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.

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).

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.

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.

Room-temperature NaI/H2O compression icing: solute-solute interactions.

In situ Raman spectroscopy revealed that transiting the concentrated NaI/H2O solutions to an ice VI phase and then into an ice VII phase at 298 K proceeds in a way different from that activated by the solute type. Unlike the solute type that raises both the critical pressures PC1 and PC2, for the liquid-VI, the VI-VII transition simultaneously occurs in the Hofmeister series order: I > Br > Cl > F ∼ 0; concentration increase raises the PC1 faster than the PC2 that remains almost constant at higher NaI/H2O molecular number ratios. Concentration increase moves the PC1 along the liquid-VI phase boundary and it finally merges with PC2 at the triple-phase junction featured at 350 K and 3.05 GPa. The highly-deformed H-O bond is less sensitive to the concentration because of the involvement of anion-anion repulsion that weakens the electric field in the hydration shells. Observations confirm that the salt solvation lengthens the O:H nonbond and softens its phonon but relaxes the H-O bond contrastingly. Compression, however, has the opposite effect from that of salt solvation. Therefore, compression recovers the polarization-deformed O:H-O bond first and then proceeds to the phase transitions. The anion-anion interaction discriminates the effect of NaI/H2O concentration from that of the solute type at an identical concentration on the phase transitions.

Size modulation electronic and optical properties of phosphorene nanoribbons: DFT-BOLS approximation.

DFT and BOLS approximations were carried out to study the electronic and optical properties of different sizes of black phosphorus nanoribbons (PNRs) with either zigzag- or armchair-terminated edges. PNRs exhibit a nearly direct bandgap, the size of which is strongly increased because of quantum effects. Meanwhile, the bandgap energies of these two kinds of edge PNRs reveal an excellent size dependency. We reconcile the size-dependence of the bandgap energy shifts for PNRs with respect to stimulated bond relaxation, and quantification of the bond length, the bond strength, and the bond nature index. Our calculations suggest that atomic under-coordination shortens the length and increases the stiffness of the P-P bond, which widens the bandgap.

Skin Bond Electron Relaxation Dynamics of Germanium Manipulated by Interactions with H2 , O2 , H2 O, H2 O2 , HF, and Au.

Although germanium performs amazingly well at sites surrounding hetero-coordinated impurities and under-coordinated defects or skins with unusual properties, having important impact on electronic and optical devices, understanding the behavior of the local bonds and electrons at such sites remains a great challenge. Here we show that a combination of density functional theory calculations, zone-resolved X-ray photoelectron spectroscopy, and bond order length strength correlation mechanism has enabled us to clarify the physical origin of the Ge 3d core-level shift for the under-coordinated (111) and (100) skin with and without hetero-coordinated H2 , O2 , H2 O, H2 O2 , HF impurities. The Ge 3d level shifts from 27.579 (for an isolated atom) by 1.381 to 28.960 eV upon bulk formation. Atomic under-coordination shifts the binding energy further to 29.823 eV for the (001) and to 29.713 eV for the (111) monolayer skin. Addition of O2 , HF, H2 O, H2 O2 and Au impurities results in quantum entrapment by different amounts, but H adsorption leads to polarization.

Nanobubble Skin Supersolidity.

Water nanobubbles manifest fascinatingly higher mechanical strength, higher thermal stability, and longer lifetime than macroscopic bubbles; thus, they provide an important impact in applications in the biomedical and chemical industries. However, a detailed understanding of the mechanism behind these mysteries of nanobubbles remains a challenge. Consistency between quantum computations and Raman spectrometric measurements confirmed our predictions that a nanobubble skin shares the same supersolidity with molecular clusters, skins of bulk water, and water droplets because of molecular undercoordination (fewer than four nearest molecular neighbors). Molecular undercoordination (coordination number Zcluster < Zsurface < Zbubble < Zbulk = 4) shortens/extends the H-O/O:H bond and stiffens/softens its corresponding stretching phonons, whose frequency shift is proportional to the square root of the cohesive energy and inversely proportional to the segmental length. The strongly polarized O:H-O bond slows the molecular dynamics and increases the viscosity. The freezing temperature is lowered by the softened O:H bond, and the melting temperature is enhanced by the stiffened H-O bond. Therefore, the supersolid skin makes the nanobubbles thermally more stable, less dense, and stiffer and slows the dynamics of their molecular motion.

Compression icing of room-temperature NaX solutions (X = F, Cl, Br, I).

In situ Raman spectroscopy revealed that transiting H2O/NaX (∼64) solutions into an ice VI phase and then into an ice VII phase at a temperature of 298 K requires excessive pressures with respect to pure water. The increase of the critical pressures varies with the solute type in the Hofmeister series order: X = I > Br > Cl > F ∼ 0. The results suggest that the solute hydration creates electric fields that lengthen and soften the O:H nonbond and meanwhile shorten and stiffen the H-O bond through O-O Coulomb repulsion. Compression, however, does the opposite to solute electrification upon the O:H-O bond relaxation. Therefore, compression of the aqueous solutions recovers the electrification-deformed O:H-O bond first and then proceeds to the phase transitions, which requires excessive energy for the same sequence of phase transitions. Ice exclusion of solute disperses the frequencies of characteristic phonons and the critical pressures with unlikely new bond formation.

Electronic binding energy and thermal relaxation of Li and LiNa atomic alloying clusters.

We examined the effects of atomic hetero- and under-coordination on the relaxation of the interatomic bonding and electronic binding energy of Li and LiNa cluster alloying using a combination of the bond-order-length-strength correlation and density functional theory calculations. We found that bond nature alteration by heterocoordination, bond relaxation by thermal excitation and atomic coordination contribute intrinsically to the core-level energy shifts with resolution of the binding energy at the atomic sites of terrace edges, facets, and bulk of the LiNa alloy nanoclusters. Our strategies may simplify the complexity of core electron binding energies in analyzing the experimental data of the irregularly coordinating atoms.

Coordination-Resolved Spectrometrics of Local Bonding and Electronic Dynamics of Au Atomic Clusters, Solid Skins, and Oxidized Foils.

By using combination of bond-order-length-strength (BOLS) correlation, the tight-binding (TB) approach, and zone-selective photoelectron spectroscopy (ZPS), we were able to resolve local bond relaxation and the associated 4f7/2 core-level shift of Au atomic clusters, Au(100, 110, 111) skins, and Au foils exposed to ozone for different lengths of time. In addition to quantitative information, such as local bond length, bond energy, binding-energy density, and atomic cohesive energy, the results confirm our predictions that bond-order deficiency shortens and stiffens the bond between undercoordinated atoms, which results in local densification and quantum entrapment of bonding electrons. The entrapment perturbs the Hamiltonian, and hence, shifts the core-level energy accordingly. ZPS also confirms that oxidation enhances the effect of atomic undercoordination on the positive 4f7/2 energy shift, with the associated valence electron polarization contributing to the catalytic ability of undercoordinated Au atoms.

Atomistic bond relaxation, energy entrapment, and electron polarization of the RbN and CsN clusters (N ≤ 58).

We systematically examined the effect of atomic undercoordination on the performance of bonds and electrons of Rb and Cs atomic clusters and their solid skins using a combination of photoelectron spectrometric analysis and density functional theory calculations. Results show that atomic coordination number reduction shortens the bonds by up to 30% for the Rb13 and Cs13 clusters, which densifies the local electrons and entraps their binding energies. Consistency between predictions and observations revealed that the Rb 4p level shifts from 13.654 eV for an isolated atom to a bulk value of 14.940 eV and the Cs 5p level shifts from 10.284 to 11.830 eV upon bulk formation. Such core-electron densification and entrapment polarize the valence charge from the inner to the outermost layer of skins, which perturbs the local Hamiltonian and hence dictates the unusual behavior of the Rb and Cs solid skins and nanocrystals.

Oxygenation mediating the valence density-of-states and work function of Ti(0001) skin.

Consistency between density function theory calculations and photoelectron spectroscopy observations confirmed predictions based on the framework of bond-band-barrier (3B) correlation notation [Sun, Prog. Mater. Sci., 2003, 48, 521-685] that an oxygen adsorbate interacts with Ti(0001) skin atoms to form a tetrahedron with creation of four valence density-of-state features: O-Ti bonding electron pairs, O nonbonding lone pairs, Ti electronic holes, and Ti antibonding dipoles. Formation of the dipoles lowers the work function of the Ti(0001) skin and electron-hole generation turns the metallic Ti(0001) into the semiconductive O-Ti(0001). Findings confirm the universality of the 3B correlation in understanding the dynamics of oxygen chemisorption and the associated valence electrons involved in the process of oxidation.

Atomic under-coordination fascinated catalytic and magnetic behavior of Pt and Rh nanoclusters.

Density functional theory (DFT) calculations with local spin density discrimination have been performed to examine the effect of atomic under-coordination on the catalytic and magnetic properties of cuboctahedral (CO) and marks decahedral (MD) structured Pt and Rh nanoclusters. Consistency between theoretical calculations and experimental observations confirmed the predictions based on the framework of bond-order-length-strength (BOLS) correlation and nonbonding electron polarization (NEP) notations. The BOLS-NEP notation suggests that the shorter-and-stronger bonds between under-coordinated atoms induce local densification and quantum entrapment of core electrons, which then polarize the otherwise conducting electrons and result in shifts of the binding energy. Such strong localization resolves the intriguing catalytic and magnetic attributes of Pt and Rh nanoclusters.

Skin-resolved local bond contraction, core electron entrapment, and valence charge polarization of Ag and Cu nanoclusters.

Consistency between density functional theory (DFT) calculations and experimental observations confirmed our predictions on the behaviour of local bonds, and the electron binding energy of cuboctahedral and Marks decahedral structures of Ag and Cu nanoclusters. The shorter and stronger bonds between under-coordinated atoms cause local densification and quantum entrapment of the core electrons, which polarize the otherwise conducting electrons (valence electrons). Such strong localization may result in extraordinary catalytic and plasmonic properties in Ag and Cu nanoclusters.

Mediating relaxation and polarization of hydrogen-bonds in water by NaCl salting and heating.

Infrared spectroscopy and contact-angle measurements revealed that NaCl salting has the same effect as heating on O:H phonon softening and H-O phonon stiffening, but has the opposite effect on skin polarization of liquid water. The mechanics of thermal modulation of O-O Coulomb repulsion [Sun, et al., J. Phys. Chem. Lett., 2013, 4, 3238] may suggest a possible mechanism for this NaCl involved Hofmeister effect, aqueous solution modulated surface tension and its abilities in protein dissolution, from the perspective of Coulomb mediation of interaction within the O:H-O bond.

Hydrogen-bond memory and water-skin supersolidity resolving the Mpemba paradox.

The Mpemba paradox, that is, hotter water freezes faster than colder water, has baffled thinkers like Francis Bacon, René Descartes, and Aristotle since B.C. 350. However, a commonly accepted understanding or theoretical reproduction of this effect remains challenging. Numerical reproduction of observations, shown herewith, confirms that water skin supersolidity [Zhang et al., Phys. Chem. Chem. Phys., DOI: ] enhances the local thermal diffusivity favoring heat flowing outwardly in the liquid path. Analysis of experimental database reveals that the hydrogen bond (O:H-O) possesses memory to emit energy at a rate depending on its initial storage. Unlike other usual materials that lengthen and soften all bonds when they absorb thermal energy, water performs abnormally under heating to lengthen the O:H nonbond and shorten the H-O covalent bond through inter-oxygen Coulomb coupling [Sun et al., J. Phys. Chem. Lett., 2013, 4, 3238]. Cooling does the opposite to release energy, like releasing a coupled pair of bungees, at a rate of history dependence. Being sensitive to the source volume, skin radiation, and the drain temperature, the Mpemba effect proceeds only in the strictly non-adiabatic 'source-path-drain' cycling system for the heat "emission-conduction-dissipation" dynamics with a relaxation time that drops exponentially with the rise of the initial temperature of the liquid source.

A common supersolid skin covering both water and ice.

Consistency in experimental observations, numerical calculations, and theoretical predictions have revealed that the skins of water and ice share the same attribute of supersolidity characterized by an identical H-O vibration frequency of 3450 cm(-1). Molecular undercoordination and inter-electron-pair repulsion shortens the H-O bond and lengthens the O:H nonbond, leading to a dual process of nonbonding electron polarization. This relaxation-polarization process enhances the dipole moment, elasticity, viscosity, and thermal stability of these skins with a 25% density loss, which is responsible for the hydrophobicity and toughness of the water skin and results in the slippery behavior of ice.

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.