Preparation, crystal structure, thermal behavior, and theoretical studies of N,N′-dinitro-4, 4′-azo-bis(1,2,4-triazolone) (DNZTO)

2.1 General

¹H, ¹³C NMR spectra were recorded on a 300 MHz (Bruker AVANCE 300) nuclear magnetic resonance spectrometer operating at 300.13 or 75.48 MHz, using [D₆]DMSO as a locking solvent. IR spectra were recorded using KBr pellets on a Bio-Rad model 3000 FTS spectrometer. ZTO was prepared according to the literature [9–11].

2.2 Synthesis of DNZTO (Scheme 1)

Scheme 1:

The synthesis route to DNZTO.

ZTO (0.500 g, 2.55 mmol) was added slowly to a mixture of 95% nitric acid (1.52 mL, 17.13 mmol) and acetic anhydride (3.28 mL, 34.67 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 1 h and poured into crushed ice. A pink solid was obtained (0.58 g, 80.0%). The final product was obtained as a white solid (0.54 g, 74.4%) recrystallized from CH₃CN-H₂O (v/v = 1:3). T_decomp = 126.55 °C (onset, 5 °C min⁻¹). – IR (KBr pellet): 3435.4, 3124.3, 3073.1, 1757.5, 1623.9, 1553.7, 1343.2, 1282.8, 1263.1, 1235.9, 1195.4, 1146.0, 1000.5, 894.4, 843.6 cm⁻¹. – ¹H NMR ([D₆]DMSO): δ = 9.260 (1H, CH) ppm. – ¹³C NMR ([D₆]DMSO): δ = 141.376 (C=O), 129.368 (C–H) ppm. – Matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry: m/z = 287.0232 [M+H]⁺, 309.0050 [M+H+Na]⁺. – Elemental analysis for C₄H₂N₁₀O₆ (M_r = 286.12): calcd. C 16.79, H 0.70, N 48.95; found: C 16.81, H 0.69, N 48.96%.

2.3 X-ray crystal structure determination

Crystals of DNZTO, suitable for single-crystal X-ray diffraction, were obtained by dissolving and retaining the compound in a minimum amount of CH₃CN at room temperature and subsequent filtration. The crystals were found to contain one molecule of CH₃CN per molecule of DNZTO. A colorless plate-like single crystal of dimensions 0.68 × 0.30 × 0.13 mm³ was mounted on a MiteGen MicroMesh using a small amount of Cargille Immersion Oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. The crystals were irradiated using graphite monochromatized MoK_α radiation (λ= 0.71073). An Oxford Cobra low-temperature device was used to keep the crystals constant at T = 153(2) K during data collection. The structure was solved and refined with the aid of the programs in the shelxtl-plus suite of programs [21, 22]. The full-matrix least-squares refinement on F² included atomic coordinates and anisotropic displacement parameters for all non-H atoms. The H atoms were included using a riding model. Table 1 summarizes important crystal structure data.

CCDC 1054257 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Crystal structure of DNZTO· Acetonitrile

The molecular and crystal structure of DNZTO · Acetonitrile is shown in Fig. 3. Selected bond lengths, angles, and torsion angles are listed in Table 2.

Fig. 3:

Molecular structure of DNZTO (two crystallographically independent molecules) and cell plot of DNZTO · Acetonitrile.

DNZTO · Acetonitrile crystallizes in the monoclinic space group P2₁/n with a cell volume of 1229.37 ų and four molecules in the unit cell. There are two crystallographically independent molecules of DNZTO each having crystallographically imposed centrosymmetry. There are only very minor differences between the independent molecules in the crystal as may be seen from the side-by-side listing of the molecular parameters in Table 2. The entire DNZTO molecules adopt a nearly planar structure with a strictly planar N₄ chain (torsion angles N(3)–N(4)–N(4)^#1–N(3)^#1 and N(8)–N(9)–N(9)^#2–N(8)^#2 = 180°), and an E-configured azo bond. The five-membered rings are only tilted to a minor extent with respect to the central N₄ chain as can be seen from the respective torsion angles in Table 2. The bond lengths N(4)–N(4)^#1 and N(9)–N(9)^#2 are 1.250(2) and 1.248(2) Å, respectively, which indicates a delocalization of the azo π bond along the N₄ moiety within DNZTO [23].

As seen from Table 2, the bond lengths N(1)–C(1) (1.2882(18) Å), C(1)–N(3) (1.3836(18) Å), N(3)–C(2) (1.3978(18) Å), N(2)–C(2) (1.4052(18) Å) in molecule I, as well as the bond lengths N(6)–C(3) (1.2891(19) Å), C(3)–N(8) (1.3826(17) Å), N(8)–C(4) (1.3948(19) Å), C(4)–N(7) (1.4004(18) Å) in molecule II are between the values for isolated C–N (1.4700 Å) and C=N (1.2730 Å) bonds, while the bond lengths N(2)–N(1) (1.3874(16) Å) and N(7)–N(6) (1.3831(16) Å) are between those of isolated N–N (1.4500 Å) and N=N (1.2500 Å) bonds. Therefore, C(1), N(1), N(2), C(2), N(3) and C(3), N(6), N(7), C(4), N(8) form a largely conjugated π system.

3.2 Thermal analysis

The thermogravimetric-differential thermal analysis (TG-DTA) curve of DNZTO is carried out at the linear heating rate of 5 °C min⁻¹ (Fig. 4). It shows that there is one exothermic peak in the decomposition process of DNZTO. The exothermic process is from 126.55 °C to 163.76 °C with the peak temperature of 149.05 °C.

Fig. 4:

TG/DTA curve of DNZTO at a heating rate of 5 °C min⁻¹.

3.3 Theoretical studies

The molecular structure and bond critical points of DNZTO are shown in Fig. 5. The structure has been fully optimized using the density functional theory (DFT) B3LYP method with the 6-311+G** basis set, which corresponds to the minimum energy points at the obtained molecular energy hypersurface (NImag = 0).

Fig. 5:

Molecular structure and bond critical points of DNZTO.

3.4 Natural bond orbital (NBO) analysis

In order to understand various second-order interactions between the filled orbitals of one subsystem, the second-order Fock matrix was established to evaluate donor (i)–acceptor (j) interaction in the NBO analysis [24]. For each donor (i) and acceptor (j), the stabilization energy E⁽²⁾ is associated with the delocalization and estimated as

E(2)=ΔEij=qiF(i, j)2/(εj−εi)

where q_i is the donor orbital occupancy, ε_j–ε_i are diagonal elements, and F(i, j) is the off-diagonal NBO Fock matrix element. NBO analysis provides the best method for interaction among bonds and also provides a convenient basis for investigating charge transfer in molecular systems [25, 26]. The larger the E² value, the more intensive is the donation tendency from electron donors to electron acceptors, and the greater is the extent of conjugation of the whole system [27]. Delocalization of electron density between occupied Lewis type (bond or lone pair) NBO orbitals and formally unoccupied (antibonding) non-Lewis NBO orbitals corresponds to a stabilizing donor–acceptor interaction. NBO analysis has been performed on the DNZTO molecule at the B3LYP/6-311+G(d, p) level in order to elucidate the intramolecular hybridization and delocalization of electron density within the molecule. The intramolecular hyperconjugative interactions of the BD N(1)–N(6) and BD* N(2)–C(3) orbital lead to strong stabilization energies of 4.94 kJ mol⁻¹. The most important interaction energy in this molecule is due to electron donation from LP(N1) to the antibonding acceptors BD* N(2)–C(3), BD* C(5)–O(9), and BD* N(6)–O(10) resulting in stabilization energies of 99.79, 191.38 and 171.58 kJ mol⁻¹, respectively. The same LP(N4) orbital with the antibonding acceptor BD* N(2)–C(3), BD* C(5)–O(9), and BD* N(8)–N(8′) leads to moderate stabilization energies of 152.58, 172.04, and 163.71 kJ mol⁻¹, respectively. The E⁽²⁾ values and types of the transitions are shown in Table 3.

3.5 Molecular electrostatic potential

The molecular electrostatic potential (MEP) is a plot of electrostatic potential mapped on the constant electron density surface displaying the electrostatic potential distribution. The different values are represented by different colors, red representing regions of most negative electrostatic potential (preferred site for electrophilic attack), blue representing regions of most positive electrostatic potential (preferred site for nucleophilic attack), and green representing regions of zero potential. To predict reactive sites for electrophilic and nucleophilic attack for DNZTO, the MEP at the B3LYP/6-311+G(d, p) level was mapped with the total electron density of the molecule. In Fig. 6, red indicates the more electron rich and blue the more electron poor areas. Furthermore, the polarization effect is clearly visible. The color code of this map is in the range between –0.0125 and 0.0125 (red and blue). Molecular shape, size, and dipole moments of the molecule provide a visual method to understand the relative polarity [28]. As can be seen from the MEP map of the molecule, the negative region is mainly localized at the O atoms of nitro groups, whereas the positive region lies in the five-membrane aromatic ring systems.

Fig. 6:

Molecular surfaces obtained using the B3LYP/6-311+G(d, p) level of DNZTO.

3.6 Total density Alpha density MEP ESP

Contour (Total density) Contour (HOMO) Contour (LUMO) Contour (ESP) are shown in Fig. 6.

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of DNZTO are shown in Fig. 7. The frontier orbital gap facilitates in characterizing the chemical reactivity and kinetic stability of the molecule. The red and green colors represent the positive and negative values for the wavefunction. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that mainly acts as an electron acceptor [29, 30]. The energy gap between the HOMO (–0.2970 a.u.) and the LUMO (–0.1435 a.u.) of the molecule is about 0.1535 a.u. The HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. Mulliken [31] has derived the wavefunctions for the ground state and excited states of the complex and the charge distribution over the atoms thus produces a way of examining the proton transfer process. The charge distributions calculated by the Mulliken method [32] for the equilibrium geometry of DNZTO are given in Table 4.

Fig. 7:

HOMO (left) and LUMO (right) of DNZTO.

3.7 Detonation performance

The theoretical parameters of DNZTO were calculated by the Kamlet-Jacobs equation [33, 34] and are presented in Table 5. Although some of the calculated values show a remarkable similarity to those of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) [35, 36] (Table 5), especially the friction sensitivity of only 10 N as compared to 120 N for RDX rules out that DNZTO can be used as RDX replacement. This could already be seen also from the low onset of thermal decomposition of 126 °C (vide supra), the value for RDX being 210 °C.