Structure, Physicochemical Properties, and Density Functional Theory

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Structure, Physicochemical Property and DFT Calculation of High-Energy Density Materials Constructed with Intermolecular Interaction: Nitro Group Charge Determines Sensitivity Xiangyu Liu, Zhiyong Su, Wenxin Ji, Sanping Chen, Qing Wei, Gang Xie, Xuwu Yang, and Sheng-Li Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5062418 • Publication Date (Web): 25 Sep 2014 Downloaded from http://pubs.acs.org on September 26, 2014

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Structure, Physicochemical Property and DFT Calculation of High-Energy Density Materials Constructed with Intermolecular Interaction: Nitro Group Charge Determines Sensitivity

Xiangyu Liu,[a, b] + Zhiyong Su,[a] + Wenxin Ji,[b] Sanping Chen* [a], Qing Wei, [a] Gang Xie,[a] Xuwu Yang, [a] and Shengli Gao [a]

[a] Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China [b] School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China + These authors contributed equally to this work.

*

Corresponding authors:

Prof. Sanping Chen Tel.: +86-029-88302604 Fax: +86-029-88302604 E-mail: [email protected]


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Abstract: Four nitro-containing energetic compounds, cocrystal of AT·DNBA (1), salt of MA·DNSA (2), salt of AG·DNBA·H2O (3) and salt of DAT·DNSA H2O (4), are synthesized and structurally characterized based on supramolecular interactions. The physicochemical properties of the compounds are theoretically and experimentally investigated in detail. The optimized structures, molecular total energies, frontier orbit energies and charge densities of 1-4 are calculated by theoretical method. The experimental results indicate that all compounds perform good thermostability and low sensitivity. Noteworthily, the values of impact sensitivity are measured to be 30, >40, 38, and >40 J, respectively, which are well corresponding to the order of nitro group charge (QNitro) calculated by Density Functional Theory (DFT). Detonation performances of 1-4 are discussed, especially, 1 and 2 exhibit superior heats of detonation (2.191 kcal·g-1 for 1, 2.214 kcal·g-1 for 2) than classical nitro-rich compounds. In addition, the non-isothermal thermokinetic parameters are obtained by Kissinger’s and Ozawa’s methods and the standard molar enthalpies of formation are calculated from the determination of constant-volume combustion energies as well.



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1. INTRODUCTION High-energy density materials (HEDMs) have attracted considerable interest in the last decades due to their potential application in various propellants, pyrotechnics and explosives.1-6 It is generally known that coordinating the relationship between high energy and sensitivity of HEDMs is the ultimate goal in the field of energetic materials.7,8 A large set of materials with new synthetic methods have emerged recently in order to meet the challengeable requirements and improve the performance of energetic materials.9-15 The pioneering scientists T. M. Klapötke16-22 and J. M. Shreeve, etc.23-35 have prepared lots of multi-component energetic compounds with excellent properties based on intermolecular interactions, respectively, such as cocrystal and salts consisted of nitrogen-rich heterocyclic or nitro-containing compounds. Recently, we have reported on the synthesis and characterization of energetic salts exhibiting high nitrogen content, favorable thermal stability and low sensitivity.36 Undoubtedly, it is an effective strategy to employ the intermolecular interactions for the construction of HEDMs, which will counteract the contradiction between detonation performance and high sensitivity in the solid state.37-40 Although abundant experiments have been performed until now,41-44 it is worth being paid more attentions to explore the structure-function relationship of energetic compounds. It is definite that energetic burst is bound up with the active group in the energetic compounds, such as the explosive properties of nitro-rich compounds [TNT (2,4,6-Trinitrotoluene), RDX (1,3,5-trinitroperhydro-1,3,5-triazine), FOX-7 (1,1-diamino-2,2-dinitroethene), etc.] are mainly dominated by nitro group (-NO2).45-51 It is reported that the nitro group is more active than other energetic groups (amidogen, nitrogen heterocyclic, etc.) due to the integrated properties including rapider release of energy, higher bond dissociation energy, greater density and more appropriate oxygen balance and nitrogen content.52-54 Based on DFT calculation, C. Y. Zhang55 and J. M. Shreeve56 have predicted that the nitro group charge (QNitro) may be closely related to the explosive trigger for covalence-typed energetic compounds. Actually, using theoretical calculation to predict and guide experimental research has long been a pursued target for scientists. What is more important, it is an advanced work to conduct research on the structure-function relationship of energetic materials based on the combination of theory and experiment, which is conducive to reveal the essential reason that effect the sensitivity of high-energy materials.


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In view of the considerations above, four new nitro-containing energetic compounds, cocrystal of AT·DNBA (1), salt of MA·DNSA (2), salt of AG·DNBA·H2O (3) and salt of DAT·DNSA H2O (4),

are

synthesized

and

structurally

characterized

in

the

present

work

(AT

=

4-Amino-1,2,4-triazole, DNBA = 3, 5-Dinitrobenzoic acid, MA = Melamine, DNSA = 3, 5-Dinitrosalicylic acid, AG = Amino guanidine, DAT = 3,5-Diamino-1,2,4-triazole). Compounds 1-4 exhibit stabilized structures due to the existence of abundance hydrogen bonding interactions, and 2 also possesses the π-π stacking interaction. Theoretical calculation is employed to study the optimized structures, molecular total energies, frontier orbit energies and charge densities of the compounds. The calculated detonation properties indicate that four compounds have the potentials of explosive. The nitro group has been proved as the active group in the multi-component energetic compounds because of the measured values of impact sensitivity are well corresponding to the values of QNitro obtained by DFT calculation. In addition, thermogravimetric analysis, thermokinetics and standard molar enthalpies of formation are investigated as well.

2. EXPERIMENTAL SECTION General caution: the compounds are energetic materials and tend to explode under certain conditions. Appropriate safety precautions (safety glasses, face shields, leather coat and ear plugs) should be taken, especially when the compounds are prepared on a large scale. Materials and instruments Chemical reagents and solvents were purchased commercially and used as received without further purification. Elemental analyses were performed on a Vario EL III analyzer fully automated trace element analyzer. FT-IR spectra were recorded on a Bruker FTIR instrument as KBr pellets. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out on a Netzsch STA 449C instrument and CDR-4P thermal analyzer of Shanghai Balance Instrument factory, respectively, using dry oxygen-free nitrogen as the atmosphere, with a flowing rate of 10 mL min-1. About 0.5 mg sample was sealed in aluminum pans in the temperature range of 25-500 oC for DSC experiments. The sensitivity to impact stimuli was determined by fall hammer apparatus applying standard staircase method using a 2 kg drop weight and the results were reported in terms of height for 50% probability of explosion (h50%).57 The friction sensitivity was determined on a



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Julius Peter’s apparatus by following the BAM method.58 The phase purity of the bulk samples were verified by X-ray powder diffraction (XRPD) measurements performed on a Rigaku RU200 diffractometer at 60 kV, 300 mA and Cu Kα radiation (λ = 1.5406 Å), with a scan speed of 5° min-1 and a step size of 0.02° in 2θ. The calculations were carried out using the Gaussian 03 suite of programs.59 The constant-volume combustion energies of the compounds were determined with a precise rotating-bomb calorimeter (RBC-type II).60 Synthesis Synthesis of 1: AT (42 mg, 0.5 mmol) was dissolved in ethanol (10 mL). After stirring for 10 min, a solution of DNBA (106 mg, 0.5 mmol in 10 mL ethanol) was added dropwise into it. The mixture was stirred at 60 oC for 2 h. The filtrate was allowed to slowly concentrate by evaporation at room temperature. After about two weeks, colorless block crystals suitable for X-ray diffraction were separated by filtration. Yield: 82% on the basis of DNBA. Anal. Calcd. For C9H8N6O6 (296.21): C, 36.46; H, 2.70; N, 28.36. Found: C, 36.43; H, 2.64; N, 28.34. IR (KBr, cm-1): 3449m, 3321m, 3235m, 3141s, 3090s, 2885m, 1877w, 1739m, 1620m, 1535s, 1466m, 1355s, 1064m, 962s, 877s, 748m, 689m, 629w, 484w. Synthesis of 2: It was synthesized similarly to 1 by using MA instead of AT, DNSA instead of DNBA, colorless block crystals suitable for X-ray diffraction were separated by filtration. Yield: 80% on the basis of MA. Anal. Calcd. For C10H10N8O7 (354.26): C, 33.87; H, 2.82; N, 31.62. Found: C, 33.85; H, 2.79; N, 31.60. IR (KBr, cm-1): 3501s, 3398s, 3221m, 2851w, 1860m, 1629w, 1518m, 1433m, 1339m, 1270m, 1185m, 1065m, 971m, 902s, 792s, 732s, 629m, 569s, 518w, 441m. Synthesis of 3: It was synthesized similarly to 1 by using AG instead of AT, and H2O instead of ethanol, colorless block crystals of 3 were obtained. Yield: 86% on the basis of DNBA. Anal. Calcd. For C8H12N6O7 (304.24): C, 31.55; H, 3.94; N, 27.61. Found: C, 31.53; H, 3.91; N, 27.58. IR (KBr, cm-1): 3455m, 3104w, 2882w, 2753w, 1684m, 1620w, 1529m, 1464m, 1346s, 1073m, 917s, 799m, 722s, 514s. Synthesis of 4: It was synthesized similarly to 1 by using DAT instead of AT, DNSA instead of DNBA and H2O instead of ethanol, colorless flake crystals suitable for X-ray diffraction were separated by filtration. Yield: 82% on the basis of DNSA. Anal. Calcd. For C9H11N7O8 (345.25):


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C, 31.28; H, 3.19; N, 28.39. Found: C, 31.26; H, 3.15; N, 28.36. IR (KBr, cm-1): 3457m, 3376m, 3189w, 2791w, 2674m, 1680w, 1458m, 1353m, 1271m, 1165s, 1002m, 920m, 838m, 686s, 557s. X-ray structure determinations For 1-4, selected single crystals were performed on a Bruker Smart Apex CCD diffractometer equipped with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) using ω and φ scan modes. The single-crystal structures were solved by direct methods using SHELXS-9761 and refined by means of full-matrix least-squares procedures on F2 with SHELXL-9762 program. All non-H atoms were located using subsequent Fourier-difference methods and refined anisotropically. In all cases hydrogen atoms were placed in calculated positions and thereafter allowed to ride on their parent atoms. Other details of crystal data, data collection parameters and refinement statistics were given in Table 1. Selected bond lengths and bond angles of compounds 1-4 were listed in Table S1 (Supporting information). Hydrogen bonding parameters were listed in Table S2 (Supporting information).

Table 1.

3. RESULTS AND DISCUSSION Crystal structure of 1: The asymmetric unit in 1 (crystallized in the monoclinic space group C2/c) contains one AT and one DNBA molecules (Figure 1a). The distances of the two C–O bonds for the neutral DNBA molecule are different obviously, the length of C–OH is 1.289(2) Å, while the length of C=O is 1.228 (2) Å. The exocyclic bond length C4–C7 is elongated to 1.519(2) Å, longer than the cyclic C–C bonds (average1.388 Å). The dihedral angle between the AT and the DNBA plane is 84.96°. The periodic repeated unit of 1 is constituted of two AT molecules and two DNBA molecules through three kinds of hydrogen bonds [N4–H11···O5 3.151(2), O6–H6···N4 3.422(2), O6–H6···N3 3.432(2) Å] (Figure 1b). Further, the C–H···O (3.0667 Å) interactions between the repeated units lead to the formation of the 1D structure (Figure 1c). Abundant intermolecular hydrogen-bond interactions are the primary drive forces for the formation of AT·DNBA cocrystal.

Figure 1.



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Crystal structure of 2: Compound 2 crystallizes in the triclinic system C2/c, and the asymmetric unit consists of one MA cation and one DNSA anion, out of which DNSA molecules transfer their protons to the nitrogen atom in MA molecules, resulting in a MA·DNSA salt (Figure 2a). The exocyclic bond lengths C2–C6 is elongated to 1.505(4) Å, which is longer than the cyclic C–C bonds (average1.386 Å). Protonated MA cation lead to a slight enhancement of the internal angles [C10–N3–C8 119.4(3)°] for 2, as compared with the neutral MA molecule (116 and 117°).63 The dihedral angle between two planes formed by MA and DNSA molecules is 15.63°. Intramolecular hydrogen bond [O5···O4 2.488(3) Å] is present in the structural network. Proton transfer from the carboxylic group in DNSA to one nitrogen atom in MA leads to the formation of charge-assistant N+–H···O–[N3···O3 2.635(3) Å] hydrogen bonding. A pair of MA cations isdimerized through N–H···N hydrogen bond [N5···N6 3.090(4) Å] similar to those observed in TMP sorbate or TMP-o-nitrobenzoate complexes (TMP = trimethoprim).64 The 3D supramolecular structure (Figure 2b) is generated through a number of N–H···O hydrogen bonds [N6···O5 2.991(4), N6···O6 2.895(4), N4···O4 3.066(3), N3···O3 2.635(3), N2···O3 3.183(4) Å] between the MA and DNSA molecules and the π-π stacking (Figure 2c) interactions [3.7020(6) Å] between two DNSA molecules.

Figure 2.

Crystal structure of 3: Compound 3 crystallizes in the monoclinic space group P2(1) with one AG cation, one DNBA anion and one lattice water molecule in the asymmetric unit (Figure 3a). The average C–N bond lengths in AG of 1.321 Å are lower than that of 1.473 Å in DNBA. The exocyclic C1–C2 bond is elongated to 1.513(3) Å, longer than the cyclic C–C bond lengths of 1.381 Å on average. The dihedral angle between the AT and the DNBA plane is 12.10°. AG connects with DNBA through three kinds of N–H···O hydrogen bonds [N1–H1···O6 2.840(3), N2–H5···O6 3.096(3) N3–H11···O2 3.478 Å], forming a twisted 1D chain that is shown in Figure 3b. Two adjacent chains combine with each other to produce a two-dimensional layer via various N–H···O hydrogen bonds [N3···O1 3.202(3), N2···O4 3.284 Å] (Figure 3c). The hydrogen bonds


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of N–H···O [N1–H2···O6 2.874(3), N4–H3···O5 2.867(3), N3-H12···O5 3.314(4) Å] connect to the adjacent layers, forming the 3D supramolecular network (Figure 3d).

Figure 3.

Crystal structure of 4: The asymmetric unit of 4 consists of one DNSA anion and one monoprotonated DAT cation, as well as one lattice water molecule (Figure 4a). The distances of two C–O bonds in the carboxylic group for DNSA molecule are obviously different, the C–OH bond length is 1.283(2) Å, while the C=O bond length is 1.233(4) Å. The exocyclic C1–C2 bond is elongated to 1.498(5) Å, longer than the cyclic C–C bond lengths (1.3915 Å on average). Intramolecular O−H···O hydrogen bond [O3···O14 2.428 Å] exsits in DNSA molecule. A pair of DAT cations are dimerized through two kinds of N–H···N hydrogen bonds [N2···N7 2.874, N9···N3 3.056 Å], which is surrounded by five DNSA anions (Figure 4b). DNSA and DAT molecules are linked by diverse N−H···O and O-H···O hydrogen bonds [N1···O4 2.753, N4···O3 2.875, N8···O11 2.936, N2···O12 3.206, N4···O12 2.916, N4···O14 2.793, O1W···O2 2.715, N9···O1W 3.086, N6···O2 2.731, N10···O1 2.892, N10···O6 2.878, N10···O13 2.986 Å] to form the 1D structure. Adjacent chains are combined as the 2D layer via the N−H···O hydrogen bonds [N5···O7 3.019, N9···O8 3.217 Å] (Figure 4c). As for the lattice water molecule, it is involved in four sets of hydrogen bonds [O2W···O9 2.882, O2W···O1W 2.879, O1W···O2W 2.765, N5···O2W 2.923 Å], further interlinking the 2D layers to construct the 3D supramolecular network (Figure 4d). Figure 4.

Thermal decomposition The thermal behaviors of 1-4 were determined using DSC and TG measurements with the linear heating rate of 5 °C min-1 under nitrogen atmosphere. Before that, in order to confirm the phase purity of the compounds, X-ray powder diffraction (XRPD) experiments has been carried out (Figure S1 in the Supporting Information). The experimental patterns are in good agreement with the simulated, indicating the phase purity of the synthesized powder products. DSC and TG curves of these energetic compounds are depicted in Figures 5 and 6.



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The DSC curves are shown in Figure 5, compound 1 presents three peaks on the curve. The endothermic peak is corresponding to the melting process and the first sharp exothermic peak shows the decomposition process of the main structure, as well as the second exothermic peak represents the condensation reaction of the residual fragments. An intense exothermic peak demonstrates the structural decomposition of compound 2. For 3, the endothermic peak responds the dehydration process and the exothermic peak displays the main decomposition process. There are two endothermic and one exothermic process for 4. The first endothermic peak is the dehydration process, and the second endothermic peak corresponds to the phase transition, and the exothermic peak reflects the main decomposition process.

Figure 5. DSC curves of compounds 1–4.

TG study for compound 1 show the decomposition process with a abrupt weight loss of about 98.85% corresponds to the collapse of the framework and decomposition of the organic components in the temperature range of 162–500 °C, In the TG curve of 2, the drastic decomposition process occurs at 280 °C, which corresponds to the collapse of framework and decomposition of organic components, accompanying the weight loss about 87.86% up to 500 °C. For 3, the first process accompanies the loss of water molecules with the weight loss of 5.07% (Calcd. 5.92%) in the temperature range of 145–163 °C, then, a flat curve of weight loss with the ratio of 92.62% corresponds to the collapse of framework and decomposition of organic components in the temperature range of 171–500 °C. For 4, the first process accompanies the weight loss of 5.49% in the temperature range of 160–206 °C, which is close to the loss of water molecules with the calculation value of 5.21%, then, an uniform drop with the weight loss of 85.50% from the temperature of 245 °C corresponds to the collapse of framework and decomposition of organic components. Obviously, the explosive performances of compounds 3 and 4 are affected by crystal lattice water molecules.

Figure 6. TG curves of compounds 1–4.

Optimized structure, mulliken atomic charge, total energy and the frontier orbital energy (hartree)


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According to the previous theoretical studies on similar energetic salts, 65-67 for instance, J. M. Shreeve et al. employs DFT-B3LYP/6-31+G(d) method to study the NO2-containing energetic salts. L. He and coworker use the same method and basis set to calculate the dinitromethanide salts, the adequacy and accuracy of the method and basis set are generally verified in the similar systems. Therefore, the DFT-B3LYP/6-31+G(d) method in Gaussian 03 package was appropriately used to optimize the structure of the compounds and compute their frequencies (Figure S2 in the Supporting Information), the crystal asymmetric units of the compounds were selected as the initial structure, single energy points were also calculated to confirm the facticity of the structure tested from X-ray diffraction. Vibration analysis indicates that the optimized structures are in accordance with the minimum points on the potential energy surface, which means no virtual frequencies, proving that the obtained optimized structures are stable. In order to obtain a better understanding of the structure of the compounds,68-69 the natural bond orbital (NBO) of compounds based on the optimized structures was carried out by using Gaussian 03. NBO analysis provides an efficient method for investigating charge distribution in molecular systems. The charge distribution data calculated by the NBO method for optimized geometries of 1-4 are tabulated in Table S3 (see in Supporting Information). NBO analysis shows that the C-NH2 carbon is more positively charged than C-NO2 and C-COOH. The electron-withdrawing effect of the -NO2 group considerably reduces the electronic charge of the connected atom, the negative charge is mainly delocalized over the carbon atoms in the benzene ring. The nitrogen atoms of the -NH2 group and the cyclic nitrogen atoms have negative charge densities, and the nitrogen atoms of the -NO2 group have essentially the same positive charge density. In 1, the N19, N20 and N21 atoms of triazole have almost the same charge densities due to the conjugative effect of triazole cyclic. In 3, the N21 and N22 atoms of amino group have the approximate negative charge density, which supports the conjugative effect between the two amino groups. Nitro compounds are very strong electron acceptors, and this ability can be represented by the net charges of the nitro group. A higher negative charge on the nitro group implies lower electron attraction and therefore more stability of the nitro compound.70-72 As known, the structure-sensitivity correlation in the field of HEDMs has been studied in the last decades. It is significant to determine whether the key safety index, sensitivity, depends on certain active group or not in the structure of energetic compounds. In recent years, a nitro group



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charge method (NGCM) is established and applied to assess and predict the properties of nitro compounds including molecular stability and impact sensitivity.55 The sensitivity of the compounds is evaluated and compared to the nitro group charges (QNitro) which are obtained by the calculation of the whole molecular atomic charges. QNitro is the algebraic sum of charges of all three atoms of a nitro group (equation 1). The average value for QNitro can be calculated by equation 2. The more negative nitro charges (QNitro) correspond to the more stable nitro compounds,55 because there is more than one nitro group in our synthesized molecules. In this Article, the QNitro values for compounds 1-4 are listed in Table 2. The QNitro are calculated as -0.358, -0.503, -0.405 and -0.517e, respectively. QNitro = QN + QO1 + QO2 Q Nitro =

(1)

1 n ∑ Q Nitro ,i n i =1

(2)

Table 2.

The structural parameters of 1-4 are compared to the X-ray diffraction data, and selected bond lengths of optimized are listed in Table S4 (see in Supporting Information). As shown in Table S4, the optimized bond lengths by the method of B3LYP are close to the experimental values. Although theoretical results deviate slightly from the experimental values, this may arise because theoretical calculations are carried out on the gas phase molecule derived from the asymmetric unit of the crystal structure, whereas the experimental results are obtained in the solid state. The calculated geometric parameters also represent a good approximation, and they can be used as a foundation to calculate the other parameters for the compounds. The above results indicate that the theoretical model and basis set carried out in the present work is appropriate to study the title compounds. Furthermore, the calculated molecular total energy under steady state would be applied to calculate the energy of detonation, as well as heat of detonation. The energy levels depending on HOMO and LUMO energies were used to assess the activity of compounds, estimating how difficult the compounds pyrolyze and explode under external stimulus. Molecular total energy (Etotal), frontier orbital energy levels (EHOMO and ELUMO), and their gaps (∆E) are calculated to be


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-1127.165993, -0.23877, -0.14792 and 0.09085 hartree for 1, -1351.301188, -0.23087, -0.11985 and 0.11102 hartree for 2. -1166.688051, -0.23595, -0.11432 and 0.12163 hartree for 3. -1334.202658, -0.23486, -0.11367 and 0.12119 hartree for 4. According to the MO theory, HOMO and LUMO are the most important factors that affect the safety performance of the compounds. The negatively occupied orbital energies indicate that the electronic state is relatively stable, and the larger energy gap represents more difficulty of the electron transitions, illustrating that the molecular structure is more stable. The results above indicate that compounds 1-4 possess favorable stable. The views of HOMO and LUMO are shown in Figure 7. For 1, the electron density of HOMO mainly focuses on AT molecule, while that of LUMO focuses on DNBA molecule, indicating that the -NO2 and -NH2 groups lead to important effect on the properties of 1. For 2 and 4, the electron densities of HOMO and LUMO are mainly focused on DNSA anion, displaying that the -NO2 group of DNSA anion is the active point of the compound. For 3, the electron density of DNBA anion in HOMO is lower than that in LUMO, while AG ion almost stays the same in both states, also showing that the -NO2 group of DNBA anion is the key factor for the property of the compound.

Figure 7.

Non-isothermal kinetics analysis for the exothermal process In our present work, Kissinger’s method73 and Ozawa’s method74,75 are used to determine the apparent activation energy (E) and the pre-exponential factor (A). The Kissinger equation (3) and Ozawa equation (4) are as follows, respectively: ln(

β T p2

AR E 1 − E R Tp

(3)

0.4567E =C RTp

(4)

) = ln

log β +

where Tp is the peak temperature; E is the apparent activation energy; A is the pre-exponential factor; R is the gas constant, 8.314 J mol-1 K-1; β is the linear heating rate and C is constant. Based on the exothermic peak temperatures measured at four different heating rates of 2, 5, 8, 10 °C min-1, Kissinger’s and Ozawa–Doyle’s method are applied to study the thermokinetics parameters for the exothermal processes in compounds 1-4. From the original data, we can



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achieve the apparent activation energy Ek and Eo, pre-exponential factor Ak and linear correlation coefficients Rk and Ro, as shown in Table 3.

Table 3.

From Table 3, it is obvious that Tp of the exothermic peak shift to higher temperatures as the heating rate increases. The E values derived from the averages of the Kissinger method (Ek) and Ozawa’s method (Eo) are calculated as 151.84, 385.44, 351.73 and 318.38 kJ mol-1 for 1-4, which shows that the exothermic processes for the compounds can not readily proceed. Besides, these linear correlation coefficients are very close to 1 and it is thus predicted that the results are credible. The Arrhenius Equations can be expressed as follows (E is the average between Ek and Eo): ln k = 12.57 – 151.84×103/(RT) for 1, ln k = 32.95 – 385.44×103/(RT) for 2, ln k = 38.58 – 351.73×103/(RT) for 3, ln k = 29.71 – 318.38×103/(RT) for 4, respectively, which can be used to estimate the rate constant of the main step of thermal decomposition process of the compounds.

Oxygen-Bomb calorimetry The constant-volume combustion energies of the compounds were determined with a precise rotating-oxygen bomb calorimeter (RBC-type II).60 Approximately 200 mg of the samples were pressed with a well-defined amount of benzoic acid (Calcd. 800 mg) to form a tablet to ensure better combustion. The recorded data were the average of six single measurements. The calorimeter was calibrated by the combustion of certified benzoic acid (SRM, 39i, NIST) in an oxygen atmosphere at a pressure of 30.5 bar. The experimental results for the constant volume combustion energies (∆cU) of energetic compounds 1–4 are -16085.56 ± 5.32, -15195.43 ± 3.13, -14363.35 ± 2.65 and -13870.07 ± 3.36 J g-1, respectively. On the basis of the formula

∆ c H mθ

= ∆cU + ∆nRT, ∆n = ng (products, g) – ng

(reactants, g), (ng is the total molar amount of gases in the products or reactants, R = 8.314 J mol-1 θ K-1, T = 298.15 K), the standard molar enthalpies of combustion ( ∆c H m ) can be derived as being

-4751.41 ± 1.58 kJ mol-1 for 1, -5370.74 ± 1.11 kJ mol-1 for 2, -4361.23 ± 0.81 kJ mol-1 for 3， -4776.87 ± 3.36 kJ mol-1 for 4. The combustion reaction equations are listed as follows: C9H8N6O6 (s) + 8 O2 (g) = 9 CO2 (g) + 4 H2O (l) + 3 N2 (g)


(5)

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C10H10N8O7 (s) + 9 O2 (g) = 10 CO2 (g) + 5 H2O (l) + 4 N2 (g)

(6)

C8H12N6O7 (s) + 7.5 O2 (g) = 8 CO2 (g) + 6 H2O (l) + 3 N2 (g)

(7)

C9H11N7O8 (s) + 7.75 O2 (g) = 9 CO2 (g) + 5.5 H2O (l) + 3.5 N2 (g)

(8)

θ The standard molar enthalpies of formation of the combustion products CO2 (g), ∆f H m (CO2, g)

θ = (-393.51 ± 0.13) kJ mol-1, H2O (l), ∆ f H m (H2O, l) = (-285.83 ± 0.04) kJ mol-1 were obtained θ from literature.76 According to Hess’s law, the standard molar enthalpies of formation ( ∆f Hm ) of

1–4 at 298.15 K are calculated as being 66.50 ± 2.53, 6.49 ± 1.65, -501.83 ± 2.18 and -333.55 ± 3.29 kJ mol-1, respectively.

Heat of detonation According to the order of H2, CO, C in forming detonation products,77 the detonation products of

1-4 are calculated as follows: C9H8N6O6 = 4 H2 + 6 CO + 3 C + 3 N2

(9)

C10H10N8O7 = 5 H2 + 7 CO + 3 C + 4 N2

(10)

C8H12N6O7 = 6 H2 + 7 CO + C + 3 N2

(11)

C9H11N7O8 = 5.5 H2 + 8 CO + C + 3.5 N2

(12)

To estimate the heat of detonation of the compounds, DFT is used to compute the energy of detonation (∆Edet), where ∆Hdet is obtained by using a linear correlation equations (∆Hdet = 1.127 ∆Edet+ 0.046, r = 0.968) 78,79 The calculated values of ∆Edet and ∆Hdet are listed in Table 4. The obtained heats of detonation were compared with the known ∆Hdet for classical high explosives (Figure 8), which indicates that the compounds 1 and 2 possess superior explosive performance.

Table 4.

Figure 8.

Characterization of detonation velocity and pressure



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Detonation velocity (D) and detonation pressure (P) are the most important parameters of the detonation characteristics of energetic materials. The detonation products produced by general explosives together with their nitrogen equivalent indices are listed in Table 5.

Table 5.

The values of D and P of an explosive can be predicted with the nitrogen equivalent equation (NE equation) shown as equations (13)-(15).80 ∑N = 100

∑

X i Ni M

(13)

D = (690 + 1160 ρ0) ∑N

(14)

P = 1.092 (ρ0 ∑N)2 – 0.574

(15)

where M represents molecular mass of an explosive; ∑N represents nitrogen equivalent of the detonation products; Ni represents nitrogen equivalent index of certain detonation product; Xi represents the mole number of certain detonation product produced by a mole explosive; ρ0 represents density of an explosive. According to equations (9)-(13), in which M(1) = 296.21 g·mol-1, M(2) = 354.26 g·mol-1, M(3) = 304.24 g·mol-1 and M(4) = 345.25 g·mol-1, total nitrogen equivalents of 1-4 are calculated through the nitrogen equivalent indexes of the detonation products. As follows:

∑ N1 =

100 × ( 4 × 0.29 + 6 × 0.78 + 3 × 0.15 + 3 × 1) = 3.14 296.21

∑N2 =

100 × (5 × 0.29 + 7 × 0.78 + 3 × 0.15 + 4 × 1) = 3.21 354.26

∑ N3 =

100 × (6 × 0.29 + 7 × 0.78 + 1 × 0.15 + 3 × 1) = 3.40 304.24

∑N4 =

100 × (5.5 × 0.29 + 8 × 0.78 + 1× 0.15 + 3.5 ×1) = 3.33 345.25

According to equations (14) and (15), in which ρ0(1) = 1.635g cm-3, ρ0(2) = 1.694 g cm-3, ρ0(3) = 1.552 g cm-3 and ρ0(4) = 1.661 g cm-3, D and P can be obtained as follows: D1 = (690 + 1160ρ02) ∑N1 = (690+1160 × 1.635) × 3.14 = 8121.94 m s−1 D2 = (690 + 1160ρ03) ∑N2 = (690+1160 × 1.694) × 3.21 = 8522.68 m s−1 D3 = (690 + 1160ρ01) ∑N3= (690+1160 × 1.552) × 3.40 = 8467.09 m s−1


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D4= (690 + 1160ρ01) ∑N4 = (690+1160 × 1.661) × 3.33 = 8713.81 m s−1 P1 = 1.092 (ρ02 ∑N1)2 – 0.574 = 1.092 × (1.635× 3.14)2 - 0.574 = 28.21 Gpa P2 = 1.092 (ρ03 ∑N2)2 – 0.574 = 1.092 × (1.694 × 3.21)2 - 0.574 = 31.72 Gpa P3 = 1.092 (ρ01 ∑N3)2 – 0.574 = 1.092 × (1.552 × 3.40)2 - 0.574 = 29.83 Gpa P4 = 1.092 (ρ02 ∑N4)2 – 0.574 = 1.092 × (1.661 × 3.33)2 - 0.574 = 32.83 Gpa The calculated P and D of compounds are larger to that of TNT and some energetic materials reported previously. The specific physicochemical details of them are listed in Table 6.

Table 6.

Sensitivity test The impact sensitivity was determined by using a Fall Hammer Apparatus. Twenty milligrams of compounds were compacted to a copper cap under the press of 39.2 MPa and hit by a 2 kg drop hammer, and calculated value of h50 represents the drop height of 50% initiation probability. The impact sensitivity values (h50) of the compounds 1 and 3 are 153 and 194 cm which corresponds to the impact energies of 30 and 38 J, while the compounds 2 and 4 don’t fire at the highest point of 200 cm which corresponds to the impact energy of 40 J. Under the same test condition, the impact sensitivity values (h50) of TNT and RDX are 77 cm (15.0 J) and 39 cm (7.5 J), which are consistent with the values from the reference.84 Hence, the impact sensitivities of the compounds are lower than that of TNT and RDX. Friction sensitivities of the compounds were measured by applying a Julius Peter’s machine using 20 mg of the sample. No friction sensitivity was observed up to 36 kg. The friction sensitivities of the compounds are lower than that of TNT (35 kg) and RDX (12 kg).85 The experimental results reveal that the compounds are insensitive to external stimulus. The impact sensitivity values for 1–4 are measured to be 30, >40, 38 and >40 J, respectively, which well responds to the order of QNitro values (-0.358e for 1, -0.503e for 2, -0.405e for 3 and -0.517 e for 4). Notably, compounds 2 and 4 with more negative nitro charges exhibit lower sensitivity values than 1 and 3, stating that the sensitivity has been a particularly close connection to the



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QNitro value. The QNitro values and sensitivities of conventional nitro compounds and compounds

1-4 are listed in Table 7. Obviously, the impact sensitivity values perform a regularity trend following the change of the QNitro values for all the compounds in the table 7, the more negative nitro charge corresponds to more stable nitro-containing energetic compound.

Table 7.

4. CONCLUSIONS In conclusion, four new nitro-containing energetic compounds are synthesized and characterized. X-ray analysis indicates that the stable structures of four compounds are restrained by intermolecular interactions in the solid state. Specially, compound 2 exhibits an outstanding decomposition temperature up to 280 oC. The optimized structures derive from DFT calculation accord with the structures determined by X-ray. The molecular total energies, frontier orbit energies and charge densities are also calculated. Calculated detonation properties illustrate that four compounds have good potentials of explosive, in particular, the compounds 1 and 2 are provided with superior heats of detonation (2.191 and 2.214 kcal g-1). The sensitivity measurements indicate that the compounds are insensitive to external stimuli. Remarkably, the measured sensitivities are well related to the QNitro values that are calculated by theoretical calculation, confirming that the -NO2 group is the active point in the nitro-containing compounds. In addition, the standard molar enthalpies of formation for compounds have been calculated to be 66.50 ± 2.53, 6.49 ± 1.65, -501.83 ± 2.18 and -333.55 ± 3.29 kJ mol-1, respectively. In a word, the present work not only obtains four high-energy compounds with favorable properties, but also shows a shining example for the combination of theory and experiment, providing the theoretical guidance for the design and synthesis of energetic materials.

ASSOCIATED CONTENT Supporting information Figures S1 and S2, Tables S1-S4. CCDC 996906, 996908, 996909 and 1000211 for 1-4, contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc. cam. ac. uk/ data _ request /cif.


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This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21373162, 21463020, 21127004, 21173168 and 21203149), and the Nature Science Foundation of Shaanxi Province (Grant Nos. 11JS110, FF10091 and SJ08B09).

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Hratchian, J. B. Cross, C.

Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03. Revision B.01, Gaussian Inc., Pittsburgh, PA, 2003. (60) Yang, X.-W.; Chen, S.-P.; Gao, S.-L. Construction of a rotating-bomb combustion calorimeter and measurement of thermal effects. Instrum. Sci. Technol. 2002, 30, 311-321. (61) Sheldrick, G. M. SHELXS-97, Program for X-ray Crystal Structure Determination, University of Göttingen, Germany, 1997. (62) Sheldrick, G. M. SHELXL-97, Program for X-ray Crystal Structure Refinement, University of Göttingen, Germany, 1997. (63) Hughes, E. W. The Crystal Structure of Melamine. J. Am. Chem. Soc. 1941, 63, 1737-1752. (64) Stanley, N.; Sethuraman, V.; Muthiah, P. T.; Luger, P.; Weber, M. Crystal engineering of organic salts: Hydrogen-bonded supramolecular motifs in pyrimethamine hydrogen glutarate and pyrimethamine formate. Cryst.



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Growth Des. 2002, 2, 631-635. (65) Joo, Y. H.; Shreeve, J. M. Nitroimino-tetrazolates and Oxy-nitroimino-tetrazolates. J. Am. Chem. Soc. 2010, 132, 15081-15090. (66) He, L.; Tao, G. H.; Parrish, D. A.; Shreeve. J. M. Impact insensitive dinitromethanide salts. Chem. Commun. 2013, 49, 10329-10331. (67) Wang, R.; Guo, Y.; Zeng, Z.; Shreeve. J. M. Nitrogen-rich nitroguanidyl-functionalized tetrazolate energetic salts. Chem. Commun. 2009, 2697-2699. (68) Liang, L.; Huang, H.-F.; Wang, K..; Bian, C.-M.; Song, J.-H.; Ling, L.-M.; Zhao, F.-Q.; Zhou, Z.-M. Oxy-bridged bis (1H-tetrazol-5-yl) furazan and its energetic salts paired with nitrogen-rich cations: highly thermally stable energetic materials with low sensitivity. J. Mater. Chem. 2012, 22, 21954-21964. (69) Garg, S.; Gao, H.-X.; Joo, Y. H.; Parrish, D. A.; Huang, Y.-G.; Shreeve, J. M. Taming of the Silver FOX. J. Am. Chem. Soc. 2010, 132, 8888-8890. (70) Samuel, P. H.-R.; Ricardo, I.-C. A systematic theoretical investigation of the relationship between heats of detonation and NBO charges and

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N NMR chemical shifts of nitro groups in nitramines and nitro paraffins.

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(79) Li, S.-H.; Wang, Y.; Qi, C.; Zhao, X.-X.; Zhang, J.-H.; Zhang, S.-W.; Pang, S.-P. 3D Energetic Metal–Organic Frameworks: Synthesis and Properties of High Energy Materials. Angew. Chem. Int. Ed., 2013, 52, 14031-14035. (80) Guo, Y.-X; Zhang, H.-S. Nitrogen equivalent and modified nitrogen equivalent equations for predicting detonation parameter of explosives-prediction of detonation velocity of explosives, Explos. Shock Waves 1983, 3, 56-66. (81) Dippold, A. A.; Klapötke, T. M.; Oswald, M. Asymmetrically substituted 5, 5′-bistriazoles–nitrogen-rich materials with various energetic functionalities. Dalton Trans. 2013, 42, 11136-11145. (82) Klapötke, T. M.; Sabaté, C. M. 1, 2, 4-Triazolium and Tetrazolium Picrate Salts:“On the Way” from Nitroaromatic to Azole-Based Energetic Materials. Eur. J. Inorg. Chem. 2008, 34, 5350-5366. (83) Klapötke, T. M.; Martin, F. A.; Mayr, N. T.; Stierstorfer, J. Synthesis and Characterization of 3, 5-Diamino-1, 2, 4-triazolium Dinitramide. Z. Anorg. Allg. Chem. 2010, 636, 2555-2564. (84) J. Köhler, R. Meyer, in: Explosivstoffe 9th ed., Wiley-VCH, Weinheim, Germany,1998. (85) Anniyappan, M.; Stalwart,M. B.; Gore, G. M.; Venugopalan, S.; Gandhe, B. R. Synthesis, characterization and thermolysis of 1,1-diamino-2,2-dinitroethylene (FOX-7) and its salts, J. Hazard. Mater. 2006, 137, 812-819. (86) Tan, B.-S.; Long, X.-P.; Peng, R.-F.; Li, H.-B.; Jin, B.; Chu, S.-J.; Dong, H.-S. Two important factors influencing shock sensitivity of nitro compounds: Bond dissociation energy of X–NO2 (X=C, N, O) and Mulliken charges of nitro group. J. Hazard. Mater. 2010, 183, 908-912. (87) Wang, R.-H.; Xu, H.-Y.; Guo, Y.; Sa, R. J.; Shreeve, J. M. Bis [3-(5-nitroimino-1, 2, 4-triazolate)]-based energetic salts: Synthesis and promising properties of a new family of high-density insensitive materials. J. Am. Chem. Soc. 2010, 132, 11904-11905. (88) Storm, C. B.; Stine, J. R.; Kramer, J. F. In Chemistry and Physics of Energetic Materials; Bulusu, S. N., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990; pp. 605-63.



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

Figure Captions: Figure 1. a) The asymmetric unit of 1 showing the atom-numbering scheme. b) The periodic repeated unit of 1. c) 1D supramolecular chain via hydrogen bonds in 1. Figure 2. a) The asymmetric unit of 2 showing the atom-numbering scheme. b) 3D structural network of 2. c) The face-to-face of DNSA anions showing the π-π stacking interaction. Figure 3. a) The asymmetric unit of 3 showing the atom-numbering scheme. b) 1D supramolecular chain of 3 via hydrogen bonds. c) 2D layer of 3. d) 3D structural network of 3. Figure 4. a) The asymmetric unit of 4 showing the atom-numbering scheme. b) A pair of DAT cations interacts with five DNSA anions in 4. c) 2D layer of 4. d) 3D structural network of 4. Figure 5. DSC curves of compounds 1–4. Figure 6. TG curves of compounds 1–4. Figure 7. View of HOMO (left) and LUMO (right) for 1(a), 2(b), 3(c) and 4(d). Figure 8. Bar chart representation of ∆Hdet for six classical explosive materials, including TATB, FOX-7, TNT, HMX, RDX and CL-20, along with predicted ∆Hdet for 1-4. Error bars correspond to the 95 % statistical-confidence level for these values. Table 1. Crystal data and structure refinement details for compounds 1-4. Table 2. QNitro and the average QNitro of compounds 1-4. Table 3. The peak temperatures of the exothermic processes at different heating rates and the thermokinetic parameters. Table 4. Calculated parameters used in the detonation reactions. Table 5. Nitrogen equivalents of different detonation products Table 6. Physicochemical properties of compounds 1–4. Table 7. The nitro group charges and sensitivities of conventional nitro compounds and compounds 1-4.


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Figure 1. a) The asymmetric unit of 1 showing the atom-numbering scheme. b) The periodic repeated unit of 1. c) 1D supramolecular chain via hydrogen bonds in 1.

Figure 2. a) The asymmetric unit of 2 showing the atom-numbering scheme. b) 3D structural network of 2. c) The face-to-face of DNSA anions showing the π-π stacking interaction.



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

Figure 3. a) The asymmetric unit of 3 showing the atom-numbering scheme. b) 1D supramolecular chain of 3 via hydrogen bonds. c) 2D layer of 3. d) 3D structural network of 3.

Figure 4. a) The asymmetric unit of 4 showing the atom-numbering scheme. b) A pair of DAT cations interacts with five DNSA anions in 4. c) 2D layer of 4. d) 3D structural network of 4.


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Figure 5. DSC curves of compounds 1–4.

Figure 6. TG curves of compounds 1–4.



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

Figure 7. View of HOMO (left) and LUMO (right) for 1(a), 2(b), 3(c) and 4(d).

Figure 8. Bar chart representation of ∆Hdet for six classical explosive materials, including TATB, FOX-7, TNT, HMX, RDX and CL-20, along with predicted ∆Hdet for 1-4. Error bars correspond to the 95 % statistical-confidence level for these values.


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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


Table 1. Crystal data and structure refinement details for compounds 1-4. Compounds

1

2

3

4

Empirical formula

C9H8N6O6

C10H10N8O7

C8H12N6O7

C9H11N7O8

Formula weight

296.21

354.26

304.24

345.25

Crystal system

Monoclinic

Monoclinic

Monoclinic

Triclinic

space group

C2/c

C2/c

P2(1)

P-1

a (Å)

18.581(3)

10.0162(17)

9.7042(13)

9.1481(13)

b (Å)

4.9297(8)

14.452(3)

4.9681(7)

11.4290(17)

c (Å)

26.800(4)

19.359(3)

13.5275

14.247(2)

α (°)

90

90

90

86.182(3)

β (°)

101.387(2)

97.485(3)

93.494(2)

86.184(3)

γ (°)

90

90

90

68.419(3)

V (Å3)

2406.6(7)

2778.4(9)

650.97(16)

1380.7(3)

Z

8

8

2

4

F(000)

1216

1456

316

712

GOF on F2

1.087

1.008

0.963

1.014

R1 =0.0586,

R1=0.1180,

R1=0.0417,

R1=0.1843,

R indices (all data) wR2 = 0.1384

wR2 = 0.1387

wR2 = 0.1107

wR2=0.1013

Final R indices

R1 = 0.0480,

R1=0.0543,

R1=0.0359,

R1=0.0647,

[I>2sigma(I)]

wR2 = 0.1306

wR2 = 0.1132

wR2 = 0.1034

wR2=0.0782

Table 2. QNitro and the average QNitro of compounds 1-4. Compound

1

2

3

4

QNitro (e)

N7(-0.363)

N12(-0.520)

N3(-0.381)

N12(-0.608)

QNitro (e)

N8(-0.352)

N15(-0.486)

N7(-0.429)

N13(-0.426)

Average QNitro (e)

-0.358

-0.503

-0.405

-0.517



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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Table 3. The peak temperatures of the exothermic processes at different heating rates and the thermokinetic parameters. Compound

1

2

Heating rate β (°C min-1)

3

4

Peaks temperatures Tp(°C)

2

251.6

306.2

184.4

253.1

5

253.2

309.7

188.7

256.7

8

267.8

313.6

190.4

261.2

10

269.1

317.4

192.5

264.2

151.33

390.39

356.83

322.07

12.57

32.95

38.58

29.71

99.58

99.63

99.36

99.79

152.34

380.49

346.62

314.68

99.67

99.21

99.43

99.56

The calculation results by Kissinger’s method Ek (kJ mol-1) -1

ln Ak(s ) Linear correlation coefficient (Rk) The calculation results by Ozawa–Doyle’s method Eo (kJ mol-1) Linear correlation coefficient (Ro)

Table 4. Calculated parameters used in the detonation reactions.

Compound

H2

CO

C

∆Edet

∆Edet

∆Hdet

∆Hdet

(hartree)

(kcal g-1)

(kcal g-1)

(kcal cm-3)

N2

1

-1127.165993

-1.1666

-113.341

-37.738

-109.447

0.896

1.904

2.191

3.583

2

-1351.301188

-1.1666

-113.341

-37.738

-109.447

1.086

1.924

2.214

3.751

3

-1166.688051

-1.1666

-113.341

-37.738

-109.447

0.222

0.459

0.563

0.874

4

-1334.202658

-1.1666

-113.341

-37.738

-109.447

0.256

0.465

0.570

0.947

Table 5. Nitrogen equivalents of different detonation products Detonation product

N2

H2O

CO

CO2

O2

C

H2

1.00

0.54

0.78

1.35

0.50

0.15

0.29

Nitrogen equivalent index


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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


Table 6. Physicochemical properties of compounds 1–4.

1

2

3

4

ANBT

81

G-NNBT

81

TAGNNBT

Formula

81

TNP82

TNT

83

RDX

83

DMAT

C9H8N6O6

C10H10N8O7

C8H12N6O7

C9H11N7O8

C4H4N8O2

C6H13N15O4

C6H19N21O4

C9H10N8O7

C7H5N3O6

C3H6N6O7

296.21

354.26

304.24

345.25

196.1

359.3

449.4

342.25

227.13

222.12

30

>40

38

>40

40

40

40

>40

15

7.5

>360

>360

>360

>360

360

360

360

>360

353

120

28.36

31.62

27.61

28.39

57.1

58.5

65.5

32.7

18.50

37.80

-86.43

-81.30

-78.89

-71.83

-65.3

-64.5

-62.3

-74.8

-73.96

-21.60

162

280

171

245

255

230

193

269

>160

210

1.635

1.694

1.552

1.661

1.61

1.75

1.75

1.639

1.654

1.800

66.50

6.49

-501.83

-333.55

267

169

888

-216

59.1

70

2.191

2.214

0.563

0.570

0.922

0.797

1.06

-

1.22

1.44

P (GPa)i

28.21

31.72

29.83

32.83

19.4

24.2

30.3

17.8

20.5

34.1

D (m s-1)j

8121.94

8522.68

8467.09

8713.81

7216

7911

8707

6876

7178

8906

Molecular Mass (g -1

mol ) Impact sensitivity a

(J)

Friction sensitivity (N)

b

N (%)c Ω (%)

d

Tdec (°C)

e

ρ0 (g cm-3)f ∆fHmθ (kJ mol-1)g ∆Hdet (kcal g-1)h

a

Impact sensitivity. b Friction sensitivity. c Nitrogen content. d Oxygen balance. e Decomposition temperature of main framework. f From X-ray diffraction.

Enthalpies of formation. h Heat of detonation. i Detonation pressure. j Detonation velocity

Table 7. The nitro group charges and sensitivities of conventional nitro compounds and compounds 1-4. compound

average QNitro (e)

IS (J)a

FS (N)b

1

-0.358

30

>360

2

-0.503

>40

>360

3

-0.405

38

>360

4

-0.517

>40

>360

-0.285

15

353

83, 86

TNT

86, 87

TATB

-0.465

50

—

HMX86, 87

-0.134

7.4

—

83, 86

-0.142

7.5

120

RDX

86,88

a

[2]

TNB

-0.278

DATB86,88

-0.404[2]

[3]

10

—

32[3]

—

b

Impact sensitivity. Friction sensitivity. TNB: 1, 3, 5-trinitrobenzene. DATB: 2, 4, 6-trinitrobenzene-1, 3-diamine.


g


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

Graphical Abstract Synopsis: The energetic compounds show low sensitivity, good thermostability and explosive potential. The nitro group charge determines the sensitivity of high-energy density compounds constructed with intermolecular Interaction, which is exampled and identified on the combination of theory and experiment.

Graphic:


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Structure, Physicochemical Properties, and Density Functional Theory

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