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Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered nitrogen heterocycles created a second σ-aromaticity in addition to the prototypical π-aromaticity. Such dual-aromatic compounds, such as the pentazole anion, were proved to have distinct chemistry in comparison to traditional π-aromatics, such as benzene, and were surprisingly unstable, susceptible to electrophilic attack, and relatively difficult to obtain. The dual-aromatics are basic in nature, but prefer not to be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding of acid−base neutralization for a reason that is unclear. Here, we carried out 63 test simulations to explore the stability and reactivity of three basic heterocycle anions (pentazole anion N5¯, tetrazole anion N4C1H1¯, and 1,2,4-triazole anion N3C2H2¯) in four types of solvents (acidic ions, H3O+ and NH4+, polar organics, THF, and neutral organics, benzene) with different acidities and concentrations. By quantum mechanical calculations of the electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, and energetics, we confirmed the presence of dual aromaticity in the heterocyclic anions, and discovered their reactivity to be a competition between their basicity and dual aromaticity. Interestingly, when the acidic ions H3O+/NH4+ are three times more in number than the basic heterocyclic anions, the anions turn to violate acid−base neutralization and remain unprotonated, and the surrounding acidic ions start to show a significant stabilization effect on the studied heterocyclic anions. This work brings new knowledge to nitrogen aromatics and the finding is expected to be adaptable for other pnictogen five-membered ring systems.

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.

An examination of the effect of B- and P-doping and codoping on the electronic structure of anatase TiO2 by performing density functional theory calculations revealed the following: (i) B- or P-doping effects are similar to atomic undercoordination effects on local bond relaxation and core electron entrapment; (ii) the locally entrapped charge adds impurity levels within the band gap that could enhance the utilization of TiO2 to absorb visible light and prolong the carrier lifetime; (iii) the core electron entrapment polarizes nonbonding electrons in the upper edges of the valence and conduction bands, which reduces not only the work function but also the band gap; and (iv) work function reduction enhances the reactivity of the carriers and band gap reduction promotes visible-light absorption. These observations may shed light on effective catalyst design and synthesis.

The effect of nitrogen doping on the behavior of field emission, surface energy and the band structure of strontium titanate (SrTi[O.sub.3]) thin films coated on silicon tip arrays is examined. The optimal 50%-nitrogen partial pressure might improve the threshold field of electron emission of the SrTi[O.sub.3] films accompanied with narrowed band gap and lowered surface energy.

The effect of surface bond-order loss on the dc conductance in a nanosolid is determined by considering the sunken part of the intraatomic trapping potential near the surface. The results show that the downshifts of the dc conductance peaks happens in an oscillatory way, depending on the strength of the perturbation and the downshifts of the peak position due to the trapping potential well depression decrease as the size of the nanosolid is increased.

An analytical solution demonstrated that a competition between bond order loss and the associated bond strength gain of the lower coordinated atoms near edge of a surface dictates the mechanics of the surface and hence, a nanosolid. It concluded that the bond loss lowers the activation energy for atomic dislocation, whereas bond strength gain enhances the energy density or mechanical strength in the region near the surface.

With the solid size reduction, the transverse optical (TO) Raman mode shifts to lower frequency and new low-frequency Raman (LFR) acoustic modes are generated and shift to higher frequency upon nanosolid Si formation. On the basis of bond-order-length-strength correlation, TO red shift arises from the cohesive bond weakening of the lower coordinated atoms near the surface region of the nanograin, and the LFR arises from intergrain interaction.

For single-walled carbon nanotubes (SWCNTs), given the known product of the Young's modulus Y and the wall thickness t and the known temperature of tip-end melting (T(sub m) = 1593 K) as well as their functional dependence on atomic coordination and bonding energy, the dimension and strength of a C-C bond in a SWCNT are determined. The solution reveals that the C-C bond is ~0.142 nm thick and ~0.125 nm long with a 68% increase in binding energy, with respect to the bulk graphite values.

The functional dependence of stress, elastic modulus, melting point, and their interdependence on the identities of a representative bond of the specimen is established for understanding the transition from the conventional Hall-Petch relationship (HPR) to the inverse HPR (IHPR) for nanostructured Ti[O.sub.2]. The reproduction of the inverse HPR has shown that the intrinsic competition has originated between the energy-density gain and the cohesive-energy remnant in the grain boundaries and the extrinsic competition between the activation and the inhibition of atomic dislocations has activated the IHPR.

A dual-shell model is developed for the size and shape induced suppression of the Curie temperature for BaTi[O.sub.3] nanocrystals, which is based on the short range bond order-length-strength correlation and the long range dipole-dipole interaction. One surface shell of three atomic layers have represented the short range interaction due to the shorter and stronger bonds between under-coordinated atoms and the other shell with thickness of [K.sub.c] has characterized the long range dipole-dipole interaction.

Atomic nonequilibrium GreenEs functions are used for examining the phonon transport in atomic wire coupled by two semi-infinite thermal contacts. The studies have shown that surface bond reconstruction has affected phonon transport significantly in the presence of anharmonicity.

The development of a local bond average (LBA) approach is reported by extending the bond-order-length-strength correlation to temperature domain for the mechanism of the thermally driven bond expansion in a solid specimen. The performance and success of the LBA approach is confirmed from a number of specimens with derived information on atomic cohesive energy.

An analytical solution is presented for correlating the elastic modulus and vibration frequency as well as their temperature dependence. The temperature dependence of the elastic modulus and vibronic frequency of Si, Ge and diamond samples have shown that the thermal softening of YoungEs modulus and optical Raman phonon frequency have come from bond expansion and bond vibration.

The discrimination of the contribution of surface-bond relaxation from the contribution of surface-bond nature alteration is discussed. Fluorination enhances the crystal binding intensity and the electron-phonon coupling.

Measurements of deep level transient spectroscopy (DLTS) and optical absorption spectroscopy were used to study the features of the density-of-states (DOS) that appeared near the valence band edge of aluminum oxide thin films. Raman spectroscopy ascertained the similarities between the observed midgap DOS feature dynamics and the oxygen lone-pair features.

The influence of different parameters on the field emission properties of carbon nitride films is studied. Studies suggest that the involvement of nonbonding (lone pair) and lone-pair-induced antibonding (dipole) states is responsible for lowering the work function and hence the electron emission threshold.

The dielectric impedance measurements of porous silicon within the frequency range of 50 Hz-1.0 MHz and temperature range of 298-798 K are examined. An interpretation of the dispersion behavior, which occurs in the real and imaginary parts of the permittivity and at low frequencies, is provided.

An atomistic model is presented for the melting behavior of a surface and a nanosolid, as well as their interdependence based on the recent 'bond order-length-strength' (bond-OLS) correlation mechanism. It is suggested that the coordination number (CN) of a surface atom reduces whereas the remaining bonds of the lower-coordianted surface atom relax spontaneously associated with the single bond energy rise.

A new approach to improve the diamond-metal adhesion with a critical scratch load upto 135 N is reported. A graded TiCN interlayer has been applied carefully, adjusting the ratio of C and N in the gas mixture to neutralize the interfacial stress due to which the diamond-metal adhesion has improved substantially.

The structural symmetry and molecular separation in water and ice remain uncertain. We present herewith a solution to unifying the density, the structure order and symmetry, the size (H-O length dH), and the separation (dOO 5 dL + dH or the O:H length dL) of molecules packing in water and ice in terms of statistic mean. This solution reconciles: i) the dL and the dH symmetrization of the O:H-O bond in compressed ice, ii) the dOO relaxation of cooling water and ice and, iii) the dOO expansion of a dimer and between molecules at water surface. With any one of the dOO, the density r(g.cm-3), the dL, and the dH, as a known input, one can resolve the rest quantities using this solution that is probing conditions or methods independent. We clarified that: i) liquid water prefers statistically the mono-phase of tetrahedrally-coordinated structure with fluctuation, ii) the low-density phase (supersolid phase as it is strongly polarized with even lower density) exists only in regions consisting molecules with fewer than four neighbors and, iii) repulsion between electron pairs on adjacent oxygen atoms dictates the cooperative relaxation of the segmented O:H-O bond, which is responsible for the performance of water and ice. [ABSTRACT FROM AUTHOR]

It has long been puzzling regarding the atomistic origin of the pressure-induced Raman optical phonon stiffening that generally follows a polynomial expression with coefficients needing physical indication. Here, we show that an extension of the bond−order−length−strength correlation mechanism and a local bond average approach to the pressure domain have led to an analytical solution to connect the pressure-induced Raman optical phonon stiffening directly to the bonding identities of the specimen and the response of the bonding identities to the applied stimulus. It is found that the pressure-induced blue-shift of Raman optical phonons arises from the bond compression and energy storage exerted by the compressive stress. Agreement between predictions and experimental measurements lead to the clarification of the detailed form for the polynomial coefficients, which provide an atomistic understanding of the physical mechanism of the external pressure induced energy gain, thermally induced bond expansion, as well as means of determining the mode atomic cohesive energy in a specimen. [ABSTRACT FROM AUTHOR]

It is intriguing that a solid containing ~101 atoms of Ga or IV-A elements (C, Si, Ge, Sn, and Pb) melts at temperatures that are higher than the melting point of the corresponding bulk solid (Tm,b) though the Tm of a solid in the 100–2 nm size range drops universally with the sample size. Consistent insight into the phenomenon of Tm oscillation (suppression followed by elevation as the solid size is reduced from bulk to subnanometre size) over the whole range of sizes remains a scientific challenge. Here we show that the Tm oscillation arises from the joint effect of bond order loss and its consequence on bond strength gain of small clusters in particular for Ga and atoms of the IV-A elements. [ABSTRACT FROM AUTHOR]

It has long been puzzling regarding the trends and physical origins of the size-effect on the elasticity of ZnO nanostructures. An extension of the atomic “coordination-radius” correlation premise of Pauling and Goldschmidt to energy domain has enabled us to clarify that the elastic modulus is intrinsically proportional to the sum of bond energy per unit volume and that the size-induced elastic stiffening arises from (i) the broken-bond-induced local strain and skin-depth energy pinning and (ii) the tunable fraction of bonds between the undercoordinated atoms, and therefore, the elastic modulus of ZnO nanostructures should increase with the inverse of feature size. [ABSTRACT FROM AUTHOR]

It has long been a puzzle regarding the atomistic origin of the thermally driven elastic softening of a solid specimen since the pioneering works of Anderson in 1966 and Watchman in 1961. Here we present an analytical solution developed from the perspective of the local bond average. It is shown that the thermally driven elastic softening of crystals can be directly related to the geometric and energetic response of the representative bonds or their average of the given specimen to the temperature variation. Reproduction of the experimental results of Ag, Au, MgO, Mg2SO4, Al2O3, KCl, Si, Ge and diamond crystals has been realized giving rise to information on the mean atomic cohesive energy without involving any hypothetical parameters used in classical thermodynamics or in continuum mechanics. [ABSTRACT FROM AUTHOR]

Summary: 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.

Summary: 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. : Computational Method in Materials Science; Density Functional Theory (DFT); Energy Materials; Materials Design Subject Areas: Computational Method in Materials Science, Density Functional Theory (DFT), Energy Materials, Materials Design

Detailed density functional theory (DFT) calculations of the adsorption energies (Ead) for oxygen on monolayer Pd on top of the Pd–Cu face-centered cubic (FCC) alloy and intermetallic B2 structure revealed a linear correspondence between the adsorption energies and the d-band center position. The calculated barrier (Ebarrier) for oxygen dissociation depends linearly on the reaction energy difference (ΔE). The O2 has a stronger adsorption strength and smaller barrier on the intermetallic Pd–Cu surface than on its FCC alloy surface. The room-temperature free energy (ΔG) analysis suggests the oxygen reduction reaction (ORR) pathways proceed by a direct dissociation mechanism instead of hydrogenation into OOH. These results might be of use in designing intermetallic Pd–Cu as ORR electrocatalysts.

By means of the nonequilibrium Green's functions and the density functional theory, we have investigated the electronic transport properties of C60 based electronic device with different intermolecular interactions. It is found that the electronic transport properties vary with the types of the interaction between two C60 molecules. A fast electrical switching behavior based on negative differential resistance has been found when two molecules are coupled by the weak π − π interaction. Compared to the solid bonding, the weak interaction is found to induce resonant tunneling, which is responsible for the fast response to the applied electric field and hence the velocity of switching.