Nonbonding Electron Delocalization Stabilizes the Flexible N8 Molecular Assembly.

Electron delocalization has an important impact on the physical properties of condensed materials. However, the L-electron delocalization in inorganic, especially nitrogen, compounds needs exploitation to improve the energy efficiency, safety, and environmental sustainability of high-energy-density materials (HEDMs). This Letter presents an intriguing N8 molecule, ingeniously utilizing nitrogen's L-electron delocalization. The molecule, exhibiting a unique lollipop-shaped conformation, can fold at various angles with very low energy barriers, self-assembling into environmentally stable, all-nitrogen crystals. These crystals demonstrate unparalleled stability, high energy density, low mechanical sensitivity, and optimal electronic thermal conductivity, outperforming existing HEDMs. The remarkable properties of these designed materials are attributed to two distinct delocalized systems within nitrogen's L-shell: π- and lone pair σ-electrons, which not only stabilize the molecular structure but also facilitate interconnected 3D networks of intermolecular nonbonding interactions. This work might pave the way to the experimental synthesis of environmentally stable all-nitrogen solids.

Discriminative Mechanical and Thermal Response of the H-N Bonds for the Energetic LLM-105 Molecular Assembly.

Molecular interactions in energetic materials form the key not only to the "structure stability, energy storage, ignition, and detonation" dynamics but also to the sensitivity to the loading of perturbation and the power intensity of radiation for the energetic substance, with the nature of the interactions remaining elusive. With the aid of perturbative Raman spectroscopy and the pressure-resolved density functional theory, we uncovered that the H-N bond of the intermolecular O:H-N bonds for LLM-105 shares the same negative compressibility and thermal expansivity of the H-O bond for the coupling O:H-O bond of water [Phys. Rep. 2023, 998, 1-68]. In contrast, the dangling H-N bond vibrating at a 3440 cm-1 high frequency does otherwise due to the absence of coupling interaction and the undercoordination-driven bond contraction. These findings should deepen our insight into interactions involving electron lone pairs and offer an efficient means for discriminating the performance of individual bonds.

Perturbative vibration of the coupled hydrogen-bond (O:H-O) in water.

Perturbation Raman spectroscopy has underscored the hydrogen bond (O:H-O or HB) cooperativity and polarizability (HBCP) for water, which offers a proper parameter space for the performance of the HB and electrons in the energy-space-time domains. The OO repulsive coupling drives the O:H-O segmental length and energy to relax cooperatively upon perturbation. Mechanical compression shortens and stiffens the O:H nonbond while lengthens and softens the HO bond associated with polarization. However, electrification by an electric field or charge injection, or molecular undercoordination at a surface, relaxes the O:H-O in a contrasting way to the compression with derivation of the supersolid phase that is viscoelastic, less dense, thermally diffusive, and mechanically and thermally more stable. The HO bond exhibits negative thermal expansivity in the liquid and the ice-I phase while its length responds in proportional to temperature in the quasisolid phase. The O:H-O relaxation modifies the mass densities, phase boundaries, critical temperatures and the polarization endows the slipperiness of ice and superfluidity of water at the nanometer scale. Protons injection by acid solvation creates the H↔H anti-HB and introduction of electron lone pairs derives the O:⇔:O super-HB into the solutions of base or H2O2 hydrogen-peroxide. The repulsive H↔H and O:⇔:O interactions lengthen the solvent HO bond while the solute HO bond contracts because its bond order loss. Differential phonon spectroscopy quantifies the abundance, structure order, and stiffness of the bonds transiting from the mode of pristine water to the perturbed states. The HBCP and the perturbative spectroscopy have enabled the dynamic potentials for the relaxing O:H-O bond. Findings not only amplified the power of the Raman spectroscopy but also substantiated the understanding of anomalies of water subjecting to perturbation.

Designing of Highly Efficient Oxygen Evolution Reaction Electrocatalysts Utilizing A Correlation Factor: Theory and Experiment.

The theoretical prediction of the catalytic activity is very beneficial for the design of highly efficient catalysts. At present, most theoretical descriptors focus on estimating the catalytic activity and understanding the enhancement mechanism of catalysts, while it is also quite important to find a factor to correlate the descriptors with preparation methods. In this work, a correlation factor, the d electron density of transition metal ions, was developed to correlate the d band center values of transition metal ions with the preparation methods of amorphization and Al introduction. According to the results of theoretical simulations, the correlation factor not only exhibited favorable linear relationships with the theoretical overpotentials of (CoFeAlx)3O4 and (CoFeAlx)3O4 + (CoFeAlx)OOH systems but also correlated with two preparation methods by altering the volume of systems. Based on theoretical guidance, the electrocatalytic activities of the prepared (CoFeAlx)3O4 specimens were gradually improved by the preparation methods of amorphization and Al introduction, and the Am-CoFeAl-2-10h specimen exhibited a low kinetic barrier of 268 mV, fast charge transfer rate, and stable electrocatalytic activity. This strategy could be applied to design highly efficient catalysts by adjusting the correlation factor of the active site with suitable preparation methods.

Applying machine learning to balance performance and stability of high energy density materials.

The long-standing performance-stability contradiction issue of high energy density materials (HEDMs) is of extremely complex and multi-parameter nature. Herein, machine learning was employed to handle 28 feature descriptors and 5 properties of detonation and stability of 153 HEDMs, wherein all 21,648 data used were obtained through high-throughput crystal-level quantum mechanics calculations on supercomputers. Among five models, namely, extreme gradient boosting regression tree (XGBoost), adaptive boosting, random forest, multi-layer perceptron, and kernel ridge regression, were respectively trained and evaluated by stratified sampling and 5-fold cross-validation method. Among them, XGBoost model produced the best scoring metrics in predicting the detonation velocity, detonation pressure, heat of explosion, decomposition temperature, and lattice energy of HEDMs, and XGBoost predictions agreed best with the 1,383 experimental data collected from massive literatures. Feature importance analysis was conducted to obtain data-driven insight into the causality of the performance-stability contradiction and delivered the optimal range of key features for more efficient rational design of advanced HEDMs.

Structure and Stability of Aromatic Nitrogen Heterocycles Used in the Field of Energetic Materials.

Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.

Water electrification: Principles and applications.

Deep engineering of liquid water by charge and impurity injection, charged support, current flow, hydrophobic confinement, or applying a directional field has becoming increasingly important to the mankind toward overcoming energy and environment crisis. One can mediate the processes or temperatures of molecular evaporation for clean water harvesting, HO bond dissociation for H2 fuel generation, solidification for living-organism cryopreservation, structure stiffening for bioengineering, etc., with mechanisms being still puzzling. We show that the framework of "hydrogen bonding and electronic dynamics" has substantiated the progress in the fundamental issues and the aimed engineering. The segmental disparity of the coupled hydrogen bond (O:HO or HB with ":" being lone pair of oxygen) resolves their specific-heat curves and turns out a quasisolid phase (QS, bound at -15 and 4 °C). Electrification shows dual functionality that not only aligns, orders, polarizes water molecules but also stretches the O:HO bond. The O:HO segmental cooperative relaxation and polarization shift the QS boundary through Einstein's relation, ΔΘDx ∝ Δωx, resulting in a gel-like, viscoelastic, and stable supersolid phase with raised melting point Tm and lowered temperatures for vaporization TV and ice nucleation TN. The supersolidity and electro structure ordering provide additional forces to reinforce Armstrong's water bridge. QS dispersion and the secondary effect of electrification such as compression define the TN for Dufour's electro-freezing. The TV depression, surface stress disruption, and electrostatic attraction raise Asakawa's molecular evaporability. Composition of opposite, compatible fields eases the HO dissociation and soil wetting. Progress evidences not only the essentiality of the coupled O:HO bond theory but also the feasibility of engineering water and solutions by programmed electrification.

Strategies for Achieving Balance between Detonation Performance and Crystal Stability of High-Energy-Density Materials.

Performance-stability contradiction of high-energy-density materials (HEDMs) is a long-standing puzzle in the field of chemistry and material science. Bridging the gap that exists between detonation performance of new HEDMs and their stability remains a formidable challenge. Achieving optimal balance between the two contradictory factors is of a significant demand for deep-well oil and gas drilling, space exploration, and other civil and defense applications. Herein, supercomputers and latest quantitative computational strategies were employed and high-throughput quantum calculations were conducted for 67 reported HEDMs. Based on statistical analysis of large amounts of physico-chemical data, in-crystal interspecies interactions were identified to be the one that provokes the performance-stability contradiction of HEDMs. To design new HEDMs with both good detonation performance and high stability, the proposed systematic and comprehensive strategies must be satisfied, which could promote the development of crystal engineering of HEDMs to an era of theory-guided rational design of materials.

Unexpected Solute Occupancy and Anisotropic Polarizability in Lewis Basic Solutions.

Density functional computation revealed that in YOH solvation (Y = Li, Na, and K), the Y+ cation locates eccentrically at the interstitial hollow site to form the Y+·4H2O unit, and the hydroxyl adds an excessive electron lone pair ":" to form a OH-·H2O unit where the hydroxide stays in the the center. The surrounding four oriented H2O neighbors interact with the Y+ through Y+ ↔ H+ repulsion and Y+:O2- attraction, causing the Y+ eccentric dislocation by some 0.80 Å. The ":" turns one O:H-O bond into the O: ⇔ :O super hydrogen bond by altering H2O·4H2O into HO-·4H2O. The repulsive Y+ ↔ H+ enlarges its separation and the attractive Y+:O shortens itself, showing the anisotropic polarization of the Y+ cation. The anisotropic Y+ polarizability, O: ⇔ :O compressibility, and the H-O bond contraction of the solute OH- distorts the solute bonding network and disperses the O:H-O segmental phonon frequencies, as confirmed spectroscopically.

Mechanism and Functionality of Pnictogen Dual Aromaticity in Pentazolate Crystals.

Our recent work (J. Phys. Chem. Lett. 2019, 10, 2378) reported the discovery of the abnormal pnictogen dual aromaticity (π and σ) in cyclo-N5- , which makes the anion unstable in nature but confers enhanced stability in sufficiently acid solution. Herein, we present systematic quantum calculations on the structures, energetics and dynamics of the pentazolate salt and metal pentazolate hydrates, focusing on the mechanism and functionality of the pnictogen dual aromaticity in these crystals, which are verified by experiments. We find that owning a net charge of -e is crucial to the formation of the dual aromaticity and the stabilization of the cyclo-N5- . The competition between the dual aromaticity and the proton affinity drives the cyclo-N5- to be unreactive to acid and remain unprotonated in these crystals. We decompose the crystal packing effect into pure mechanical compression and interspecies nonbonding interactions, and figure out that the type and number of the adjacent counterions of the cyclo-N5- anion, instead of the compression effect, accounts for the protonation state reversion in the vacuum and in the crystal. The current work supports our original conclusion (Science 2018, 359, eaas8953) and is expected to provide compelling evidence against the current debate on the cyclo-N5- stability (Science 2018, 359, eaao3672; J. Phys. Chem. Lett. 2018, 9, 7137; J. Am. Chem. Soc. 2019, 141, 2984).

Waste bones derived nitrogen-doped carbon with high micropore ratio towards supercapacitor applications.

Fabrication of high-performance electrodes from waste biomass has attracted increasing attention among the energy storage and conversion field. In this work, we have synthesized nitrogen-doped activated carbon by a simultaneous pyrolysis/activation method from waste bones. It is found that the specific surface area and pore structure of as-synthesized carbon depends on the carbonization temperature (500-800 °C), and the highest specific surface area is 1522 m2 g-1. The electrochemical properties of Pork bone, Blackfish bone, Eel bone based activated carbon (PBAC, BFAC, EBAC) mainly depend on their micro-/mesoporosity. Three samples PBAC-600, BFAC-600 and EBAC-600, which have higher ratio of micropore surface area and nitrogen content, exhibit enhanced specific capacitance of 263, 302 and 264F g-1 in 6 M KOH electrolyte. Furthermore, the assembled symmetric supercapacitors of PBAC-600 can deliver energy density as high as 7.0 and 26.2 Wh Kg-1 in the aqueous and ionic liquid electrolyte, respectively. Such excellent performance can be attributed to the microporous structure, reasonable pore size distribution and nitrogen self-doping of the activated carbon. This research indicates that waste bones have great potential for mass fabrication of the activated carbon electrodes for energy storage applications.

Stabilization of the Dual-Aromatic cyclo-N5- Anion by Acidic Entrapment.

Pentazole anion, the best candidate for full-nitrogen energetic materials, can be isolated only from acidic solution for unclear reasons, which hinders the high-yield realization of a full-nitrogen substance with higher energy density. Herein, we report for the first time the discovery of the dual aromaticity (π and σ) of cyclo-N5-, which makes the anion unstable in nature but confers additional stability in acidic surroundings. In addition to the usual π-aromaticity, similar to that of the prototypical benzene, five lone pairs are delocalized in the equatorial plane of cyclo-N5-, forming additional σ-aromaticity. It is the compatible coexistence of the inter-lone-pair repulsion and inter-lone-pair attraction within the σ-aromatic system that makes the naked cyclo-N5- highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo-N5- become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise, the cyclo-N5- cannot be productively isolated. The dual aromaticity discovered in cyclo-N5- is expected to be universal for pnictogen five-membered ring systems.

Hydration of Hofmeister ions.

Water dissolves salt into ions and then hydrates the ions to form an aqueous solution. Hydration of ions deforms the hydrogen bonding network and triggers the solution with what the pure water never shows such as conductivity, molecular diffusivity, thermal stability, surface stress, solubility, and viscosity, having enormous impact to many branches in biochemistry, chemistry, physics, and energy and environmental industry sectors. However, regulations for the solute-solute-solvent interactions are still open for exploration. From the perspective of the screened ionic polarization and O:H-O bond relaxation, this treatise features the recent progress and a perspective in understanding the hydration dynamics of Hofmeister ions in the typical YI, NaX, ZX2, and NaT salt solutions (Y = Li, Na, K, Rb, Cs; X = F, Cl, Br, I; Z = Mg, Ca, Ba, Sr; T = ClO4, NO3, HSO4, SCN). Phonon spectrometric analysis turned out the f(C) number fraction of bonds transition from the mode of deionized water to the hydrating. The linear f(C) ∝ C form features the invariant hydration volume of small cations that are fully-screened by their hydration H2O dipoles. The nonlinear f(C) ∝ 1 - exp.(-C/C0) form describes that the number insufficiency of the ordered hydrating H2O dipoles partially screens the anions. Molecular anions show stronger yet shorter electric field of dipoles. The screened ionic polarization, inter-solute interaction, and O:H-O bond transition unify the solution conductivity, surface stress, viscosity, and critical energies for phase transition.

Unprecedented O:⇔:O compression and H↔H fragilization in Lewis solutions.

Charge injection in terms of lone pairs ':', protons, and ions upon acid and base solvation mediates the hydrogen bonding network and properties of Lewis solutions, and is ubiquitously important in many subject areas of Chemical Physics. This work features the recent progress and future trends in this aspect with a focus on the solute-solvent interactions and hydrogen bond (O:H-O or HB) transition from the vibration mode of ordinary water to the hydrating states. A combination of the O:H-O bond cooperativity notion, differential phonon spectrometrics, calorimetric detection, and quantum computations clarified the solute capabilities of O:H-O bond transition in HX and YOH (X = Cl, Br, I and Y = Li, Na, K) solutions. The H+ and the lone pair do not stay alone to move or shuttle freely between adjacent H2O molecules, but they are attached to a H2O molecule to form (H3O+ and OH-)·4H2O tetrahedral motifs, which transits an O:H-O bond into the H↔H anti-HB point breaker in acidic solutions and into the O:⇔:O super-HB compressor and polarizer in basic solutions, respectively. H↔H disrupts the solvent network and surface stress, having the same effect of liquid heating on HB bond relaxation and thermal fluctuation on surface stress. The O:⇔:O compression lengthens and weakens the solute H-O bond, which heats up the solution during solvation. The H-O bonds due to H3O+ contract by 3% and due to OH- shrink by 10%. The Y+ and X- ions perform in the same manner as they do in salt solutions to form hydration shells through electrostatic polarization and hydrating H2O dipolar screen shielding. Focusing more on the O:H-O bond transition would be even more promising and revealing than on the manner and mobility of lone pair and proton transportation.

Electronic structure of two-dimensional In and Bi metal on BN nanosheets.

The electronic structures of two-dimensional (2D) indium (In) and bismuth (Bi) metal on BN nanosheets are systematically studied using hybrid density functional theory (DFT). We found that 2D In and Bi metal effectively modulate the band gap of a BN nanosheet. We also found that the indirect band gap of the 2D In and Bi metal electronic structures are 0.70 and 0.09 eV, respectively. This modulation originates from the charge transfer between the 2D metal and BN nanosheet interfaces, as well as from the electron redistribution of the In/BN and Bi/BN heterojunctions of the s and p orbitals. Our results provide an insight into 2D In/BN and Bi/BN heterojunctions, which should be useful in the design of 2D In and Bi metal-semiconductor-based devices.

Supersolidity of undercoordinated and hydrating water.

Supersolidity of ice, which was proposed in 2013 and intensively verified since then [C. Q. Sun et al., Density, Elasticity, and Stability Anomalies of Water Molecules with Fewer than Four Neighbors, J. Phys. Chem. Lett., 2013, 4, 2565-2570; C. Q. Sun et al., Density and phonon-stiffness anomalies of water and ice in the full temperature range, J. Phys. Chem. Lett., 2013, 4, 3238-3244], refers to the water molecules being polarized by molecular undercoordination, which is associated with the skin of bulk ice, nanobubbles, and nanodroplets (often called confinement), or by the electrostatic field of ions in salt solutions [X. Zhang et al., Mediating relaxation and polarization of hydrogen-bonds in water by NaCl salting and heating, Phys. Chem. Chem. Phys., 2014, 16(45), 24666-24671; C. Q. Sun et al., (H, Li)Br and LiOH solvation bonding dynamics: molecular nonbond interactions and solute extraordinary capabilities, J. Phys. Chem. B, 2018, 122(3), 1228-1238]. From the perspective of hydrogen bond (O:H-O or HB with ":" representing the lone pairs on O2-) cooperative relaxation and polarization, this review features the recent progress and recommends future trends in understanding the bond-electron-phonon correlation in the supersolid phase. Supersolidity is characterized by a shorter and stiffer H-O bond, longer and softer O:H nonbond, deeper O 1s energy band, and longer photoelectron and phonon lifetimes. The supersolid phase is less dense, viscoelastic, and mechanically and thermally more stable. Furthermore, O:H-O bond cooperative relaxation offsets the boundaries of structural phases and increases the melting point while lowering the freezing temperature of ice, which is known as supercooling and superheating.

Arsenene nanoribbon edge-resolved strong magnetism.

We proposed a mechanism to induce strong magnetism of up to 10.92 emu g-1 in hexagonal-phase arsenene nanoribbon (AsNR) from the perspective of edge quantum entrapment. Consistency between bond-order-length-strength correlation (BOLS) theory and density functional theory (DFT) calculations verified that: (i) the edge bond contraction of 9.54% deepened the edge potential well of AsNR, (ii) a net charge of 0.06 e- transferred from the inner region to the edge; and (iii) the edge quantum well polarized the unpaired electron and the net spin (antiferromagnetic or ferromagnetic depending on the width) is localized at the zigzag edge. The finding sheds a light on applications of AsNR in magnetic storage devices.

Phonon Spectrometric Evaluation of the Solute-Solvent Interface in Solutions of Glycine and Its N-Methylated Derivatives.

From the perspective of O:H-O bond cooperativity, we analyzed the solute capability of transiting the O:H-O bond from the mode of ordinary water to the hydration state and its consequence on the solution viscosity and surface stress. Phonon spectrometric results suggest that glycine and its N-methyl derivatives strongly affect the surrounding solvent molecules through H ↔ H repulsion and dipolar polarization. The H ↔ H interproton repulsion disrupts the surface stress, and the polarization enhances the solution viscosity.

CuS/RGO hybrid photocatalyst for full solar spectrum photoreduction from UV/Vis to near-infrared light.

To make full use of the solar energy, it remains a great challenge for semiconductor photocatalysts to harvest the full solar light spectrum from ultraviolet (UV) to visible even the near infrared (NIR) wavelength. Here we show firstly the CuS/RGO (reduced graphene oxide) hybrid photocatalyst synthesized via a microwave assisted method with full solar light (UV-Vis-NIR) active for efficient Cr(VI) reduction. The CuS/RGO displays high absorption and catalytic activity in the UV, visible and even the NIR light regions. As co-catalyst, RGO can separate and inhibit the recombination of charge carriers, consequently improving the catalytic activity. Only 1wt% RGO emersions can reduce 90% of Cr(VI) under the radiation of light over the full spectrum. Findings may provide a new strategy and substance to expand the utilization range of solar light from UV to visible even the NIR energy.