In 2020, S. V. Bondarchuk proposed by density functional theory that poly(1,5-tetrazolediyl) — a hypothetical polymer whose covalent backbone is a chain of tetrazole rings linked at the ring nitrogen (position 1) and ring carbon (position 5), giving the repeat unit CN4_4 with 82.35 wt% nitrogen — is a dynamically and mechanically stable monoclinic (P21P2_1) crystal at ambient pressure, with density near 2.0 g cm−3^{-3}, a large positive enthalpy of formation (about +376 kJ mol−1^{-1} per repeat unit), and detonation performance comparable to the explosive RDX. The proposal, nicknamed 'CarNit4', is purely computational: the crystal came from a deliberately restricted eigenvector-following search, the thermochemistry and band gap were obtained with single functionals, and no sample has ever been synthesized. The open problem is to establish whether this predicted material actually exists: synthesize an ambient-recoverable CN4_4-backbone polymer and validate its predicted structure and properties, or demonstrate — experimentally or by a systematic polymorph and stability analysis — that no such metastable material is realizable. No ambient-stable CN4_4-backbone polymer has been reported.

Background

Energetic materials store chemical energy that can be released by extremely fast decomposition, and they underlie explosives and propellants. A persistent design goal in this field is the 'green' high-energy-density material (HEDM): a substance with a large positive enthalpy of formation (the heat absorbed when a compound forms from its elements — large positive values mean energy is stored in the compound), high density, and decomposition products that are mainly benign nitrogen gas. Tetrazoles — five-membered aromatic rings containing one carbon and four nitrogen atoms — are privileged building blocks because they combine a very high nitrogen fraction with a large positive heat of formation, and very many tetrazole-based energetic salts and molecular explosives have been reported. In nearly all known tetrazole-containing polymers (for example the energetic binder poly(vinyl tetrazole)), the tetrazole ring hangs as a side group on a conventional carbon chain; the ring itself is not the load-bearing polymer backbone.

Bondarchuk (2020) asked what happens if the tetrazole rings themselves form the chain. Poly(1,5-tetrazolediyl) is the hypothetical polymer obtained by connecting tetrazole rings through the ring nitrogen at position 1 and the ring carbon at position 5, giving a purely organic backbone with repeat unit CN4_4, 82.35 wt% nitrogen, and an oxygen balance of −47%-47\% (oxygen balance measures how oxygen-deficient a compound is relative to complete combustion of its carbon to CO2_2; a strongly negative value means the material needs an external oxidizer to burn completely). Using periodic density functional theory (DFT) — the standard first-principles method for crystal electronic structure, here with the PBE generalized-gradient approximation functional and the HSE06 hybrid functional — the paper reported a geometry-optimized monoclinic crystal in space group P21P2_1 with density about 2.0 g cm−3^{-3}; a phonon dispersion calculation (phonons are the collective lattice vibrations; absence of imaginary vibrational modes signals dynamical stability) showing no unstable modes; a positive-definite elastic stiffness tensor, satisfying the Born–Huang mechanical stability criteria; a computed band gap near 4.2 eV; computed IR and Raman spectra including characteristic inter-ring C–N marker vibrations; and a standard enthalpy of formation of about +376 kJ mol−1^{-1} per repeat unit. Feeding these quantities into the empirical Kamlet–Jacobs relations (which estimate detonation velocity and pressure from density and heat of formation) and the NASA CEA equilibrium-chemistry code (which computes combustion products and propulsive performance), the paper concluded that CarNit4 as a single explosive performs close to RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine, a standard military explosive), and that formulations with external oxidants sharply improve its propulsive metrics.

Everything about CarNit4, however, is computational. The P21P2_1 structure came from a deliberately restricted eigenvector-following crystal-structure search (a method that walks from an initial guess along unstable vibrational directions to nearby stationary points) rather than an exhaustive exploration of the polymorph landscape; the enthalpy of formation and band gap were each computed with a single functional, and the paper itself notes that different standard functionals diverge for such quantities; and no polymer with a pure CN4_4 backbone has ever been reported. The adjacent experimental chemistry underlines the gap rather than closing it: polymethylenetetrazole, synthesized and characterized in 2024, is a real energetic polymer with tetrazole rings in the backbone, but the rings are interrupted by CH2_2 spacer units; energetic polytetrazenes, synthesized in 2021, place nitrogen-rich trans-2-tetrazene units in the backbone, but these are not 1,5-linked tetrazole rings; and compressed 1H-tetrazole (CH2_2N4_4) amorphizes above roughly 14 GPa and polymerizes under compression — apparently through carbon–nitrogen bonding — only reversibly, with no polymeric product recovered at ambient conditions. Whether the tetrazole ring itself can serve as the backbone of an ambient-stable energetic polymer is therefore a well-defined, entirely open question.

Problem Statement

Establish experimentally whether the computationally predicted polymeric energetic material CarNit4 — a polymer whose covalent backbone consists of 1,5-linked tetrazole rings with repeat unit CN4_4 — exists as a synthesizable solid, or refute that possibility. Concretely, a resolution of the question takes one of two forms. (1) Realization: devise and carry out a synthesis (ambient wet chemistry, solid-state chemistry, or high-pressure synthesis with recovery) of the CN4_4-backbone polymer, recover it at ambient conditions as a persistent (meta)stable solid — that is, one that survives recovery to ambient temperature and pressure (including decompression after a high-pressure route) and persists without decomposing over a laboratory timescale sufficient for it to be isolated and characterized — and characterize it: elemental composition against the CN4_4 stoichiometry, spectroscopic evidence of an intact 1,5-tetrazolediyl backbone (ruling out CH2_2-spacer products such as polymethylenetetrazole and decomposition products such as carbon nitride), thermal persistence, and, where the sample is crystalline, diffraction data compared with the predicted P21P2_1 polymorph and its computed IR/Raman spectra and density. (2) Refutation: demonstrate, by systematic experiment or by a comprehensive crystal-structure-prediction and stability study that covers the polymorph landscape beyond the single eigenvector-following candidate, that no poly(1,5-tetrazolediyl) polymorph is recoverable and metastable at ambient conditions in the sense of branch (1). The scientific claim at stake is the source paper's core prediction: that a tetrazole ring can serve as the load-bearing backbone of an ambient-stable energetic polymer whose computed detonation performance is RDX-class. The target of the problem is the ambient-condition existence of such a backbone — its realization and validation under (1), or its refutation under (2) — not the independent reproduction of the computed performance metrics.

References

  1. Sergey V. Bondarchuk, "CarNit4 – A new polymeric energetic material based on poly(1,5-tetrazolediyl)", Polymer 186 (2020) 122048. DOI: 10.1016/j.polymer.2019.122048, https://doi.org/10.1016/j.polymer.2019.122048
  2. Dexiang Gao, Xingyu Tang, Xuan Wang, Xin Yang, Peijie Zhang, Guangwei Che, Jun Han, Takanori Hattori, Yajie Wang, Xiao Dong, Haiyan Zheng, Kuo Li, Ho-kwang Mao, "Phase transition and chemical reactivity of 1H-tetrazole under high pressure up to 100 GPa", Physical Chemistry Chemical Physics 23 (2021) 19503–19510. DOI: 10.1039/d1cp02913d, https://doi.org/10.1039/d1cp02913d
  3. Wenbo Li, Xiaoli Huang, Kuo Bao, Zhonglong Zhao, Yanping Huang, Lu Wang, Gang Wu, Bo Zhou, Defang Duan, Fangfei Li, Qiang Zhou, Bingbing Liu, Tian Cui, "A Novel High-Density Phase and Amorphization of Nitrogen-Rich 1H-Tetrazole (CH2N4) under High Pressure", Scientific Reports 7 (2017) 39249. DOI: 10.1038/srep39249, https://doi.org/10.1038/srep39249
  4. Ljubica Brenjo, Aleksandar Oklješa, Matija Tomšič, Berta Barta Holló, Jovica Nešić, Elvira Tóth, Črtomir Podlipnik, "Polymethylenetetrazole: Synthesis, Characterization, and Energetic Properties", Molecules 29 (2024) 3389. DOI: 10.3390/molecules29143389, https://doi.org/10.3390/molecules29143389
  5. John Eymann, Lionel Joucla, Guy Jacob, Jean Raynaud, Chaza Darwich, Emmanuel Lacôte, "Energetic Nitrogen-Rich Polymers with a Tetrazene-Based Backbone", Angewandte Chemie International Edition 60 (2021) 1578–1582. DOI: 10.1002/anie.202008562, https://doi.org/10.1002/anie.202008562
  6. Sergey V. Bondarchuk, "Theoretical evaluation of hexazinane as a basic component of nitrogen-rich energetic onium salts", Molecular Systems Design & Engineering 5 (2020) 1003–1011. DOI: 10.1039/d0me00007h, https://doi.org/10.1039/d0me00007h