The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence

Article pubs.acs.org/JPCA

The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence for Benzenemethanimine Formation Rui M. Pinto,*,† Mauro Guerra,† Grant Copeland,§ Romeo I. Olariu,∥ Paula Rodrigues,⊥ M. Teresa Barros,‡ M. Lourdes Costa,† and António A. Dias*,† †

Laboratório de Instrumentaçaõ , Engenharia Biomédica e Física da Radiaçaõ (LIBPhys-UNL), Departamento de Física, Faculdade de Ciências e Tecnologia, FCT, and ‡REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal § Department of Chemistry, The University of Southampton, Southampton SO17 1BJ, U.K. ∥ Faculty of Chemistry, “Al. I. Cuza” University of Iasi, 11 Carol I, 700506, Iasi, Romania ⊥ ITQB, Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, 2780-901 Oeiras, Portugal S Supporting Information *

ABSTRACT: We study the gas-phase pyrolysis of benzyl azide (BA, C6H5CH2N3) using ultraviolet photoelectron spectroscopy (UVPES) and matrix-isolation infrared (IR) spectroscopy, together with electronic structure calculations and Rice−Ramsperger− Kassel−Marcus (RRKM) calculations. It is found that BA decomposes via N2 elimination at ca. 615 K, primarily yielding benzenemethaninime. Other end products include HCN and C6H6. N-Methyleneaniline is not detected, although its formation at higher temperature is foreseen by RRKM calculations.

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INTRODUCTION Organic azides undergo decomposition when heated, usually through N2 elimination.1,2 The energy release can reach 378 kJ/ mol in azide polymers.3 This property has led to a widespread range of applications, from diesel−azide fuel blends to rocket propellants and airbags.4,5 Organic azides are also used as precursors for compounds with high nitrogen content such as carbon nitride nanomaterials.6,7 Their decomposition often proceeds through several steps involving reaction intermediates, and these species can be observed in spectroscopic studies performed under controlled conditions. Controlled heating can be used to obtain species which otherwise would not be easily accessible through synthetic methods.8 Using appropriate spectroscopic techniques,9−11 such elusive intermediates can be characterized in situ, and knowledge of their electronic structure can be promptly obtained.12−15 Extensive studies by our group on the thermal decomposition of organic azides using UV photoelectron spectroscopy (UVPES) and infrared (IR) matrix isolation spectroscopy have led to the establishment of two main decomposition mechanisms12−14,16,17 (see Scheme 1). The first, termed a Type 1 mechanism, involves the loss of N2 and the formation of an imine. In this mechanism, a 1,2 H-shift always occurs, but it is not established if it starts after or before the N2 loss. In the first case, a singlet nitrene is left from the N2 elimination, and then it converts into a more stable imine, through a 1,2 H-shift. In the second case, the 1,2 H-shift promotes the loss of N2 and © XXXX American Chemical Society

Scheme 1

leads to the formation of the imine, in a synchronous way. So far, no nitrenes have been detected from thermal decomposition of organic azides, and the available experimental and theoretical evidence suggests that the 1,2 H-shift to form an imine and the N2 elimination occur synchronously. The second mechanism, termed Type 2, involves the formation of a cyclic transition state or intermediate, which originates from the transfer of a H atom or an alkyl group to an electron-deficient Received: March 13, 2015 Revised: April 21, 2015

A

DOI: 10.1021/acs.jpca.5b02453 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

preheated to ca. 325 K, to obtain sufficient vapor pressure. The sample was degassed with repeated freeze−pump−thaw cycles prior its injection into the system. To study the pyrolysis of BA, a radio-frequency (rf) induction heating system positioned inside the ionization chamber of the spectrometer was used to heat the flowing sample vapor from RT to 1775 K. A graphite susceptor wrapped in carbon felt and enclosed in an insulating alumina tube was positioned centrally inside a water-cooled copper coil carrying the rf current, above the ionization region. Further technical details of the assembly can be found elsewhere.24,25 Spectra were recorded at different furnace temperatures, which were measured using a K-type (Ni−Cr/Ni−Al) thermocouple in contact with the internal wall of the susceptor (≤1600 K) and an optical pyrometer focused on the outside of the susceptor (≥1600 K). At room temperature, spectral calibration was achieved by admitting a small amount of argon (partial pressure ca. 1−2 × 10−5 mbar) into the reaction chamber. During pyrolysis, bands of the major pyrolysis products, such as N2 and HCN, and a small amount of residual H2O vapor in the chamber were used to calibrate the energy scale. BA was prepared according to the procedure described earlier.20 Azides are potentially explosive and therefore must be handled with all due precautions. Care was taken to avoid unwanted reactions at all stages in the preparation and handling of the azide samples. Matrix-Isolation IR Spectroscopy. The matrix-isolation IR spectrometer uses a conventional closed-cycle cryostat (Air Products, model CSW-202) and an IR grating spectrophotometer (PerkinElmer, model 983G), together with a CsI deposition window, maintained at ca. 12 K. Nitrogen (N2) was used to form the inert-gas matrix. Sample gas passes through a silica tube before being deposited. This tube is located inside an alumina tube furnace which can be resistively heated using molybdenum resistance windings. Matrix ratios were estimated to be in excess of 1000:1 (inert gas:sample). Typical deposition times were 30−60 min at a specific furnace temperature, and the matrix IR spectra were recorded in the 4000−500 cm−1 range. Further details can be found elsewhere.12 Note that to achieve total decomposition in the PE and matrix IR studies, the temperatures were different as the flow system, pressures, and furnace arrangement were different. For a given degree of decomposition, the matrix IR temperatures were generally lower. Computational Details. The initial search for different benzyl azide conformers was performed using second-order Møller−Plesset perturbation theory (MP2) with the valence double-ζ polarized basis set 6-31G(d), including all electrons (i.e., no frozen core). Energy minima found from a relaxed energy scan were further optimized and confirmed using MP2/ 6-311++G(d,p) calculations yielding all real harmonic frequencies. Relative energies of the conformers were computed with the Gaussian-3 (G3) composite method.26 Basis set superposition error (BSSE) was not accounted for. Estimates for the relative populations of the conformers, at room temperature, were obtained using the Boltzmann distribution formula. Outer-valence (9−19 eV) photoelectron spectra of BA and possible intermediates were simulated from the results of electron propagator theory (EPT) applied to the MP2/6-311+ +G(d,p) optimized structures of each conformer. Vertical ionization energies (VIEs) obtained within the outer-valence Green’s function (OVGF) and partial third-order (P3)

N atom of the azide. The cyclic structure can decompose to give N2 and other dissociation products. Herein, the pyrolysis of benzyl azide (BA, PhCH2N3) is investigated using gas-phase UVPES and IR matrix isolation spectroscopy. Compared to higher reactivity organic azides,18 BA stability is enhanced due to the benzene ring. This property allied to the structural flexibility in the CC−NN dihedral angle could favor cyclization reactions over other rearrangements. Cyclicization directly competes with imine formation that usually follows N2 elimination. In the pyrolysis of 2azidoethanol19 and 2-azidoacetamide,13 for example, during N2 elimination the nitrene RN is immediately stabilized by a 1,2-H shift, and no cyclic intermediate is formed. However, the electron-deficient terminal N atom can also bind to remote sites of the molecule. Any structure thus formed can be identified as a stable compound or a short-lived intermediate between the nitrene and subsequent product. Cyclic intermediates have been observed in the case of ethyl azidoacetate19 and could be formed in the pyrolysis of BA and its methylated derivatives. BA has been studied recently using UVPES and mass spectrometry.20 However, matrix-isolation studies are lacking and investigation of BA pyrolysis is scarce. In 1964, Kreher and Kühling,21 using gas chromatography, have identified Nbenzylideneaniline, PhCHNPh, among the decomposition products, probably arising from bimolecular reactions in the decomposition process. According to the authors, benzyl nitrene, PhCH2N••, rearranges to both benzenemethanimine, PhCHNH, and N-methyleneaniline, PhNCH2, which then interact to form PhCHNPh. In this last step, methylenimine (CH2NH) is also formed. The formation of cyclic intermediates was not investigated or presented as an alternative to the imine or aniline pathways. In this work, we monitor the pyrolysis of BA (see Scheme 2) and propose pathways accounting for the observed species. We Scheme 2. Initial Steps in the Pyrolysis of Benzyl Azide

also perform electronic structure and Rice−Ramsperger− Kassel−Marcus (RRKM) calculations22,23 to understand the decomposition and calculate branching ratios. Our goal is to understand the mechanisms behind BA pyrolysis, considering the role of the imine, N-methyleneaniline, and cyclic intermediates in the overall unimolecular decomposition process.

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METHODS UV Photoelectron Spectroscopy. UV photoelectron spectra of benzyl azide were recorded using He(I) radiation (21.22 eV), with a spectrometer specifically designed for hightemperature pyrolysis studies. Typical operating resolution was 30−35 meV, as measured for the (3p)−1 ionization of argon. BA was admitted as a vapor into the ionization region through a stainless steel valve, from a glass vial of the liquid sample B


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The Journal of Physical Chemistry A schemes,27−29 using 6-311++G(d,p) basis sets, were convoluted by Lorentzian functions (fwhm = 0.4 eV) proportional to the corresponding pole strengths (PS). A 0.4 eV width accounts for the vibrational envelope associated with each ionization and fits reasonably well the experimental band profile. For each conformer, a spectrum was generated by summing each function. Final spectra result from the sum of each conformer contribution, weighted by its specific Boltzmann population ratio (BPR). Vibrational analysis using density functional theory (DFT) with the B3LYP/6-311++G(d,p) functional30 were used to simulate IR spectra, following a similar approach. Wavenumbers corresponding to each normal mode were scaled by 0.9679 to account for anharmonicity31 and dressed with Lorentzian functions (fwhm = 5 cm−1). The potential energy surface for the pyrolysis process was investigated at the MP2(full)/6-31G(d) level of theory and further refined with G3, following an earlier procedure.16 Thermochemical properties are calculated according to statistical mechanical principles using the software ChemRate (v.1.5.8),32 from room temperature (RT) to 1800 K. Lowfrequency vibrational modes that could be assigned to internal rotations were treated as hindered rotors33 using barrier heights calculated at the MP2/6-311++G(d,p) level. The rate constants for every reaction pathway were then obtained by solving the master equation (ME) implemented in ChemRate. Decomposition of stabilized intermediates is considered via a steadystate solution of the ME. Since in our experiments the pressure inside the induction oven was not monitored, we estimated an average pressure of 3.5 × 10−3 mbar in the photoelectron experiments based on pressures measured elsewhere in the ionization region. No buffer gas was used during the pyrolysis experiments, and to describe the collisional energy transfer, an exponential-down model was used with ⟨ΔEdown⟩ = 500 cm−1. Lennard-Jones collision parameters (σ and ϵ/κ) were estimated from literature values for similar molecules. All electronic structure calculations presented in this work were carried out using the Gaussian 09 software.34

Figure 1. He(I) photoelectron spectrum of BA, recorded at room temperature (a), and simulated outer-valence photoelectron spectrum, based on P3/6-311++G(d,p) results (b). The stick spectrum corresponds to the lowest energy conformer (BA-G). The asterisk marks a controlled N2 leak.

Table 1. Experimental and Calculated Vertical Ionization Energies (VIEs, eV) for BA

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RESULTS AND DISCUSSION Electronic Structure and Vibrational Analysis. Previously, we have investigated the electronic structure of BA using ab initio methods35 and photoelectron spectroscopy.20 Assignment of the He(I) photoelectron spectrum was performed on the basis of Koopmans’ theorem (KT) applied to HF/6-311++G(d,p) orbital energies. This neglects the effects of electron reorganization and correlation energy change on ionization. Moreover, only the lowest energy conformer was considered. We now extend the analysis by computing VIEs of each conformer using more accurate methods based on electron propagator theory (EPT). The two conformers of BA, gauche (BA-G) and trans (BA-T), are associated with minima in the rotational energy surface for the CC−NN dihedral angle.36 Considering the Gibbs free energies computed with the G3 method, BA-T is at ca. 4.4 kJ/mol above the gauche conformer. This value corresponds to Boltzmann population ratios (BPRs) of 85% for BA-G and 15% for BA-T at room temperature. Although less abundant, the trans conformer should not be overlooked in the subsequent analysis. Experimental and simulated photoelectron spectra of BA are shown in Figures 1a and 1b, respectively. Table 1 lists observed and computed VIEs assigned to bands A-E together with the character of each molecular orbital (MO). VIEs obtained with the partial third-order approximation (P3) lead to mean

band

MO

VIE

calcda

character

A

HOMO 34a 33a 32a 31a 30a 29a 28a 27a 26a

9.28 ± 0.01

9.26 9.44 9.90 11.36 12.33 12.34 12.73 14.07 14.37 14.58

π1,Ph π3,Ph π*N3 σ*N3 σPh πPh σPh σPh, πNN σPh, πNN σPh

B C D

E

a

9.63 ± 0.01 10.96 ± 0.01 11.88 ± 0.04

14.13 ± 0.09

From P3 results on the BA-G conformer.

absolute differences (MAD) with experimental values of 0.28 eV, improving on earlier results obtained with the Hartree− Fock (HF) method (MAD = 1.08 eV). Both the renormalized outer-valence Green’s function (OVGF) and OVGF B approaches were also used, resulting in MADs below those obtained with P3 (0.22 and 0.25 eV, respectively). However, OVGF predicts a triply degenerate HOMO level, failing to describe bands A and B correctly. P3 thus provides the best compromise between qualitative and quantitative description of the outer-valence spectrum. Figure 2a shows the IR spectrum of BA isolated in a N2 matrix (at ca. 12 K), together with the simulated spectrum obtained from vibrational analysis using B3LYP (Figure 2b). The latter includes contributions from both G and T C


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Figure 3. IR spectrum (1500−1200 cm−1) of BA, isolated in N2 matrix maintained at 12 K (a), and simulated IR spectrum, based on scaled B3LYP/6-311++G(d,p) results (b).


conformers (17/3 ratio) and closely matches the experimental spectrum. Table 2 presents the strongest observed absorption Table 2. Observed IR Bands of BA Isolated in a N2 Matrix (12 K) and Corresponding Calculated Wavenumbers (cm−1) and Intensities (km mol−1)a mode

assignt

v44 v39 v38 v32 v29 v17 v14 v13 v12

ν(H,Ph)a ν(CH2)s ν(N3)a w(CH2) ν(N−N2) ν(C−N3) w(H,Ph) w(H,Ph) δ(CNN)

a b

obsd wavenumber

calcdb wavenumber

calcdb intensity

2099 1352 1254 880 757/737 700 670

3079 2933 2156 1337 1257 859 739 689 659

23 37 522 15 152 25 25 53 29


ν, stretching; w, wagging; δ, scissoring; a, asymmetric; s, symmetric. B3LYP/6-311++G(d,p) results on BA-G (scaled by 0.9679).

appearance of the characteristic N2 band at 15.60 eV and a decrease in intensity in band C. As with other organic azides, this behavior is associated with the N2 elimination from the azide chain. From 615 to 1195 K, the first and second set of bands undergo noticeable transformations, indicating formation of new products. From 730 K upward, band C is no longer present: the N2 elimination is almost complete, and the intensity for the N2 first band remains unaltered. At 1310 K, the band at 13.60 eV, followed by a set of closely spaced bands, signals the formation of HCN. Simultaneously, the first and second bands begin to acquire some structure, altering their shape. The first band appears slightly shifted toward higher IE. HCN formation progresses until 1775 K, accompanied by a decrease in the intensity of the two broad bands between 8 and 13 eV. Even at this temperature, H2O is present (VIE = 12.62 eV), probably being released from the inner walls of the ionization chamber and reaching the ionization region. The first stage of pyrolysis results in either the formation of an imine by migration of a nearby H atom (a type 1 mechanism) or formation of a cyclic TS/intermediate, by transfer to or binding of a remote atom or group (a type 2 mechanism). From the UVPES monitoring of the pyrolysis

bands and computed vibrational frequencies of BA-G. Specific spectral features (i.e., double lines around 750 cm−1), which are not predicted by B3LYP, can be associated with matrix effects. A band at 1597 cm−1 is assigned to the v2 mode of water.37 At 1085 cm−1, another strong band arises which is predicted at 1075 cm−1 by B3LYP at considerable lower intensity; therefore, it contains another contribution other than just BA-G, and we prefer to leave it unassigned until further work is performed. The presence of the higher energy conformer (BA-T) is subtle and limited to specific regions: between 1500 and 1200 cm−1 (Figure 3), the peak at 1282 cm−1 can be assigned to the T conformer, on the basis of B3LYP simulations. Around 650 cm−1 (Figure 4), a small and broad peak can also be assigned to BA-T (computed at ca. 630 cm−1). In spite of the low population of the trans conformer, comparison between the observed and simulated spectra indicates that both conformers, T and G, contribute to the overall spectral pattern. Pyrolysis Monitored by UVPES. The process of pyrolysis was monitored by UVPES at each step of increasing temperature of the furnace. Figure 5 shows the spectral evolution between room temperature and 1775 K. The gasphase thermal decomposition of BA starts at 615 K, with the D


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The Journal of Physical Chemistry A Ph• + Ph• → C12H10

(4)

Biphenyl, C6H6, N2, and HCN have already been studied by UVPES; therefore, reference spectra and VIEs can be found in the literature.39,40 N-Methyleneaniline was studied by DiStefano et al.,41 using He(I) photoelectron spectroscopy. Since PhHCNH has not been studied by UVPES, we have simulated its outer-valence ionization region employing the same procedure used in the analysis of the photoelectron spectrum of BA. The spectrum of PhNCH2 was also simulated using EPT procedures. Figures 6a and 6b show estimated contributions of both compounds to the experimental spectra, at the beginning (730 K) and at the end (1535 K) of the pyrolysis.

Figure 5. He(I) photoelectron spectra of the pyrolysis process of BA, recorded at increasing furnace temperature.

process of BA, two reactions for the initial stage of decomposition can be proposed:

Figure 6. He(I) photoelectron spectra of the pyrolysis process of BA (gray), recorded at 740 (a) and 1535 K (b) and contributions from possible decomposition products N2 (dark gray), HCN (black), PhHCNH (imine, light gray) and PhNCH2 (N-methyleneaniline, lighter gray hatched).

TS1

Ph−CH 2−N3 ⎯⎯⎯→ Ph−HCNH + N2

(1)

TS2

Ph−CH 2−N3 ⎯⎯⎯→ Ph−NCH 2 + N2

(2)

where TS1 and TS2 are the singlet transition states leading to the formation of Ph−HCNH (benzenemethanimine) and Ph−NCH2 (N-methyleneaniline), respectively. Based on the experimental evidence and results of electronic structure calculations, the formation of HCN, at a later stage, arises most likely from the decomposition of benzenemethanimine through TS3, leaving benzene and hydrogen isocyanide (which isomerizes into HCN):

At an intermediate stage of the pyrolysis process (T = 1195 K, Figure 5), no traces of benzene or biphenyl can be unambiguously assigned. Although the overall spectral pattern of biphenyl seems to fit the pyrolysis spectrum of BA at 1195 K, it lacks the first ionization band located at ≈7.9 eV.39 Benzene also was undetected. HCN was also not seen at this stage, and this is expected to be produced with benzene (reaction 3). On the other hand, simulations for PhHCNH fit reasonably well the pyrolysis spectra obtained at moderate (T = 740 K) and high temperatures (T = 1535 K), as can be seen in Figures 6a and 6b. UVPES-monitored pyrolysis strongly indicates that only this intermediate is formed. In Figure 6b, the first two bands at 8.79 and 9.39 eV compare well with the values obtained by DiStefano et al.41 (8.73, 9.38 eV) and with our calculations (8.69, 9.24 eV). However, DiStefano et al. have also observed a third band at 10.30 eV, which was assigned to the nitrogen lone-pair orbital and was partially overlapped by

TS3

Ph−HCNH ⎯⎯⎯→ C6H6 + HNC

(3)

38

A recent study by Shukla et al. on C6H5 + C6H6/C6H5 gasphase reactions at high temperatures shows that benzene is converted mainly into biphenyl (C12H10) as early as 1140 K, and that from 1380 to 1500 K, small quantities of phenylethyne (C8H7) and naphthalene (C10H8) are also formed. Biphenyl is known to be formed from two phenyl radicals, following the reaction E


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Figure 7. IR spectrum (4000−500 cm−1) of BA, isolated in N2 matrix maintained at 12 K (a), and products of pyrolysis at 625 (b) and 675 K (c). Features marked with an asterisk arise from intermediates forming between 625 and 675 K (see text for details).

Figure 8. IR spectrum of pyrolysis products after heating BA to 625 K, isolated in N2 matrix maintained at 12 K (top), and simulated IR spectra of benzenemethanimine and N-methyleneaniline, based on scaled B3LYP/6-311++G(d,p) results (bottom). The strongest modes are depicted, and unknown products are marked with an asterisk (see text for details).

of other organic azides, also studied via matrix-isolation IR, such as methyl azidoformate (635 K)14 and 3-azidopropionitrile (645 K).17 The feature rich spectrum shown in Figure 7b is provisionally assigned to an imine, N-methyleneaniline, or an unknown compound formed after N2 elimination. With increasing furnace temperature (Figure 7c, 675 K), the intensity of the unknown set of features already found at 625 K increases, while some new peaks arise. HCN stretching modes are clearly detectable at 3287 cm−1 and near 747 cm−1, indicating that the compound formed at 625 K is now decomposing. Taking into consideration reaction 3, formation of HCN leaves behind benzene which has an active mode centered at 678 cm−1. Thus, we have performed more detailed investigations in the 770−670 cm−1 range, at ca. 675 K, comparing the results with benzene’s reference spectrum. As can be seen in the inset of Figure 7c, a band arises near 680 cm−1 which could be assigned to the strongest mode of benzene. A few weak bands observed at 675 K (2235, 2138, 1525, 548 cm−1) are not easily assigned to any of the other products arising from proposed reactions (Ph•, HCNH, CH2N•). These are marked with an asterisk in Figure 7c.

another band at lower ionization energy. In our work, we observe a shoulder at ≈10 eV, but not a clear band at 10.30 eV; therefore, we cannot confirm the presence of PhNCH2 and are obliged to leave the spectrum measured at high temperature partially assigned to PhHCNH and some unknown species. This latter species could result from the molecular rearrangement of the imine or from bimolecular reactions between the main products of the pyrolysis. More sensitive spectroscopic techniques,42,43 which are outside the scope of our work, would be required to complete the assignment. The computed VIEs of PhHCNH and PhNCH2 are shown in Table S2 (see Supporting Information). Pyrolysis Monitored by Matrix-Isolation IR. To investigate further which products are involved in the thermal decomposition of BA, we have used matrix-isolation IR spectroscopy. Spectra obtained between RT and 675 K are shown in Figure 7. Traces of water can be detected from RT conditions upward, probably due to desorption from the heated inlet system or residual water in the parent azide. Near 625 K, BA is fully decomposed, as revealed by the absence of the peak for the N3 stretching mode at 2099 cm−1. The temperature for complete decomposition follows that obtained in the pyrolysis F


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The Journal of Physical Chemistry A In Figure 8, a comparison between the product spectrum at 625 K and the simulated IR spectra of benzenemethaninime and N-methyleneaniline is presented. The calculated spectrum of the imine fits reasonably well the observed IR bands, and the assignment is straightforward. Most intense experimental features arise at 1635 (strong, indicated as s), 1582 (medium, indicated as m), 1451 (m), 1391 (m), 1152 (s), 970 (s), 830 (s), 728 (m), and 685 (m) cm−1; a summary of these results and their assignment is given in Table 3. A band at 970 cm−1 Table 3. Observed IR Bands of BA Pyrolysis Products (625 K) Isolated in a N2 Matrix (12 K) and Corresponding Calculated Wavenumbers (cm−1) and Intensities (km mol−1)a mode

assignt

obsd wavenumber

calcdb wavenumber

calcdb intensity

v37 v36 v33 v32 v30 v29 v28 v27 v26 v24 v23 v21 v12 v11 v10 v9 v7

ν(H,Ph)a ν(H,Ph)a ν(CH) ν(CCN)a ν(CC,Ph)s γ(H,Ph) δ(H,Ph) γ(HCNH) γ(H,Ph) δ(HCNH) δ(H,Ph) δ(HCNH) tw(H) δ(H,Ph) w(H) w(H) γ(H)

3087 3070 2907 1635 1582 1494 1451 1391 1322 1213 1182 1152 830 797 728 684 622

3086 3075 2907 1634 1565 1475 1432 1381 1310 1207 1162 1134 817 786 708 675 608

16 19 57 126 12 2 13 19 7 11 7 57 54 20 20 35 24

Figure 9. Potential energy surface for the pyrolysis of BA, leading to the formation of benzenemethaninime, N-methyleneaniline, and a cyclic isomer, computed using the G3 method.

elimination from trans-BA, evolving via a 170 kJ/mol barrier (TS2) and leading to the formation of PhNCH2. Formation of the product PhNCH2 is slightly less energetic (+36 kJ/mol) than the imine formed from TS1. A third channel arising from N2 elimination of BA-G was found, leading to a cyclic structure via TS4 at 278 kJ/mol. Energy-wise, this pathway is unlikely to occur, adding to the fact that no evidence of a cyclic compound was observed. Benzene and HCN could arise from PhCHNH, via 1,2-H shift between HCNH and the proton deficient carbon atom of benzene. The high activation energy, involving a transition structure located 380 kJ/mol above PhCHNH + N2 (TS3), agrees with the high temperature observed for HCN formation in UVPES (>1195 K) and matrix-isolation IR (>675 K). Decomposition of PhNCH2 could also lead to HCN and benzene but requires more energy than the previous route. At higher temperature, both pathways would lead to HCN traces in the spectra. Rice−Ramsperger−Kassel−Marcus (RRKM) theory was applied to the pyrolysis reaction, from the first steps of decomposition to HCN (or HNC) formation. Figure 10 presents branching ratios for all reactants, intermediates and products from RT to 1775 K. It can be seen that N2 elimination starts at the same temperature for BA-G and BA-T and is almost complete near 975 K. Imine and N-methyleneaniline go over 10% in the 705−755 K range. Above 1025 K, PhCHNH

a ν, stretching; w, wagging; δ, scissoring; tw, twisting; γ, rocking; a, asymmetric; s, symmetric. bB3LYP/6-311++G(d,p) results on imine (scaled by 0.9679).

was also observed in the pyrolysis of 2-azidoethanol19 and left unassigned at that time. In the pyrolysis of 2-azidoacetamide13 such a band was also observed and assigned to NH3. NH3 formation from 2-azidoacetamide pyrolysis has been proposed as a result of a concerted reaction, starting readily after imine formation (via type 1 mechanism) and involving one H-shift and two C−C bond breaks, to give in a single-step NH3, HCN, and CO as products. However, since we cannot envision a similar pathway for NH3 formation during BA pyrolysis, we provisionally leave the peak at 970 cm−1 unassigned. Despite that, we can rule out N-methyleneaniline from the products observed until 675 K on the basis of B3LYP/6-311++G(d,p) vibrational analysis. This corroborates with the UVPES results, where N-methyleneaniline cannot be unambiguously assigned. Discrepancies between the temperature onset for imine formation monitored with UVPES and matrix IR, 725 vs 625 K, arise mainly from differences in the experimental conditions of the two methods, e.g., different pressure conditions inside the furnaces. Computational Modeling and RRKM Analysis. The potential energy landscape for the pyrolysis of BA is presented in Figure 9. N2 elimination starting from the gauche conformer of BA involves a 166 kJ/mol barrier (TS1), leading to the formation of benzenemethanimine at −229 kJ/mol, in a strongly exothermic reaction. This channel competes with N2

Figure 10. Branching ratios for the gas-phase thermal decomposition of BA until ca. 1800 K, calculated using RRKM theory on MP2 results. G


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40308/2007 and SFRH/BPD/92455/2013, respectively. The authors thank J. P. Santos for providing the computational resources. We also thank J. M. Dyke for valuable discussions and support with the photoelectron experiments.

amounts to 60% and PhNCH2 to 40%, suggesting that the latter could be in fact detectable by UVPES. However, comparison with the spectra in Figure 5 suggests that (i) the PhNCH2 yield from RRKM calculations is overestimated at moderate/high temperature and (ii) PhCHNH undergoes isomerization to an unknown compound at higher temperature, giving rise to the transformation of the first band seen in the UVPES spectra from 730 to 1535 K. Also, according to RRKM calculations, HCN and benzene amount only to 10% at 1575 K, near the temperature at which HCN is in fact detectable by UVPES (see Figure 6b). Since HCN and HNC are interchangeable (barrierless isomerization) above 1275 K, the HNC branching ratio should be interpreted as an HCN yield. Overall, RRKM and the electronic structure calculations performed provide a good description of BA pyrolysis when compared with the UVPES data and support benzenemethanimine formation as the preferable channel of decomposition. All the evidence points toward the first stage of the decomposition occurring via a type 1 mechanism to give an imine and N2.

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CONCLUSIONS The pyrolysis of benzyl azide was studied up to 1775 K, using UV photoelectron spectroscopy and matrix-isolation IR spectroscopy. Results have been interpreted using ab initio and DFT electronic structure calculations. RRKM calculations were performed to further investigate the pyrolysis outcome. It was found that BA decomposes via N2 elimination through a type 1 mechanism at ca. 615 K, primarily yielding benzenemethaninime. The latter was characterized using UVPES and matrix-isolation IR. Other end products include HCN and probably C6H6. Cyclic intermediates were not detected, neither N-methyleneaniniline (which is foreseen by RRKM), but experimental evidence points to the formation of an unknown compound at higher temperature, which comes either from a high-energy channel involving bimolecular reactions or from imine isomerization.

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ASSOCIATED CONTENT

S Supporting Information *

Energy-minimized Cartesian coordinates, energies and structures (Table S1) of all compounds under study, calculated at the G3 level of theory; VIEs of PhHCNH and PhNCH2, computed with the P3 method (Table S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b02453.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone +351 212 948 576; Fax +351 212 948 549; e-mail [email protected] (R.M.P.). *Phone +351 212 948 576; Fax +351 212 948 549; e-mail [email protected] (A.A.D.). Present Address

R.M.P.: INESC MN, Rua Alves Redol, 9, 1000-029 Lisboa, Portugal. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M.P. and M.G. acknowledge the support of Fundaçaõ para a Ciência e a Tecnologia (FCT), under Contracts SFRH/BD/ H


Article

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The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence

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