High-Energy-Density Materials: An Amphoteric N-Rich Bis(triazole) and Salts of Its Cationic and Anionic Species

The synthesis and characterization of the N-rich bis(triazole) compound 1H,4′H-[3,3′-bis(1,2,4-triazole)]-4′,5,5′-triamine (C4H7N9) with a N content of 69.6% by weight is reported. The compound exhibits a rich acid–base behavior because it can accept up to two protons, forming a monocation and a dication, and can lose one proton, forming an anion. Measurement of the acid constants has shown that there exist well-defined pH intervals in which each of the four species is predominant in solution, opening the way to their isolation and characterization by single-crystal X-ray analysis as salts with different counterions. Some energetic salts of the monocation or dication containing oxidizing inorganic counterions (dinitramide, perchlorate, and nitrate) were also prepared and characterized in the solid state for their sensitivity. In particular, the neutral compound shows a very remarkable thermal stability in air, with Td = 347 °C, and is insensitive to impact and friction. Salts of the dication with energetic counterions, in particular perchlorate and nitrate, show increased sensitivities and reduced thermal stability. The salt of the monocation with dinitramide as the counterion outperforms other dinitramide salts reported in the literature because of its higher thermal stability (Td = 230 °C in air) and friction insensitiveness.

NMR and mass spectra of 1

Acid-base equilibria, UV-VIS spectra, and UV-VIS titrations
In this section, 1 will also be indicated as HL, the singly protonated form of 1 as H 2 L + , the doubly protonated form as H 3 L ++ , and the deprotonated form as L -. The protolytic equilibria of 1 were studied by UV-VIS absorption spectroscopy in 0.5 M NaCl, as the ionic medium. The 1 stock solutions were prepared starting from the solid. Deionized and doubly distilled water was used to prepare all aqueous solutions. NaCl (Fluka, dried overnight at 120 °C) was used to prepare the ionic medium solutions. For each experimental point at 2.5<pH<10, the equilibrium free proton concentration was evaluated from the measured electromotive force at the ends of the galvanic cell GE/TS/RE, where TS indicates the Test solution, GE is the glass electrode and RE is a reference electrode (0.5 M NaCl|Hg 2 Cl 2 |Hg(Pt)) placed outside but electrically connected to TS through a salt bridge. The Nernst potential of the cell, E(mV), can be written as E= E 0 + 59.16 log [H + ] + E j where E j = liquid junction potential due to the replacement of Na + with H + . 1 The evaluation of the constant of the glass electrode, E 0 , was performed in the first stage of each experiment by a coulometric titration, using the Gran method. 2 The test solutions at 0.3<pH<2 were obtained analytically. All the experiments were carried out in air in a thermostat, at 25.00 ± 0.03 °C. Potentiometric experimental data were collected by means of an automatic data acquisition system based on Hewlett-Packard (HP) instrumentation. The glass membrane electrodes reversible to protons were supplied by Metrohm. Highly precise (± 0.02 mV) emf measurements were made by adapting the impedance of the glass electrode through operational amplifiers. Coulometric variations of the solution composition were carried out using a Hewlett Packard "DC Power Supply". The intensity of the current in the electrolysis circuit was measured from the potential drop at the ends of a calibrated resistance; the current density was set at about 1 mA/cm 2 . Absorption spectra were recorded with a Varian Cary 50 UV-Vis spectrophotometer using 1 cm cell. The primary spectrophotometric data (Absorbance, pH, ) were elaborated both graphically 3 and numerically, by the HYPERQUAD program. 4 In Figure S5, the continuous curves were obtained by fitting the experimental points with an equation that relates the Absorbance of the solution (at a given wavelength) with the pH, the analytical concentration of 1, and the equilibrium constants, by assuming the law of additivity of absorbances (Bouger-Lambert-Beer equation). The Bouger-Lambert-Beer equation is now explicitly derived for the system 1 (HL).
From the Lambert-Beer law, assuming additivity of the absorbances, it is (1) in which b is the optical path of the cell (1 cm) and is the molar extinction coefficient of the species i at the given wavelength. The analytical concentration of 1 (HL), C, is given by The acid-base conditional equilibrium constants are From these, it is possible to express the concentration of all species but one (e. g. the neutral one), as a function of the equilibrium constants and the concentration of H 3 O + : Putting (4) in (2) Putting (4) and (6) in (1) (10) and, finally, the Bouger-Lambert-Beer equation The UV-VIS spectra, at constant total concentration and different pH, are reported in Fig. S4 for 1.

Fig. S4
. UV−VIS absorption spectra of 1 at constant total concentration, C = 5.01 × 10 −5 M, in NaCl 0.5 M recorded at 0.6 ≤ pH ≤ 9.9. The spectra have been grouped in three set of curves for easier lecture.
The plots of Absorbance vs pH at fixed wavelength and the fitting of the points using the Bouger-Lambert-Beer equation are reported in Fig. S5. Based on the distribution diagram ( Fig. 2 of the typescript), the isosbestic points at =282 nm in Fig.   S4(a), at 244 nm in Fig. S4(b), and at 259 nm in Fig. S4

DSC Analysis
The DSC curves of energetic compounds 1, 2, 4 and 5 are reported in Fig. S6.

Computational Part (Energetics)
In order to be able to calculate the detonation parameters of 1 and of the corresponding salts, the enthalpy (H ) was calculated quantum chemically with the CBS-4M method. 5 The CBS method begins with a HF/3-21G(d) calculation to optimize the structure and for the calculations of the zero-point energy. Then, using a larger basis set, the so-called base-energy is calculated. A MP2 /6 -31 + G calculation with a CBS extrapolation gives the perturbation-theory correct energy, which takes the electron correlation into account. A MP4 (SDQ) /6 -31 + (d,p) calculation is used, to estimate the correlation contributions of a higher order. The most widely used CBS-4M version today is a reparametrization of the original CBS-4 version, which contains additional empirical correction terms (M stands for "minimal population localization" here).
The enthalpies of the gaseous species M can then be calculated using the method of the atomization energies: 6-8 To be able to convert the standard enthalpies of formation (g) for the gas-phase into values for ∆ 0 the condensed phase, for covalent molecules (NG) we additionally require the enthalpy of sublimation ΔH sub. (for solids) or the enthalpy of vaporization ΔH vap. (for liquids). Both values can be estimated using the Trouton's rule, in which T m is the melting point of the solid and T b is the boiling point of the liquid: 10 Neutral triamine (compound 1 of the typescript) is a solid and has a m.p. of 351 °C (= 624 K). Therefore, the enthalpy of sublimation is calculated to be ΔH sub. (1) = 117 kJ mol -1 (28.0 kcal mol -1 ).
In the case of ionic solids of the type AB, AB 2 or A 2 B, the lattice energy (ΔU L ) and lattice enthalpy (ΔH L ) can be calculated by using the Jenkin's method. [11][12][13][14] Only the molecular volumes of the ions are required. These can be most easily obtained from single crystal X-ray diffraction data: Here | z + | and | z -| are the dimensionless charges of the cations and anions and ν is the number of ions per 'molecule' ( The lattice energy ΔU L can easily be converted into the corresponding lattice enthalpy ΔH L :