HN3 Mixtures: The First Observation

The gas-phase ion chemistry of BF3/HN3 mixtures was investigated by the joint application of mass ... HN3, and their FT-IR and UV spectra were recorde...
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J. Phys. Chem. B 2006, 110, 4492-4499

Gas-phase Ion Chemistry of BF3/HN3 Mixtures: The First Observation of [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) Ions Federico Pepi,*,† Andreina Ricci,† and Marzio Rosi‡ UniVersita` degli studi di Roma “La Sapienza”, Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente AttiVe, Piazzale Aldo Moro 5, 00185 Roma, Italy, and UniVersita` di Perugia, Istituto di Scienze e Tecnologie Molecolari del CNR, c/o Dipartimento di Chimica, Via Elce di Sotto 8, 06123 Perugia, Italy ReceiVed: October 24, 2005; In Final Form: December 13, 2005

The gas-phase ion chemistry of BF3/HN3 mixtures was investigated by the joint application of mass spectrometric techniques and theoretical methods. The addition of BF2+ to HN3 led to the first observation of [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) ions in the gas phase. Consistent with collisionally activated dissociation (CAD) mass spectrometric results, theoretical calculations performed at the B3LYP and CCSD(T) levels identified the F2B-NH-N2+, F2B-NH+, FB-N3+, and FBN+ ions as the most stable isomers on the [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) potential energy surfaces. The F2B-NH+ and FBN+ ions, characterized by a triplet ground state, are formed from F2B-NH-N2+ and FB-N3+ through a spin-forbidden decomposition process. It is worth noting that F2BNH-N2+ is the protonated form of difluoroboron azide, BF2N3, a neutral molecule that has never been experimentally detected. The application of theoretical and experimental methods allowed evaluation of the unknown PA of BF2N3, whose best theoretical estimate 171.2 ( 3 kcal mol-1 at the CCSD(T) level is comparable with the experimental one, 170.1 ( 3 kcal mol-1. The main interest of all these ionic species is represented by their possible application in boron nitride (BN) physical and chemical vapor deposition.

Introduction Boron azides have been known for more than fifty years. Since the first synthesis of boron triazide B(N3)3, reported by Wiberg in 1954,1 a large number of studies have been conducted on the production and the spectroscopic characterization of variously substituted boron azides.2 In particular, the formation of boron halide azides was first reported by Paetzold et al.,3 who synthesized (BCl2N3)3 via the reaction of BCl3 with LiN3. The intermediate product, BCl2N3, stabilizes immediately owing to the irreversible formation of trimeric dichloroboron azide (BCl2N3)3, as confirmed by its crystal structure, which was determined by Mu¨ller in 1971.4 Boron azides containing iodine in place of chlorine were also obtained by Dehnicke et al. from the reaction of BI3 with IN3 in benzene, but the resulting BI2N3 could not be isolated in pure condition.5 In recent studies, BCl2N3 and BCl(N3)2 were isolated in a low-temperature argon matrix from the reaction of BCl3 and HN3, and their FT-IR and UV spectra were recorded.6-7 In 1972, Wiberg8 first reported the preparation of (BF2N3)3, recently characterized by Klapo¨tke et al., using vibrational and multinuclear NMR spectroscopy.9 The same authors calculated the thermodynamic stability of BF2N3 with respect to the corresponding dimerization and trimerization reactions.10 All attempts to crystallize (BF2N3)3 failed, and both the monomer and dimer have never been experimentally detected. The extensive studies on boron azides are motivated by the fact that these molecules have been shown to be good precursors * Corresponding author. E-mail: [email protected]. Telephone: +390649913119. Fax +390649913602. † Universita ` degli studi di Roma “La Sapienza”, Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive. ‡ Universita ` di Perugia, Istituto di Scienze e Tecnologie Molecolari del CNR.

for boron nitride (BN) film deposition. Boron nitride exists in two crystalline forms: a cubic structure (cBN) with physical and chemical properties approaching those of diamond and a hexagonal structure (hBN) with softness comparable with carbon graphite. Because cBN can be both n- and p-type doped, it represents a potential new material for electronic devices. Boron nitride can be deposited by several methods such as chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and physical vapor deposition (PVD).11-12 In many of these techniques, the deposition of BN is achieved by ion bombardment. In particular, it has been demonstrated that B+ and N+ ion deposition is a unique method for obtaining the controllable growth of high-purity cubic-BN films.13 Recently, the synthesis of thick adherent boron nitride films was carried out by bias-assisted direct current (DC) jet plasma CVD or microwave plasma (MW) CVD, employing a mixture of N2, BF3, H2, and a noble gas.14-15 Furthermore, the synthesis of boron nitride nanotubes has recently attracted considerable attention because their physical properties are complementary to carbon nanotubes.16 In this paper, we report the first experimental observation of gaseous [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) ions synthesized from the reaction of BF2+ or B2F5+ with HN3 under typical mass spectrometric chemical ionization (CI) conditions. The structure of these species was studied by the joint application of mass spectrometric techniques and theoretical methods. Because these ions contain the BN moieties coordinated to good living groups such as HF and N2, they could prove, at least in principle, excellent projectile ions in boron nitride deposition techniques. Furthermore, the knowledge of the formation and decomposition mechanisms of these new ionic species can prove useful for the comprehension of the unknown ionic processes involved in boron nitride deposition.

10.1021/jp0560922 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006

Gas-phase Ion Chemistry of BF3/HN3 Mixtures

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Finally, an additional interest in the gas phase chemistry of these ions derives from their possible application in the semiconductor industry. The formation of silicon layers doped with boron atoms is commonly achieved by ion implantation techniques using B+ or BF2+. In particular, the use of larger ions containing boron atoms may be crucial for obtaining boron deposition with depths of less than 100 nm as required in silicon technology for ultrashallow junctions.17 Experimental Section All the gases used were purchased from Matheson Gas Products Inc., with a stated purity of 99.9 mol %. HN3 was synthesized by reacting NaN3 in excess stearic acid.18 Hydrazoic acid is explosive and toxic and should be handled with extreme attention. Mass Spectrometric Experiments. Triple-quadrupole mass spectrometric experiments were performed with a TSQ 700 instrument from ThermoFinnigan Ltd. The ions generated in the chemical ionization (CI) source were driven into the collision cell, actually a RF-only hexapole, containing the neutral reagent. The collisionally activated dissociation (CAD) spectra were recorded, utilizing Ar as the target gas at pressures up to 1×10-5 Torr and at collision energies ranging from 0 to 50 eV (laboratory frame). The charged products were analyzed with the third quadrupole, scanned at a frequency of 150 amu s-1. FT-ICR measurements were performed by using an Apex TM 47e spectrometer from Bruker Spectrospin AG equipped with an external ion source operating in the CI mode. The ions, generated in the external source, were transferred into the resonance cell, isolated by broad band and “single shot” ejection pulses and thermalized by collisions with Ar introduced in the cell by a pulsed valve. The pressure of the neutral reactant introduced in the cell, ranging from 1 × 10-8 to 1 × 10-7 Torr, was measured by a Bayard-Alpert ionization gauge, whose reading was corrected for the relative sensitivity to the various gases used according to standard procedures.19 The pseudo-firstorder rate constants were obtained by plotting the logarithm of the BH+t/BH+t)0 ionic intensities ratio as a function of the reaction time. Then the bimolecular rate constants were determined from the number density of the neutral molecules deducted from the gas pressure. Average dipole orientation (ADO) collision rate constants, KADO, were calculated as described by Su and Bowers.20 Reaction efficiencies are the ratio of the experimental rate constant, Kexp, to the collision rate constant, KADO. The uncertainty of each rate constant is estimated to be about 30%. Computational Methods. Density functional theory, using the hybrid21 B3LYP functional,22 was used to optimize the geometry of relevant species and evaluate their vibrational frequencies. Although it is well-known that density functional methods using nonhybrid functionals sometimes tend to overestimate bond lengths,23 hybrid functionals as B3LYP usually provide geometrical parameters in excellent agreement with experiment.24 Single-point energy calculations at the optimized geometries were performed by using the coupled-cluster singleand double-excitation method,25 with a perturbational estimate of the triple-excitation [CCSD(T)] approach,26 in order to include extensively correlated contributions.27 Transition states were located by using the synchronous transit-guided quasi-Newton method attributed to Schlegel and co-workers.28 The 6-311+G(2d) basis set was used.29 Zero-point energy corrections evaluated at the B3LYP/6-311+G(2d) level were added to CCSD(T) energies. The 0 K total energies of the species of interest were corrected to 298 K by adding translational, rotational, and

Figure 1. (a) BF3/Ar CI spectrum, (b) BF3/Ar CI spectrum of hydrazoic acid.

vibrational contributions. The absolute entropies were calculated by standard statistical-mechanistic procedures from scaled harmonic frequencies and moments of inertia relative to B3LYP/ 6-311+G(2d) optimized geometries. All calculations were performed by using Gaussian 03.30 Results Formation of [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) Ions. Figure 1a displays the high-pressure FT-ICR CI mass spectrum of boron trifluoride diluted in argon (1:3). A similar spectrum was observed in the CI source of the triple-quadrupole (TQ) mass spectrometer. The spectrum is dominated by the peaks at m/z 48/49, m/z 67/68, and m/z 115, 116, and 117, the isotopologue ions expected for boron-containing species, corresponding to BF2+, BF3+, and B2F5+, respectively. The two peaks at m/z 88/89 correspond to the [BF2Ar]+ addition product. As shown in Figure 1b, the addition of a small amount of hydrazoic acid to the CI plasma leads to the appearance of the ionic species at m/z 91/92, m/z 71/72, and m/z 63/64, formally corresponding to the [BF2,HN3]+, [BF,N3]+, and [BF2,NH]+ ions, respectively. The mass attributions were confirmed by FTICR high-resolution measurements. The CI spectrum also shows the HN3+ ion at m/z 43 produced by a charge exchange reaction and the H2N3+ ions at m/z 44 generated by proton-transfer processes. To identify the formation pathway of the [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) ions, all the possible ionic precursors, BF2+, B2F5+, BF3+, H2N3+, and HN3+, were isolated in the FT-ICR cell and allowed to react with neutral hydrazoic acid or boron trifluoride. The H2N3+ and HN3+ ions do not react with BF3, whereas charge transfer is the only process observed in the reaction between BF3+ and HN3. The role of these species in the formation of the [BFnNxHn-1]+ ions can be ruled out. As evident from Figure 2, the reaction of BF2+ and B2F5+ with neutral HN3 leads to the formation of the [BF2,HN3]+ ions at m/z 91/92. At the low-pressure conditions of the ICR cell, the inefficient thermalization of the [BF2,HN3]+ adduct produced via the BF2+ ions leads to the formation of higher intensities of the [BF2,NH]+ fragment ion due to the loss of a N2 molecule. Low intensities of the [BF2,HN3]+ ions appear only after longer reaction times. The [BF,N3]+ ions at m/z 71/72 were never

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Figure 2. Time profiles of the ionic intensities of the products obtained from the reaction of BF2+ (a) and B2F5+ (b) with HN3.

detected in the FT-ICR cell. However, their origin from the [BF2,HN3]+ adduct, through the loss of an HF molecule, can be derived from collisionally activated decomposition (CAD) mass spectra (vide infra). The presence of the H2N3+ ions at m/z 44 can be justified by the occurrence of proton-transfer reactions from the ionic products, [BF2,HN3]+ and [BF2,NH]+, to neutral hydrazoic acid. Structural Characterization of the [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) Ions. Collisionally activated dissociation (CAD) mass spectrometry has been used to obtain structural information on the [BFnNxHn-1]+ ions. [BF2,HN3]+ Ions. Formally, the addition of HN3 to BF2+ can involve the attack of both the terminal nitrogen atoms of hydrazoic acid, leading to the isomers 1 and 2. An additional protomer, 3, could be generated from 1 and 2 through a proton shift on a fluorine atom of the BF2 moiety.

F2B-NH-N2 1

F2B-N2-NH 2

HF2BN3 3

The energy-resolved CAD spectrum of [11BF2,HN3]+, recorded at collision energies ranging from 0 to 50 eV (laboratory frame), is reported in Figure 3. The spectrum is dominated by the loss of the HN3 moiety, leading to the 11BF2+ fragment, which accounts for 80% of the whole ionic pattern at a collision energy of 50 eV. The intensity of this fragment increases continuously with increasing collision energy. It is interesting to note that the easier fragmentation channel is the formation of the [11BF2,NH]+ ion, through the loss of a N2 molecule, which appears at a collision energy of ca. 3 eV. The intensity of this fragment reaches a maximum at a collision energy of 10-11 eV and then decreases, probably generating BF2+. The presence of the [11BF,N3]+ ion in the CAD spectrum indicates its formation from the decomposition of the [BF2,HN3]+ adduct through the loss of an HF molecule. The mass attribution of these fragments was confirmed by the shift of one mass unit observed in the CAD spectrum of the corresponding [10BF2,HN3]+ ions.

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Figure 3. Energy-resolved TQ/CAD spectrum of the [11BF2,HN3]+ ions.

TABLE 1: ICR Reactivity of [BF2,HN3]+ Ions base

PA (kcal mol-1)

proton-transfer efficiency (%)

CO COS H2O CF3CN H2S CF3COOH HN3 CH3CN

142.0 150.2 165.0 164.7 168.2 170.1 180.7 186.2

11.0 29.0 100 100

Additional structural information was obtained by examining the reactivity of the [BF2,HN3]+ adduct by FT-ICR mass spectrometry. The [BF2,HN3]+ ions, formed in the external CI ion source, transferred into the resonance cell and isolated after collisional thermalization with Ar, were allowed to react with selected nucleophiles introduced into the cell at low pressure. The only reaction observed was proton transfer. Table 1 reports the proton affinities (PA) of the bases used31-33 and the proton-transfer reaction efficiencies (RE) measured. No reaction was observed with bases having a PA lower than 168.2 kcal mol-1, whereas 100% efficiency was observed with HN3. It is reasonable to assume that the gas-phase proton affinity of BF2N3 can be positioned between the PA of H2S (168.2 kcal mol-1) and the PA of HN3 (180.7 kcal mol-1). This large PA range can be restricted, considering that the efficiency of the proton-transfer approaches 30% in the case of CF3COOH (PA ) 170.1 kcal mol-1). Given the limited accuracy of these measurements, the PA of BF2N3 can be roughly estimated as 170.1 ( 3 kcal mol-1. [BF2,NH]+ Ions. By taking into account that these ions are generated by the loss of the N2 molecule from the [BF2,HN3]+ adduct, two isomers, characterized by F2BNH+ and HF2BN+ connectivity, can be postulated. The F2BNH+ isomer is generated by the loss of the nitrogen molecule from isomer 1, whereas HF2BN+ derives from the corresponding fragmentation of protomer 3. The energy-resolved CAD spectrum of the [11BF2,NH]+ ions recorded at collision energies ranging from 0 to 40 eV is reported in Figure 4.

The loss of the 11BF2+ ion is the easier fragmentation channel, appearing at a collision energy of ca. 2 eV. This fragment dominates the spectrum, accounting for 60% of the whole ionic pattern at 40 eV. Only at higher collision energies do two minor fragmentation channels appear, leading to the formation of the [11BFN]+ and [11BF,NH]+ fragments, respectively, due to the loss of HF and F. [BF,N3]+ Ions. It is reasonable to assume that this species, arising from the loss of a HF molecule from the [BF2,HN3]+ ions, is characterized by the FB-N3 connectivity, although a possible formation of a cyclic structure cannot be ruled out. The energy-resolved CAD spectrum of the [11BF,N3]+ ions recorded at collision energies ranging from 0 to 40 eV is reported in Figure 5. The spectrum is dominated by the easy dissociation into the 11BFN+ ion generated by the loss of a N2 molecule. Only at higher collision energies does the 11BF+ fragment corresponding to the loss of the N3 moiety start to appear. Theoretical Calculations. To rationalize the structurally diagnostic results obtained by the CAD spectra, theoretical calculations were performed by using an approach based on the density functional theory using the hybrid B3LYP functional. Additional calculations were carried out by using the coupledcluster single- and double-excitation method with a perturbational estimate of the triple-excitation CCSD(T) approach. The optimized structures of BF2N3 and its protonated species [BF2N3]H+ are shown in Figure 6. The protonation of BF2N3 gives rise to three different isomers, 1, 2, and 3, two of them protonated on a nitrogen atom and one protonated on a fluorine atom. The relative energies of these species, computed at 298 K both at the B3LYP and CCSD(T) levels of theory, are reported in Table 2, together with the barrier heights for their isomerization processes. Species 1, 2, and 3 are singlet in their ground states. Species 1 is computed to be the most stable one, being more stable than 2 by 19.9 (26.0) kcal mol-1 at the B3LYP (CCSD(T)) level of theory and more stable than 3 by 50.1 (54.3) kcal mol-1. The formation of 1 via the addition of BF2+ to HN3 is exothermic by 79.7 (84.2) kcal mol-1, whereas the same species is obtained from the reaction of B2F5+ and HN3 in a process exothermic by 53.1 (57.3) kcal mol-1 at the B3LYP (CCSD-

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Figure 4. Energy-resolved TQ/CAD spectrum of the [11BF2,NH]+ ions.

Figure 5. Energy-resolved TQ/CAD spectrum of the [11BF,N3]+ ions.

(T)) level of theory. The interconversion between ions 1 and 2 requires overcoming a significant activation barrier, computed to be 107.9 kcal mol-1 at the B3LYP level of theory (103.1 kcal mol-1 at the CCSD(T) level), whereas the isomerization of structure 1 into 3 is characterized by a barrier height of 53.5 (58.1) kcal mol-1. The proton affinity of BF2N3 on the nitrogen atom bound to the boron is computed to be 169.4 ( 3 kcal mol-1 and 171.2 ( 3 kcal mol-1, at the B3LYP and CCSD(T) levels of theory, respectively. Table 2 also reports some selected dissociation reactions of species 1, 2, and 3. We considered spin-allowed reactions with the exception of the dissociation of 1 and 3 into N2 and F2BNH+ and HF2BN+, respectively, where we have also reported the spin-forbidden dissociation into the triplet ground state of F2BNH+ and HF2BN+. The loss of a N2 molecule leading to the formation of the F2BNH+ ions in the triplet state is the easiest dissociation

channel of isomer 1, requiring 46.2 (37.7) kcal mol-1 at the B3LYP (CCSD(T)) level of theory. The corresponding spinallowed process is endothermic by 72.9 (70.0) kcal mol-1, an energy comparable with the endothermicity of the loss of the entire HN3 group computed to be 79.7 (84.2) kcal mol-1. Isomer 1 can lose the HF molecule through the isomerization into 3, a process requiring 63.5 (69.2) kcal mol-1 at the B3LYP (CCSD(T)) level of theory. The only fragmentation channel characteristic of isomer 2 is the dissociation into BF2+ and HN3, found to be endothermic by 59.8 (58.2) kcal mol-1 at the B3LYP (CCSD(T)) level of theory. The favorite dissociation reaction for isomer 3 is the loss of the HF molecules, requiring only 13.4 (14.9) kcal mol-1. The [BF2,NH]+ fragment shows two different structures, the first one protonated on the nitrogen atom, F2BNH+, and the second one protonated on a fluorine atom, HF2BN+. Both structures show a singlet excited state and a triplet ground state. These structures and their relative energies are reported in Figure

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Figure 6. Optimized geometries of the [BF2,HN3]+ ions and of neutral BF2N3. Bond lengths are in angstroms and bond angles in degrees.

TABLE 2: Energy Changes (kcal mol-1, 298 K) Computed at the B3LYP (CCSD(T)) Level of Theory for Selected Dissociation and Isomerization Reactions of [BF2,HN3]+ Ions process

∆H°

BF2 + HN3 f 1 B2F5+ + HN3 f 1 + BF3 1f 2 1f 3 2f 3 1 f F2BN3 + H+ 1 f F2BNH+ (1A) + N2 1 f F2BNH+ (3A′′) + N2 1 f BF2+ + HN3 1 f FBN3+ + HF 2 f BF2+ + HN3 3 f HF2BN+ (1A′) + N2 3 f HF2BN+ (3A′′) + N2 3 f FBN3+ + HF

-79.7 (-84.2) -53.1 (-57.3) 19.9 (26.0) 50.1 (54.3) 30.2 (28.3) 169.4 (171.2) 72.9 (70.0) 46.2 (37.7) 79.7 (84.2) 63.5 (69.2) 59.8 (58.2) 75.0 (61.3) 42.8 (31.4) 13.4 (14.9)

+

barrier height

107.9 (103.1) 53.5 (58.1) 56.5 (56.8)

7, whereas some selected dissociation energies of these species are reported in Table 3. The triplet ground state of the F2BNH+ ion is more stable than the singlet by 26.7 (32.3) kcal mol-1. Besides, F2BNH+ is more stable than the HF2BN+ isomer by 46.7 (47.9) kcal mol-1, and its isomerization to HF2BN+ requires overcoming a significant activation barrier, computed to be 63.8 kcal mol-1 at the B3LYP level of theory (68.5 kcal mol-1 at the CCSD(T) level). The spin-allowed formation of the NH fragment is the easiest dissociation channel of the F2BNH+ ion in the triplet ground state, being endothermic by 51.8 (51.3) kcal mol-1 at the B3LYP (CCSD(T)) level of theory. The loss of the HF molecule and of the F atom from this species requires higher energies, computed to be 75.3 and 86.9 kcal mol-1 at the B3LYP level of theory, respectively. The F2BNH+ ion in the singlet excited state loses NH with only 25.1 (19.0) kcal mol-1, whereas the spin-allowed process leading to NH in the singlet excited state requires a higher energy, 61.0 (54.9) kcal mol-1 at the B3LYP (CCSD(T)) level of theory.

Figure 7. Optimized geometries of the fragments [BF2,NH]+ and [BF,N3]+. Relative energies at 298 K calculated at the B3LYP (CCSD(T)) level of theory are shown. Bond lengths are in angstroms, angles in degrees, and energies in kcal mol-1.

TABLE 3: Energy Changes (kcal mol-1, 298 K) Computed at the B3LYP (CCSD(T)) Level of Theory for Selected Dissociation and Isomerization Reactions of [BF2,NH]+ and [BF,N3]+ Ions process

∆H°

F2BNH+ (1A) f BF2+ + NH (3Σ-) F2BNH+ (3A′′) f BF2+ + NH (3Σ-) F2BNH+ (1A) f BF2+ + NH (1∆) F2BNH+ (1A) f FBNH+ + F F2BNH+ (3A′′) f FBNH+ + F F2BNH+ (3A′′) f HF2BN+ (3A′′) F2BNH+ (1A) f FBN+(3A′) + HF F2BNH+ (3A′′) f FBN+ (3A′) + HF F2BNH+ (1A) f FBN+ (1A′) + HF HF2BN+ (1A′) f FBN+ (3A′) + HF HF2BN+ (3A′′) f FBN+ (3A′) + HF HF2BN+ (1A′) f FBN+ (1A′) + HF FBN3+ f BF+ + N3 FBN3+ f FBN+ (3A′) + N2 FBN3+ f FBN+ (1A′) + N2

25.1 (19.0) 51.8 (51.3) 61.0 (54.9) 60.2 (58.7) 86.9 (91.0) 46.7 (47.9) 48.6 (44.8) 75.3 (77.1) 90.7 (82.9) -3.5 (-0.8) 28.6 (29.2) 38.6 (37.3) 138.0 (135.5) 58.0 (45.6) 100.1 (83.7)

barrier height

63.8 (68.5)

The HF2BN+ ion in the triplet ground state loses the HF molecule through a process endothermic by only 28.6 (29.2) kcal mol-1. In Figure 7, we also reported the optimized structure of the [BF,N3]+ ion in the singlet ground state and, in Table 3, the energies of its characteristic fragmentations. The spin-forbidden loss of a N2 molecule leading to the formation of FBN+ in the triplet ground state is the easiest dissociation channel of linear FBN3+, requiring 58.0 kcal mol-1 at the B3LYP level of theory. The corresponding spin-allowed process requires 100.1 kcal mol-1. The FBN3+ ion loses the entire N3 group at higher energies (138.0 kcal mol-1 at the B3LYP level). Finally, all attempts to optimize a FBN3+ cyclic structure failed. Discussion The boron trifluoride chemical ionization of hydrazoic acid leads to the first observation of [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) ions according to the following reaction scheme. The addition

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of BF2+ to HN3 leads to [BF2,HN3]+ ions, which, excited by the large exothermicity of their formation process, probably, in the low-pressure regime of the mass spectrometer CI source, do not survive but generate the [BF2,NH]+ and [BF,N3]+ fragment ions. On the contrary, the reaction involving the B2F5+ leads to the observation of the [BF2,HN3]+ adducts through a process that is less exothermic and more entropically favored by the loss of the BF3 moiety. The mutually supporting results obtained from the energyresolved TQ-CAD mass spectra, FT-ICR reactivity, and theoretical calculations allow the structural characterization of these species and the identification of the ionic pathways responsible for their formation. The energy-resolved CAD spectrum of the [BF2,HN3]+ adduct shows that this species loses the N2 group at a collision energy lower than that necessary to lose the HN3 and HF molecules. According to theoretical calculations, this evidence states that the [BF2,HN3]+ ionic population certainly contains the F2BNHN2+ ions characterized by isomer 1 connectivity and excludes the formation of isomer 3. In fact, isomer 1 is the only species in which the loss of the N2 group is easier than the loss of the HN3 and HF molecules, whereas isomer 3 loses the HF molecule with a dissociation energy of only 13.4 kcal mol-1. However, the dissociation into [BF,N3]+ and HF observed in the CAD spectrum necessarily involves the isomerization of 1 into 3 and its subsequent dissociation, a process requiring 63.5 kcal mol-1, i.e., an energy comparable with that characteristic of the loss of the entire HN3 group. On the basis of these energetic considerations, the possible existence of isomer 2 cannot be excluded. The loss of HN3, the only fragmentation characteristic of this species, requires an energy (59.8 kcal mol-1 ) that is higher than that necessary for isomer 1 to lose the N2 molecule and comparable with that needed to lose HF. Proof of the exclusive formation of 1 could be derived from the FT-ICR experiments carried out to evaluate the PA of BF2N3. The rough experimental value of 170.1 ( 3 kcal mol-1, obtained by the bracketing method, is quite comparable with the value calculated at the B3LYP level of 169.4 kcal mol-1 (171.2 kcal mol-1 at the CCSD(T) level). The presence in the [BF2,HN3]+ ionic population of isomer 2, less basic than 1 by 19.9 kcal mol-1, would have led to the observation of the protontransfer reaction to bases having PA lower than 168.0 kcal mol-1. On this basis, the presence of isomer 2 in the ionic population isolated in the FT-ICR cell can be excluded, although no definitive conclusion can be drawn about its possible existence in the ionic population generated in the CI source. It is worth noting that the F2BNH-N2+ ions represent the protonated form of difluoroboron azide, BF2N3, a neutral species that has never been experimentally detected. Concerning the [BF2,NH]+ ion structure, it is reasonable to assume that this species, generated by the loss of the N2 molecules from isomer 1, is characterized by the F2B-NH connectivity. In particular, this ion is formed in the triplet ground state. According to the energy-resolved CAD spectrum, the BF2NH+ ion in the triplet ground state loses the BF2+ group at lower energies than that of the HF and F moiety. The formation of singlet F2B-NH+ can be excluded, considering the high energy necessary to generate this excited electronic state from isomer

Pepi et al. 1 decomposition. Furthermore, also, the existence of the isomer characterized by the HF2BN+ connectivity can be ruled out because this species shows an easy fragmentation into FBN+ and HF, a process observed in the CAD spectra only at high collision energies. The [BF,N3]+ ions are characterized by a linear FB-N3 structure, in that theoretical calculation failed to reveal a stable cyclic structure. CAD mass spectrum shows that the easiest dissociation product observed in the decomposition of linear FB-N3+ ions is the FBN+ ion. According to theoretical calculations, the latter species is characterized by a triplet ground state. It is interesting to note that the newly formed boron-nitrogen bond in the F2B-NHN2 and FB-N3+ ions is stronger than the N-N2 bond of the hydrazoic acid skeleton because these species preferably lose the N2 moiety rather than the whole HN3 or N3 group. From this point of view, these ions could be ideal candidates for boron nitride deposition, considering that, in the collision with the solid surface, they can generate FBN+ through the loss of HF and N2. Attempts to deposit boron nitride films by soft-landing these ionic species on different solid substrates are under way in our laboratory, utilizing a TQ mass spectrometer suitably modified to deposit low-energy mass-selected ion beams.34 Conclusions The addition of BF2+ to hydrazoic acid leads to the first observation of [BFnNxHn-1]+ (n ) 1, 2; x ) 1, 3) ions. The joint application of mass spectrometric techniques and theoretical methods allows four new, never previously observed, ionic species to be identified and structurally characterized: F2BNHN2+, F2B-NH+, FB-N3+, and FBN+. The F2B-NH+ and FBN+ ions are characterized by a triplet ground state. The formation of the F2B-NH-N2+ ion is particularly important because this species represents the protonated form of difluoroboron azide, BF2N3, a neutral molecule never experimentally detected. In accordance with its structure, protonated difluoroboron azide under FT-ICR conditions behaves as a Brønsted acid undergoing exclusively H+ transfer to selected bases. In this context, the application of theoretical and experimental methods allows the evaluation of the unknown PA of BF2N3, whose best theoretical estimate, 171.2 ( 3 kcal mol-1 at the CCSD(T) level, is comparable with the experimental value, 170.1 ( 3 kcal mol-1, obtained utilizing the FT-ICR “bracketing” method. All these species represent promising precursors for boron nitride deposition through conventional methods involving high-energy ion beams. Attempts to deposit boron nitride films using low-energy ion beams are under way in our laboratory. Acknowledgment. Work was carried out with the financial support of the University of Rome “La Sapienza” and Italian Ministero dell’ Universita` e della Ricerca Scientifica (MIURFIRB). References and Notes (1) Wiberg, E.; Michaud, H. Z. Naturforsch. 1954, 96, 497. (2) Fraenk, W.; Habereder, T.; Hammerl, A.; Klapo¨tke, T. M.; Krumm, B.; Mayer, P.; No¨th, H.; Warchhold, M. Inorg. Chem. 2001, 40, 1334 and references herein. (3) Paetzold, P. I. Z. Anorg. Allg. Chem. 1966, 345, 79. (4) Mu¨ller, U. Z. Anorg. Allg. Chem. 1971, 382, 110. (5) Kreuger, N.; Dehnicke, K. Z. Anorg. Allg. Chem. 1978, 444, 71. (6) Johnson, L. A.; Sturgis, S. A.; Al-Jihad, I. A.; Liu, B.; Gilbert, J. V. J. Phys. Chem. A 1999, 103, 686. (7) Travers, M. J.; Gilbert, J. V. J. Phys. Chem. A 2000, 104, 3780.

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