Unusual Protonation of the Hydroxylammonium Cation Leading to the

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Unusual Protonation of the Hydroxylammonium Cation Leading to the Low Thermal Stability of Hydroxylammonium-Based Salts Zhipeng Lu, Ying Xiong, Xianggui Xue, and Chaoyang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11136 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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The Journal of Physical Chemistry

Unusual Protonation of the Hydroxylammonium Cation Leading to the Low Thermal Stability of Hydroxylammonium-based Salts Zhipeng Lu, †,‡ Ying Xiong, † Xianggui Xue, † and Chaoyang Zhang*†$ †

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-327, Mianyang,

Sichuan 621900, China. ‡ $

Department of Mathematics and Physics, Officers College of CAPF, Chengdu, 610213, China. Beijing Computational Science Research Center, Beijing 100048, China.

Abstract Energetic ionic salts (EISs) are a class of thriving and promising energetic materials (EMs) that can possess excellent properties and performances comparable to common conventional EMs composed of neutral molecules. As EMs, their response mechanisms to external stimuli are strongly responsible for their safety and thus are highly concerned about. Nevertheless, insight into these mechanisms remains still lack. We find in the present work a bimolecular reaction between two same sign charged ions during heating dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX50), a typical EIS that are attracting increasing attention with a high potential of practical applications. That is, the protonation of NH3OH+, or a reaction between two cations, occurs and serves as a dominant initial step in the thermal decay of TKX-50. This is a rare case as a bimolecular reaction can usually hardly take place between two ions with same sign charges (two anions or two cations), due to their electrostatic repulsion preventing their sufficient approaching each other to induce the reaction. The protonation proceeds by a H+ transfer from a NH3OH+ to its neighboring one, and subsequently decompose NH3OH+ to the final stable products of NH3 and H2O simultaneously to collapse the crystal lattice of TKX-50. This new finding can well explain the experimental observations of the prior decay of NH3OH+ to the bistetrazole-1,1'-diolate anion when TKX-50 heated at a constant temperature of 190 oC and the relatively low thermal stability of NH3OH+ based EISs relative to others. Thereby, we propose a strategy to avoid a ready proton transfer and subsequent decomposition to enhance the thermal stability of EISs. This work is hopefully to richen the insight into both the decay mechanism of EISs and the mechanism of the reactions between same sign charged ions. 1 Environment ACS Paragon Plus

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1. INTRODUCTION Safety is one of the two most important properties/performances concerned for energetic materials (EMs) and contains the most complicated issues in the field. Numerous experiments and simulations have shown many factors responsible for the safety.1,2 For example, from the viewpoint of structures, they include molecular stability1-4, molecular stacking mode5-11, polymorph, crystal perfection, crystal shape, crystal size12,13, interfacial characteristics14, and so forth. In addition, it is well known that the safety can strongly be related to stimulation styles. Furthermore, the statuses of samples and testing conditions can also influence the measurement values of the safety. In a word, the safety is one of the largest challenges in the field of EMs, from the aspect of science, technology or engineering. In principle, the micro-mechanism of the evolution of an EM against external stimuli is a base to understand its safety.15 In practice, the safety is usually experimentally evaluated by sensitivity, reflecting the response degree of an EM to an external stimulation during manufacture, storage, transportation and usage: the higher sensitivity represents the higher sensitivity. Among the external stimuli, heat is thought to be the most general one. The thermal stability is always considered for assessing the sensitivity for an EM, because heat can sever as a primary stimulation or a subsequent one transferred from other styles of stimuli. Regarding heat, we have to mention the hot spot theory 16

, which is an extensively accepted and well known concept in the field of EMs and is strongly

related to heat. It is deemed that any stimulation, such as heating, impact, friction, shock or electrostatic impulse, is finally transferred to heat to analyze molecules, form and grow hot spots, and combust and detonate an EM finally. As a matter of fact, because heating an EM enhances atomic vibrations and induces molecular decay from the break of the weakest bond, the strength of the weakest bond, denoted by bond dissociation energy (BDE), is usually adopted as an indicator of thermal stability, in combination with its easy accessibility by simple quantum chemical calculations on single molecules. Besides, the core of the recent proposals of the face-to-face π-π crystal packing5,6 and the reversible hydrogen transfer facilitating low impact sensitivity

17

are just

that they can buffer against external mechanical stimuli to avoid temperature elevating enough to proceed to final combustion and detonation. Anyhow, the thermal response of an EM against 2 Environment ACS Paragon Plus

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external stimuli is an unavoidable point to reveal its safety mechanism. And the details of thermal decomposition are usually regarded as a basic starting-point to understand or evaluate sensitivity.

Figure 1. Common cations and anions composing of EISs.

Energetic ionic salts (EISs) appear now in prosperity18, even though they are not new as they have come into being for a long time (e.g., ammonium perchlorate, AP). The current prosperity may be attributed to several factors. For example, as the cations and anions illustrated in Figure 1, the components and structures of the ions are largely extended from the traditional ones like AP, i.e., they are no longer inorganic only, and can largely enhance energy density. Meanwhile, by means of ionization, the molecular stability, in particular, of the nitrogen-rich compounds, can to a large extent be strengthened in contrast to the original neutral molecule. Besides, combining the ionization (to increase intermolecular interactions to elevate packing coefficients) with appropriate coformers to improve oxygen balance, the energy output of EISs can increase, as both packing density and oxygen balance strongly influence the energy output of EMs. Furthermore, with a sense of combinational chemistry, numerous new EISs can be prepared once a new cation or anion has been synthesized. All these can push the booming of EISs. As a matter of fact, 3 Environment ACS Paragon Plus


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dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) 19, as an excellent representative of EISs, has been verified to outperform many conventional EMs composed of neutral molecules and in pilotscale experiments. As a class of EMs, the sensitivity mechanism of EISs is also highly concerned about. Regarding this, the thermal decomposition mechanism of EISs should be seen as a starting-point to reveal the origin of their properties and performances. Presumably, in contrast to the conventional EMs composed of neutral molecules, the thermal decomposition mechanism of EISs is more complicated. For example, for a conventional EM, the initial step can start in a monomolecular (one case) or a bimolecular (one case) manner; while, as to an EIS, the initial reaction can take place in five possible ways, two ways of monomolecular reactions (the monomolecular decay of a cation or an anion) and three ways of bimolecular reactions (the bimolecular decay between two cations themselves, between two anions themselves, or between a cation and an anion). Currently, as a whole, the insight into the thermal decomposition mechanism of EISs is still much poorer than that of common EMs, and most of them focus on TKX-50. For example, An et al proposed an initial step for the TKX-50 decay, the proton transfer from NH3OH+ (HA+) to C2O2N82− (BTO) to form NH2OH (HA) and C2HO2N8−. The protonated intermediate C2HO2N8− is less thermally stable than C2O2N82−.20 By means of ab initio molecular dynamics (AIMD) simulations and experiments, we confirmed that a heat-induced phase transition occurs prior to the proton transfer, and compression prohibits the proton transfer and thereby the thermal decomposition too. Meanwhile, we developed a strategy for stabilizing the unstable systems like HA through ionization and separation. 21 Besides, Sinditskii et al deemed that the decomposition of TKX-50 is determined by decomposition of free HA formed in the dissociation reaction, and the first stage includes partial decomposition of tetrazole moieties also. They found that an intermediate decomposition product is (NH4)2C2O2N8, which further decomposes in the second stage at elevated temperatures. 22 We still focus on the thermal decomposition mechanism of TKX-50 in this work, as well as those of hydroxylammonium-3,3′-dinitramino-4,4′-bifurazane (HA-DNABF) 23, guanidinium-5,5′bis (tetrazole-2-oxide) (G-BTO)24 and guanidinium-3,3′-dinitramino-4,4′-bifurazane (G-DNABF) 23.

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These four EISs possess same cations (HA+ or G+) or anions (BTO2- or DNABF2-), facilitating the comparison from a viewpoint of component. By means of AIMD simulations and calculations, and Roman spectra and thermal decomposition analyses, we find a new pathway for decaying TKX-50, i.e., NH3OH+ + H+ (from HA+ or C2HO2N8−)→NH3OH22+→NH3+ + H2O+→NH3 + H2O. This new finding of the protonation of HA+, not only well explains the experimental observations of the complete decay of HA moieties prior to the tetrazole ones when heating TKX-50 at a constant temperature of 190 oC, and the relatively low thermal stability of HA-based EISs to others, but also supplies a typical case of reactions between species with same signed charges, which occurs rarely due to electrostatic repulsion preventing their approaching each other to induce the reactions. All these are expected to richen the insight into the underlying mechanism of thermal stability of TKX50, as well as of other EISs with an increasing amount, and deepen the insight into the reactions between two same sign charged ions. 2. EXPERIMENTS AND CALCULATIONS 2.1 Experiments. According to recent studies, we know that the heat-induced phase transition of TKX-50 happens at ~180 oC21; the temperature peaks of decomposition by differential scanning calorimetry (DSC) measurements vary in a range of 210–250 °C, depending on heating rates19,25,26; and lots of gas is produced when TKX-50 heated in the range of 180-200 oC for 200 minutes22. To clarify which temperature starts the thermal decay of TKX-50, we performed again the similar experiments on the refined TKX-50 samples. In particular, we paid much attention to the case of TKX-50 heated at a constant temperature of 190 oC, the average of 180 and 200 oC, among which a lot of gas can be produced given enough heating time. 22 Sample preparation. TKX-50 crystals were prepared by the method proposed by Klapötke et al 19

and refined with a purity of >99%. Raman spectra analysis. The Raman spectra of TKX-50 heated in a range of 23–210 °C were in

situ collected using a double-grating Jobin Yvon spectrometer and a thermoelectric-cooled charge



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coupled device detection system (Princeton Instruments PIXIS 100BR). The 514.5 nm line from an argon ion laser (CVI Melles Griot 43 Series Ion Laser) was adopted for Raman excitation. An Olympus microscope lens with a focal distance of 20 mm and a numeric aperture of 0.35 was employed to focus laser beams on the sample. The Raman system is capable of measuring Raman frequencies of as low as 70 cm−1 with a spectral resolution of ~2 cm−1. Temperature was controlled by a controller of Linkam TS1500 with a resistive heater wrapped around the ceramic sample cup and some thermocouples with an accuracy of 1 K. The incident power was set to below 40 mW to avoid any photochemical damage to the TKX-50 samples. TGA-DSC measurements. The TGA-DSC measurements in a range of 23–400 °C were performed on a TA Instruments Discovery with a TKX-50 sample of ~1.5 mg and a heating rate of 5 °C/min. A nitrogen gas flow of 20 mL/min was maintained through the furnace during measurements. The TGA-DSC traces were analyzed using TROIS Analysis Software of TA Instruments. In particular, as mentioned above, we paid much attention to the case of constant temperature heating at 190 oC for three hours. 2.2 Ab Initio Calculations Table 1. Comparison of Experimental and Calculated Crystallographic Parameters of the Four EISs. EISs

TKX-50

G-BTO

HA-DNABF

Methods

Expt.

PBE+D2

Expt.

PBE+D2

Expt.

PBE+D2

Expt.

PBE+D2

Formula

C2H8N10O4

C2H8N10O4

C4H12N14O2

C4H12N14O2

C4H8N10O8

C4H8N10O8

C6H12N14O6

C6H12N14O6

CCDC

872232

-

895965

-

962865

-

962854

-

Crystal System

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Space Group

P21/c

P21/c

P21/c

P21/c

P21/n

P21/n

P21/n

P21/n

a (Å)

5.4408(6)

5.5589

3.6052(3)

3.5146

4.1911(2)

4.1776

4.9871(3)

5.2375

b (Å)

11.7514(13)

11.3567

17.1233(13)

17.4617

11.3885(4)

11.2837

9.4870(5)

9.2852

c (Å)

6.5612(9)

6.4025

9.4577(8)

9.3805

11.5404(4)

11.6919

14.9381(9)

14.5781

α (○)

90

90

90

90

90

90

90.00

90

β (○)

95.071(11)

96.293

97.727(9)

99.715

95.323(4)

94.204

91.218(5)

89.080

γ (○)

90

90

90

90

90

90

90.00

90

Z

2

2

2

2

2

2

2

2

V (Å3)

417.86(9)

401.76

584.06(8)

567.43

548.45(4)

549.66

706.60(7)

708.87


G-DNABF

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ρcalc. (g·cm-3)

1.877

1.952

1.639

1.687

1.963

1.959

1.769

1.763

All AIMD simulations were carried out using VASP27 with first-principle pseudopotentials constructed by projected augmented waves, in which the H1s, C2s2p, N2s2p, and O2s2p orbitals are considered as valence states, and the exchange–correlation functional is treated with the generalized gradient approximation (GGA) following the Perdew, Burke and Ernzerhof formulation (PBE). 28-30 A plane-wave basis set with an energy cutoff of 850 eV was used in the simulations. PBE+D2 method of Grimme 31 was employed to correct long-range dispersion interactions. To validate the methods feasible to the four EISs, TKX-50, HA-DNABF, G-BTO and G-DNABF, we performed full structural relaxation of their primitive unit cells determined experimentally using the conjugate gradient method with 3×1×3 Monkhorst–Pack k-point (Γ-centered) sampling in reciprocal space to ascertain the reliability of the applied methods. The Brillouin zone (BZ) integration was carried out by the linear tetrahedron method with Blöchl’s correction.32 The selfconsistent convergence criteria of energy were set to 1×10−7 and 1×10−6 eV for electronic and ionic relaxations, respectively. As listed in Table 1, the lattice parameters optimized by PBE+D2 agree well with the experimentally observed ones, verifying its reliability to the four EISs. In the following calculations, PBE+D2 method was employed in AIMD simulations. The AIMD simulations were conducted with a canonical ensemble (NVT) and Nosé–Hoover thermostat, and supercells enlarged from the optimized unit cells of TKX-50 (2×1×2), G-BTO (3×1×1) and G-DNABF (3×1×1). For each AIMD simulation, only the gamma point was sampled in the Brillouin zone, the Verlet algorithm was adopted to integrate Newton’s equations of motion with a time step of 0.25 fs, the structure was first equilibrated at 23 oC for 1 ps and subsequently heated to assigned temperature in 0.25 ps, and maintained at this temperature for given time. The electron temperature was incorporated through the Fermi–Dirac distribution at the corresponding temperature. To analyze the chemical specie evolution in the AIMD simulations, the atomic connectivity was determined by bond distance cutoffs and a lifetime criterion to avoid miscounting the short-term fluctuations in bonds above/within the cutoffs. Cutoffs do not actually break/form a bond. Two atoms were counted as bonded if their distance is less than 1.33 times of the normal 7 Environment ACS Paragon Plus


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covalent bond length33, and must survive at least 20 and 45 fs for bonds involving and not involving H atoms, respectively.34 These time scales were chosen based on the characteristic vibrational frequencies of the related chemical bonds by considering that the bond between two atoms should sustain at least a vibrational period to complete one vibration. 3. RESULTS AND DISCUSSION 3.1 Thermal Decay Mechanism of TKX-50.

Figure 2. Raman spectra and photomicrographs of heated and annealed TKX-50 at various temperatures.

We first pay attention to the evolution of Raman spectra and appearance of TKX-50 heated from 23 to 200 oC and annealed from 200 to 23 oC at a rate of 5 oC/min. As illustrated in Figure 2, many new vibrational peaks appear first at ~180 oC. These peaks have been assigned to the formation of a new heat-induced phase of TKX-50, meta-TKX-50, in our recent work

21

. During temperature

increasing from 180 to 200 oC, the intensities of these new vibrational peaks are enhanced (see SI of Supporting Information (SI)); while, when temperature maintains at 200 oC, these peaks are continuously weakened, implying the reduction of the sample. After heating at 200 oC for 60


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minutes, the sample is annealed to 23 oC. It is interesting to find that the peaks belonging to HA+ disappear, remaining those of BTO∙2H2O (bitetrazole-1,1'-diolate-dihydrate) and ABTOX (diammonium-1H,1H'-5,5'-bitetrazole-1,1'-diolate). It shows that HA is completely decomposed prior to the BTO rings. That the TKX-50 decay starts at 180 oC can also be confirmed by the microphotographs of the TKX-50 appearance in Figure 2. Below 180 oC, the TKX-50 crystal appears transparently; some nucleuses are formed at 180 oC; and the crystal becomes more and more unclear with temperature increasing. Even though TKX-50 is annealed to 23 oC, the crystal appears blurredly, due to the disorder caused by the partial chemical decay.

Figure 3. DSC and TGA profiles (a) and photomicrographs (b) of TKX-50 heated from 23 to 400 oC.

From above Roman and microphotograph detections, we can find that the HA+ is decayed while the BTO ring remains within a temperature range of 180-200 oC 22. To consolidate this, we carried out another thermal decomposition experiment, a DSC-TGA measurement, among which a moderate temperature of between 180 and 200 oC, 190 oC, was selected for a constant temperature heating. As shown in Figure 3(a), the temperature first increases from 23 to 190 oC at a rate of 5 o

C/min, keeps at 190 oC for three hours, and thereafter increases to 400 oC at a same rate. From the

TGA profile in the figure, it can be seen that the weight loss begins after the temperature reaches 9 Environment ACS Paragon Plus


190 oC for several minutes. Thereafter, the weight loss proceeds tardily, as the weight only loses ~16 % in the three hours. The subsequent temperature increasing causes the rapid loss from 210 oC, in agreement with the DSC temperature peak reported previously19,21. Together with the weight loss, heat releases. And the heat release rate varies with the weight loss rate. Meanwhile, the evolution of the crystal appearance of TKX-50 in Figure 3(b) can also reflect the evolution of TKX-50 against heating. From left to right and from top to bottom of the figure, the crystal heating continues: the edges of the TKX-50 crystal appear clearly at 170 oC; the crystal becomes blurred at 180 oC; and at 190 oC, it becomes more unclear, and some bubbles appear as time proceeds, suggesting the appearances of gaseous and liquid substances in this case. Wholly, the crystal appearances in this case don’t show any drastic reaction, in agreement with the gentle variations of DSC and TGA profiles in Figure 3(a). All these observations show that TKX-50 can already have started to be analyzed before temperature reaches to 210 oC, consistent with the previous report of Sinditskii et al, in which lots of gas was found when TKX-50 heated in the range of 180-200 oC for 200 minutes. 22 16

+

(a)

NH3OH

12

C2O2N8

2-

8

Number of Fragments

4 10 0 8 6 4 2 0 12

(b)

NH2OH

-

C2HO2N8

C2H2O2N8

NH3

H2O

N2

N2O

NH4

+

(c)

8 4 0 6

(d)

4

3

Energy (×10 eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 -1.150

(e)

IV

-1.16 -1.17 -1.18

I

0

III

II

1

2

3

V

4

5

6

7

8

9 10 11 12 13 14 15

Time (ps) Figure 4. Evolutions of key chemical fragments and potential energy for TKX-50 heated at 1800 K.


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To reveal the underlying mechanism of TKX-50 against heating, we performed an AIMD NVT simulation at 1800 K for 15 ps on a 2×1×2 supercell enlarged from the optimized unit cell of TKX50.19 As time proceeds, the reactants of HA+ and C2O2N82- decrease continuously (Figure 4(a)). HA and C2HO2N8-, as the proton transferred intermediates, increase first to maximums and decrease thereafter; and C2H2O2N8 appears with a quantity of one only, showing that C2O2N82- can seldom accept H+ fully (Figure 4(b)). In Figures 4(c) and 4(d), H2O, N2 and N2O increase successively as final stable products, and their delay time differs from one another, i.e., H2O, N2 and N2O appear in an increasing order of delay time; NH3 waves in a quality range of 0-3; and NH4+ increases gently. Figure 4(e) indicates the process of a first increase and a subsequent decrease of the potential energy evolution of TKX-50 against heating, belonging to a typical process of the first heat absorption and subsequent release of EMs. Wholly, from the viewpoints of the evolutions of reactants, typical intermediates and final products, and potential energy, we can conclude that the thermal decay of TKX-50 undergoes five successive stages: Stage I (0-0.8 ps), the main reaction is that H+ is transferred from HA+ to C2O2N82- to form HA and C2HO2N8-. This has been evidenced in some previous studies. Meanwhile, a few HA+ ions can each capture a free H+ to produce NH3 and H2O (this will be discussed in the next section). Because of the endothermic nature of the phase transition and the proton transfer, with a small quantity of heat release by decomposing a few of HA+, this period is endothermic, with potential energy increasing. Stage II (0.8-5.3 ps), HA+ is protonated to produce NH3 and H2O; meanwhile, the proton transferred product HA is decayed into NH3, H2O and N2, showing an exothermic nature, with potential energy reducing. At this stage, NH4+ is formed by the protonation of NH3, settling a base for forming ABTOX. Also, the formation of H2O sets a base for producing BTO·2H2O. This stage would correspond to the gentle decomposition observed in the TGA-DSC measurement of TKX-50 heated at a maintained temperature of 190 oC (Figure 3(a)). Also, the simulated results of this stage can well explain the Raman spectra evolution of TKX-50 against heating in Figure 2. Stage III (5.3-8.7 ps), N2 increases by the fission of the tetrazole rings. Stage IV (8.7-11.4 ps), one of the products of the tetrazole rings



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fission, N2O is formed and increases. The observed later formation of N2O than that of N2 is attributed to the latter requires a higher energy barrier. Because the tetrazole ring fission into N2 and N2O releases much heat, potential energy of Stages III and IV decreases rapidly, corresponding to the fast mass loss and the first peak temperature on the DSC profile in Figure 3(a). Stage V (11.4 ps-final), the final stage involves the decay of the intermediate of ABTOX and BTO·2H2O into N2 and H2O, and other reactions. It can explain the second peak temperature within 240-275 oC on the DSC profile in Figure 3(a). From above discussion, we can find that the simulation results can well elucidate the experimental observations. That is, by means of the Raman spectra and TGA-DSC measurements, and the AIMD simulation, the thermal decay of TKX-50 can be partitioned into five stages: the heat-induced phase transition and the proton transfer, the further proton transfer and the HA decay to NH3, H2O and N2, ABTOX and BTO·2H2O formation, the tetrazole ring fission into N2 and N2O, and ABTOX and BTO·2H2O decay to final stable products. Furthermore, in combination with our previous result of heat-induced phase transition occurring at 180 oC 21 and the HA decay determined by DSC-TGA measurements at 190 oC, we can confirm that some physical and chemical changes have already taken place prior to the violent decomposition at ~210 oC. It is interesting to find the protonation of HA+ in our AIMD simulation on heated TKX-50. In this section, we will pay attention to the protonation. As usual, due to electrostatic repulsion, hardly two ions with same signed charges can approach enough to react with each other. Nevertheless, for the protonation of HA+, as a bimolecular reaction between two cations, it indeed occurs, regardless of electrostatic repulsion. As demonstrated in Figure 5, we captured the protonation by two pathways: one is by the double HA+ (Figure 5(a)), and the other is by HA+ and C2HO2N8- (Figure 5(b)). By such protonation, H2O+ and NH3+ are formed as active particles, setting a base for producing ABTOX and BTO·2H2O subsequently. It should be a new finding as a pathway in the thermal decay of TKX-50. Such pathway may also take place in other HA-based EISs.


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Figure 5. Snapshots showing the protonation of HA+ and the decay of protonated products into H2O+ and NH3+. C, H, N and O atom are represented in grey, white, blue and red, respectively. These representations are considered in following figures. And the transferred H atoms are heighted in green.

Figure 6. Potential energy (in kcal/mol and black) surface of NH3OH++H+→NH3OH22+→NH3+ +H2O+, and bond lengths (in blue) and atomic NBO charges (in red) of all species.

Given that the protonation has been observed in the AIMD simulation, its thermodynamics and kinetics require to be clarified. In our previous work, the energy barrier for the proton transfer in solid TKX-50 from HA+ to C2O2N82- to form HA and C2HO2N8- was predicted to be 39.4 kcal/mol, with an energy increase of 38.6 kcal/mol of the proton transferred products.17 Here, simply, the related energetics in gaseous state was explored at the level of M06-2X/aug-cc-Pvdz35,36. As demonstrated in Figure 6, in the gaseous state, the protonation to the final NH3+ and H2O+ is much



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energetically allowed with a heat release of 129.9 kcal/mol, with a premise of the reaction occurring with a free H+, which is produced by heating. The protonation of HA+ to NH3OH22+ (HA2+) requires overcoming a very shallow barrier of 2.2 kcal/mol, with a heat release of 78.2 kcal/mol thereafter. It suggests the easiness of the protonation. This should be understandable, as the O atom of HA+ is negatively charged and possesses two lone electron pairs, which make it to be readily chemically combined with H+. Even though there is a barrier (65 kcal/mol, calculated by a crossing points of energy-interatomic distance curves of single and triple state) for NH3OH22+ → NH3+ + H2O+, NH3OH+ + H+→NH3+ + H2O+ can take place simultaneously once the protonation occurs, as this barrier is lower 13.2 kcal/mol than the total energy of the primary reactants of HA+ and H+. Even though we have not carried out transition state searching in the condensed TKX-50 to ascertain the barrier of NH3OH++NH3OH+ → NH3OH22++NH2OH (1) or NH3OH++C2HO2N8- → NH3OH22++ C2O2N82- (2) as we dealt with NH3OH+ + C2O2N82-→NH2OH + C2HO2N8- (3) before, we can also deduce that reaction 1 possesses a barrier a bit higher than that of reaction 3. This should be reasonable, because the key steps of both reactions are the partition of H+, which requires the most energy of the barriers. Due to the longer O…H distance between two neighboring HA+ (2.36 Å) than that between neighboring HA+ and C2O2N82- (1.74 Å) 19, and the electrostatic repulsion between H+ and HA+, while the attraction between H+ and C2O2N82-, the barrier of reaction 1 is a bit higher than that of reaction 3. Furthermore, we check the bond lengths and atomic NBO charges of all related species. In Figure 6, there are three types of bonds, O-H, N-H and O-N bonds. During the protonation and the decay of HA2+, the bond lengths of each kind of bonds vary a bit, i.e., 0.04, 0.02 and 0.03 Å for the O-H, N-H and O-N bonds, respectively. Because the O-N bond is the weakest of HA+ and HA2+, attention to it should be paid. Comparing the O-N bond length of HA+ and HA2+, we can find that it increases slightly (0.03 Å), due to the increases of net positive charges on the OH (OH2) and NH3 groups, which increase the electrostatic repulsion. This repulsion should contribute to the heat release after the O-N bond break. 14 Environment ACS Paragon Plus

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Besides, regarding to the conversion of NH3+ and H2O+ into NH3 and H2O, respectively, it is in fact the electron transfers. In our AIMD simulation, it is found that NH3 and H2O are instantly formed once the protonation of HA+ is finished, and the lattice structure is collapses shortly. That is, the electron transfer is completed through the collision among NH3+ and H2O+, and other chemical particles like HA, C2O2N82-, C2O2HN8- and various intermediates. 3.2 Effect of the protonation of HA+ on thermal stability. Table 2. Comparison of Thermal Decomposition Temperatures (Tdec, oC) of Series of EISs. Cations HA+ NH4+ N2H5+ G+ AG+ DAG+ TAG+ Anion BTO2- 24

210 290

220

274 228 -

205 312

234

316 251 208

BT2O2- 38

172 265

-

331 255 -

217*

DNBTO2- 39 217 257* 228

329 246 -

207*

AFTA- 40

BT

*

210

2- 37

207*

213 277

216

-

DNABF

141 230

230

280 215 -

203

NTX- 41

157 173

-

211 185 174

153

166 195

180

171 200 -

-

2-23

DPNA *

- 42

258 231

*

216

Crystallized as a hydrate.

Salification is an efficient way to stabilize some unstable substances.43 For example, relative to the pure HA, the thermal stability of all observed HA-based EISs are enhanced, as the thermal decomposition temperatures (Tdec) of these EISs are much elevated from 33 oC, Tdec of pure HA. While, as listed in Table 2, with respect to a same anion, for example, BTO2-, its HA-based EIS possesses the lowest Tdec. This case is universal in other salts with various anions. It shows the HAbased salts are universally less thermally stable than other salts with same anions. In Table 2, except from the NTX-based salts, HA-based salts are always the least thermally stable in other EISs. The least thermal stability may be attributed to the protonation of HA+. To verify this, we first pay attention to the intermolecular interactions in HA-based salts, as they are a base for discussing the stability. Besides TKX-50, HA-DNABF, G-BTO and G-DNABF are also considered for comparison, as these four EISs possess same anions or same cations. For HA-based salts, recent work showed there are extensive and moderate, even strong intermolecular HBs in them, attributed to that HA+ can serve as both HB acceptors and donors. 44,45 Besides, as demonstrated by blue dash 15 Environment ACS Paragon Plus


Page 16 of 26

in Figure 7, HBs can also exist among HA+ themselves, as in TKX-50 and HA-DNABF. While, such case of HBs among cations themselves does not happen in the two G-based EISs, G-BTO and G-DNABF. The compact HBs in HA-based salts make ions stacked compactly44,45, with higher packing coefficients and packing densities, relative to other EISs. This should be one of the reasons for choosing HA+ as cations in preparation.

Figure 7. Views of crystal packing along the [001] faces of (a) HA-BTO, (b) G-BTO, (c) HA-DNABF and (d) GDNABF. The hydrogen bonds (HBs) around an assigned an anion, between two cations in unit cells, and remaining HBs are denoted in green, blue and black, respectively.

It is generally believed that the strong intermolecular interactions contribute to high thermal stability. Because covalent interactions are usually stronger than noncovalent ones, it is deems that the break of the noncovalent interactions takes place before that of the covalent interactions, and the strong intermolecular interactions facilitate high thermal stability. Nevertheless, it is not the case for the four EISs. The HB dissociation energy of TKX-50, HA-DNABF, G-BTO and G-DNABF (per asymmetry with one anion and two cations) are predicted to 274, 241, 211 and 178 kcal/mol, respectively, without obvious relation on their Tdec, 210, 141, 274 and 280 oC, respectively (Table 16 Environment ACS Paragon Plus

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S1 of SI). It suggests other mechanisms responsible for the thermal decay of these EISs. As a matter of fact, besides intermolecular interactions (e.g., intermolecular HBs), the molecular stability of the anions and cations, the easiness of bimolecular reactions to decay between double anions, between double cations, or between an anion and a cation can also dominate the thermal stability43. 110

TKX-50 (HA-BTO)

100

Potential Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


G-BTO

90

G-DNABF 80 70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Time (ps) Figure 8. Potential energy evolutions of the three EISs heated at 2500 K. The arrows point to the maximums of the potentials.

Relative to the Tdec of the neutral precursors of HA (33 oC), G (50oC), BTO (214 oC) and DNABF (80 oC), only TKX-50 (210 oC) possesses a lower Tdec than its anion precursor, while HADNABF (141 oC), G-BTO (274 oC) and G-DNABF (280 oC) possess higher Tdec than both their anion and cation precursors, respectively. It implies that the thermal decay of TKX-50 may proceed by a different mechanism from the other salts. To verify this, we performed another three AIMD simulations on the thermal decomposition of TKX-50, G-BTO and G-DNABF with the canonical ensemble too for 15 ps. All the simulation temperatures were set to be a same temperature of 2500 K, as G-BTO and G-DNABF possess relatively high Tdec in practice. That we didn’t simulate the thermal decay of HA-DNABF is attributed to that its relatively low thermal stability can simply be understandable as both its anion and cation precursors are not thermally stable. Meanwhile, because TKX-50 and G-BTO are both the HA-based EISs, they may be suffered from a similar mechanism responsible for the initial decay once this decay starts from a bimolecular reaction between HA+ themselves. As demonstrated in Figure 8, the first increase and subsequent reduction curves of the 17 Environment ACS Paragon Plus


potential energy of the three EISs show the lower thermal stability of TKX-50 than those of G-BTO and G-DNABF, as TKX-50 possesses a shorter delay time for potential decrease at a same temperature of 2500 K. This agrees well with the experimentally determined Tdec. Furthermore, it verifies again the reliability of applied MD simulation methods to the EISs. 12 +

CH6N3

8

2-

C2O2N8

Number of Fragments

4 80 CH5N3

6 2 200 16 12 8 4 60

C2ON5

C2HON5 N2

CH3N2

3

C2HO2N8

CH4N3

4

4

Energy (×10 eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H2O

NO

CH2N2

NH3

2 -1.160 IV

-1.18 -1.20 -1.22

II

I

0

1

2

3

III

4

5

6

7

8

9 10 11 12 13 14 15

Time (ps) Figure 9. Evolutions of key chemical fragments and potential energy for G-BTO heated at 2500 K.

The details of TKX-50 against heating at 2500 K are demonstrated in Figure S1 of SI. As a whole, the case of 2500 K is similar to that of 1800 K (Figure 4), with a difference in reaction rate, i.e., the higher temperature of 2500 K leads to the higher rate as usual. Regarding G-BTO, we can also find four stages during its thermal decay in Figure 9. The first stage (Stage I) is endothermic with the proton transfer from CH6N3+ to C2O2N82- to produce intermediates of CH5N3 and C2HO2N8as the initial step for the thermal decomposition. At this stage, small quantities of N2、NO and C2HON5 by the decomposition of the intermediate of C2HO2N8- appear. The heat begins to release at the second stage, with large quantities of N2 and H2O as final products, and some NO as an 18 Environment ACS Paragon Plus

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intermediate. The number of CH6N3+ waves from this stage to the final within the timescale of simulation, showing that it has not completely been decayed. This case is much different from that of NH3OH+. That is, as illustrated in Figures 4 and S1 of SI, HA+ disappears at 8 and 1.2 ps at 1800 and 2500 K, respectively. At the third stage, some NH3, CH2N2 and CH3N2 appear as the result of the CH5N3 decay, and the quantities of N2 and H2O increase continuously. Finally, reactions proceed by consuming intermediates. 12 CH6N3

8

+

C4O6N8

2-

Number of Fragments

4 0 6 4

CH5N3

C3O3N5

CON2

CO2N3

2 12 0 8

CH2N2

NO2

N2O

NO

4 16 0 12 8

3

Energy (×10 eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2O

N2

CO2

NH3

4 -1.390 III

-1.41

-1.45

II

I

-1.43 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

Time (ps) Figure 10. Evolutions of key chemical fragments and potential energy for G-DNABF heated at 2500 K.

With respect to G-DNABF, it is not found that the proton transfer from CH6N3+ to C4O6N82-, different from the case of G-BTO and TKX-50. Three stages can be clarified for the G-DNABF decay. At the first stage, CH6N3+ is deprotonated to produce CH5N3 (deduced by the increase of its quantity), and C4O6N82- is decomposed into C3O3N5, CON2 and NO2. Furthermore, C3O3N5, CON2, NO2 and CH5N3 react with one another to produce CH2N2, NO and N2O. At the final stage, further



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reaction proceeds with the formation of more and more final products of N2 and H2O. Wholly, there is no obvious protonation of anions as the initial step for thermally decaying G-DNABF.

Figure 11. The hydrogen bonds around an assigned an anion (green) and between two cations (blue) of (a) HABTO, (b) G-BTO, (c) HA-DNABF and (d) G-DNABF.

Comparing the thermal decay mechanisms of the three EISs, we can find the deprotonation of the cations always take place. Nevertheless, the deprotonation consequences of the three EISs are different from one another: for TKX-50, H+ can be combined with both the cation of HA+ and the anion of C2N10O22- for further reactions; H+ can only be linked with the anion of C4O6N82- in the case of heating G-BTO; and when heating G-DNABF, it isn’t observed that H+ is compulsorily combined with either the cation of CH6N3+ or the anion of C4O6N82-. Recent studies showed that the H+ transfer from HA+ to C2N10O22- in TKX-50 should overcome a barrier of 39.4 kcal/mol at the theory level of PBE/D217. On this basis, the subsequent decomposition of gaseous C2HN10O2should overcome another barriers of 37.2 and 59.5 kcal/mol to produce N2 and N2O on the level of B3LYP/6-311++G(d,p)

20

, respectively. It is just the considerably high barriers for subsequent

decomposition that makes the initial proton transfer reversible17, facilitating the low impact


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sensitivity of TKX-50. While, presumably, the proton transfer to HA+ should require a higher barrier that that to C2N10O22-. However, this transfer features a consequence of heat release, opposite to the case of the transfer to C2N10O22-, in which further energy is required prior to subsequent decomposition. Thus, the protonation of HA+ can dominate the initial decay steps. Even though we didn’t perform a MD simulation on HA-DNABF, we deduce that the protonation of HA+ can also take place. As illustrated in Figure 11, the HB distances of double neighboring HA+ in TKX-50 and HA-DNABF are 2.36 and 1.84 Å, respectively. The shorter HB distance in HADNABF will facilitate the proton transfer, or the protonation of HA+. This can be one of the reasons for the low thermal stability of HA-DNABF. It also agrees with a recent strategy proposed to separate unstable species to avoid bimolecular reactions and enhance thermal stability43.

Figure 12. Energetics (standard enthalpies) of the weakest bonds in various molecules, and pKa (in brackets)46 of protonated molecules.

From above discussion, we can know that the protonation of HA+ can take place when heating HA-based EISs, as a new finding of the mechanism responsible for the initial decay. Meanwhile, we may be concerned about the stability of HA+, the accessibility of the deprotonation of HA+ to supply H+, and the energetics of HA2+. We calculated the energetics (standard enthalpy changes) of all the weakest bonds in related molecules. As demonstrated in Figure 12, for all neutral precursors of the cations that appear usually in current EISs, their molecular stability increase with strengthening the weakest bonds after protonation. Monomolecularly, ionization indeed enhances 21 Environment ACS Paragon Plus


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the molecular stability. On the other hand, according to pKa values in the figure, we find that HA+ possesses the smallest pKa, showing its highest ability to supply H+. N2H5+ and NH4+ follow HA+, and all guanidine cations possess high pKa. Regarding the protonation of the cations, it is ready to understand that NH4+ and G+ should not be protonated, as there is no lone pair on their N atoms. After protonation, all six cations each with +2 charges are energetically unstable, i.e., more or less, heat will release when they are each decomposed into double radicals. There are the most heat releases for HA2+ and N2H62+, showing their lowest thermodynamic stability. Comprehensively, HA+ possesses the highest ability to partition H+, and also be protonated to decompose itself. This is a main reason for its prior thermal decay to BTO2- in TKX-50, and also for the low thermal stability of HA-based EISs shown in Table 1. N2H5+ follows HA+, showing the relatively low thermal stability too; while, for the remaining cations, the protonation occurs hardly, generally resulting in their high thermal stability. Therefore, by means of the proposed mechanism, we can well understand the thermal stability of many EISs, despite the usual complexity of thermal decomposition. 4 CONCLUSIONS In summary, we performed thermal decomposition experiments and AIMD simulations on a typical EIS of TKX-50, as well as AIMD simulations on other two EISs (G-BTO and G-DNABF), to reveal underlying mechanism of their decay. It is interesting to find that the protonation of HA+ proceeds as an initial step for thermal decay when heating TKX-50. This new finding should be of great significance, attributed to two factors. The first is that the protonation of HA+ is just a case of a bimolecular reaction that occurs rarely between two same sign charged ions, due to that the electrostatic repulsion between the same sign charged ions prevents them from approaching each other enough to induce a reaction. We have clarified this case as that the O atom of HA+ possesses a high ability to combine free H+ with slight steric hindrance, i.e., the O atom is negatively charged with two lone electron pairs. The second is that reaction pathways of NH3OH+ + H+→NH3OH22+→ NH3+ + H2O+→NH3 + H2O can well explain the experimental observation of TKX-50 heated at a


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constant temperature of 190 oC: the complete decay of HA is finished before that of tetrazole rings during heating TKX-50 to be decomposed. Furthermore, by means of this new finding, we can well understand the lower thermal stability of HA and N2H5+–based EISs (compared to NH4+ salts), attributed to the similar mechanism of protonation of cations. ■ ASSOCIATED CONTENT Supporting Information Raman peaks of 1,1'-BTO·2H2O and ABTOX, geometries and AIM analyses of the four EISs, and evolutions of key chemical fragments and potential energy for TKX-50 heated at 2500 K. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author C. Y. Zhang, email: [email protected]; Tel: 86-816-2493506. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS We greatly appreciate the financial support from the National Natural Science Foundation of China (U1530262, 21673210 and 11602241) and Scientific Challenge Project of China. ■ REFERENCES (1) Politzer. P.; Alper, H.E. in Computational Chemistry: Reviews of Current Trends; Leszczynski, J., Ed; World Scientific: River Edge, NJ, pp 271-286, 1999; and references therein. (2) Zeman S.in Politzer and Jane S.Murray (Eds.): Energetic Materials, Part 2, Elsevier B. V. Amsterdam: pp 25~52, 2003; and references therein. (3) Zhang, C.; Shu, Y.; Huang, Y.; Zhao, X.; Dong, H. Investigation of Correlation between Impact Sensitivities and Nitro Group Charges in Nitro Compounds. J. Phys. Chem. B, 2005, 109, 8978-8982. (4) Zhang, C.; Shu, Y.; Wang, X.; Zhao, X. A New Method to Evaluate the Stability of the Covalent Compound: By the Charges on the Common Atom or Group. J. Phys. Chem. A, 2005, 109, 6592-6596. (5) Zhang, C.; Wang, X.; Huang, H. π-stacked Interactions in Explosive Crystals: Buffers against External Mechanical Stimuli. J. Am. Chem. Soc. 2008, 130, 8359-8365. (6) Ma, Y.; Zhang, A.; Zhang, C.; Jiang, D.; Zhu, Y.; Zhang, C. Crystal Packing of Low Sensitive and High Energetic Explosives. Cryst. Growth Des., 2014, 14, 4703-4713. (7) Ma, Y.; Zhang, A.; Xue, X.; Jiang, D.; Zhu, Y.; Zhang, C. Crystal Packing of Impact Sensitive High Energetic 23 Environment ACS Paragon Plus


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Table of Contents Graphic Unusual Protonation of the Hydroxylammonium Cation Leading to the Low Thermal Stability of Hydroxylammonium-based Salts

Keywords: reaction between two same sign charged ions, protonation, energetic crystal, thermal stability, energetic ion salts, ab initio simulation.


Unusual Protonation of the Hydroxylammonium Cation Leading to the

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