One Metal and Forty Nitrogens. Ab Initio Predictions for Possible New High-Energy Pentazolides

High-nitrogen-content energetic BNn+ (n = 4-16) clusters

Nitrogen-rich materials have attracted considerable attention in recent years as potential high-energy-density materials (HEDMs). However, their metastability poses substantial challenges for synthesis under ambient conditions. Here, we employ a novel strategy to explore energetic and structural features of the nitrogen-rich BNn+ (n = 4-16) clusters by doping with the light non-metal boron. A series of boron-doped nitrogen clusters, with nitrogen content ranging from 83% to 96%, were obtained by laser ablation and analyzed via time-of-flight mass spectrometry. The results indicate that the BN6+ cluster is dominant in the mass spectrum, while the BN12+ cluster exhibits the highest abundance after cooling the cluster source with liquid nitrogen. To further confirm the experimental results, extensive structure searches for BNn+ (n = 4-16) clusters are conducted using the CALYPSO method and density functional theory (DFT) calculations. The calculations show that BN6+ (singlet) exhibits a planar [B(NN)3]+ three-branched geometry with D3h symmetry, while BN12+ (singlet) adopts a [(linear-N6)B(N3)2]+ branched geometry with C1 symmetry. Stability analyses reveal the relatively high structural stabilities of BN6+ and BN12+ clusters and agree well with the experimental findings. Notably, the gas-phase enthalpy of formation of the BN12+ cluster is remarkably high, -1385.10 kJ mol-1, suggesting its potential as a high-energy building block for HEDM construction. The present findings enhance the structural diversities of non-metal-doped nitrogen clusters, and offer new perspectives for the rational design and synthesis of nitrogen-rich energetic materials.

Genesis of Polynitrogen Compounds Employing Silicon Substituted cyclo[18]carbon: A DFT Investigation

Activation of molecular N2 and its catalytic ability to form NH3 using C17Si has been already reported. This current study reports the formation of exclusive polynitrogen clusters (N4 and N5) on the C17Si ring. The clusters are generated using N2 and N3 respectively. Physical and chemical property analyses of the clusters show that the N5 cluster exhibits greater stability than N4. The former is seen to experience reduced molecular strain compared to the latter owing to its co-planar geometry. The thermodynamic calculations of the systems further show that the formation of the N5 cluster is spontaneous compared to N4 on the C17Si ring.

Revisiting the Hitherto Elusive Cyclohexanehexone Molecule: Bulk Synthesis, Mass Spectrometry, and Theoretical Studies

The cyclohexanehexone (C₆O₆) octahydrate molecule was claimed to be synthesized as early as 1862. However, the chemical in the 1862 study and the chemicals used in most of the existing studies and sold by most chemical vendors are actually dodecahydroxycyclohexane dihydrate (C₆(OH)₁₂·2H₂O). Here we revisit our bulk synthesis method of C₆O₆ by the dehydration of the C₆(OH)₁₂·2H₂O material, and report the mass spectrum of C₆O₆ that has been highly challenging to obtain owing to its high sensitivity toward ambient conditions. A new home-built electrospray ionization mass spectrometry setup in a glovebox is utilized to detect C₆O₆ in the form of C₆O₆H^-. Tandem mass spectrometry MSⁿ (n = 2-4) presents consecutive losses of CO molecules, further confirming the structure of C₆O₆. Theoretical calculations are performed to recover the chemical bonding of C₆O₆ and to rationalize the synthetic method. This work provides a benchmark understanding of the historically elusive C₆O₆.

Cis-Trans Isomerization and Thermal Decomposition Mechanisms of a Series of Nx (x = 4, 8, 10, 11) Chain-Catenated Energetic Crystals

Nitrogen-rich compounds based on heteroaromatic rings with different lengths of nitrogen chains are at the forefront of the energetic materials field. We studied the decomposition processes and reaction kinetics of a series of N_x (x = 4, 8, 10, 11) chain-catenated energetic crystals at various temperatures (2400-3000 K) based on a combinational strategy based on density functional tight binding molecular dynamics (DFTB-MD) simulations and density functional theory (DFT). The results show that the thermal decomposition and reaction kinetics are dependent on both the temperature and nitrogen chain's length. There are two sequential stages in the initial decomposition process for the crystals N₈ and N₁₀: (i) competition between cis-trans isomerization and initial unimolecular decomposition and (ii) subsequent complicated global decomposition reactions. Increasing either the temperature or nitrogen chain's length will accelerate the competition and make initial decomposition dominate. However, cis-trans isomerization does not occur in the crystals N₄ and N₁₁. The dominant initiation paths for N₄, N₈, and N₁₀ occur in the heterocycle and in the bond between the heterocycle and azo group, while that for N₁₁ is ring elimination. The decomposition reactions exhibit a clear first-order kinetics character. The energy paths based on DFT calculations are determined as an addition to the DFTB-MD results. Our findings provide insights into the comprehensive understanding of thermal decomposition behaviors of nitrogen chain-catenated and even all-nitrogen energetic materials.

Identification of octahedral coordinated ZrN12+ cationic clusters by mass spectrometry and structure searches

Cationic zirconium-doped nitrogen clusters were produced by laser ablation of a Zr : BN mixture target and were detected by TOF mass spectrometry. It is found that the mass peak of the ZrN12+ cluster is dominant in the spectrum. The ZrN12+ cluster was further dissociated with 266 nm photons. Extensive structure searches of a cationic ZrN12+ cluster indicate that the ground state structure of ZrN12+ consists of a central Zr atom and six N2 pairs with Oh symmetry. The calculated binding energy of the ZrN12+ cluster is about 0.96 eV, which is in accordance with the result of the photodissociation experiment. The neutral ZrN12 cluster has almost the same geometry, but with D3h symmetry. NBO analysis showed that the molecular orbitals of ZrN12+/0 clusters are mainly composed of Zr 4d and N 2p orbitals. These findings provide rich information for understanding the geometries and the electronic properties of zirconium-doped N clusters, which will offer valuable guidance for the exploration of other metal doped nitrogen clusters.

Predictions on High-Power Trivalent Metal Pentazolate Salts

High-energy-density materials (HEDMs) have been intensively studied for their significance in fundamental sciences and practical applications. Here, using the molecular crystal structure search method based on first-principles calculations, we have predicted a series of metastable energetic trivalent metal pentazolate salts MN₁₅ (M= Al, Ga, Sc, and Y). These compounds have high energy densities, with the highest nitrogen content among the studied nitrides so far. Pentazolate N₅^- molecules stack up face-to-face and form wave-like patterns in the C222₁ and Cc symmetries. The strong covalent bonding and very weak noncovalent interactions with nonbonded overlaps coexist in these ionic-like structures. We find MN₁₅ molecular structures are mechanically stable up to high temperature (∼1000 K) and ambient pressure. More importantly, these trivalent metal pentazolate salts have high detonation pressure (∼80 GPa) and velocity (∼12 km/s). Their detonation pressures exceeding that of TNT and HMX make them good candidates for high-brisance green energetic materials.

Recent advances in the syntheses and properties of polynitrogen pentazolate anion cyclo-N5− and its derivatives

This review summarizes recent developments and advances in pentazole chemistry, including substituted-pentazole precursors, strategies for the preparation of pentazolate anion, derivatives of pentazolate anion and their bonding properties.

Synthesis and characterization of the pentazolate anion cyclo-N5ˉ in (N5)6(H3O)3(NH4)4Cl

Pentazole (HN5), an unstable molecular ring comprising five nitrogen atoms, has been of great interest to researchers for the better part of a century. We report the synthesis and characterization of the pentazolate anion stabilized in a (N5)6(H3O)3(NH4)4Cl salt. The anion was generated by direct cleavage of the C–N bond in a multisubstituted arylpentazole using m-chloroperbenzoic acid and ferrous bisglycinate. The structure was confirmed by single-crystal x-ray diffraction analysis, which highlighted stabilization of the cyclo-N5ˉ ring by chloride, ammonium, and hydronium. Thermal analysis indicated the stability of the salt below 117°C on the basis of thermogravimetry-measured onset decomposition temperature.

Trapping N5 rings and N3 chains on the outer surface of fullerene C60: a theoretical study

The capture of N3-chains and N5-rings on the outer surface of C60 was studied using density functional calculations. For the neutral N5-ring, it was found that a N5-ring trapped by a C60 cage becomes more stable than an isolated N5-ring radical, and a C60-N5 compound with a C-N bond at an exohedral position of C60 is more stable than an isomer with the N5-ring encapsulated in C60. Such stability arises from the reduction in molecular strain energy, and charge transfer from C60 to N5. Dynamics calculations indicate that capture of the N5-ring on the outer surface of C60 is a barrierless process. Furthermore, the trapping sites of more N5-rings on the C60 were determined using condensed Fukui functions, where the N5-rings prefer to be trapped on the surface to form addition products across 6,6-junctions. Based on the optimized geometries of C60-(N5) n (n = 2, 6, 10), their chemical stabilities were found to be comparable with that of C60 in terms of the gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals. Similar phenomena were found for an N3-chain wrapped on the surface of C60. However, the results of the average adsorption energies show that C60 can capture N5-rings more effectively than N3-chains.

Experimental observation of TiN12+ cluster and theoretical investigation of its stable and metastable isomers

TiN n+ clusters were generated by laser ablation and analyzed experimentally by mass spectrometry. The results showed that the mass peak of the TiN12+ cluster is dominant in the spectrum. The TiN12+ cluster was further investigated by photodissociation experiments with 266, 532 and 1064 nm photons. Density functional calculations were conducted to investigate stable structures of TiN12+ and the corresponding neutral cluster, TiN12. The theoretical calculations found that the most stable structure of TiN12+ is Ti(N2)6+ with Oh symmetry. The calculated binding energy is in good agreement with that obtained from the photodissociation experiments. The most stable structure of neutral TiN12 is Ti(N2)6 with D3d symmetry. The Ti-N bond strengths are greater than 0.94 eV in both Ti(N2)6+ and its neutral counterpart. The interaction between Ti and N2 weakens the N-N bond significantly. For neutral TiN12, the Ti(N3)4 azide, the N5TiN7 sandwich structure and the N6TiN6 structure are much higher in energy than the Ti(N2)6 complex. The DFT calculations predicted that the decomposition of Ti(N3)4, N5TiN7, and N6TiN6 into a Ti atom and six N2 molecules can release energies of about 139, 857, and 978 kJ mol-1 respectively.

Chain or ring: which one is favorable in nitrogen-rich molecules N6XHm, N8XHm, and N10XHm (X = B, Al, Ga, m = 1 and X = C, Si, Ge, m = 2)?

A series of nitrogen-rich molecules N6XHm, N8XHm, and N10XHm (X = B, Al, Ga, m = 1 and X = C, Si, Ge, m = 2) consisting of N3 and N5 radicals, are systematically investigated by using B3LYP and B3PW91 DFT methods. It is found that for the nitrogen-rich molecules, the structures with N3-chains (N5-ring) are more stable than those containing a N3-ring (N5-chain). This result could be well-explained by the intrinsic stability of the N3 and N5 radicals and their charge distribution in nitrogen-rich molecules. The dissociation energies further indicate that the B-doped and C-doped structures are the most stable among the molecules with three elements of group 13 and 14, respectively. Energy decomposition analysis shows the bond of boron-nitrogen is stronger than that of carbon-nitrogen. Detailed bonding analysis demonstrates that the B-N bond is determined by σ and π interactions between the B and N atoms, whereas C-N bonds by only σ interactions. These results imply that the boron atom is more suitable than the carbon atom for building the nitrogen-rich molecules studied in this article.

Spin–spin coupling constants in homonuclear polynitrogen species

Quantum chemical calculations provide new insights into the dependence of J(N,N) coupling tensors on bonding environment in a series of polynitrogen species including N(5)(+).