Insensitive ionic bio-energetic materials derived from amino acids

Energetic salts/ionic liquids have received increasing attention as fascinating energetic materials, and the use of renewable compounds is a promising approach to developing energetic materials. Until recently, biomolecules have been used as raw materials to develop neutral energetic compounds, whereas research focused on ionic energetic materials obtained from natural bio-renewable frameworks is scarce. This work systematically investigates ionic bio-energetic materials (IBEMs) derived from sustainable natural amino acids. In addition to combustibility, high density, good thermal stability, and one-step preparation, these IBEMs demonstrated apparent hypotoxicity and insensitivity. Moreover, a theoretical examination was performed to explore their appropriate properties. The intriguing results of this study indicates that IBEMs are potential bio-based energetic materials.

OTf) were all assessed under the same condition, and all present results analogous to those of the amino acids, which indicates that the corresponding Cl − , Br − , TFA − , HSO 4 − and OTf − anions are not suitable for designing IBEMs. The combustibility performances of TNT, TNP, RDX and HMX were also tested and showed sustainable combustion with a mild flame, corresponding to their popular applications in propellants and explosives. As a comparison, the combustion of most IBEMs in this work is clearly more vigorous than that of TNT, TNP, RDX and HMX.
Thermal stability is vital to the performance of energetic materials.
[AA]ClO 4 and [AA]NO 3 are thermally stable at room temperature and can be maintained without any decomposition for more than two years. [AA] ClO 4 exhibit good thermal stability with decomposition temperatures (T d ) greater than 210 °C, which exceed the criterion of 200 °C 45,46 (Table 1). The highest thermal stability out of all samples is found for [Gly]ClO 4 (263 °C). The T d values of [AA]NO 3 are in the range of 125-183 °C, which are far beyond that of the typical NBEM and NG (50 °C). The melting points (T m ) or glass transition temperatures (T g ) of [AA]ClO 4 and [AA]NO 3 are between −46 °C and 159 °C. The initial melting point analysis reveals that [AA]ClO 4 materials generally have significantly lower melting points than their nitrate analogues, among which many can be classified as ionic liquids. The thermal data also showed that certain IBEMs display a liquidus range at temperatures above 150 °C. The low T m and good thermal stability indicate that these materials are easily molded, similar to TNT. Differential thermal analysis (DTA) is a routine method used to characterize energetic materials 17,18 . Heat flow was measured under a nitrogen environment at a heating rate of 10 °C min −1 . The DTA profile of [Gly]ClO 4 and [Gly]NO 3 is illustrated in Fig. 4. A notably large and sharp exothermic peak of [Gly]ClO 4 is observed at a peak temperature of approximately 273 °C, which could be related to the oxidation-reduction reaction between glycine cation and ClO 4 − .  4 ) were determined to verify the accuracy of the theoretical results, and the relative error is less than 5%. Therefore, the theoretical results are in good    (8)).
[Ala]NO 3 crystallizes in the orthorhombic space group P2 1 2 1 2 1 with four cation and four anion moieties in the unit cell. The packing structure of [Ala]NO 3 is built up by hydrogen bonding and interactions between cations and anions along the a axis (Fig. 3d). Each NO 3 − anion is surrounded by three alaninium cations oriented toward the oxygen atoms (O3, O5). The donor-acceptor contact distance range is 2.618(6) to 2.893(6) Å.
The localized orbital locator (LOL) that is dependent on the kinetic-energy density reveals the electronic shell structure of the compounds 52  To explain the H-bonding, the quantum theory of atoms in molecules (QTAIM) analysis was used [56][57][58] . The topological criteria for the existence of hydrogen bonding is the bond path between the hydrogen atom and acceptor with the bond critical point (BCP) occurring at the minimum electron density ρ(r) (ρ BCP ) and an appropriate  gradient, the Laplacian of the electron density ∇ 2 ρ(r) (∇ 2 ρBCP ) (Fig. 8). Table 2 lists the H-bonds as determined by the existence of a BCP and identifies these as weak and medium H-bonds in the [Gly]ClO 4 crystal. The O4-H4-O8 bond with ρ BCP = 0.0247 e bohr −3 (∇ 2 ρBCP = 0.1074 e bohr −5 ) lies well within the medium H-bonding range. These hydrogen bonds in the crystal might contribute to closer packing to obtain the high density of [Gly]ClO 4 . Table 3 lists the H-bonds as determined by the existence of a BCP and identifies these as medium and strong H-bonds in the [Ala]NO 3 crystal. The O6-H6-O3 bond with ρ BCP = 0.0424 e bohr −3 (∇ 2 ρBCP = 0.1323 e bohr −5 ) lies well within the strong H-bonding range. The natural bond orbital (NBO) analysis is also a key approach to the study of H-bonding 57,58 . The interactions between the empty acceptor-hydrogen (A-H) antibonding orbital (σ * ) and the filled lone pair orbital (n) on the donor (D) are shown in Fig. 8. The orbital overlap represents the charge transfer from the occupied donor lone-pair orbital into the empty antibonding orbital. The stabilization energy E (2) (n→σ*) associated with the amount of electron density donated from the occupied donor lone-pair orbital to the empty antibonding orbital is obtained. The values for E (2) (n→σ*) in [Gly]ClO 4 range from 10.71 to 30.19 kJ mol −1 ( Table 3) The toxicity of energetic materials is an important concern for the public. To assess the acute lethal toxicity of IBEMs on aquatic animals, diluted aqueous solutions of selected high-quality energetic amino acid salts/ionic liquids were administered to the model species Macrobrachium nipponense. Macrobrachium nipponense is a dominant species in the freshwater and stream ecosystems and is a sensitive bio-indicator of pollutants 59 Fig. 9. The mortality increases with the initial concentration of IBEMs. The median lethal concentrations (LC 50 ), the most important toxicological parameter, were calculated according to the lethal rates of Macrobrachium nipponense. The value of LC 50 for an examined compound is the dose required to kill half of the members of a tested population after a specified test period. It is obvious that the longer the test duration, the shorter are the median lethal concentrations ( 49) mg L −1 , which exceed the low toxicity critical value of 100 mgL −160 . These IBEMs are apparently hypotoxic to Macrobrachium nipponense in the acute lethal toxicity assay, although in large amounts, perchlorate interferes with iodine uptake into the thyroid gland 61,62 .

Conclusion
In conclusion, sixteen amino acid salts/ionic liquids, including [AA]ClO 4 and [AA]NO 3 materials, were systematically studied as ionic bio-energetic materials (IBEMs). We present a new approach for the development of bio-energetic materials.
[AA]ClO 4 and [AA]NO 3 were easily prepared from bio-renewable amino acids and possessed high densities as well as low sensitivities. The combustion of these compounds is more intense than that of certain conventional NEMs. The energetic properties of several IBEMs are comparable to those of TNT, TNP, RDX and HMX. These IBEMs preserve the benign environment from the natural source and display excellent hypotoxicity. Our work suggests that IBEMs are promising candidates for bio-based energetic materials.

Experimental Section
General Methods. All chemicals were obtained commercially as analytical-grade materials and used as received. Solvents were dried using standard procedures. The ionic bio-energetic materials [AA]X were synthesized by direct acidification from amino acids and corresponding acids in water for 24 h and subsequently dried    Table 2. Hydrogen bonds in [Gly]ClO 4 crystal. 1 1 + X, + Y, −1 + Z; 2 1−X, 1−Y, 1−Z; 3 1 + X, + Y, −1 + Z.  Table 3. Hydrogen bonds in [Ala]NO 3 crystal. 1 1/2 − X, −1 − Y, 1/2 + Z; 2 −1 + X, + Y, + Z; 3    by evaporation of water in air followed by vacuum drying, as described in our previous studies 38 . Infrared spectra (IR) were recorded on a NEXUS 670 FT-IR spectrometer on KBr pellets. 1 H and 13 C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 and 100 MHz, respectively, with d 6 -DMSO as the locking solvent. The 1 H and 13 C chemical shifts are reported in ppm relative to TMS. Coupling constants are given in Hertz. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE elemental analyzer. Decomposition temperatures were characterized using a thermogravimetric analyzer (TGA) on a NETZSCH TG 209F1 calorimeter. The heat flow of these materials was obtained by differential thermal analysis (DTA) using a NETZSCH TG 209F1 calorimeter. Measurements were accomplished by heating the samples at a heating rate of 10 °C min −1 from 25 to 600 °C. Melting points were determined by differential scanning calorimetry (DSC) on a TA Q20 calorimeter calibrated with standard pure indium. Measurements were performed at a heating rate of 10 °C min −1 with a nitrogen flow rate of 20 mL min −1 . The reference sample was an Al container with nitrogen. The densities were measured at 25 °C using a pycnometer. The experimental enthalpy of combustion was measured using a Parr 6725 bomb calorimeter (static jacket) equipped with a Parr 207 A oxygen bomb. Initial safety testing of amino acids salts/ionic liquids with respect to impact and friction were performed using the BAM method. The sensitivity towards impact (IS) was tested by the action of a falling weight from different heights. The friction sensitivity (FS) was determined by rubbing a small amount of material between a porcelain plate and a pin with different contact pressures. Macrobrachium nipponense were obtained from local commercial suppliers and transported to a plastic aquarium equipped with an air pump to aerate the water. The temperature was maintained at 20.0 ± 0.5 °C. Macrobrachium nipponense were acclimated for one week and those with body length 5 ± 0.5 cm were used in acute toxicity tests in the initial experiments. Stock solutions were prepared in deionized water. Laboratory tests were conducted to determine the median lethal concentration (LC 50 ) for Macrobrachium nipponense. Ten animals were randomly sampled and placed in plastic beakers. After 24 h acclimatization, Macrobrachium nipponense were exposed to different concentrations of [ 3 were removed from the flask, and a suitable crystal was selected and attached to a glass fiber. The data were collected by a New Gemini, Dual, EosS2 diffractometer with graphite-monochromated Cu-Kα radiation (λ = 1.54184 Å). The crystal was held at 293 K during data collection. Using Olex2 64 , the structure was solved with the ShelXT 65 structure solution program using Direct Methods and refined with the XL 66 refinement package using least squares minimization. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. Crystal data and structure refinement for [Gly]ClO 4 and [Ala]NO 3 are given in Table 5. Details of the data are given in Tables S1-S4. The LOL, QTAIM and NBO analysis of the cluster conformers in the [Gly]ClO 4 crystal structure at the M062X/6-311++G (d, p) level was performed by the Gaussian09 (Revision A.02) suite of programs 47 . The Gaussian output wfn files were used as inputs for Multiwfn to perform the QTAIM analysis 53 . The LOL analysis map was drawn by Multiwfn. The AIM topological analysis diagram was drawn by Multiwfn and VMD 71 . The NBO was plotted by Multiwfn and VMD using the Gaussian output fch files.

D H A d(D-H)/Å d(H-A)/Å d(D-A)/Å <(DHA)/°ρ BCP /e bohr
Detonation property. The detonation parameters were calculated using the Kamlet-Jacobs equation 72  where P is the detonation pressure (GPa), D is the detonation velocity (km s −1 ), ρ m is the packed density (g cm −3 ), φ is the characteristic value of explosives, N is the moles of gas produced per gram of explosives (mol g −1 ), M is an average molar weight of detonation products (g mol −1 ), and Q is the maximum estimation heat of detonation (cal g −1 ).