N5 Ring Research Articles

The structural, electronic properties, pressure response and decomposition mechanism of bispentazole (N10) with first principles calculations

Bispentazole (N10) is one of the recently-found full-nitrogen energetic materials, which have remarkable performance in the pursuit of higher energy and greener explosive. Therefore, the structural, electronic properties, pressure response, and decomposition mechanism of four types of N10, namely P21-N10, I-N10, V-N10, and P-N10, are investigated using first-principles calculations. Our results show that all the structures are thermodynamic stability, and P21-N10 and P-N10 are dynamical stability. All N10 structures belong to insulators with indirect band gaps. The results show that the N-N bond on the five-membered cyclic ring, which is on the opposite side of the N-N bond connected to the two five-membered cyclic rings, is relatively weak. From the decomposition pathway of the N10 molecule, it can be observed that the product N2 is gradually separated from two cyclic N5 rings in the molecule. The result demonstrated that the N-N bond in full-nitrogen materials can release energy by breaking of N-N bond to form the stable N2. The calculated three reaction energy barriers means that the decomposition temperature of N10 is lower than that of N8 and N6. Finally, the released energy as 168.0 kcal/mol indicates that the N10 molecule can release a lot of energy when it decomposes.

Prediction of novel tetravalent metal pentazolate salts with anharmonic effect

In recent decades, pentazolate salts have gained considerable attention as high energy density materials (HEDMs). Using the machine-learning accelerated structure searching method, we predicted four pentazolate salts stabilized with tetravalent metals (Ti-N and Zr-N). Specifically, the ground state MN20 (M = Ti, Zr) adopts the space-group P4/mcc under ambient conditions, transforming into the I-4 phase at higher pressure. Moreover, the I-4-MN20 becomes energetically stable at moderate pressure (46.8 GPa for TiN20, 38.7 GPa for ZrN20). Anharmonic phonon spectrum calculations demonstrate the dynamic stabilities of these MN20 phases. Among them, the P4/mcc phase can be quenched to 0 GPa. Further ab-initio molecular dynamic simulations suggest that the N5 rings within these MN20 systems can still maintain integrity at finite temperatures. Calculations of the projected crystal orbital Hamilton population and reduced density gradient revealed their covalent and noncovalent interactions, respectively. The aromaticity of the N5 ring was investigated by molecular orbital theory. Finally, we predicted that these MN20 compounds have very high energy densities and exhibit good detonation velocities and pressures, compared to the HMX explosive. These calculations enrich the family of pentazolate compounds and may also guide future experiments.

Free and Complexed Fluoropentazoles: Anomalous Ring Charge and Appreciable Kinetic Stability.

Fluoronitrogens are strongly related to high-energy density materials. Fluoropentazole (cyclo-FN5) with hitherto the highest N/F ratio at ambient pressure belongs to the family of well-known and 119-year-old pentazoles. Due to the presence of the overwhelmingly electronegative F element, cyclo-FN5 is the only pentazole to date that contains a cationic N5 ring. However, also due to F's large electronegativity, the reported ring-destruction barrier of cyclo-FN5 is nearly the lowest (6.7 kcal/mol), which has made its synthesis and characterization quite difficult. Here, cyclo-FN5 was re-predicted to bear an almost doubled ring-destruction barrier (∼12-14 kcal/mol) at the CBS-QB3, G4, W1BD, CCSD(T)/CBS//CCSD/cc-pVTZ, and CCSD(T)/CBS//CCSD(T)/aug-cc-pVTZ levels. Promisingly, upon further complexation with Lewis acid(s), the ring-destruction barrier of cyclo-FN5 might reach 23 kcal/mol, which is comparable to that of the well-known and already synthesized arylpentazoles (∼20 kcal/mol), and the positive charge on the N5 ring can be increased to 0.66 |e|.

Pressure-stabilized high-energy-density material YN10

Polynitrogen compounds have been intensively studied for potential applications as high energy density materials, especially in energy and military fields. Here, using the swarm intelligence algorithm in combination with first-principles calculations, we systematically explored the variable stoichiometries of yttrium–nitrogen compounds on the nitrogen-rich regime at high pressure, where a new stable phase of YN10 adopting I4/m symmetry was discovered at the pressure of 35 GPa and showed metallic character from the analysis of electronic properties. In YN10, all the nitrogen atoms were sp 2-hybridized in the form of N5 ring. Furthermore, the gravimetric and volumetric energy densities were estimated to be 3.05 kJ g−1 and 9.27 kJ cm−1 respectively. Particularly, the calculated detonation velocity and pressure of YN10 (12.0 km s−1, 82.7 GPa) was higher than that of TNT (6.9 km s−1, 19.0 GPa) and HMX (9.1 km s−1, 39.3 GPa), making it a potential candidate as a high-energy-density material.

High-energy-density pentazolate salts: CaN10 and BaN10

The search for high energy density materials (HEDMs) in polymeric nitrogen compounds has gained considerable attention. Previous theoretical predictions and experiments have revealed that metal ions can be used to stabilize the pentazolate (N 5 − ) anion. In this work, by employing a machine learning-accelerated crystal structure searching method and first-principles calculations, we found that the new pentazolate salts, CaN10 and BaN10, are energetically favorable at high pressures. Phonon dispersion calculations reveal that they are quenchable at ambient pressure. Ab initio molecular dynamics simulations verify their dynamic stability at finite temperature. Bader charge and electron localization function illustrates that alkaline earth atoms serve as electron donors, contributing to the stability of N5 rings. Bonding calculations reveal covalent bonds between nitrogen atoms and weak interactions between N5 rings. Similar to other pentazolate salts, these polymeric nitrides have high energy densities of approximately 2.35 kJ/g for CaN10 and 1.32 kJ/g for BaN10. The predictions of CaN10 and BaN10 structures indicate that these salts are potential candidates for green nitrogen-rich HEDMs.

To Be or Not To Be Protonated: cyclo-N5- in Crystal and Solvent.

Pentazole (HN5) and its anion ( cyclo-N5-) have been elusive for nearly a century because of the unstable N5 ring. Recently, Zhang et al. reported the first synthesis and characterization of the pentazolate anion cyclo-N5- in (N5)6(H3O)3(NH4)4Cl salt at ambient conditions ( Science 2017, 355, 374 ). However, whether the cyclo-N5- in (N5)6(H3O)3(NH4)4Cl salt is protonated or not has been debated ( Huang and Xu, Science, 2018, 359, eaao3672 ; Jiang et al. Science, 2018, 359, aas8953 ). Herein, we employed ab initio molecular dynamics (AIMD) simulations, which can well present the dynamic behavior at realistic experimental conditions, to examine the potential protonated state of cyclo-N5- in both crystal and dimethyl sulfoxide (DMSO) solvent. Our simulations revealed that the protonation reaction of (N5)6(H3O)3(NH4)4Cl → (N5)5(N5H)(H2O)(H3O)2(NH4)4Cl is thermodynamically spontaneous according to Δ G < 0, and the small energy barrier of 12.6 kJ/mol is not enough to prevent the partial protonation of cyclo-N5- due to the temperature effect; consequently, both deprotonated and protonated cyclo-N5- exist in the crystal. In comparison, the DMSO solvent effect can remarkably reduce the difference of proton affinities among cyclo-N5-, H2O, and NH3, and the temperature effect can finally break these hydrogen bonds and lead to the deprotonated cyclo-N5- in DMSO solvent. Our AIMD simulations reconcile the recent controversy.

Simple Route to Metal cyclo-N5– Salt: High-Pressure Synthesis of CuN5

A naked cyclo-N5– salt has yet to be recovered to ambient conditions, precluding its application as a high-energy density material for explosive or propulsion applications. Here, we suggest a simple route for the synthesis of a metal cyclo-N5– salt via compressing CuN6. Using first-principles calculations with structural search, we predict a CuN5 compound with a cyclo-N5– anion that is energetically favorite in the pressure range of 50–100 GPa. At ambient conditions, CuN5 comprises alternant-connected cyclo-N5– anions and Cu+ ions, forming a zigzag chain. The copper cyclo-N5– salt is thermodynamically stable with a 2.5 eV band gap at ambient conditions. Further analysis of electronic properties reveals that the Cu atoms not only contribute electrons to change the bonding state of N5 rings but also use empty outer-shell orbitals to accommodate lone pair electrons of N atoms, forming coordinate bonds to stabilize the system. This expands the known cyclo-N5– salt and indicates a simple route to its synthesis.

Modulation of the flavin-protein interactions in NADH peroxidase and mercuric ion reductase: a resonance Raman study.

NADH peroxidase (Npx) and mercuric ion reductase (MerA) are flavoproteins belonging to the pyridine nucleotide:disulfide oxidoreductases (PNDO) and catalyzing the reduction of toxic substrates, i.e., hydrogen peroxide and mercuric ion, respectively. To determine the role of the flavin adenine dinucleotide (FAD) in the detoxification mechanism, the resonance Raman (RR) spectra of these enzymes under various redox and ligation states have been investigated using blue and/or near-UV excitation(s). These data were compared to those previously obtained for glutathione reductase (GR), another enzyme of the PNDO family, but catalyzing the reduction of oxidized glutathione. Spectral differences have been detected for the marker bands of the isoalloxazine ring of Npx, MerA, and GR. They provide evidence for different catalytic mechanisms in these flavoproteins. The RR modes of the oxidized and two-electron reduced (EH2) forms of Npx are related to very tight flavin-protein interactions maintaining a nearly planar conformation of the isoalloxazine tricycle, a low level of H-bonding at the N1/N5 and O2/O4 sites, and a strong H-bond at N3H. They also indicate minimal changes in FAD structure and environment upon either NAD(H) binding or reduction of the sulfinic redox center. All these spectroscopic data support an enzyme functioning centered on the Cys-SO-/Cys-S- redox moiety and a neighbouring His residue. On the contrary, the RR data on various functional forms of MerA are indicative of a modulation of both ring II distortion and H-bonding states of the N5 site and ring III. The Cd(II) binding to the EH2-NADP(H) complexes, biomimetic intermediates in the reaction of Hg(II) reduction, provokes important spectral changes. They are interpreted in terms of flattening of the isoalloxazine ring and large decreases in H-bonding at the N5 site and ring III. The large flexibility of the FAD structure and environment in MerA is in agreement with proposed mechanisms involving C4a(flavin) adducts.

Screening benzylpentazoles for replacing PhN5 as cyclo-N5 − precursor by theoretical calculation

Benzylpentazole (B) and its derivatives (B-NH 2 ~ B-NF 2 ) with electron-donating group (−NH2, −NHMe, −NMe2, and−OH) and electron-withdrawing group (−NO2, −CN, −CF3 and −NF2) were studied using density functional theory to assess their potentials for replacing phenylpentazole (PhN 5 ) as cyclo-N5 − precursor. The pyrolysis mechanisms of the N5 ring and the C─N bond connecting pentazole and benzyl, bond lengths, and atomic charge have been investigated for revealing the effects of substituents on stability and assessing the possibilities of benzylpentazoles as cyclo-N5 − precursor. Substituents especially electron-withdrawing groups lower the N5 ring stability and the C─N bond stability of B simultaneously. Compared to PhN 5 , benzylpentazoles mostly possess higher activation energy for the N5 ring decomposition (81.2 ~ 101.7 kJ/mol), lower C─N bond dissociation energy (385.6 ~ 452.8 kJ/mol), and higher chemical stability (7.83 ~ 8.49 eV). B and B-NMe 2 with the highest N5 ring stability, appropriate C─N bond stability, and chemical stability are the most potential candidates for replacing PhN 5 as cyclo-N5 − precursor.

High-Pressure Synthesis of a Pentazolate Salt

The pentazolates, the last all-nitrogen members of the azole series, have been notoriously elusive for the last hundred years despite enormous efforts to make these compounds in either gas or condensed phases. Here, we report a successful synthesis of a solid state compound consisting of isolated pentazolate anions N5–, which is achieved by compressing and laser heating cesium azide (CsN3) mixed with N2 cryogenic liquid in a diamond anvil cell. The experiment was guided by theory, which predicted the transformation of the mixture at high pressures to a new compound, cesium pentazolate salt (CsN5). Electron transfer from Cs atoms to N5 rings enables both aromaticity in the pentazolates as well as ionic bonding in the CsN5 crystal. This work provides critical insight into the role of extreme conditions in exploring unusual bonding routes that ultimately lead to the formation of novel high nitrogen content species.

Topology and Structure/Function Correlation of Ring- and Gate-forming Domains in the Dynamic Secretin Complex of Thermus thermophilus

Secretins are versatile outer membrane pores used by many bacteria to secrete proteins, toxins, or filamentous phages; extrude type IV pili (T4P); or take up DNA. Extrusion of T4P and natural transformation of DNA in the thermophilic bacterium Thermus thermophilus requires a unique secretin complex comprising six stacked rings, a membrane-embedded cone structure, and two gates that open and close a central channel. To investigate the role of distinct domains in ring and gate formation, we examined a set of deletion derivatives by cryomicroscopy techniques. Here we report that maintaining the N0 ring in the deletion derivatives led to stable PilQ complexes. Analyses of the variants unraveled that an N-terminal domain comprising a unique βββαβ fold is essential for the formation of gate 2. Furthermore, we identified four βαββα domains essential for the formation of the N2 to N5 rings. Mutant studies revealed that deletion of individual ring domains significantly reduces piliation. The N1, N2, N4, and N5 deletion mutants were significantly impaired in T4P-mediated twitching motility, whereas the motility of the N3 mutant was comparable with that of wild-type cells. This indicates that the deletion of the N3 ring leads to increased pilus dynamics, thereby compensating for the reduced number of pili of the N3 mutant. All mutants exhibit a wild-type natural transformation phenotype, leading to the conclusion that DNA uptake is independent of functional T4P.

Theoretical investigations on the stability of alkali metal substituted phenylpentazole.

The alkali metal (M=Li, Na, and K) para-substituted (M-1), meta-substituted (M-2) or ortho-substituted (M-3) derivatives of phenylpentazole (PhN5) were studied using density functional theory. The substituted metals improve the energy barrier for decomposition of the N5 ring of PhN5 by 19.3 ∼ 65.0 kJ/mol. M-3 has the ionic N-M bond, which is not found for M-1 and M-2. M-1 and M-2 have similar electrostatic potentials and dispersion interactions between metal and N5 ring. The comparable intramolecular interactions of M-1 and M-2 lead to similar N5 ring stability. Compared to M-1 and M-2, M-3 has a more negative charge on N5 ring and stronger dispersion interaction. The stronger intramolecular interactions of M-3 result in the higher N5 ring stability. For M-1 and M-2, different metals have slight affects on N5 ring stability. For M-3, N5 ring stability decreases in the order of Li > Na > K. The substituted metal lowers E(g) of PhN5.

Theoretical investigations on stability of pyridylpentazoles, pyridazylpentazoles, triazinylpentazoles, tetrazinylpentazoles, and pentazinylpentazole searching for a replacement of phenylpentazole as N5 (-) source.

Stabilities of pyridylpentazoles, pyridazylpentazoles, triazinylpentazoles, tetrazinylpentazoles, and pentazinylpentazole were studied using density functional theory to assess their potentials as the source of pentazole anion (N5 (-)) for replacement of phenylpentazole (PhN 5 ). Replacing the aryl group of PhN 5 by six-member heterocycle weakens pentazole ring. Compared to PhN 5 , title molecules have longer N-N bonds and lower activation energy (E a,1) needed for the N5 ring breaking. E a,1 decreases with the increasing number of nitrogen atoms of heterocycle. The ortho nitrogen of heterocycle most obviously lowers the stability of pentazole. The central C-N bond dissociation energies (BDEs) of title molecules are lower than that of PhN 5 . For the molecule with 0~1 ortho-nitrogen, H rearrangement happens during the central C-N bond breaking. The energy (E a,2) required for H rearrangement is considerably smaller than the corresponding BDE. ΔE a,2 (E a,2(PhN5) - E a,2 = 7.5~35.7 kJ mol(-1)) is larger than ΔE a,1 (E a,2(PhN5) - E a,2 = 4.6~15.5 kJ mol(-1)), while ΔE a,2/E a,2(PhN5) (2~9.5 %) is smaller than ΔE a,1/E a,1(PhN5) ( 4.4~15.0 %). The larger ΔE a,1/E a,1(PhN5) suggests that title molecules can not be the better N5 (-) than PhN 5 .

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.

Novel lithium-nitrogen compounds at ambient and high pressures.

Using ab initio evolutionary simulations, we predict the existence of five novel stable Li-N compounds at pressures from 0 to 100 GPa (Li13N, Li5N, Li3N2, LiN2, and LiN5). Structures of these compounds contain isolated N atoms, N2 dimers, polyacetylene-like N chains and N5 rings, respectively. The structure of Li13N consists of Li atoms and Li12N icosahedra (with N atom in the center of the Li12 icosahedron) – such icosahedra are not described by Wade-Jemmis electron counting rules and are unique. Electronic structure of Li-N compounds is found to dramatically depend on composition and pressure, making this system ideal for studying metal-insulator transitions. For example, the sequence of lowest-enthalpy structures of LiN3 shows peculiar electronic structure changes with increasing pressure: metal-insulator-metal-insulator. This work also resolves the previous controversies of theory and experiment on Li2N2.

Theoretical studies on stability and pyrolysis mechanism of salts formed by N5 − and metallic cations Na+, Fe2+ and Ni2+

The salts formed by N5 − and metallic cations (Na+, Fe2+ and Ni2+) may be potential candidates for superior energetic materials and were studied with the density functional theory B3LYP method and ab initio molecular orbital theory MP2 method using the 6-31G* and LanL2DZ basis sets. Thermal dissociations of these salts are initiated by breaking of the N5 ring, and those of Fe(N5)2 and Ni(N5)2 proceed sequentially through two transition states. In gas phase, the activation barriers (E as, in kJ/mol) of thermal dissociations decrease in the order of N5 − > NaN5 > HN5 > Ni(N5)2 > Fe(N5)2. Products of initial pyrolysis of these salts are N2 and metallic azide. The frontier orbital energy gaps (in eV) are N5 − (8.27) > HN5 (7.40) > NaN5 (5.10) > Fe(N5)2 (4.92) > Ni(N5)2 (3.43). The more stable salt has the smaller electron transfer between the cation and anion.

Theoretical studies on the stability of phenylpentazole and its substituted derivatives of –OH, –OCH3, –OC2H5and –N(CH3)2

Phenylpentazole (PhN5) and its derivatives with 1–3 electron donating substituents (–OH, –OCH3, –OC2H5 and –N(CH3)2) were studied using density functional theory. The pyrolysis mechanism and effects of substituents on stabilities were discussed. The activation energies (Eas, 362–402 kJ mol−1) for the cleavage of the C–N bonds linking the aryl and the pentazole are far larger than those (109–117 kJ mol−1) for the breaking of the N–N bonds in the pentazole ring. Decomposition of the pentazole ring should be the initial step of pyrolysis of arylpentazole and its derivatives. The pentazole ring in PhN5 is stabilized by substituents which increase the electron density and strengthen the delocalization of the N5 ring. The abilities of these substituents to improve Ea and to decrease the frontier orbital energy gap (Eg) have the same order: –N(CH3)2 > –OC2H5 > –OCH3 > –OH.

Dinitrogen Release from Arylpentazole: A Picosecond Time-Resolved Infrared, Spectroelectrochemical, and DFT Computational Study

p-(Dimethylamino)phenyl pentazole, DMAP-N5 (DMAP = Me2N-C6H4), was characterized by picosecond transient infrared spectroscopy and infrared spectroelectrochemistry. Femtosecond laser excitation at 310 or 330 nm produces the DMAP-N5 (S1) excited state, part of which returns to the ground state (τ = 82 ± 4 ps), while DMAP-N and DMAP-N3 (S0) are generated as double and single N2-loss photoproducts with η ≈ 0.14. The lifetime of DMAP-N5 (S1) is temperature and solvent dependent. [DMAP-N3](+) is produced from DMAP-N5 in a quasireversible, one-electron oxidation process (E1/2 = +0.67 V). Control experiments with DMAP-N3 support the findings. DFT B3LYP/6-311G** calculations were used to identify DMAP-N5 (S1), DMAP-N3(+), and DMAP-N in the infrared spectra. Both DMAP-N5 (S1) and [DMAP-N5](+) have a weakened N5 ring structure.

What are the roles of N3 and N5 rings in designing polynitrogen molecules?

A series of polynitrogen molecules composed by multiple cyclic-N(3) and/or cyclic-N(5) are systematically investigated by using DFT and MP2 methods. It is found that all energetically favored structures appear with a corner to corner connection between two rings for the cyclic-N(3) and/or the cyclic-N(5). Furthermore, straight interaction between the N(3)-rings would decrease the stability of cyclic-N(3) relative to the cyclic-N(3) radical and thus not favor the formation of a stable structure containing multiple cyclic-N(3), while the interaction between the N(5)-rings can improve the stability of cyclic-N(5) and tend to produce a stable structure composed by multiple cyclic-N(5). For the stable structure consisting of cyclic-N(3) and cyclic-N(5), the charge is transferred from cyclic-N(3) to cyclic-N(5), facilitating the stability of N(5), and the attacked site of the structure by the radicals is analyzed by condensed Fukui function. Finally, the stabilities of the structures N(3)(N(5))(3) and N(5)(N(5))(5) are investigated and demonstrated to be attributed to the charge transfer and the coordination number of N(3)-ring and N(5)-ring.

Potential high-energy pentazolides: HB(N5 )3 M1∼2 (N5 )3 BH (M = Be, Mg, Ca, Zn, and Cd)

HB(N5)3M1∼2(N5)3BH (M = Be, Mg, Ca, Zn, and Cd) have been investigated as potential high-energy density materials in this article by means of density functional theory. They are all of high-energy content and show kinetic stability as to the breakup of N5 ring, especially HB(N5)3Mg(N5)3BH. The MN bondings in the mononuclear compounds are stronger than those in the corresponding dinuclear ones, except in the Be analogues. Furthermore, except HB(N5)3Zn2(N5)3BH, all the dinuclear compounds are shown to be energetically stable with respect to disproportionation. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010