The Decomposition Mechanism of Hexanitrohexaazaisowurtzitane

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

The Decomposition Mechanism of Hexanitrohexaazaisowurtzitane (CL-20) by Coupled Computational and Experimental Study Macharla Arun Kumar, Parimi Ashutosh, and Anuj A Vargeese J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01197 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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The Journal of Physical Chemistry

The Decomposition Mechanism Hexanitrohexaazaisowurtzitane (Cl-20) By Computational And Experimental Study

of Coupled

Macharla Arun Kumar‡, Parimi Ashutosh‡, Anuj A.Vargeese* Advanced Center of Research in High Energy Materials (ACRHEM), University of Hyderabad, Hyderabad 500 046, India Ph: +91-40-2313-8708; Fax: +91-40-23012800 * Email: [email protected] ‡ The two authors contribute equally to this work Abstract: A

novel

degradation

pathway

of

2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-

hexaazaisowurtzitane (CL-20) was identified using computational and experimental methods. Density functional theory (DFT) calculations were employed to obtain its uni-molecular degradation pathway and experimental, Ultra high Performance Liquid Chromatographycoupled-High Resolution Mass spectrometry (UPLC-HRMS), Thermogravimetry-coupledFourier Transform Infrared spectrometry (TG-FTIR), Thermogravimetry (TG), and Differential Scanning Calorimetric (DSC), data was used to validate the computationally deduced degradation pathways. Based on the indications from computational and experimental results, the cleavage of the strained fragment from CL-20 was identified instead of NO2 or HONO elimination as in conventional high energy materials. This fragmentation results in the formation of two energetic species, dinitro dihydropyrazine and dinitroformimidamide, which makes CL20 one of the most powerful energetic material. This novel degradation pathway of CL-20 will be useful in understanding the decomposition of cage molecules, design of new practical energetic molecules and development/improvement of thermokinetic codes used for energetic property calculations.

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Introduction High-energy-density materials, such as polynitropolyaza-caged compounds, are challenging materials for synthetic chemists and engineers because of their highly strained bond angles. Nonetheless, they are widely used in propellants and explosives. First synthesized by Nielsen in 1987,1 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), commonly known as CL-20, is one of the most powerful yet stable energetic materials developed till date.2,3 CL-20 has been of interest to researchers in the civil, defence, and space industries as an energetic ingredient preferable to conventional energetic materials.4 Its cage structure allows strain energy to be trapped in the molecule, while the high oxygen balance gives it a high energy density and consequently a high detonation velocity.5 Apart from the strain energy, the presence of six nitro groups in CL-20 provides an excellent oxidizer to fuel ratio that allows the hydrocarbon backbone to be oxidized, as in conventional explosives.6 The combination of the strain energy and the energy released by oxidation leads to a superior detonation velocity (VoD) and enthalpy of formation (∆fH). In 4,10-Dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0. 05,9.03,11]dodecane (TEX) electron densities are associated with oxygen atoms in the five membered ring (Figure S1). Whereas in the CL-20, the electron density is higher inside the cage (Figure 1), which induces the molecule to degrade in a unique manner. Although some reports are available,7,8,9 CL-20 is a relatively new molecule; therefore, its decomposition behaviour and degradation pathways are yet to be studied in detail. The existing reports provide some perspective, but none provide a comparison of the theoretical pathway to the experimental findings.7,8,9,10 Developing an


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understanding of this complex chemical process, and estimating the influence of various factors on the decomposition pathway, is scientifically and technologically demanding.10,11

Figure 1. Optimized structure and Molecular Electro Static Potential (MESP) diagram of CL–20, the outer blue colour appearance indicates the lower electron density outside the cage The chemical decomposition pathway, kinetic parameters, and thermodynamic properties are the key factors in understanding the ignition and combustion process as well as the product distribution during explosions.12 Numerous methods have been used to study and understand the decomposition pathways of molecules and compounds; these include LASER-induced breakdown spectroscopy (LIBS),13 T-Jump/time-of-flight mass spectrometry, mass spectrometry (MS),14 T-Jump Fourier transform infrared (FTIR) spectroscopy, thermogravimetric-FTIR (TGFTIR), thermogravimetric-MS (TG-MS), and computational methods.7,8,10,15 However, several issues remain uncharted in the time domain. A recent ReaxFF-lg reactive molecular dynamics study of CL-20 showed the formation of NO2 by N-NO2 bond cleavage as the initial step and N2, H2O, CO2, and H2 as the final decomposition products.10 Considering the difficulty of elucidating the condensed phase decomposition pathway, initially we used DFT calculations to deduce the gas-phase uni-molecular degradation pathway



of CL-20. We then used experimental data from ultra-high-performance liquid chromatographycoupled high-resolution mass spectrometry (UPLC-HRMS), TG-FTIR, thermogravimetry (TG), and differential scanning calorimetry (DSC) to validate the computational results. Earlier, using similar methodologies, we have carried out kinetic11,16,17 and computational studies12,18 to understand the decomposition mechanism in HEMs and mass spectrometry to detect the intermediates.19

Experimental and Computational Methods Materials: The CL-20 is an energetic material, and the sample required for the study was kindly provided by High Energy Materials Research Laboratory (HEMRL), Pune. The sample was stored by dispersing in deionized water and recrystallized before subjecting it to the experimental studies. For the UPLC-MS analysis, LC-MS-grade acetonitrile, ammonium acetate and water were purchased from Sigma-Aldrich Pvt. Ltd. Caution!!!CL-20 is a moderately sensitive energetic compound and is dangerous. Although we have encountered no difficulties during handling, CL-20 should be handled with extreme care and appropriate standard safety precautions. Mechanical actions involving scratching or scraping of CL-20 must be avoided. Computational Methodology: The calculations are done in gas phase at room temperature (298.15oK) using Gaussian 09 program.20 The density functional theory method B3LYP 6311++** is used for optimization of the structures. Symmetrical structures are ignored while performing calculations. The bond cleavage is assumed to be homolytic in nature. The optimized minima structures are found to be with no imaginary frequencies. The bond dissociation energies (BDEs) are calculated as mentioned by Bach et al.19,21 The detonation properties of CL-20 is


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obtained from EXPLO5 (V6.03).22 The total energies of the structures, the BDEs for the respective paths, and the detonation performance parameters and their respective detonation products calculated by EXPLO5 program are given in supplementary information. Although we have checked all possible structures for calculation, only least energy structures are mentioned in this work to avoid complexity. Thermogravimetric analysis (TGA): Thermogravimetric (TG) analysis of CL-20 was carried out under a flowing nitrogen atmosphere in a TA instruments SDT Q600 TG/DSC instrument. In all experiments, 1-1.5 mg of sample was loaded in an open 90 µL alumina pan and heated. Nitrogen at a flow rate of 100 mL min-1 was used as the purge gas. UPLC-HRMS analysis: For the UPLC analysis of CL-20, the sample was heated in TG-DTA instrument up to 220°C (CL-20-220), 225°C (CL-20-225), 230°C (CL-20-230), or 235°C (CL20-235) at 3°C/min heating rate and quenched. Further, the quenched sample was analyzed on a Waters ACQUITY H-Class UPLC (Waters Corporation). All the samples were dissolved in LCMS grade acetonitrile:water (1:1) solvent and made up in Waters certified LC-MS 2 mL amber glass vials. A reverse phase HSS T3 C18 column (2.1 × 100 mm, 1.8 µm particle size) maintained at 35 ºC was used for chromatography. The mobile phases consisted of A (water in 0.1% ammonium acetate) and B (acetonitrile). A gradient elution was done at a flow rate of 0.35 mL min-1 with an injection volume of 1 µL. The gradient was as follows: 30 to 40% B (0-1 min), 40 to 80% B (1-6 min), 80% B (6-8 min), 80 to 30% B (8-8.5 min), and 30% B (8.5-12 min). The in situ mass acquisition was performed on a Waters Xevo G2-XS QTOF (Waters Corporation) using electrospray ionization (ESI) in negative mode. Data acquisition was controlled with UNIFI 1.8 informatics platform (Waters Corporation). The data range was from 50-1000 Da. Leucine enkephalin (200 pg/µL) was used as the lock mass (m/z 554.2615). The



source temperature applied was 120oC and the desolvation gas flow was at 1000 L/h with a desolvation temperature of 450°C. The capillary voltage was at 2.5 kV and the cone voltage was 30V. All the tandem mass spectrometry (MS/MS) was recorded using argon as the collision gas with collision energy of 20 V [20]. Evolved gas analysis-Simultaneous TG-FTIR: The ex situ TG-FTIR experiments were carried out using a TA instruments SDT Q600 TG/DSC coupled to a TG-FTIR cell placed in the Bruker Tensor II FTIR Spectrometer. Experiments were conducted in nitrogen at a purge rate of 100 mL min-1. Approximately 2 mg of sample was loaded in an open 90 µL alumina pan and heated. The transfer line temperature was maintained at 150°C and IR cell temperature was maintained at 180°C. FTIR data collection was manually triggered at the pre-determined temperature and the IR data was collected at 40 seconds interval with a wavenumber resolution of 1cm-1. DSC analysis: DSC analysis was carried out on a Perkin Elmer DSC8000 instrument. About 1 mg sample was loaded into a closed aluminum pan and heated at 3°C/min under nitrogen flow maintained at 40 mL/min.

Results and Discussion Computational Studies In molecules, including CL-20, the weakest bonds, such as N-NO2 and C-NO2, are prone to dissociate when subjected to an energy supply. However, as mentioned elsewhere in the literature, at higher temperatures HONO elimination is preferred over NO2 elimination.23 HONO formation is attributed to heavy vibration of the C-H bond, which allows NO2 to capture the proton as it leaves, due to the hydrogen-oxygen interaction. In addition, the molecular electrostatic potential (MESP) of CL-20 (Figure 1) shows the entrapment of electron density


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inside the cage. Accordingly, we examined three possibilities: 1) NO2 elimination, 2) HONO elimination, and 3) strained fragment elimination, and performed DFT (Density Functional Theory) calculations. The structures of the intermediate species and the decomposition paths are given in Scheme 1. The total energies of the optimized structures are given in Table S1. The BDEs are provided in Table S2. For NO2 elimination path, the weakest bonds, N-NO2, are targeted and dissociated from their possible locations, i.e., from the six and five-membered rings. The calculated BDEs suggest that it is easier for NO2 to be removed from the five-membered ring than the six-membered ring. For dissociation from the five-membered ring, the energy required is 132.84 kJ·mol-1, while dissociation from the six-membered ring requires 152.79 kJ·mol-1. In addition, the total energy data also suggests that removing the nitro group from the five-membered ring is more feasible. Therefore, we presume that dissociation from the five-membered ring is favoured. This leads to the formation of A1, and from A1, another NO2 elimination is prompted independently from the five and six-membered rings. The calculations were done ignoring symmetrical structures; the total energy data suggests that, if the nitro group is eliminated from the position diagonal to the previous elimination, then the total energy of the structure is minimum compared to other possibilities. Thus, A1 settles as A2; this process requires a BDE of 167.24 kJ·mol-1. This particular elimination allows the nitrogen and carbon atoms to gain double bond character, cleaving the C-C bridge. From A2, the path can be bifurcated: one where degradation continues from the six-membered ring, and one from the five- membered ring. If the degradation is from the six-membered ring, then it follows the minimal total energy criteria and settles as A3; A3 then loses another nitro group, and settles as A4, from A4 it settles to A5, and finally, after losing all of the nitro groups, it settles to the final structure A6. Eventually, A6 could decompose to N2,



HCN, and carbon. The BDE data suggest that this particular path contributes less to the energy release; hence, it is unlikely that a molecule such as CL-20 would follow it. On the other pathway from A2, the degradation progresses by N-NO2 cleavage at the position of the fivemembered ring and settles as A7. Further cleavage allows A7 to settle as A8. The nitrogen atoms on opposite sides come close to each other (Figure S2), which leads to the subsequent degradation, where A8 breaks to form F1, HCN, and N2. This process is believed to contribute to the energy released from CL-20, as F1 is relatively an energetic species. In molecules such as CL-20, where the cage is associated with electron density gradients and which have capability to form hydrogen bonds, HONO elimination is a reasonable possibility and we performed DFT calculations accordingly. The computational calculations and the total energy values show that HONO elimination is more plausible from the five-membered ring than the six-membered ring. In CL-20, through the HONO elimination, the carbon atom and the neighboring nitrogen atom gain double bond character, and settles as B1. Subsequent HONO elimination allows B1 to settle as B2, in which the other bridge carbon also gains double bond character. From B2, another HONO elimination is unlikely as the distance between the hydrogen and the oxygen atoms is too great. Thus, the N-NO2 bond is cleaved and the structure settles as B3. From B3, another N-NO2 cleavage allows the structure to settle as B4. In B4, the nitrogen atoms come closer as they try to retain the single bond characteristics (Figure S2). From B4, we presume that there are two possible routes, one in which the elimination of the nitro groups continues to form B5, and from B5 (Figure S2) the final nitro group cleaves and settles as B6; and one in which the top strained fragment cleaves from the six-membered ring to settle as B7 and F1. If path B4 through B6 is considered, then, as the BDE suggests, 289.59 kJ·mol-1 of energy will be released during the conversion from B5 to B6. On the other hand, for the path B4


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through B7, the energetic F1 formation caters to energy release in the later stages of decomposition. While the other two paths focus on the weakest bonds and their cleavage, this path is solely focused on the actual possibilities for CL-20. As the MESP of CL-20 suggests, the electron density is higher inside the cage, indicating inherent gradient and interactions. When subjected to degradation, either by thermal or other means, this could cause the C-N cleavage, which would eventually cleave the top strained fragment from CL-20. This cleavage produces two daughter intermediates, C1 and F1, and both are energetic species that would contribute to the overall energy release. However, C1 is highly unstable so it would immediately break to form C2, another energetic species that would contribute to the energy release in the later stages of decomposition.



O2 N

NO2 elimination

O 2N N

A1

N O 2N

NO2 N N NO2

NO 2 N

N

N

N

N

NO 2

CL-20 HONO elimination

N

O2 N

N

N

NO2

N O2 N

A2

NO 2 N

N

N

N

N

NO 2 O2N N

A3

N O 2N

A4

O2 N

N

N N NO2

N

N

N

N

A5

N

N N

N

N N

N N

N

NO2 N N

NO 2

N

N N

A7

NO2

N N

NO2 N

N N

N

N

A8*

N

NO2

N

N

N

F1

NO2 NO2

N

N

N

N

N

N

N

N

O 2N

N + N

NO2

NO2

C1

F1 NO2

B2

NO2

2O N 2

N

N

N NO2 + N NO2

N

F1

C2

N

B3

NO2 N

N

N

N

N

NO2

NO2

NO2

N

N

B1

N

N

N

N

NO2

N

N

B4*

NO2 N

NO2

+ N2 +2HCN

N

N

O 2N

N

NC N

N

N

N

N

NO2 N

N

N

N

N

N

N

NO2

NO2 NO2

N

Fragment elimination

NO2

NO2 NO2

N

NO2

NO2

A6

N

NO 2

NO2 N

O2N

NO2 N

NO2

N

O2 N

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CN N N

B5*

B7

+ N NO2

F1 B6

Scheme 1. Unimolecular degradation pathways of CL-20 (* The optimized 3D structures of A8, B4, and B5 were given in Figure S2) UPLC-HRMS Studies To understand the decomposition stages TG measurements of CL-20 were carried out up to 220, 225, 230, and 235 °C (Supplementary material, Figure S3) and then quenched. The samples remained in the crucible was collected and analyzed using UPLC-HRMS. UPLC-HRMS data of CL-20, CL-20-220, CL-20-225, CL-20-230, and CL-20-235 are shown in Table 1, and


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Figure S4-S10. The LC data for CL-20 showed a single peak at a retention time of 4.34 minutes with >99 % purity (Figure S4). CL-20 is an electronegative molecule that ionizes most easily in the negative polarity ionization mode; hence, the data for all of the samples were collected in the ESI negative mode. Figure S5 show the negative mode ESI-MS spectrum of the chromatographic peak at 4.34 minutes. The results show the ions equivalent to [M-H]- at m/z 437.0155 and adduct ions at m/z 472.9918, 474.9892, 500.0106, 911.0038, 913.0.35, and 938.0393 that correspond to [M+35Cl]-, [M+37Cl]- M+NO3]-, [2M+35Cl]-, [2M+37Cl]-, and [2M+NO3]-, respectively. UPLC-HRMS data for CL-20-220 was similar to that of CL-20 (Figure S6). Two separate peaks with different retention times were identified in the chromatograms of CL-20-225 and CL20-230 at 4.22 (0.32 %) and 4.34 min (99.68 %) (Figures S7, S9). The mass spectrum of the peak eluting at 4.22 min contained species at m/z 456.9957, 458.9903, 484.0142, 879.0266, 881.0106, and 906.0328, corresponding to [(M-O)+35Cl]-, [(M-O)+37Cl]-, [(M-O)+NO3]-, [(2M-O2)+35Cl]-, [(2M-O2)+37Cl]-, and [(2M-O2)+NO3]-, respectively (Figure S8). The second peak, eluting at 4.34 min, was identified as CL-20. No peak was observed in the UPLC-MS data for CL-20-235 (Figure S10), indicating the complete dissociation or breakdown of the compound into smaller species below detectable limits. MS/MS Analysis MS/MS analysis of the CL-20 adduct ion at m/z 500.01 was conducted in order to determine the fragmentation routes that lead to the formation of various product ions (Figure S11). The spectra showed major product ions at m/z 251.0270, 205.0352, 171.0143, 159.0406, 132.9971, 105.9852, 88.9928, and 61.9802 (Table 2). Using the data, we identified two major fragmentation routes. A tentative breakdown pattern for this major series of diagnostic product ions is presented in Scheme 2. CL-20 eliminates 2HNO2, 2NO2, and H to afford the product ion



[B4-H]- at m/z 251.0270. The [B4-H]- ion eliminates another NO2 to produce the [B5-H]- at m/z 205.0352. Similarly [B5-H]- ion can also lose NO2 to produce the [B6-H]- at m/z 159.0406. The results also revealed C-N cleavage, which produces [F1-H]- at m/z 171.0143 as well as a highabundance of [C2]- at m/z 132.9971. The product ion [C2]- eliminates HCN to produce the dinitramide ion at m/z 105.9852. Table 1. The major ion species found in the analysis of CL-20, CL-20-220, CL-20-225, CL-20230, and CL-20-235 Compound Retentio Observed Tentative peak ID Elemental Calculated n time mass (m/z) (M = CL-20) composition mass (Da) (min) CL-20 4.34 437.0155 [M-H]C6H5N12O12 437.0149 M 438.0228 C6H6N12O12 472.9918 [M+35Cl]472.9916 C6H6N12O1235Cl 37 37 474.9892 [M+ Cl] 474.9887 C6H6N12O12 Cl 500.0106 [M+NO3]500.0106 C6H6N13O15 35 35 911.0038 [2M+ Cl] 911.0145 C12H12N24O24 Cl 913.0135 [2M+37Cl]913.0116 C12H12N24O2437Cl 938.0393 [2M+NO3]938.0334 C12H12N25O27 CL-20-220 4.33 Same as CL-20 35 CL-20-225 4.22 456.9957 [(M-O)+ Cl]C6H6N12O1135Cl 456.9967 37 458.9903 [(M-O)+ Cl] C6H6N12O1137Cl 458.9938 484.0142 [(M-O)+NO3]484.0157 C6H6N13O14 35 35 879.0266 [(2M-O2)+ Cl] 879.0247 C12H12N24O22 Cl 881.0106 [(2M-O2)+37Cl]881.0218 C12H12N24O2237Cl [(2M-O2)+NO3] 906.0328 906.0437 C12H12N25O25 4.34 Same as CL-20 4.23 Same as CL-20-225 CL-20-230 4.34 Same as CL-20 CL-20-235 No data obtained Table 2. The major fragmented ion species found in the MS/MS analysis of CL-20 adduct ion at m/z 500.01 Observed mass Elemental composition Calculated mass (m/z) (Da) 500.0108 C6H6N13O15 [M+NO3] 500.0106 251.0270 C6H3N8O4 [B4-H]251.0277 205.0352 C6H3N7O2 [B5-H]205.0348 171.0143 C4H3N4O4 [F1-H] 171.0154 159.0406 C6H3N6 [B6-H]159.0419 132.9971 CHN4O4 [C2] 132.9998 105.9852 [N3O4]105.9889


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CHN2O3 [HCN+NO3][NO3]-

88.9928 61.9802

88.9987 61.9878

NO2

NO2

O2N N N O2N

N N NO2

NO2

-2HNO2 -2NO2 -H

N N NO2

m/z 500.0106 [M+NO3]-

N

N

N

N

N

N

-NO2

NO2

N

N

N

N

N

N

-NO2

NO2

m/z 205.0352 [B5-H]-

m/z 251.0270 [B4-H]-

N

N

N

N

N

N

m/z 159.0406 [B6-H]-

NO2 N

2 ON N 2

N

NO2

m/z 132.9971 [C2]-

N NO2

N

m/z 171.0145 2 O2N NO2 m/z 105.9852 [F1-H]-

2 HCN m/z 88.9928 [HCN+NO3]-

Scheme 2. Fragmentation pathways observed for the ESI-MS/MS of the CL-20 adduct ion at m/z 500.01 TG-FTIR Studies TG-FTIR technology was applied to analyse the gaseous decomposition products. The differential thermogravimetry (DTG) data confirmed a two-stage decomposition of CL-20 (Figure 2). The FTIR spectra of the gas phase decomposition products of CL-20 are shown in Figure 3 and Figure S12. This analysis confirmed the presence of NO2 (1630 and 2902 cm-1), N2O (2230 and 1289 cm-1), NO/CO (1956–1800 cm-1), H2O (3990–3381 and 2115–1258 cm-1), and HCN (3340 and 669 cm-1) during the first and second stages of CL-20 decomposition. During the initial stages of decomposition, the main products observed were NO2 and CO2; as the reaction rate increased, NO2 became the predominant species, as indicated by the increasing absorbance value of NO2. Subsequently, NO2 remained a major component throughout the decomposition phenomena. Over the time, the concentration of other species, such as N2O, NO, H2O, and HCN, became detectable. Hence, the decomposition of CL-20 releases N2O, CO2, and very small traces of NO, H2O, and HCN. The later stages of CL-20 decomposition (238–254 °C) show only CO2 (Figure S13), indicating that the left-over carbon being oxidized by the traces of oxygen.



Figure 2. TG-DTA-DTG curve of thermal decomposition of CL-20 at heating rate of 3° C min-1 0.912

0.931

0.950

0.969

0.988

(oC )

254 250 246 242 238 234 230 226 222 218 214 210 206 202

Te m pe ra tu re

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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4000

3500

3000

2500

2000 -1

1500

1000

Wavenumber (cm )

Figure 3. The temperature/time resolved TG-FTIR spectra of the CL-20 thermal decomposition products

Figure 4. DSC curve of thermal decomposition of CL-20 at heating rate of 3° C min-1 Thermokinetic Calculations The detonation performance parameters of CL-20 were calculated (Table S3) using the thermokinetic code EXPLO5 V6.03, which predicted CO2, HCOOH, H2O, CO and traces of other possible products (Figure S14). Another interesting finding in our experimental study was the presence of HCN and absence of HCOOH during the decomposition. In fact, this finding


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contributes to the improvement in the values predicted by the thermokinetic codes used for energetic property calculations. Thermal Analysis of CL-20 In order to compare the elucidated pathway with the composite decomposition behavior, the TG-DTA and DSC data of CL-20 decomposition were analyzed. The thermal properties of CL-20 were obtained using TG-DTA and DSC between 200 and 250 °C, as shown in Figure 2. Two-stage decomposition was observed from the thermal analysis, and a corresponding major exothermic decomposition peak was observed at 238 °C in the DTA curve and at 240 °C in the DSC curve. However, further examination of the DSC curve (Figure 4) suggested two-stage decomposition with two convoluted or overlapping peaks at 231 and 240 °C. The results suggest that the majority of the initial mass loss is due to the loss of the nitro groups and the formation of recombination products, leading to the formation of CO2, HCN, and CO. In the later stages, the remaining product is mainly carbon, which is oxidized to yield CO2.

Combined Analysis of Computational and Experimental Results The combined analysis of the computational, UPLC-MS, TG-FTIR, TG-DTA and DSC revealed several interesting results. The presence of a high intensity m/z peak at 132.9971 [C2](Figure S11) in the MS/MS data confirms that degradation occurs through a strained fragment elimination path. Some of the intermediate species from the HONO elimination path were also observed in the experimental MS/MS data at m/z 251.0270 [B4-H]-, m/z 205.0352 [B5-H]-, and m/z 159.0406 [B6-H]-, but at relatively low intensities. The high energy release during the second stage of the decomposition of CL-20 is possibly due to the formation of the intermediate species C2 and subsequent high energy density species such as dinitramide. All of the



computationally obtained paths lead to the formation of F1; however, the combined UPLCHRMS and computational data make it evident that the strained fragment elimination path is the most probable path. Since the DFT calculations are carried out for obtaining the uni-molecular degradation pathway and HRMS identifies the molecular fragments (ie., m/z), many of the molecular intermediates predicted by DFT calculations found to agree with the fragments confirmed by HRMS. The NO2 elimination path and HONO elimination path seem energetically favorable in the primary stages, however, the total energy input required for degradation is relatively large, which is not the case for the strained fragment elimination path. In addition, the formation of two extremely unstable and energetic species in the strained fragment elimination path would make this path more exothermic than the other two paths.

Conclusions DFT calculations and experimental techniques were adopted to investigate the degradation pathway of CL-20. Among the possible degradation pathways considered, the strained fragment elimination was identified as the most favorable pathway. CL-20 complexes with Cl- or NO3- and is detected in the ESI negative mode in MS analysis. The major ion in the spectrum was found at m/z 500.0106, corresponding to [M+NO3]-. The MS/MS analysis of m/z 500.01 showed a major product ion [C2]- at m/z 132.9971 and other product ions at m/z 171.0143, 105.9852, 88.9928, 61.9878, 251.0277, 205.0352, and 159.0406 corresponding to [F1H]-, [N3O4]-, [HCN+NO3]-, [NO3]-, [B4-H]-, [B5-H]-, and [B6-H]-. From the computational and experimental studies, we confirm that the condensed phase degradation of CL-20 leads to the formation of F1, C2, and dinitramide, while a small percentage leads to the formation of B4, B5, and B6. The elucidated mechanism indicates a non-conventional pathway for the degradation of


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energetic caged molecules. The high electron density trapped inside the cage induces the electron density gradient and prompts the removal of fragment C1 from CL-20, which immediately breaks down to form C2. The findings indicate a unique degradation mechanism through fragmentation; a behavior unique to the strained cage molecule, CL-20. The thrust of our work is to obtain a better understanding of chemical processes and reaction mechanisms that occur during decomposition and use this understanding to draw better correlation with the combustion behavior of isowurtzitane cage molecules.

Supporting Information Total energies of intermediates, BDEs for the required paths, 3D optimized structures of A8, B4, and B5, TG-DTA of CL-20 stopped at 220oC, 225oC, 230oC, and 235oC, UPLC-HRMS analysis of CL-20, TG-FTIR analysis of CL-20, and EXPLO5 data of CL-20.

Acknowledgements The research was funded by Defence Research and Development Organization (India) in the form of grand in aid to Advanced Centre of Research in High Energy Materials (ACRHEM), ERIP/ER/1501138/M/01/319/D(R&D).

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m/z 500.0108

X

NO2 Elimination

X

HONO Elimination

✔

Fragment Elimination

+ m/z 171.0145

m/z 132.9971

Do strained cage molecules follow conventional degradation paths?


The Decomposition Mechanism of Hexanitrohexaazaisowurtzitane

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