Proposed Proton-Transfer Mechanism for the Initial Decomposition

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Proposed Proton-Transfer Mechanism for the Initial Decomposition Steps of BTATz Galit Parvari, Moran Levi, Maya Preshel Zlatsin, Larisa Panz, Dan Grinstein, Levi Gottlieb, Chagit Denekamp, and Yoav Eichen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12217 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

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Proposed Proton-Transfer Mechanism for the Initial Decomposition Steps of BTATz Galit Parvari1‡, Moran Levi1‡, Maya Preshel Zlatsin1, Larisa Panz1, Dan Grinstein1, Levi Gottlieb2*, Chagit Denekamp2*, Yoav Eichen1* 1

Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Technion City, Haifa 3200008, Israel RAFAEL, Advanced Defense Systems LTD, POB 2250 Haifa 3102101, Israel ‡ Authors of equal contribution to this work. 2

ABSTRACT

The first steps in the gas-phase decomposition mechanism of N3,N6-bis (1H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6diamine, BTATz, anions and the kinetic isotope effects in these processes were studied using combined multi-stage mass spectrometry (MS/MS) and computational techniques. Two major fragmentation processes, the exergonic loss of nitrogen molecules and the endergonic loss of hydrazoic acid, were identified. The observation of a primary isotope effect supported by calculations, suggests that the loss of a nitrogen molecule from the tetrazole ring involves proton migration, either to, or within the terazole ring, as a rate-determining step. The fragmentation of a hydrazoic acid occurs through an asymmetrical retro pericyclic reaction. Calculations show the relevance of these mechanisms to neutral BTATz. Our findings may contribute to the understanding of decomposition routes in these nitrogen-rich energetic materials and allow tailoring their reactivity and decomposition pathways for better control of performance. INTRODUCTION

Triazole, tetrazole and tetrazine rings are frequently used as building blocks for a wide range of nitrogenrich energetic materials and gas-generators.1,2,3,4,5,6,7,8,9,10 Thus, understanding the mechanism, kinetics and thermodynamics of the decomposition reaction of materials, such as N3,N6-bis(1H-tetrazol-5yl)-1,2,4,5-tetrazine-3,6-diamine, BTATz11,12,13, is important for designing new and improved materials.

Figure 1. Molecular structure of BTATz. Nitrogen positions are shown for one of the tetrazole rings.

Experimental work14,15,16, as well as high-level calculations17,18,19 revealed possible decomposition mechanisms as well as the role of tautomerism in the thermal decomposition of tetrazole ring derivatives. For example, the thermal decomposition of 5-AT and 5-methyl

tetrazole was studied by Pinto and coworkers, using UV photoelectron spectroscopy and theoretical studies.20,21 Measurements of gaseous decomposition products of 5-AT at 245 ˚C revealed the formation of N2, HCN, NH2CN and HN3. According to the G2MP2 calculations, the less stable 1H-5-AT tautomer eliminates N2 in a two-step reaction. First, 5-AT isomerizes into azido methane imidamide, H2NC(NH)N3, with an energy barrier of 22.9 kcal mol-1. In a second step, this species rearranges and fragments into H2NNCNH and a nitrogen molecule with a barrier of 35.0 kcal mol-1. The 1H-5-AT tautomer can also generate HN3 and NH2CN through a barrier of 49.0 kcal mol-1. The more stable 2H-5-AT tautomer decomposes, to release N2 and NH2CNNH through a barrier of 35.8 kcal mol-1. The 2H-5-AT tautomer can also generate HN3 and NH2CN through a barrier of 54.5 kcal mol-1. Paul et. al. studied the unimolecular thermal decomposition of 1H-5-AT and 2H-5-AT tautomers using quantum chemical calculations at the CCSD(T)/aVTZ level of theory. They found that the decomposition products N2 and metastable CH3N3 are generated through activation barriers of 40.4 and 36.7 kcal/mol respectively.19 Gas-phase decomposition of isolated molecules or ions can also be used to gain insight into decomposition


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mechanisms. Piekiel and Zachariah studied the decomposition of several energetic materials containing amino tetrazolium moieties under ulra-high heating rates (~105 K/sec), using time-of-flight mass spectroscopy. 22 They reported on the formation of HN3 and N2 products during decomposition. Sivabalan used pyrolysis GC-MS techniques to show that the decomposition of the tetrazolyl ring in hydrazinium azo tetrazolate dihydrate yields HCN and N2.23 Kiselev and coworkers conducted a theoretical study on the tautomerism and thermal decomposition of non-substituted tetrazole ring.17 The barriers of monomolecular tautomeric transformations were found to be considerably higher (∼50-70 kcal/mol) compared to concerted double H atom transfer reactions in the H-bonded complexes of tetrazole ring tautomers (∼18-28 kcal/mol). This emphasizes an important difference in the decomposition mechanisms between the gas-phase and condensed phases. Their calculations show that the two primary decomposition pathways lead to the formation of HN3 and N2, with the latter being the preferred kinetic product for 1H and 2H tetrazole rings. Yet another work showing the significance of intermolecular hydrogen atom migration was performed on crystalline 5AT at high temperatures and pressures using a combined pressure dependent X-ray crystallography and molecular dynamics calculations.24 As demonstrated in the literature referenced above, the observed decomposition products of tetrazolyl groups depend on their substituents, the decomposition conditions and also on the choice of method. BTATz, first reported in 2004 by Chavez et. al.25, is an energetic material that is composed of a central tetrazine ring connected to two terminal tetrazolyl rings through an amine bridge. Its decomposition is characterized by flameless release of large amounts of nitrogen gas.26 Sikderet.al. followed the thermal decomposition of BTATz using DSC and TG-FTIR, and observed the formation of what are likely NH3 and HCN decomposition products.26 More recently, Sinditskii and coworkers studied the combustion mechanism of BTATz.27 Based on thermal decomposition kinetic data obtained from DSC measurements, they concluded that the less thermostable tetrazole rings28 are the first to undergo thermal decomposition through release of molecular nitrogen. This is followed by the decomposition of the more stable tetrazine moiety. They report on the solid-residue decomposition products as observed by IR, upon heating BTATz to 465°C. These products were identified as ammonium azide, polyamine and 2,5,8-triamino-tri-s-triazine, melem. The mechanism they offer involves N-N bond cleavage

within the terazole ring to produce an azide and an imine on the tetrazole ring carbon, so no loss of nitrogen molecules occurs in the first step. In the second step, polymerization occurs through the loss of three nitrogen molecules, including one that has at least one nitrogen atom from the main tetrazine ring. Here we report on the early steps in the decomposition of BTATz anions in the gas-phase, as revealed from multi-stage mass-spectrometry (MS/MS) studies of the system, using minimal energy required for the process. As evidenced by mass spectrometry, in the gas phase, BTATz commences its decomposition mainly from the tetrazolyl groups in pathways that are consistent with some of the reported decomposition pathways of 5amino tetrazoles. However, HN3 has not yet been reported as a decomposition product of BTATz. Comparison of the MS2 spectra of BTATz-H4 and its isotopologue BTATz-D4 show the existence of an important isotope effect in the different decomposition pathways, suggesting a central role of hydrogen atom migration in the rate-determining step. The measurements were accompanied by calculations to assist the elucidation of the thermodynamics, kinetics, and kinetic isotope effects of decomposition. Based on these results we propose a mechanism that is based on intramolecular proton transfer steps for the decomposition routes involving molecular nitrogen loss. This proposed mechanism is relevant at least for anionic and possibly neutral BTATz. Tetrazole decomposition pathways involving proton migration, mostly tautomerism and the decomposition pathways of the different tautomers, have been previously taken into consideration as part of decomposition mechanisms.17,18,20 However the pathways we show herein have not been reported yet. Furthermore, we find that tetrazole systems which are part of BTATz have different decomposition pathways than 5-AT. Previous studies on proton-involving decomposition processes did not lead to the loss of two nitrogen molecules. The present report first presents crystallographic data on the structure of BTATz and its clusters. It then shows the MS2 and MS3 spectra measured for BTATz anions, along with the suggested structures of these fragments. Calculations (G4) accompany this section, providing the relative energies of these structures and the transitions between them. We then suggest mechanisms for the transitions discussed in the previous section, and accompany these with calculated activation energies (b3lyp/6-311++g(d,p)). Support for the suggested mechanisms is provided from MS2 measure-

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ments showing isotope effects. We then expand the discussion to neutral BTATz, for which we also provide data derived from calculations. EXPERIMENTAL

Materials All starting materials and solvents described in the manuscript were purchased from Sigma-Aldrich. Solvents and starting materials were used as received unless noted. BTATz was prepared in four steps from commercially available materials, according to modified literature procedures. Triamino guanidinium mono hydrochloride:29 Guanidinium hydrochloride (2.38 g, 2.5 mmol) was dissolved in 30 mL 1,4-dioxane in a 100 mL round bottom flask. Hydrazine hydrochloride (3.75 g, 7.5 mmol) was added to the solution and the combined mixture was refluxed for four hours, then cooled to room temperature. The crude product, in the form of a white precipitate, was filtered and rinsed with two portions of 5 mL cold 1,4-dioxane, affording pure triamin guanidinium mono hydrochloride as white crystalline powder in 90% yield (3.16 g, 2.26 mmol). MP: 240 ˚C; 13C NMR (D2O): δppm 159.5; MS (TOFES+): m/z 105.07 ([M-Cl]+), 105.07, 247.12 ([M2-HCl]+). 3,6-Bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine:29 Triamino guanidinium mono hydrochloride (2 g, 1.4 mmol) was dissolved in 30 mL distilled water and the solution was cooled to 4 ˚C. Acetyl acetone (2.8 g, 2.8 mmol) was then added dropwise over 30 minutes to the solution while stirring. The combined solution was heated to 85 ˚C for 5 h, and then cooled to room temperature. The crude product, in the form of a yellow precipitate, was rinsed with two portions of 5 mL distilled water. The product was used for the next step without any further purification. MP: 150 ˚C; 1H NMR (CDCl3): δppm 8.02 (2H, s), 5.97 (2H,s), 2.49 (6H,s), 2.22 (6H,s); 13C NMR (CDCl3): δppm 13.47, 13.79, 109.86, 142.30, 145.78, 149.95; MS (TOFES+): m/z 273.15 ([M+H]+). 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetrazine:29 The crude product of the preceding step was suspended in 25 mL acetic acid, then 1 g sodium nitrite was added to the mixture and the suspension stirred for two hours at room temperature. 30 g crushed ice were added to the solution, inducing the precipitation of 3,6-bis(3,5-dimethyl1H-pyrazol-1-yl)-1,2,4,5-tetrazine in the form of a reddish powder. The powder was further washed with two portions of 10 mL distilled water, affording a pure product in 60% yield for the two consecutive steps (1.16 g, 4.7 mmol). MP: 226 ˚C; 1H NMR (CDCl3): δppm 6.2 (2H,s), 2.72 (6H,s), 2.39 (6H,s); 13C NMR (CDCl3): δppm 13.86, 14.65, 111.89, 143.78, 154.47, 159.33; MS (TOFES+): m/z 271.1 ([M+H]+).

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N3,N6-di(1N-tetrazol-5-yl)-1,2,4,5-tetrazine-2,6-diamine (BTATz-H4):11b 3,6-bis (3,5-dimethyl- 1H-pyrazol-1-yl)1,2,4,5-tetrazine (1.8 mmol, 0.5 g) and 5-AT monohydrate (4.8 mmol, 0.5 g) were dissolved in 8 mL sulfolane and the solution was stirred for 25 hrs at 135 ˚C, and then cooled to room temperature. Addition of 20 mL ethyl acetate induced crystallization of the product in the form of an orange powder. Washing the crude with two portions of 20 mL cold ethanol afforded the pure product, BTATz, in 61% yield (0.28 g, 1.1 mmol). The product was recrystallized from DMSO by slow addition of acetone. The 1H and 13C NMR spectra show only the peaks of the pure product, and the high-resolution mass-spectrum shows only species that are relevant to BTATz. MP: 312.26 °C (DSC peak, decomposes); 1H NMR (DMSOd6): δppm 12.5 (br-s); 13C NMR (DMSO-d6): δppm 151.4, 158.6; HRMS (TOFES-): Calc. for C4H3N14: m/z 247.0665, found: m/z 247.0533 ([M-H]-). Tetradeuterio N3,N6-di(1N-tetrazol-5-yl)-1,2,4,5-tetrazine-2,6-diamine (BTATz-D4): BTATz-D4 was prepared by dissolving BTATz-H4 in slightly basic D2O ([D2O]>>>[BTATz]), then precipitating it by neutralization of the solution. The H/D replacement was confirmed by HRMS. When using BTATz-D4, freshly prepared solutions in deuterated solvents were used to prevent isotope scrambling. Apparatus NMR spectra were recorded on Bruker-AV-300, BrukerAVIII-400 and Bruker-AVIII-600 spectrometers. Mass spectrometry measurements were carried out on a Bruker Maxis impact QTOF system in an ESI negative mode, direct probe. MS conditions: Ionization mode: ESI, negative; Capillary: 4000 V; Nebulizer: 2 Bar; Dry Gas: 5 l/min nitrogen; Dry temp: 200 ˚C; Vaporizer Temp.: 450 ˚C. Collision-induced dissociations (CID, MSn) were conducted, in a collision cell following mass selection and preceding the high-resolution daughter ion analysis in the ToF analyzer. Nitrogen was used as the collision gas. The laboratory collision energy was varied between 3 and 10 eV. Data were processed by Bruker otofControl software using MRM (Multiple Reaction Mode). Crystal structures were obtained using Kappa CCD diffractometer. The single-crystal material was immersed in Paratone–N oil and was quickly fished with a glass rod, mounted on the diffractometer and placed under a cold stream of nitrogen. Data collection was performed using monochromated MoKα radiation, λ=0.71073 Å, using φ and ω scans to cover the Ewald sphere.30 Accurate cell parameters were obtained with the amount of indicated reflections.31 The structure was solved by direct methods (SHELXS-97)32 and refined by full-matrix least-squares methods against F2 (SHELXL-97).33 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were refined isotropically on calculated positions using a riding model with their Uiso values constrained to 1.5 times the Ueq


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of their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other carbon atoms. Software used for molecular graphics: Mercury 3.5.34 Thermodynamic and kinetic calculations were performed using the Gaussian 09 Rev. D.01 program.35 All species were calculated as single molecules in the gas-phase. Molecular structures and their respective energies (stationary points, with frequency analyses showing positive values) were first calculated in b3lyp/6-31g(d,p) level of theory, and the output was subjected to further optimization in G4 level of theory. The energy reported in the diagrams is taken directly from the output, as the “G4 Free Energy” value (room temperature). Activation energies were calculated at b3lyp/6311++g(d,p) level, and were obtained by the subtraction of the energy of the starting material from the energy of the transition-state, TS. The transition-state, calculated under the “modredundant” transition-state option for optimization, was verified to have only a single imaginary frequency, corresponding to the type of bond cleavage/formation relevant to the reaction. The input structure for this transition-state was the stationary point structure of the highest energy found from a relaxed potential energy surface scan (PES scan). In this scan, all geometries but one were free to optimize. The frozen geometry (interatomic distance) was held at a value that was increased or decreased stepwise throughout the scan. For proton migration from the nitrogen atom bridge to the tetrazole ring carbon, the distance between this proton and the tetrazole carbon was shortened. For proton migration from the tetrazole ring to the tetrazine ring, the distance between this proton and its starting-material bound nitrogen atom was lengthened. For the routes of pericyclic ring opening leading to HN3 fragmentation, the distance between two nitrogen atoms (N3-N4) within the tetrazole ring was lengthened. For cases where a nitrogen molecule fragments from the central tetrazine ring, the distance between one of the tetrazine nitrogen atoms and its starting-material bonded carbon atom, was lengthened. Kinetic isotope effect (KIE) calculations were performed using the Quiver36 program. The input for this program was based on two output (log) files from the G4 calculations in Gaussian: the “ground-state” (starting material) and the “transition-state”. The reported KIE size is taken directly from the Quiver output file (analysis section), for the ‘uncorrected’ value, with isotopologue 1, H, being the reference, and isotopologue 2, D, providing the reported value in relation to the reference.

RESULTS AND DISCUSSION

BTATz clusters in MS and in the crystal The electrospray ion source generates different clusters of BTATz molecular anions, [Mn-H]-, with m/z 247, 495, 743, 991 and 1239, corresponding to n=1, 2, 3, 4 and 5. This observed clustering most likely originates

from intermolecular hydrogen bonding between BTATz molecules, as a single BTATz can act both as a hydrogen-bond donor as well as an acceptor. The crystal structure of BTATz•DMSO, Figure 2, reveals two such bonds between each pair of BTATz molecules, situated in roughly the same plane, with a distance of 2.895 Å between the nitrogen atom bridge and N4 of the tetrazole ring, and a N-H-N angle of 170.86˚. In this manner, each BTATz molecule is bound to two BTATz molecules, forming long chains. Another insight provided by the crystal structure is that the tetrazole ring in BTATz is in its 1H-tetrazole tautomer. In gas-phase 5-AT, the 2H tautomer has been calculated20 to be about 3 kcal/mol more stable than the 1H tautomer. the reason for the greater stability of the 1H tautomer in BTATz probably stems from the existence of two hydrogen bonds between the protons of the tetrazole rings and nitrogen atoms of the tetrazine ring, dNtetrazine-Ntetrazole=2.358 Å, αNtetrazine-H-Ntetrazole=114.77˚. Our calculations indicate that the 1H tautomers of BTATz anions are more stable than those of the 2H tautomers. In light of this and the crystal structure data, we assume for the rest of the discussion hereon that the predominant form of BTATz features the tetrazoles in the 1H tautomer.

Figure 2. Top and side views of two BTATz molecules in the BTATz•DMSO crystal. The two molecules are bound through two hydrogen bonds. See SI for more crystallographic data.

Gas-phase decomposition pathways of BTATz anions, MS/MS fragmentation patterns and calculated energetics thereof. Collision-induced dissociation of these supramolecular anionic clusters reveals that they all fragment into the supramolecular cluster [M(n-1)-H] and the molecular species [M-H] with the negative charge localized on either of them, showing no strong preference.



Figure 3. Isomeric molecular anions of BTATz, [MB]- and [MT]-.

The [M-H]- anion of BTATz may exist as two different tautomers: the product of deprotonation of N1 of the tetrazolyl group, [MT]-, and the product of deprotonation at the NH bridge tethering the tetrazolyl and tetrazine rings, [MB]-. Calculations (G4) predict [MB]- to be 7.5 kcal/mol more stable than [MT]-. Therefore, for the following discussion, the energy of [MB]- is taken as the reference zero point for the energies of all other species discussed, Figure 3. The MS2 spectrum recorded for the [M-H]- ion of BTATz, Figure 4, reveals two main decomposition products upon activation with the minimal energy required for fragmentation (3.0 eV). These are the products of elimination of either hydrazoic acid, HN3, or two N2 molecules, resulting in the formation of anions of m/z 204 and 191 respectively. A minor peak at m/z 219 reveals the loss of a single nitrogen molecule, and another minor peak at m/z 148 reveals the consecutive elimination of both HN3 and two N2 molecules.

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Both [MB]- and [MT]- can fragment to lose nitrogen molecules and hydrazoic acid. However, [MT]- can only lose hydrazoic acid distally to the charged tetrazole ring, requiring the hydrogen for the HN3 formation. To complement the data from the MS2 measurements, calculations were carried out on all the possible fragmentation pathways for these reactions, for both [MB]-, Figure 5, and [MT]-, Figure 6. Elimination of hydrazoic acid from [MT]- forms [MT-HN3]d-, which is located at +26.2 kcal/mol. [MB]- may lose HN3 either from the charge proximal or charge distal tetrazole rings.

Figure 5. Calculated (G4) free energy values (in kcal/mol) for the first fragmentation of [MB]-. The fragmenting species, either N2 or HN3, is detailed on the arrow. The resulting m/z value of the organic remainder anion is also provided.

Figure 6. Calculated (G4) free energy values (in kcal/mol) for the first fragmentation of [MT]-. The fragmenting species, either N2 or HN3, is detailed on the arrow. The resulting m/z value of the organic remainder anion is also provided.

Figure 4. a. MS2 spectrum measured for isolated [M-H]- ions of m/z 247; b. enlarged m/z 185 – 210 section of Figure 4.a.

This yields two different isomeric products, with the formal charge located on the bridging nitrogen atom, [MB-HN3]p- and [MB-HN3]d-, located at +11.9 and +20.4 kcal/mol respectively. Conversely, all the possible pathways that involve the loss of two nitrogen molecules from the tetrazole rings


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are exergonic, with larger free energy values obtained for the release of the second nitrogen molecule. [MT]decomposes stepwise by losing N2 from either the charge proximal or charge distal tetrazole rings in a slightly exergonic process, yielding [MT-N2]p- and [MT-N2]d-, located at -3.4 and -2.8 kcal/mol respectively. This step is followed by a second, more exergonic, loss of N2, forming [MT-2N2]p- and [MT-2N2]d-, located at -25.8 and -29.4 kcal/mol respectively. In a similar manner, [MB]- may lose the first N2 molecule from either the charge proximal or charge distal rings, yielding [MB-N2]p- and [MB-N2]d-, located at -3.4 and 7.0 kcal/mol respectively. This step is followed by a second, more exergonic, loss of N2, forming [MB2N2]p- and [MB-2N2]d-, located at -25.8 and -36.1 kcal/mol respectively. MS3 measurements were also recorded for the product ions of m/z 204 ([M-HN3]-, three possible isomers) and m/z 191 ([M-2N2]-, three possible isomers, since [MB2N2]p- and [MT-2N2]p- are identical). The MS3 spectra depicted in Figure 7 reveal the consecutive fragmentation of these ions. The MS3 spectrum of the m/z 204 species, Figure 7 left, reveals three major peaks located at m/z 176, m/z 161, and m/z 148, corresponding to the loss of one nitrogen molecule, one hydrazoic acid and two nitrogen molecules respectively. The MS3 spectrum of the m/z 191 species, Figure 7 right, shows the formation of only one major species, m/z 148, without the formation of m/z 176 and 161 fragments that are observed in the MS3 fragmentation pattern of the m/z 204 species.

Figure 7. MS3 spectra measured for ions of m/z 204 (left) and 191 (right).

Figure 8 depicts the possible routes for these observed secondary and tertiary fragmentations. The m/z 204 species consists of three possible isomers, derived from the previous fragmentations of [MB]- and [MT]- as discussed above. [MT-HN3]d- can lose two nitrogen molecules from its anionic tetrazole ring in a strongly exergonic step, leading to m/z 148 species located at -4.9

kcal/mol, a[MB-HN3-2N2]-. This latter species can also be obtained by two other fragmentation pathways, either by another m/z 204 species, [MB-HN3]d-, losing two nitrogen molecules in a slightly less exergonic step, or by m/z 191 species, [MT-2N2]p-, losing an HN3 fragment in an endergonic step. The third m/z 204 species, [MB-HN3]p-, cannot produce this product unless it first rearranges the anionic charge from its nitrogen tethering bridge to the tetrazole ring.

Figure 8. Calculated (G4) free energy values (in kcal/mol, in red) for second and third fragmentations of the MS2 fragment ions of m/z 191 and m/z 204. The fragmenting species, either N2 or HN3, is detailed above the arrow. The resulting m/z value of the organic remainder anion is also provided.

Rather, when this species loses two nitrogen molecules, it produces a much more stable m/z 148 product, with its anionic charge on the tethering nitrogen bridge, located at -23.8 kcal/mol, b[MB-HN3-2N2]-. This product can also be generated by [MB-2N2]d-, m/z 191, losing a hydrazoic acid from its uncharged tetrazole ring, in a mild endergonic step. The third m/z 191 anion, [MT-2N2]d-, cannot lose HN3 as its tetrazole ring is negatively charged, lacking the proton needed for the process. Furthermore, since the MS3 spectrum does not show further loss of nitrogen molecules for any of the m/z 191 species, [MT-2N2]d- is not shown in Figure 8. The m/z 176 species may be the result of any of the m/z 204 anions losing a single N2 molecule, and is demonstrated in Figure 8 as part of the pathway of decomposition of [MB-HN3]d-. The third species observed in the MS3 spectrum, m/z 161, is the least stable of all calculated species, located at +32.5 kcal/mol, and is the result of a BTATz anion losing two hydrazoic acids. Since this can only occur from the terminal tetrazol rings, this fragment has a high symmetry with a central tetrazine ring and two terminal nitriles, with the charge located on one of the tethering nitrogen



bridges and the other being protonated. This species can be obtained by a second HN3 elimination from [MB-HN3]p- or from [MB-HN3]d-, with an endergonic reaction of +20.6 kcal/mol and +12.1 kcal/mol respectively. This multiplicity in routes of formation likely explains the relative large abundance of this species observed in the mass spectra. Two main conclusions arise from the calculation analyses: a) the loss of hydrazoic acid, both from [MB](from either ring) and from [MT]-, is always endergonic; b) the loss of a nitrogen molecules is always exergonic. Additionally, the loss of the second nitrogen molecule is more exergonic than the first; c) the loss of nitrogen molecules is probably the driving force for the self-propagating decomposition reaction of this energetic material. It could also explain why MS2 spectra show the loss of a single nitrogen molecule as a very minor peak, with the product of the first loss of N2 continuing decomposition, losing the second nitrogen molecule. Interestingly, the loss of all possible four N2 molecules, two from each tetrazole ring, resulting in the formation of m/z 135, is not observed under our experiment conditions. Possibly, the resulting species is in itself unstable and further decomposes without leaving a noticeable trace. Alternatively, as gas phase reactions are mostly kinetically controlled, the barrier for this process is simply too high. Exploring the decomposition mechanisms in BTATz anions The loss of a nitrogen molecule has previously been reported as a primary step in the photochemical decomposition of different tetrazole derivatives.37 Previous reports on non-substituted 1H-tetrazole discuss the elimination of a nitrogen molecule originating from nitrogen atoms at positions N2 and N3 of the ring.17 This fragmentation mechanism of the tetrazole ring is highly unlikely to take place in our case. Calculations carried out in order to explore this fragmentation process through a stepwise elongation of the N2-N3 bond in BTATz anions ([MB]d-, [MB]p-, [MT]d- and [MT]p-) failed to show this as a competitive fragmentation pathway. Furthermore, our calculations (b3lyp-6311++g(d,p)) show that following the elimination of the first N2 from the terazole ring (from positions 1 and 2 or 3 and 4), the activation energy for the loss of the second N2 molecule from the same site is considerably lower than that of the first elimination (see kinetics chapter below). Thus, along with the larger exergonity of the loss of the second nitrogen molecule from the same tetrazole ring (discussed above), it appears to be

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a preferred route in BTATz. This is also supported by experimental results. MS2 and MS3 measurements show species, which have lost two subsequent N2 molecules and consecutively an additional HN3 molecule. If the two N2 molecules had each been eliminated from a single tetrazole ring, there would have not been a likely source for the HN3, as it requires an intact tetrazole ring to eliminate from. Another alternative is that one of the N2 molecules is eliminated from the central tetrazine ring (see neutral BTATz sections below), with the other from one of the tetrazoles and the HN3 from the other tetrazole ring. However, this possibility is also unlikely, as our calculations indicate that such a loss from the central ring causes the remaining molecular anion to fragment into two nearly identical species, which do not show further loss of N2 in the MS3 spectra. Our hypothesis was that proton transfer processes could play a key role in the dissociation of molecular nitrogen from the tetrazole ring. We therefore studied computationally the effect of proton migration in both anion isomers [MB]- and [MT]-. Since two types of protons are available, both on the tethering nitrogen bridge and on the N1 nitrogen atom of the tetrazole ring, migrating these protons to the tetrazole ring carbon atom produces six possible routes. All six routes were studied by rudimentary relaxed PES scans (b3lyp/631g(d,p)), in which the distance between the relevant proton and the nearest tetrazole ring carbon atom was shortened stepwise. Two routes, in which the nitrogen bridge proton is transferred to the neutral tetrazole ring carbon atom, show only an increase of energy due to this migration. In one case, the negative charge is on the distal tetrazole ring while in the other on the distal nitrogen bridge.

Figure 9. Loss of a N2 molecule as a result of proton migration, on a [MT]- species as an example for proton migration-induced fragmentation in BTATz. a. Structure of the calculated TS (b3lyp/6-311++g(d,p)). b. Proposed mechanism.


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However, all four remaining proton migration processes result with spontaneous loss of N2, and were therefore subjected to higher level PES scans (b3lyp/6311++g(d,p)). An example for the transition-state, that is obtained based on one of these scans, along with the corresponding proposed mechanism, is presented in Figure 9. In a similar manner, we explored the fragmentation of HN3 from the anion isomers [MB]- and ([MT]-. Since there is no likely proton migration leading to this decomposition, the PES scans (b3lyp/6-311++g(d,p)) lengthened the bond between the N3 and N4 atoms of the tetrazole ring. All three possible routes, simulating an asymmetric retro 1,3-cycloaddition mechanism, lead to the spontaneous cleavage of the C-N1 bond within the tetrazole ring, and the subsequent loss of HN3. An example for the TS obtained based on one of these scans, along with the corresponding proposed mechanism, is provided in Figure 10.

Figure 10. Loss of an HN3 molecule from the tetrazole ring, on a [MT]- species as an example of HN3 loss in BTATz. a. Structure of the calculated TS (b3lyp/6-311++g(d,p)). b. Proposed mechanism.

Computational kinetic studies for the different routes of the offered decomposition mechanisms Since the gas-phase phenomena we observe in the MS2 measurements are all kinetically controlled, we characterized the energies of activation for the eleven transitions discussed in the previous section. The decomposition pathways suggested for 5-AT21, passing through azido methane imidamide, HRNC(NH)N3 intermediates is unlikely to account for the observed MS decomposition pattern. We therefore endeavored to explore alternative decomposition pathways that comply with our experimental results, even at the price of passing through higher barriers. Four proton-transfer processes leading to the dissociation of the first N2 molecule, their following respective four dissociations of a second N2 molecule from the same

site, Figure 11, and the three possible dissociations of HN3 molecules, Figure 12. All transition-states were derived from the highest-energy step in the PES scans, and in all cases, the negative frequency was verified to correspond with the appropriate bond cleavage/formation. Unless noted otherwise, all values in this section are derived from calculations at b3lyp/6311++g(d,p) level.

Figure 11. Activation energies for the proposed N2 loss mechanisms in the four paths for BTATz anions. The values are based on the sum of electronic and thermal free energies. The values in parentheses are based on the sum of electronic and zero-point energies. All values were calculated at b3lyp/6311++g(d,p) level (except for the one marked by *, which was calculated at b3lyp/6-31g(d,p) level).

Of the four routes involving proton migration leading to the loss of the first N2 molecule, the simplest is that of [MT]-, where the charge-proximal nitrogen-bridge proton is transferred to the anionic tetrazole ring, Figure11, path a. We found a single step with a single TS, at ∆G‡=66.2 kcal/mol relative to [MT]-.The other decomposition pathway for [MT]- involves the migration of the distal, non-charged, tetrazole ring proton, Figure 11, path b. In this case, proton migration includes two steps. The first involves proton-transfer from the tetrazole ring to the tetrazine ring, ∆G‡=13.4 kcal/mol. This step is followed by a second proton migration from the nitrogen-bridge to the carbon atom of the



newly negatively charged tetrazole ring, in a manner similar to the migration described above in the first route, Figure 11, path a., ∆G‡=66.7 kcal/mol. The third and fourth routes were found for [MB]-. One involves the migration of the proton bound to the N1 atom in the charge distal tetrazole ring, Figure 11, path c. This path is similar to path b, both in steps and in activation energies, ∆G‡=11.0 and 66.9 kcal/mol for the inter-ring migration and the following proton-transfer from the bridging amine to the carbon atom of the tetrazole ring, respectively. The fourth path provided a single step process with a single TS (b3lyp/6-31g(d,p)), in which the proton on the N1 atom of the tetrazole ring proximal to the anionic nitrogen bridge migrates to the carbon atom of the same tetrazole ring. Unfortunately, the b3lyp/6-311++g(d,p) level of calculation failed to provide an appropriate TS, as instead the proton migrates to the nearest nitrogen atom of the tetrazine ring. In light of the small difference in activation energies produced by the two bases for other pathways, we report the free energy of activation found in the lower level calculation, ∆G‡=58.3 kcal/mol (b3lyp/6-31g(d,p)). In all four pathways the dissociation of the second N2 molecule by cleavage of the carbon-nitrogen bond in the same site, was found to have similar, yet considerably lower free energies of activation compared to the loss of the first N2 molecule, ∆G‡=11.6, 13.0, 10.6 and 11.6 kcal/mol for paths a, b, c and d (the later being identical at this stage to path a), respectively.

Figure 12. Activation energies for the proposed mechanisms for HN3 loss in the three paths for BTATz anions. The values are based on the sum of electronic and thermal energies. The values in parentheses are based on the sum of electronic and zero-point energies. All values were calculated at b3lyp/6311++g(d,p) level.

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Only three pathways are possible for the loss of HN3 from the molecular anion of BTATz, since [MT]- offers only one tetrazole ring, distal to the charged tetrazole ring, as a source for this fragmentation process. All three pathways were found to have a single step with a single TS, ∆G‡=51.8, 51.6 and 46.7 kcal/mol for HN3 loss from: a) the tetrazole ring distal to the charged tetrazole ring in [MT]-, Figure 12, path a; b) from the tetrazole ring distal to the charged nitrogen bridge in [MB]-, Figure 12, path b; and c) from the tetrazole ring proximal to the charged nitrogen bridge in [MB]-, Figure 12, path c, respectively. Clearly, these energies of activation are considerably lower than those of the proton-migration dependent fragmentation of the first N2 molecule. Along with the findings that the loss of HN3 from anionic BTATz and its fragments is always endergonic, this renders these products kinetic products, and explains their abundance in the MS2 spectra. It also stressed that there are a larger number of pathways leading to the loss of N2, which likely also play a role explaining why it is observed in the measurement despite their disadvantageous kinetic parameters in our measurements. Isotope effect in the decomposition of BTATz anion The above discussed proton migration, leading to the loss of the first N2 from the tetrazole ring, involves cleavage of an N-H bond with simultaneous H-C bond formation. The loss of this nitrogen molecule should therefore show a significant isotope effect. Calculations (Quiver) involving proton-transfer decomposition in [MT]- and [MB]-, pathways a and d in Figure 9, predict a primary kinetic isotope effect of kH/kD~ 4. Furthermore, calculations also predict a smaller isotope effect for the loss of HN3 from the tetrazole ring, kH/kD~ 1.2 for all three possible decomposition pathways of [MB]p-, [MB]d- and [MT]d-. MS2 measurements reveal a primary kinetic isotope effect for the loss of one N2 molecule and no or little isotope effect for the loss of HN3. This was performed by comparing the relative signal intensity, [M-X]-/[M]- with X being N2 or H/DN3, for the two isotopologues, BTATz-H3- and BTATz-D3-, under the same experimental conditions. While the loss of HN3 shows kH/kD~1 to 1.2 at 3 eV and 4 eV respectively, the loss of one N2 molecule shows kH/kD~2.0 to 2.4 at 3 eV and 4 eV respectively. Correlation to neutral BTATz The results presented in this work deal with the anionic forms of BTATz, but several associations with the respective neutral form can be drawn. Calculations on


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neutral BTATz (b3lyp/6-311++g(d,p)) indicate that a proton migration from the tethering nitrogen bridge to the carbon atom of the adjacent tetrazole ring, but not a migration from the N1 of the tetrazole ring to this tetrazole ring carbon, leads to spontaneous loss of an N2 molecule. The transition-state for this process is similar to that found for [MT]-, and the free energy of activation for this process is ∆G‡=83.2 kcal/mol. The second nitrogen molecule is released from the same site with an activation of ∆G‡=13.3 kcal/mol. This finding reveals that the mechanism for the proton-migration dependent elimination of nitrogen molecules does not require the presence of a negative charge. However, the presence of this charge lowers the energy barrier for the first N2 molecule release by about 15-25 kcal/mol, depending on the anionic isomer and the reaction site relative to the formal charge, proximal vs. distal. This can be explained by the negative charge facilitating the proton migration, a role which hydrogen bonds within the solid could play in its absence. The exergonic loss of the first N2 is calculated at -19.8 kcal/mol (G4), about 10-15 kcal/mol larger than for the molecular anions, depending on the anionic isomer and the reaction site relative to the formal charge. This shows that while the anionic form is easier to activate, the neutral form is expected to be more energetic in its performance. Neutral BTATz was also found to lose HN3 through N3N4 bond lengthening (b3lyp/6-311++g(d,p)), with an activation free energy of ∆G‡=49.6 kcal/mol, and a TS similar both in geometry and energy to those found for the anions. This process is endergonic by 16.1 kcal/mol (G4), rendering the release of HN3 a kinetic process, similar to the findings for the anions. Examining the ∆H‡ in comparison to the ∆G‡ for both decomposition process, shows (b3lyp/6-311++g(d,p)) values of ∆H‡=50.3 kcal/mol and ∆G‡=48.9 kcal/mol for HN3 loss, and ∆G‡=82.6 kcal/mol and ∆G‡=83.2 kcal/mol for N2 loss. Therefore, for the loss of HN3 entropy contributes slightly to lowering the free energy of activation, and will increase its contribution with increasing temperature, whereas for N2 decomposition entropy increases the free energy of activation, likely due to the high order of the transition state. These values should be used with caution, as they are close to the error of the calculations. Interestingly, following the exploration of N2 dissociation from the tetrazine ring in BTATz, by lengthening a single tetrazine C-N bond, it was found that the resulting loss of a single N2 molecule is also accompanied by the formation of two cyano-aminotetrazoles, N-(1H-tetrazol-5-yl)cyanamide, m/z=110.0. The energy of activation for this process (b3lyp/6-

311++g(d,p)) is ∆G‡=54.8 kcal/mol, and the process is strongly exergonic, ∆G=-50.4 kcal/mol. In our experimental system, a very small peak (less than 5% intensity compared to the intensity of the highest intensity peak) of m/z=109 can be found in the MS spectra, corresponding to the anion of this product ([M-H]+). The presence of a negative charge in anionic BTATz, is calculated (b3lyp/6-311++g(d,p)) to lower the energy of activation for the loss of N2 from the tetrazine ring to ∆G‡=47.0 kcal/mol in [MT]- and further to ∆G‡=42.3 kcal/mol in [MB]-. However, it is noted that in this latter case the product, besides the released N2, is a molecular anion which still has a single N-N bond between the two nitrogen atoms of the former tetrazine moiety. For both these anions, the release of the second N2 molecule from the same site is unlikely, as multiple C-N bonds would need to be cleaved. This is in contrast to the mechanism of N2 loss from the tetrazole ring, where the energy of activation for the consecutive N2 release is considerably lower. Another support for our assumption that the loss of N2 from the tetrazine is suppressed in our case is that in the MS3 we do not observe signals corresponding to the loss of two HN3 and a single N2 fragment. Conclusions The gas-phase decomposition of BTATz anions and the kinetic isotope effects in these processes were studied by MS/MS techniques. The two main fragment types we observe are the products of the anion losing nitrogen and hydrazoic acid. Supported by calculations, we propose that the N2 decomposition routes involve proton migrations to and within the terazole ring, leading to the spontaneous fragmentation of the first nitrogen molecule. We found that this step is always slightly exergonic, while the second release of a nitrogen molecule from the same ring is considerably more exergonic, likely providing the energy required for the self-propagating reaction for this energetic material. This second N2 loss is characterized by a lower activation energy, and is therefore expected to yield mainly the product of the double N2 loss. The fragmentation of a hydrazoic acid, which is slightly lower in activation energy, is endergonic and thus is unsupportive of a self-sustained reaction. We therefore propose that the decomposition of BTATz, in aspects of rate and energy can be modified by controlling the ratio of N2 to HN3 decomposition. For example, methylation of the N1 carbon could lead to potentially higher performance of this energetic material. Neutral BTATz has been calculated to have similar proton-migration driven N2 decomposition pathways with higher activation energies. Possibly, our findings could contribute



to the understanding of decomposition routes in these nitrogen-rich energetic materials and perhaps help tailoring their reactivity and decomposition pathways for better control of performance.

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[email protected].

The reader is encouraged to review the supporting information accompanying this paper which includes 1H and 13C NMR spectra, crystallographic data, MS2 spectra of kinetic isotope effects, and calculation results (G4, b3lyp/6-311++g(d,p) and Quiver). The cif file of BTATz•DMSO is deposited in the Cambridge Crystallographic Database, CCD, Entry number: CCDC 1587968.

Author Contributions

ACKNOWLEDGEMENT

The manuscript was written through contributions of all authors.‡These authors contributed equally.

This project was funded by the Israeli MOD (Grant number 4440528923) and by the Technion Center for Security Science and Technology (CSST) funds. We thank Prof. Amnon Stanger from the Schulich Faculty of Chemistry for his helpful contribution.

AUTHOR INFORMATION Corresponding Author

SUPPORTING INFORMATION

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Proposed Proton-Transfer Mechanism for the Initial Decomposition

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