A General Model of Impact Sensitivity for Nitrogen-Rich Energetic Materials: A Combined Incremental Theory and Genetic Function Approximation Study.

In this paper, we report the first example of impact sensitivity prediction based on the genetic function approximation (GFA) as a regression method. The prediction is applicable for a wide variety of chemical families, which include nitro compounds, peroxides, nitrogen-rich salts, heterocycles, etc. Within this work, we have obtained 7 empirical models (with 27-32 basis functions), which all provide 0.80≤R2≤0.83 and 7.2 J≤RMSE≤7.8 J (for 450 training set compounds) and 0.64≤R2≤0.70 and 11.2 J≤RMSE≤12.4 J (for 170 test set compounds). The models were developed using Friedman Lack-of-Fit as a scoring function, which allows avoiding an overfitting. All the models have simple descriptors as basis functions and include linear splines. Furthermore, the applied descriptors do not require expensive calculation procedures, namely, non-empirical quantum-chemical calculations, complex iterative procedures, real space electron density analysis, etc. Most descriptors are based on structural and topological analysis and a part of them require very cheap semi-empirical PM6 calculations. The prediction takes a few minutes as an average, and most of the time is for the structure preparation and manual calculation of the descriptor "Increment", which is based on our recent incremental theory.

Applications of Boron Cluster Supramolecular Frameworks as Metal-Free Chemodynamic Therapy Agents for Melanoma.

Chemodynamic therapy (CDT) is a highly targeted approach to treat cancer since it converts hydrogen peroxide into harmful hydroxyl radicals (OH·) through Fenton or Fenton-like reactions. However, the systemic toxicity of metal-based CDT agents has limited their clinical applications. Herein, a metal-free CDT agent: 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT)/ [closo-B12 H12 ]2- (TPT@ B12 H12 ) is reported. Compared to the traditional metal-based CDT agents, TPT@B12 H12 is free of metal avoiding cumulative toxicity during long-term therapy. Density functional theory (DFT) calculation revealed that TPT@B12 H12 decreased the activation barrier more than 3.5 times being a more effective catalyst than the Fe2+ ion (the Fenton reaction), which decreases the barrier about twice. Mechanismly, the theory calculation indicated that both [B12 H12 ]-· and [TPT-H]2+ have the capacity to decompose hydrogen into 1 O2 , OH·, and O2 -· . With electron paramagnetic resonance and fluorescent probes, it is confirmed that TPT@B12 H12 increases the levels of 1 O2 , OH·, and O2 -· . More importantly, TPT@B12 H12 effectively suppress the melanoma growth both in vitro and in vivo through 1 O2 , OH·, and O2 -· generation. This study specifically highlights the great clinical translational potential of TPT@B12 H12 as a CDT reagent.

Grammar of Impact Sensitivity: An Incremental Theory.

In this paper, we report the first attempt to quantify impact sensitivity using the second-order incremental approach based on the structural features of explosives. It has been found that impact height (h50) can be expressed via a multiplicative incremental exponential form, in which the exponents are characteristic coefficients of structural increments multiplied by their numbers in the molecule. The method was developed on a large array of experimental data (450 molecules and salts) of different energetic materials, namely, nitro compounds, peroxides, nitrogen-rich salts, heterocycles, etc., while testing of the model was performed for 170 compounds. The results demonstrate a noticeable correlation with the experimental h50 values. Thus, the corresponding R2 and RMSE for the training and test sets are 0.56 (12.5 J) and 0.63 (18.8 J), respectively. In this work, we use 53 individual structural increments, but their number can be extended, and the corresponding coefficients can be refined; this allows for increasing the prediction accuracy on-the-fly. The calculation algorithm is discussed, and the corresponding examples are presented. The performed machine-based regression analysis using genetic function approximation, multiple linear regression, and artificial neural network has proven the reasonability and informativity of the proposed incremental theory. Thus, the developed approach significantly extends our understanding of the impact sensitivity phenomenon and translates it into the category of one that can be calculated by a pocket calculator.

Radical pair formation due to compression-induced electron transfer in crystals of energetic salts.

An interesting effect was observed when studying explosive and non-explosive crystalline ionic materials at high pressures. A wide benchmark set of 76 crystals of different families was studied using the state-of-the-art methods at ambient pressure and in extremes (at 20, 50 and 100 GPa). It was found that hydrostatic compression leads to an electron transfer from the anion to the cation, which was carried out with different efficiencies for explosive and non-explosive salts. The measure of this electron transfer is reflected in the Hirshfeld charges (q) on cations, which decreased with the rise of pressure. Non-explosive materials are generally resistant to this effect, while explosives are much more susceptible. Thus, at 100 GPa, all the studied energetic salts demonstrate qcat < +0.1e, while for the non-explosive salts qcat > +0.1e. This value can be considered as a conditional boundary between explosive and non-explosive salts. The observed effect is in accord with the Szigeti's dielectric theory as well as with the electrophilicity/electronegativity equalization principle. In the present paper, we develop a mechanism of the explosive decomposition based on the assumption about formation of a radical pair as a result of the following reaction: . The study of such radicals revealed their intrinsic instability, which generally reflects either in a dissociative structure or in the presence of strongly weakened trigger bonds.

Bipentazole (N10): A Low-Energy Molecular Nitrogen Allotrope with High Intrinsic Stability.

In this Letter, we report a crystal structure prediction and characterization of a molecular nitrogen allotrope N10 (bipentazole) using state-of-the-art computational methods. To date, in the form of a P21 space group crystal, this allotrope is the most stable predicted form of nitrogen, other than N2, in the pressure range 0-42 GPa. Its metastability at ambient conditions was justified using phonon dispersion and mechanical properties calculations as well as ab initio molecular dynamics simulations. Due to a high intrinsic stability caused by aromaticity, bipentazole may appear to be the first nitrogen allotrope stable enough for a large-scale synthesis at ambient conditions. The calculations of propulsive characteristics revealed that bipentazole is an excellent "green" energetic material. A potential strategy for the synthesis of this compound is offered and rationalized. The unique electronic structure of bipentazole makes it a strongly electrophilic all-nitrogen reagent, which can exhibit unusual chemistry.

Beyond molecular nitrogen: revelation of two ambient-pressure metastable single- and double-bonded nitrogen allotropes built from three-membered rings.

In this study, we present a crystal structure prediction and characterization for two novel single- and double-bonded nitrogen allotropes (denoted as CubN and DobN) bearing three-membered nitrogen cycles. These materials are ambient-pressure metastable robust solids with strong cohesive energies. Due to the presence of strained nitrogen cycles, these allotropes are high-lying phases, which retain high energy densities at least up to 200 GPa, when CubN becomes lower in energy than ζ-N2. The studied allotropes are indirect wide-bandgap semiconductors. A detailed spectral characterization of these materials is also presented herein. Due to their extremely high heats of formation and crystal densities, CubN and DobN demonstrate excellent detonation properties that are comparable to those of previously reported phases (cg-N and TrigN). Propulsive properties (including specific impulse and characteristic velocity) of CubN and DobN as solid monopropellants were also estimated. Thus, if synthesized, the present nitrogen allotropes will have great potential for practical applications and achieving fundamental knowledge about nitrogen as a chemical element.

Impact sensitivity of aryl diazonium chlorides: Limitations of molecular and solid-state approach.

The mechanism of the compression-induced decomposition of aryl diazonium chlorides is proposed on the basis of quantum-chemical calculations of both the isolated cations and crystalline materials. The electron transfer from the anion to the cation, followed by the crystal decomposition, is observed with the rise of pressure. Taking the known nature of the structural changes in cations undergone upon reduction, five structural, vibrational and electronic determinants of impact sensitivity are found. Thus, a correlation (R2 = 0.79) between the experimentally known impact sensitivity of 40 different aryl diazonium cations and the developed empirical function Ω, which includes the above-mentioned parameters, is obtained. Meanwhile, an abnormal impact sensitivity of 4-nitrobenzenediazonium chloride (4 J) compared to the parent benzenediazonium chloride (3 J) is rationalized on the basis of first-principles calculations of the latter and its three nitro derivatives. Using our recently proposed solid-state criteria of impact sensitivity, another empirical function Ω was developed being able to predict impact sensitivity of these four salts with very good confidence (R2 = 0.97).

Quantification of Impact Sensitivity Based on Solid-State Derived Criteria.

An attempt was made to develop a general description of impact sensitivity. For this purpose a set of 24 well-known, as well as recently synthesized, C-H-N-O-Cl explosives covering the wide range of impact sensitivity ( h50 = 9-320 cm) was studied using first-principles calculations at different external pressures. To quantify impact sensitivity, a theoretical approach was developed based on the solid-state derived criteria, which include triggering pressure, average number of electrons per atom, crystal morphology, energy content and melting temperature. These criteria follow from the theoretical consideration of the crystal compression caused by an impact event. Apart of the compression, the influence of crystal habit shapes and energy content are also discussed. The main idea is in the electron flow probability from valence to conduction bands in a solid. To support the developed theoretical background, the corresponding numerical illustration is presented in the paper. The obtained empirical correlation exhibits a significant regression coefficient ( R2 = 0.83). Furthermore, the found criteria have complementary character. When using them individually, the correlation becomes poor or even vanishes. Thus, a sensitive to impact explosive is expected to be more easily convertible to the metal upon compression, to possess a spherical crystal habit and to have a greater number of electrons per atom as well as a high energy content and a low melting temperature. Consequently, an insensitive explosive has the inverse characterization.

Solvatochromic effect in absorption and emission spectra of star-shaped bipolar derivatives of 1,3,5-triazine and carbazole. A time-dependent density functional study.

A series of three star-shaped compounds containing both donor (carbazole) and acceptor (2,4,6-triphenyl-1,3,5-triazine) moieties linked through various linking bridges was studied theoretically at the linear response TD-DFT level of theory to describe their absorption and fluorescence spectra. The concept of a localized charge-transfer excited state has been applied successfully to explain the observed strong solvatochromic effect in the emission spectra of the studied molecules, which can be utilized for the fabrication of color tunable solution-processable OLEDs. The concept is in particularly applicable to donor-acceptor species with a C 3 symmetry point group where the static dipole moment changes dramatically upon electronic excitation. An important peculiarity of the studied molecules is that they are characterized by non-zero values of the HOMO and LUMO orbitals in the same common part of molecular space that provides a large electric dipole transition moment for both light absorption and emission. Graphical abstract Star-shaped C 3 symmetry point group derivatives for color tunable OLEDs.

Super high-energy density single-bonded trigonal nitrogen allotrope-a chemical twin of the cubic gauche form of nitrogen.

A new ambient-pressure metastable single-bonded 3D nitrogen allotrope (TrigN) of trigonal symmetry (space group R3[combining macron]) was calculated using density functional theory (DFT). A comprehensive characterization of this material, comprising thermodynamic, elastic, and spectral (vibrational, UV-vis absorption, and nuclear magnetic resonance) properties, was performed. Using high-throughput band structure calculation, the TrigN phase was characterized as an insulator with an indirect band gap of 2.977 eV. Phonon dispersion calculations justified that this structure is vibrationally stable at ambient pressure. The calculated Raman activities at the Γ-point demonstrated a rich pattern, whereas no relatively intense transitions were observed in its IR absorption spectrum. The TrigN material is almost transparent to visible light as well as to ultraviolet A and B. The main absorption peaks appeared within the range of 50-200 nm. The electron arrangement of the nitrogen nuclei in the studied nitrogen allotrope is much denser compared to that of the molecular nitrogen, which is in agreement with the calculated magnetic shielding tensor values. Robust mechanical stability is revealed from the elastic constants calculation. Due to strong anisotropy, the values of the Young's moduli vary from 281 to 786 GPa. A huge amount of internal energy is enclosed in the TrigN material. Upon decomposition to molecular nitrogen, the energy release is expected to be 11.01 kJ g-1 compared to the value of 10.22 kJ g-1 for the cubic gauche form of nitrogen. The TrigN allotrope possesses unique detonation characteristics with a detonation pressure of 146.06 GPa and velocity of 15.86 km s-1.

Theoretical study of relationships between structural, optical, energetic, and magnetic properties and reactivity parameters of benzidine and its oxidized forms.

Structural, topological, optical, energetic, and magnetic properties and reactivity parameters of benzidine, its radical cation, and its dication as well as molecular complexes of the benzidine dication with the F(-), Cl(-), Br(-), I(-), NO3(-), HSO4(-), and H2PO4(-) anions were calculated at the B3LYP/6-311++G(2d,2p) level of theory in the CH2Cl2 medium. The CAM-B3LYP functional (as the most reliable one) and the 6-311++G(3df,3pd) basis set were used for the UV-vis absorption spectra prediction. The obtained spectral results are in a good agreement with available experimental data. A number of the calculated global and local molecular properties, including several recently developed ones, (in general, more than 20 parameters), namely, λmax, the bond lengths and orders (l and LA,B), adiabatic ionization energy (IEad), global electrophilicity index (ω), condensed electrophilic Fukui functions (f(+)) and dual descriptor (ΔfA), van der Waals molecular volume, nuclear independent chemical shifts (NICS) and QTAIM topological parameters were estimated in the critical points of the C(1)-C(1'), C(2)-C(3), and C(4)-N bonds as well as at the ring critical point. These quantities were found to be in a strong linear dependence (R(2) > 0.99 in most cases) with the number of detached electrons (Nel) from the benzidine molecule up to formation of the dication (Nel = 2). On one hand, a position of the long-wave absorption band (λCT) corresponding to the anion-to-cation charge transfer in the neutral complexes of the benzidine dication with anions, correlates with the Mulliken electronegativity of the anion (R(2) = 0.8646) and its adiabatic ionization energy (R(2) = 0.8054). On the other hand, the correlations with the anion charge in the complexes and the anion isotropic polarizability are rather poor (R(2) = 0.6392 and 0.3470, respectively). On the ground of the obtained strong relationships, one may recommend the calculated molecular properties as potentially preferable descriptors for the benzidine-based compounds in terms of the QSAR methodology.

State-dependent global and local electrophilicity of the aryl cations.

Two alternative approaches­vertical and adiabatic­are used to estimate global and local electrophilicity (ω and ωk+) indexes for a series of aryl cations in both the ground and first excited electronic states using the well-known Parr scheme. The energy parameters used in these methods are obtained by the B3LYP/6-311++G(2d,2p) calculations of the aryl cations and of their oxidized and reduced forms in acetonitrile medium. The ground state ω values are lower than those for the excited state, which is in accord with the maximum hardness principle. Analysis of the ω indexes calculated with more reliable adiabatic approach reveals a dependence of the ground and first excited state ω indexes on the singlet­triplet energy gap of the aryl cations. A plot of the above dependence has a hyperbola-like shape; thus, the maximum (ground state) and minimum (first excited state) ω indexes correspond to the aryl cation, for which the singlet­triplet splitting is close to zero. Moreover, the ωk+ index distribution at the ipso-carbon atoms does not obey the maximum hardness principle, since it depends on spin multiplicity, not on the electronic state spatial type. For many singlet ground state aryl cations, the ωk+ indexes at the ipso-carbon atom are lower when calculated in the excited triplet state; that is due to a strong ω delocalization onto two electrophilic centers. This explains a higher chemoselectivity of the triplet aryl cations in reactions with the π-nucleophiles compared to the corresponding singlet arylium species. Applicability of the adiabatic approach for calculation of the ω and ωk+ indexes is supported by the experimental data on the nucleophile-independent parameter E for the singlet and triplet state of the p-Me2NC6H4+ cation.