 1Energy changes (kcal mol–1, 0 K) computed at the B3LYP/6-311+G(2d) (CCSD(T)/6-311+G(2d)) level of theory for selected dissociation and isomerization reactions forthe system BFþ2 /CH4

Process DE Barrier height

BFþ2 þ CH4 ! BF2CHþ4 ð1Þ �26.1 (�27.3)B2Fþ5 þ CH4 ! BF2CHþ4 ð1Þ þ BF3 0.3 (�0.4)BF2CHþ4 ð1Þ ! BFCHþ3 ð3Þ þ HF 21.6 (21.3)BF2CHþ4 ð1Þ ! BFCH3 � � �HFþ ð2aÞ 7.5 (6.5) 29.6 (32.2)BFCH3 � � �HFþ ð2aÞ ! BFCHþ3 ð3Þ þ HF 14.1 (14.8)BFCH3 � � �HFþ ð2aÞ ! BFCH3 � � �HFþ ð2bÞ �1.7 (�1.8) 0.8 (1.0)BFCHþ3 ð3Þ ! CHþ3 þ BF 116.0 (108.4)BFCHþ3 ð3Þ ! CH3 þ BFþ 141.9 (140.5)BFCHþ3 ð3Þ ! BCHþ3 þ F 176.7 (174.1)BFCHþ3 ð3Þ ! HFBCHþ2 ð4Þ 107.0 (105.3) 120.0 (121.6)

Table 2Energy changes (kcal mol–1, 0 K) computed at the B3LYP/6-311+G(2d) (CCSD(T)/6-311+G(2d)) level of theory for selected dissociation and isomerization reactions forthe system [BFCH3]+/NH3

Process DE Barrier height

BFCHþ3 þ NH3 ! BFCH3NHþ3 �62.9 (�66.3)BFCH3NHþ3 ! BFCH4Nþ þ H2 63.4 (65.8) 63.5 (65.9)BFCH3NHþ3 ! BFHCH3NHþ2 40.0 (40.5) 57.8 (60.9)BFHCH3NHþ2 ! BCH3NHþ2 þHF 10.1 (12.2)BFCHþ3 þ NH3 ! BFCH4Nþ þH2 0.5 (�0.5)BFCHþ3 þ NH3 ! CH2NHþ2 þ BFþ H2 44.9 (36.3)BFCH3NHþ3 ! BCH3NHþ2 þ HF 50.1 (52.7)BFCH3NHþ3 ! CH2NHþ2 þ BFþ H2 107.8 (102.6)

F. Pepi et al. / Chemical Physics Letters 461 (2008) 21–27 25

this isomerization process shows a barrier of 120.0(121.6) kcal mol�1 at B3LYP (CCSD(T)) level of theory.

Table 2 reports the energy for the reaction of the BFCHþ3 ionswith ammonia together with the energy of some selected dissoci-ation of the [BFCH3,NH3]+ adduct. The optimized structures of the[BFCH3,NH3]+ ion and its dissociation products are reported inFig. 4.

The addition of BFCHþ3 to NH3 is exothermic by 62.9(66.3) kcal mol�1. The BFCH3NHþ3 ion may lose an H2 molecule giv-ing rise to the cyclic BFCH4N+ ion in a process endothermic by 63.4(65.8) kcal mol�1 at the B3LYP (CCSD(T)) level of theory. This pro-cess shows a barrier which is only slightly higher (by 0.1 kcalmol�1) than the endothermicity of the reaction.

The BFCH3NHþ3 ion may also isomerize into BFHCH3NHþ2 charac-terized by a structure in which a proton of the ammonia is shiftedto the fluorine atom of the BF moiety. This process is endothermicby 40.0 (40.5) kcal mol�1 and is characterized by an energy barrierof 57.8 (60.9) kcal mol�1 at the B3LYP (CCSD(T)) level of theory.The loss of the HF molecule from the BFHCH3NHþ2 ion is endother-mic by 10.1 (12.2) kcal mol�1 at the B3LYP (CCSD(T)) level andleads to the formation of the pseudo-linear fragment BCH3NHþ2 .

4. Discussion

In the following discussion, for simplicity we will refer only toCCSD(T) energies.

The formation of BF2CHþ4 and BFCHþ3 ions was accomplished byBF3 chemical ionization of methane according to the followingreaction scheme.

The mutually supporting results obtained from the energy resolvedTQ-CAD mass spectra and theoretical calculations allow the struc-tural characterization of these species and the identification of theionic pathways responsible for their formation.

The thermoneutral reaction of the B2Fþ5 ion with CH4 leads to theobservation of the BF2CHþ4 adducts in the triple quadrupole colli-sion cell. Conversely, the addition of bare BFþ2 ions to CH4, exother-mic by 27.3 kcal mol�1, in the low pressure regime of the triplequadrupole cell was not observed. In this case, the BF2CHþ4 adductinitially formed, excited by the exothermicity of its formation pro-cess and by the translational energy of the BFþ2 reactant ions, doesnot survive but generates the BFCHþ3 fragment through the loss ofan HF molecule.

The BF2CHþ4 ion may exist in two isomeric forms, isomer 1, char-acterized by the BF2—CHþ4 connectivity and the HF—BFCHþ3 isomer2, less stable than 1 by 6.5 kcal mol�1. The isomerization of 1 into2 requires 32.2 kcal mol�1. Considering the exothermicity of thereactions of BFþ2 and B2Fþ5 with CH4, at least in principle, only isomer1 should be formed in the mass spectrometer CI source by B2Fþ5whereas excited BFþ2 ions could led also to isomer 2 formation.

The BF2CHþ4 CAD spectrum shows that this species loses a meth-ane molecule at a collision energy comparable to that necessary forthe loss of the HF molecule. According to theoretical calculations,these results demonstrate that the BF2CHþ4 ionic population con-tains only ions characterized by isomer 1 connectivity. In fact, themost stable isomer 1 is the species responsible of the formationof the BFþ2 fragment ion characterized by a dissociation energy of27.3 kcal mol�1. However, the dissociation into BFCHþ3 and HF canbe attributable to the isomerization of 1 into 2 and its subsequentdissociation, a process requiring at least 32.2 kcal mol�1, i.e. an en-ergy comparable with that characteristic of the BFþ2 fragment for-mation.

The existence of ion characterized by isomer 2 connectivity canbe excluded since this species loses the HF molecule through a pro-cess endothermic by only 14.8 kcal mol�1.

Nevertheless, the exclusive presence of isomer 1 in the BF2CHþ4 pop-ulation can be also deduced by observing its reactivity with ammonia.The transfer of the BFþ2 group to NH3 is a classical ligand exchangereaction attributable only to the BF2—CHþ4 ion–molecule complex.

Fig. 4. Optimized geometries of the [BFCH3NH3]+ ion and its dissociation products.Bond lengths are in angstroms and bond angles in degrees.

26 F. Pepi et al. / Chemical Physics Letters 461 (2008) 21–27

Concerning the BFCHþ3 fragment, this ion is generated by the loss ofthe HF molecules from the BF2CHþ4 population and is characterized bythe F—B—CHþ3 connectivity. The existence of isomer 4 can be ruled outon the basis of energetic consideration since the barrier for its forma-tion from 3 (121.6 kcal mol�1) is considerably higher than the exo-thermicity of the process leading to 3 formation (6.0 kcal mol�1).

BFCH3+ + NH3

0.0 (0.0)

-62.9 (-66.3)

-5.1 (-5

ΔE0 (kcal mol-1)

BFCH3NH3+

+0.6 (-0.4

Fig. 5. Theoretical potential energy diagram of the reaction between [FB–CH3]+ ions andthermodynamic parameter are in kcal mol�1.

The reactivity of F—B—CHþ3 ions with ammonia finds relevant analo-gies with that of similar species such as HBCHþ3 ; PtCHþ3 and bare CH3

+

ions.The potential energy diagram of the reaction between BFCHþ3 and

NH3 is reported in Fig. 5.The first step of this reaction is the deeply exothermic interaction

between the electron deficient boron atom of BFCHþ3 and the nitrogenatom of ammonia. The CH3—BF—NHþ3 ion, excited by the large exo-thermicity of its formation process loses an HF molecule through ashift of an ammonia proton to the F atom of the BF moiety. This pro-cess generates the linear CH3—B—NHþ2 ion in a reaction exothermicby 13.6 kcal mol�1 having a barrier of 60.9 kcal mol�1.

However, CH3—BF—NHþ3 can eliminate an H2 molecule in a thermo-neutral process having a negligible barrier that gives rise to theBFCH4N+ ion characterized by a cyclic structure where a new car-bon–nitrogen bond is formed.

The study of the mechanism of the H2 loss by using ND3 is precludedby the complete H/D scrambling observed in the [BFCH3–ND3]+ ion/molecule complex generated in the triple quadrupole collision cell.Moreover, this evidence shows the existence of a long-livedCH3—BF—NHþ3 complex analogously with what already observed inthe reaction between CHþ3 and NH3 in SIFT condition [41]. Further-more, theoretical calculations indicate that the loss of the H2 moleculeoccurs by a 1–2 elimination mechanism, the only process that maylead directly to the stable cyclic BFCH4N+ isomer characterized bythe CH2–NH2 connectivity. The 1,1 elimination channel, in absenceof rearrangements processes, would yield the CH–NH3 or CH3–NHisomers, the formation of which cannot be supported by theoreticalcalculations.

Compared to transition-metal ion, in the case of methane activationby BFþ2 the formation of the immino ion, CH2NHþ2 , was not observedsince the dissociation of the BFCH4N ion into BF, and CH2NHþ2 is endo-thermic by 36.8 kcal mol�1 .

Attempts to study methane activation by fluorinated metallic spe-cies belonging to the third group, such as AlFþ2 or GaFþ2 , are in progressin our laboratory.

.4)

-22.9 (-25.8)

-12.8 (-13.6)

BFHCH3NH2+

BCH3NH2+ + HF

BFCH4N+ + H2

+0.5 (-0.5))

NH3 computed at the B3LYP/6-311+G(2d) (CCSD(T)/6-311+G(2d)) level of theory. All

F. Pepi et al. / Chemical Physics Letters 461 (2008) 21–27 27

5. Conclusions

The joint application of mass-spectrometric and ab initio theo-retical methods allowed the identification and structural charac-terization of the BF2CHþ4 and BFCHþ3 ions obtained from the BF3/CI of methane. The BF2CHþ4 ions is characterized by the F2B—CHþ4connectivity. The loss of an HF molecule from this species gives riseto the F—B—CHþ3 ion. The reaction of BFCHþ3 with NH3 leads to theobservation of cyclic FBCH4N+ ion an interesting new species char-acterized by the formation of a carbon–nitrogen bond. From thispoint of view, the reaction between BFþ2 and CH4 may representan example of methane activation by non-metallic ions.

Acknowledgments

Work carried out with the financial support of the University ofRome ‘La Sapienza’ and Italian Ministero dell’ Università e dellaRicerca Scientifica (MIUR-FIRB).

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