Trinitromethyl-triazolone (TNMTO): a highly dense oxidizer

Abstract

A scalable synthesis of 5-(trinitromethyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (TNMTO) is possible from commercially available 2-methylpyrimidine-4,6-diol. It exhibits high density (1.90 g cm⁻³) with comparably low thermal stability (T_d = 80 °C) and positive oxygen balance (OB^co = 20.51%, OB^CO₂ = 0.0%). TNMTO has an attractive combination of detonation properties (P = 35.01 GPa, D = 8997 ms⁻¹) and propulsive properties (I_sp(neat) = 251.85 s, ρI_sp(neat) = 478.52 gs cm⁻³, ). These are superior to ammonium dinitroamide (ADN), 2,2,2-tetranitroacetimidic acid (TNAA) and ammonium perchlorate (AP), making it a potential green oxidizer in solid rocket propulsion.

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1. Introduction

Solid propellant, a composition of propellant/fuel, binder, plasticizer, oxidizer, and curing agents, is a new emerging field in high energy density materials (HEDMs).^1,2 These materials produce excellent propulsive thrust by generating high-temperature gaseous products upon deflagration during combustion. Many research groups contribute to developing novel solid propellants by designing their core components, such as propellants/fuel, binders, plasticizers, oxidizers, and curing agents. However, the two most important components of solid propellants are the fuel and oxidizer. In the last few decades, ammonium perchlorate-based (AP) composite propellants (with GAP/HTPB/Al) have been extensively studied in space-related applications. Ammonium perchlorate (AP) has an excellent oxygen balance (34%) with a high density (ρ = 1.95 g cm⁻³) and is very frequently used as an oxidizer to produce the best thrust in rocket propulsion systems.³ However, it produces extremely toxic gaseous products (HCl, Cl₂, NO, N₂O, etc.) during combustion, continuously contaminating the environment.⁴ Therefore, this is the ideal time to develop an improved oxidizer for rocket propulsion. Recently, some interesting green oxidizers (chlorine free), namely bis(3-nitro-1-(trinitromethyl)-1H-1,2,4-triazol-5-yl) methanone (BNTNMTMO, 1),⁵ 4,4′,5,5′-tetranitro-2,2′-bis-(trinitromethyl)-2H,2′H-3,3′-bipyrazole (TNBTNMBP, 2),⁶ 1,3-dinitro-6-(trinitromethyl)-1,2,3,4-tetrahydro-1,3,5-triazine (DNTNMTHT, 3),⁷ (Z)-N,2,2,2-tetranitroacetimidic acid (TNAA, 4),⁸ and 5-azido-10-nitro-1H,5H-bis(tetrazolo)[1,5-c:5′,1′-f]-pyrimidine (ANBTzP),⁹ have been developed, which have shown potential for replacing AP since they exhibit excellent energetic properties as oxidizers are listed in Fig. 1. On the other hand, ammonium dinitramide (ADN)¹⁰ and hydrazinium nitroformate (HNF)¹¹ are well-known chlorine-free energetic materials; however, they also have some drawbacks, including their high negative enthalpies of formation, high sensitivities, and hygroscopic nature. Nitrotrizolone (NTO, 5) was synthesized a century ago, and is a unique molecule that has many desirable features, including high density, high detonation velocity, and pressure with its unexpected insensitivity toward the mechanical stimulus, which makes it an interesting energetic material. Thus, it has been extensively utilized in munitions.¹² Recently, we have reported some different analogues of NTO.¹³ In a continuing effort to explore its chemistry, our goal has been to synthesize novel high-energy-density materials (oxidizers, propellants, and explosives) with excellent properties. In the series of new oxidizers, we now report the synthesis of TNMTO, 6 as a green and highly dense oxidizer with balanced performance for rocket propulsion.

Fig. 1 Representative oxidizers with trinitro-methyl (–C(NO₂)₃) moieties^5–8 and NTO.¹²

2. Results and discussion

2.1. Synthesis

Commercially available 2-methylpyrimidine-4,6-diol 7, which was first nitrated with a 1 : 1 nitrating mixture (98% H₂SO₄ and 100% HNO₃) at 0 °C to isolate pure compound 8,¹⁴ for the first time, in pure form. Subsequently 8 was treated with hydrazine (40% solution in H₂O) to give 9 in excellent yield. Then 9 under acidic conditions (12 M HCl) formed 5-(dinitromethyl)-2,4-dihydro-3H-1,2,4-triazol-3-one 10 ¹³ in excellent yield. Finally, compound 10 was further nitrated with 100% HNO₃ to give 5-(trinitromethyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (TNMTO, 6) as shown in Scheme 1. When compound 8 was treated with aqueous NH₃ (30% solution), excellent yields of 11 resulted. Its structure was confirmed by single crystal X-ray analysis.

Scheme 1 Synthesis of TNMTO.

2.2. Crystal structure

Compound 8 yields FOX-7¹⁵ on hydrolysis (see Fig. S24 and S25, ESI†). TNMTO (6) was crystallized by slow recrystallization from DCM and exhibits a monoclinic P21/n space group. Its crystal density is 1.902 g cm⁻³ at 100 K with a single molecule in the asymmetric unit (Z = 4, Z′ = 1), as seen in Fig. S5–S9 (see ESI†). TNMTO crystals have a network of intermolecular H-bonds and π–π-interactions, which contribute to the higher density of 1.902 g cm⁻³ at 100 K and an experimental density of 1.900 g cm⁻³ at 25 °C. The C–C bond length (C2 = C3 = 1.484 Å) for compound TNMTO is slightly shorter than TNAA (C1 = C2 = 1.534 Å). The torsion angle of N3–C2–C3–N4 is 136.6°, N3–C2–C3–N5 is 103.8°, N3–C2–C3–N6 is 14.4° and that of N1–C2–C3–N4 is 49.9°, N1–C2–C3–N5 is 69.7°, N1–C2–C3–N6 is 172.09° suggesting that the molecule is rigidly packed (Fig. 2). The H-bonding interplay has a maximum 3.1 Å D–D distance and a minimum 110° angle present in TNMTO: N1–O1: 2.77 Å, N2–O1: 2.846 Å, N2–O6: 2.988 Å, O3–O7: 2.825 Å.

Fig. 2 (a) Single crystal X-ray structure; and (b) packing diagram; (c) H-bonds of compounds 6, 9 and 11·H₂O.

Compound 9 was crystallized by slow recrystallization from CH₃CN and exhibits a monoclinic P21/n space group (see Fig. 2 and Table S11, ESI†). Compound 11 was crystallized by slow recrystallization from CH₃CN and exhibits a monoclinic P21/n space group (see Fig. 2 and Table S12, ESI†).

2.3. Physicochemical properties

The enthalpy of formation of TNMTO was estimated using Gaussian 03 suite of programs¹⁸ with the help of the isodesmic method (see Table 1 and Fig. S1, ESI†). Subsequently, corresponding detonation and propulsive performances were evaluated with and experimental densities by using EXPLO5 V6.06 software.¹⁹TNMTO exhibits a negative , −64.47 kJ mol⁻¹. However, it is superior to TNAA (−134.60 kJ mol⁻¹), AP (−295.80 kJ mol⁻¹), and ADN (−134.60 kJ mol⁻¹). Oxygen balance (OB, Ω), the degree to which all hydrogen can be converted to H₂O and all carbon into CO or CO₂, is listed in Table 1. TNMTO has a high positive OB at 20.51% with a high density of 1.900 g cm⁻³, comparable to well-known oxidizers TNAA and ADN. The efficiency of fuels/propellants can be measured in terms of their specific impulse (I_sp). The propulsive power of TNMTO was evaluated at two different compositions (i) neat compound, and (ii) composite formulation with aluminium (Al). Results revealed that, as a neat compound, TNMTO exhibits a propulsive power superior to that of TNAA, ADN, AP, K₂BDAF and K₂DNABT. While in the composite formulation, it performed better than TNAA, K₂BDAF, AP, ADN and illustrate slightly lower density specific impulse (ρI_sp) than K₂DNABT (Table 1 and Fig. 3).

Fig. 3 Comparison of propulsive properties.

Table 1 Comparison of physicochemical properties of TNMTO, TNAA, AP, ADN, K₂BDAF and K₂DNABT

Compound	TNMTO, 6	TNAA	AP	ADN	K 2 BDAF	K 2 DNABT
a  Ref. 8. b  Ref. 1, 3 and 8. c  Ref. 1 and 5. d  Ref. 16. e  Ref. 17. f Molecular formula. g Formula weight. h Oxygen balance (based on CO). i Oxygen balance (based on CO2). j Oxygen content in %. k Nitrogen and oxygen contents in %. l Calculated enthalpy of formation. m Measured densities, gas pycnometer at 25 °C. n Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 °C min^−1). o Calculated detonation velocity. p Calculated detonation pressure. q Heat of detonation. r Impact sensitivity. s Friction sensitivity. t Bond dissociation energies of trigger bond. u Φ H–L = HOMO–LUMO energy gaps. v I sp = specific impulse of neat compound. w ρI sp = density specific impulse of neat compound. x Characteristic velocity. y I sp = specific impulse at 88% compound and 12% Al. z ρI sp = density specific impulse at 88% compound and 12% Al. aa Specific impulse were calculated at ambient pressure of 0.1 MPa, chamber pressure of 7 MPa, an isobaric pressure of 70 bar and initial temperature of 3300 K. ab This work.
Formula^f	C3H2N6O7	C2HN5O9	NH4ClO4	NH4N(NO2)2	C6K2N10O10	C2K2N12O7
FW(g mol^−1)^g	234.08	238.98	117.49	124.06	450.30	334.30
Ω^CO (%)^h	20.51	43.50	34.04	25.80	10.66	14.35
Ω^CO2 (%)^i	0.0	30.12	27.23	25.80	–7.10	–4.8
O (%)^j	48	60	54	45	36	19
N + O (%)^k	83.74	89.53	66.39	96.75	66.63	69.42
ΔHf (kJ mol^−1)^b^,^l	−64.47	–134.60	–295.80	–149.72	110.10	326.40
ρ (g cm^−3)^c^,^m	1.90	1.87	1.95	1.81	2.04	2.11
T d (°C)^n	80	137	200	159	229	200
P (GPa)^o	35.01	23.00	15.80	23.60	30.10	31.70
D (ms^−1)^p	8997	7503	6368	7860	8138	8330
−Q [kJ kg^−1]^q	5626	3215	1435	2754	5514	4959
IS (J)^r	≥10	≥19	≥15	3–5	≥2	≥1
FS (N)^s	≥120	≥20	≥360	64–72	≥20	≤1
BDEs (kJ mol^−1)^t	58.15	102.11	—	—	—	—
Φ H–L (eV)^u	3.549	5.385	—	—	—	—
I sp (s)^v^,^aa	251.85	208.61	156.63	202.14	227.26^ab	210.49^ab
ρI sp (gs cm^−3)^w^,^aa	478.52	390.11	305.43	365.87	463.61^ab	444.13^ab
C* (ms^−1)^x^,^aa	1501	1289	977	1267	1398^ab	1258^ab
I sp (s)^y^,^aa	258.58	241.32	232.00	256.25	242.39^ab	239.17^ab
ρI sp (gs cm^−3)^z^,^aa	509.41	468.55	468.85	482.42	509.40^ab	518.21^ab

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Mulliken's charges are shown in Fig. 4, C3–C18 is the most polar bond and N14 is the most negatively charged N-atom in the molecule.

Fig. 4 Mulliken charges on TNMTO, 6.

The electrostatic potentials (ESP) of TNMTO, 6 were analysed by B3LYP/6-311++G(d,p) method and plotted with Multiwfn and VMD software.²⁰ the negative fraction (blue) and a positive fraction (red) located on nitro groups and triazolone ring, respectively and indicate the comparatively more and less active sites on the molecule surface (Fig. 5a).

Fig. 5 (a) Electrostatic potential (ESP) maps of TNMTO, 6. (b) HOMO–LUMO molecular orbitals.

Since ESP gives the indication of impact sensitivity, the high electron availability (more negative ESP surface), resulted in less sensitivity. The ratio of negative surface area for TNMTO was calculated to be 50.10, and that is higher than NTO (47.38), RDX (44.26) and TNT (42.12) (see Table S3, ESI†). Based on the balance of charges, the stability order is HMX > RDX > TNT > NTO > TNMTO (see Table S3, ESI†). The kinetic stability of TNMTO was predicted as HOMO and LUMO energy gaps, and order of kinetic stabilities is TNMTO < TNAA < TNT < RDX < HMX.

Furthermore, the relationship between the quantum mechanical electron density (ρ) and reduced density gradient (s = 0.5(3π²)^1/3)|∇ρ|/ρ^4/3 for TNMTO was obtained and shown as noncovalent interaction (NCl) plots. Where, reduced density gradient spikes at zero, negative and positive density values of sign (λ²)ρ(r) signify the van der Waals (VDW) interactions, H-bond interactions, steric effect, respectively (Fig. 6). Most of these close contacts are in the range of 2.770 Å to 2.988 Å, confirming the presence of van der Waals (VDW) interactions and H-bond interactions in the molecule.

Fig. 6 Non-covalent interaction (NCI): (a) reduced density gradient (RDG) and (b) scatter diagram of compound TNMTO, 6 at B3LYP/6-311++G(d,p) level.

The stability of TNMTO crystals is supported by various types of interactions (O–O (40.5%), O–N (20.2%) and O–H (30.0%)), which revealed with the help of Hirshfeld surfaces analysis (calculated with CrystalExplorer 21.5 software),²¹ as shown in Fig. 7.

Fig. 7 (a) Hirshfeld surfaces in crystal stacking for compound TNMTO, 6. (b) Arrangement in crystal packing. (c) 2D-fingerprint and individual contribution of major close contacts.

The thermal stabilities of the new compounds were measured at two different heating rates (5 °C min⁻¹ and 10 °C min⁻¹) using DSC and TGA (see Fig. S26–S42, ESI†) and the results are also agreed supported with their bond dissociation energies (BDEs) (Table 1). The bond dissociation energies (BDEs) of the trigger bond of TNMTO is 58.15 kJ mol⁻¹ which is significantly lower than that of TNAA (102.11 kJ mol⁻¹) (Table 1). TNMTO is solid at room temperature and decomposes at 80 °C on heating without melting (see Fig. S26, ESI†). The mechanical stabilities of TNMTO were determined by measuring their friction sensitivities (FS) and impact sensitivities (IS) using friction tester and BAM drop hammer techniques²² and found to be 10 J and 120 N, respectively, which are comparable to RDX (120 N). These indicate that TNMTO is more stable than ADN (IS = 3–5 J and FS = 64–72 N).

Detonation properties of TNMTO were estimated by using the EXPLO5 program (v. 6.06).¹⁹TNMTO exhibits superior detonation properties (P = 35.01 GPa, D = 8997 ms⁻¹) than RDX (P = 34.07 GPa, D = 8867 ms⁻¹), TNAA (P = 23.00 GPa, D = 7503 ms⁻¹), ADN (P = 23.60 GPa, D = 7860 ms⁻¹), K₂BDAF (P = 30.10 GPa, D = 8138 ms⁻¹), K₂DNABT (P = 31.70 GPa, D = 8330 ms⁻¹), and AP (P = 15.80 GPa, D = 6368 ms⁻¹). The heat of detonation (Q, energy) and characteristic velocity (C*, efficacy) of TNMTO (−Q = 5626 kJ kg⁻¹, C* = 1501 ms⁻¹) are significantly higher than TNAA (−Q = 3215 kJ kg⁻¹, C* = 1289 ms⁻¹, AP (−Q = 1435 kJ kg⁻¹, C* = 977 ms⁻¹), ADN (−Q = 2754 kJ kg⁻¹, C* = 1267 ms⁻¹), K₂BDAF (−Q = 5514 kJ kg⁻¹, C* = 1267 ms⁻¹) and K₂DNABT (−Q = 4959 kJ kg⁻¹, C* = 1267 ms⁻¹) (Table 1, Fig. S45 and 46, ESI†).

3. Conclusion

A scalable synthesis of TNMTO has been developed. TNMTO was fully characterized by various techniques such as FTIR, NMR, elemental analysis and single crystal X-ray analysis. This new compound exhibits a very high density (1.90 g cm⁻³), supported by intermolecular H-bonds and π–π-interactions. However, the cyclic urea moiety makes it more hygroscopic. Based on calculated detonation properties (P = 35.01 GPa, D = 8997 ms⁻¹) and ballistic properties (I_sp(neat) = 251.85 s, ρI_sp(neat) = 478.52 gs cm⁻³, ), it has potential to replace AP and ADN as a green oxidizer in solid rocket propulsion. TNMTO exhibits greater stability toward mechanical stimuli and comparably lower thermal stability than ADN.

Author contributions

S. L. investigation, methodology, conceptualization and manuscript writing. R. J. S. X-ray data collection and structure solving. S. L. and J. M. S. conceptualization, manuscript writing – review and editing, supervision.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The Rigaku Synergy S Diffractometer was purchased with support from the National Science Foundation MRI program (1919565). We are grateful for the support of the Fluorine-19 fund.

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Footnote

† Electronic supplementary information (ESI) available. CCDC numbers are 2268205–2268207. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt02232c

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