Fluxional halogen bonds in linear complexes of tetrafluorodiiodobenzene with dinitrobenzene.

The fluxional nature of halogen bonds (XBs) in small molecular clusters, supramolecules, and molecular crystals has received considerable attention in recent years. In this work, based on extensive density-functional theory calculations and detailed electrostatic potential (ESP), natural bonding orbital (NBO), non-covalent interactions-reduced density gradient (NCI-RDG), and quantum theory of atoms in molecules (QTAIM) analyses, we unveil the existence of fluxional halogen bonds (FXBs) in a series of linear (IC6F4I)m(OONC6H4NOO)n (m + n = 2-5) complexes of tetrafluorodiiodobenzene with dinitrobenzene which appear to be similar to the previously reported fluxional hydrogen bonds (FHBs) in small water clusters (H2O)n (n = 2-6). The obtained GS ⇌ TS ⇌ GS ' $$ \mathrm{GS}\rightleftharpoons \mathrm{TS}\rightleftharpoons {\mathrm{GS}}^{\hbox{'}} $$ fluxional mechanisms involve one FXB in the systems which fluctuates reversibly between two linear CI···O XBs in the ground states (GS and GS') via a bifurcated CI O2N van der Waals interaction in the transition state (TS). The cohesive energies (Ecoh) of these complexes with up to four XBs exhibit an almost perfect linear relationship with the numbers of XBs in the systems, with the average calculated halogen bond energy of Ecoh/XB = 3.48 kcal·mol-1 in the ground states which appears to be about 55% of the average calculated hydrogen bond energy (Ecoh/HB = 6.28 kcal·mol-1) in small water clusters.

First-Principles Investigation on Ru-Doped Janus WSSe Monolayer for Adsorption of Dissolved Gases in Transformer Oil: A Novel Sensing Candidate Exploration.

Using first-principles theory, this work purposes Ru-doped Janus WSSe (Ru-WSSe) monolayer as a potential gas sensor for detection of three typical gas species (CO, C2H2, and C2H4), in order to evaluate the operation status of the oil-immersed transformers. The Ru-doping behavior on the WSSe surface is analyzed, giving rise to the preferred doping site by the replacement of a Se atom with the formation energy of 0.01 eV. The gas adsorption of three gas species onto the Ru-WSSe monolayer is conducted, and chemisorption is identified for all three gas systems with the adsorption energy following the order: CO (-2.22 eV) > C2H2 (-2.01 eV) > C2H4 (-1.70 eV). Also, the modulated electronic properties and the frontier molecular orbital are investigated to uncover the sensing mechanism of Ru-WSSe monolayer upon three typical gases. Results reveal that the sensing responses of the Ru-WSSe monolayer, based on the variation of energy gap, to CO, C2H2, and C2H4 molecules are calculated to be 1.67 × 106, 2.10 × 105, and 9.61 × 103, respectively. Finally, the impact of the existence of O2 molecule for gas adsorption and sensing is also analyzed to uncover the potential of Ru-WSSe monolayer for practical application in the air atmosphere. The obtained high electrical responses manifest strong potential as a resistive sensor for detection of three gases. The findings hold practical implications for the development of novel gas sensing materials based on Janus WSSe monolayer. We anticipate that our results will inspire further research in this domain, particularly for applications in electrical engineering where the reliable detection of fault gases is paramount for maintaining the integrity and safety of power systems.

First-Principles Study of the Structural, Mechanical, Electronic, and Thermodynamic Properties of AlCu2M (M = Ti, Cr, Zr, Sc, Hf, Mn, Pa, Lu, Pm) Ternary Intermetallic Compounds.

Based on the first principles, the structural stability, mechanical characteristics, electronic structure, and thermodynamic properties of AlCu2M (M = Ti, Cr, Zr, Sc, Hf, Mn, Pa, Lu, Pm) are investigated. The calculated results indicate that the AlCu2Pa crystal structure is more stable and that AlCu2Pa should be easier to form. All of the AlCu2M compounds have structural stability in the ground state. Elastic constants are used to characterize the mechanical stability and elastic modulus, while the B/G values and Poisson ratio demonstrate the brittleness and ductility of AlCu2M compounds. It is demonstrated that all computed AlCu2M compounds are ductile and mechanically stable, with AlCu2Hf having the highest bulk modulus and AlCu2Mn having the highest Young's modulus. AlCu2Mn has the highest intrinsic hardness among AlCu2M compounds, according to calculations of their intrinsic hardness. The electronic densities of states are discussed in detail; it was discovered that all AlCu2M compounds form Al-Cu and Al-M covalent bonds. Additionally, we observe that the Debye temperature and minimum thermal conductivity of AlCu2Mn and AlCu2Sc are both larger than those of others, indicating stronger chemical bonds and higher thermal conductivities, which is consistent with the elastic modulus results.

The Effect of Nb Doping on the Properties of Ti-Al Intermetallic Compounds Using First-Principles Calculations.

The crystal structures, stability, mechanical properties and electronic structures of Nb-free and Nb-doped Ti-Al intermetallic compounds were investigated via first-principles calculations. Seven components and eleven crystal configurations were considered based on the phase diagram. The calculated results demonstrate that hP8-Ti3Al, tP4-TiAl, tP32-Ti3Al5, tI24-TiAl2, tI16-Ti5Al11, tI24-Ti2Al5, and tI32-TiAl3 are the most stable phases. Mechanical properties were estimated with the calculated elastic constants, as well as the bulk modulus, shear modulus, Young's modulus, Poisson's ratio and Pugh's ratio following the Voigt-Reuss-Hill scheme. As the Al content increases, the mechanical strength increases but the ductility decreases in the Ti-Al compounds. This results from the enhanced covalent bond formed by the continuously enhanced Al-sp hybrid orbitals and Ti-3d orbitals. Nb doping (~5 at.% in this study) keeps the thermodynamical and mechanical stability for the Ti-Al compounds, which exhibit slightly higher bulk modulus and better ductility. This is attributed to the fact that the Nb 4d orbitals locate near the Fermi level and interact with the Ti-3d and Al-3p orbitals, improving the metallic bonds based on the electronic structures.

Sc@B28-, Ti@B28, V@B28+, and V@B292-: Spherically Aromatic Endohedral Seashell-like Metallo-Borospherenes.

Transition-metal-doped boron nanoclusters exhibit unique structures and bonding in chemistry. Using the experimentally observed seashell-like borospherenes C2 B28-/0 and Cs B29- as ligands and based on extensive first-principles theory calculations, we predict herein a series of novel transition-metal-centered endohedral seashell-like metallo-borospherenes C2 Sc@B28- (1), C2 Ti@B28 (2), C2 V@B28+ (3), and Cs V@B292- (4) which, as the global minima of the complex systems, turn out to be the boron analogues of dibenzenechromium D6h Cr(C6H6)2 with two B12 ligands on the top and bottom interconnected by four or five corner boron atoms on the waist and one transition-metal "pearl" sandwiched at the center in between. Detailed molecular orbital, adaptive natural density partitioning (AdNDP), and iso-chemical shielding surface (ICSS) analyses indicate that, similar to Cr(C6H6)2, these endohedral seashell-like complexes follow the 18-electron rule in bonding patterns (1S21P61D10), rendering spherical aromaticity and extra stability to the systems.

Magnetically Doped Molybdenum Disulfide Layers for Enhanced Carbon Dioxide Capture.

Carbon capture and storage (CCS) technologies have the potential for reducing greenhouse gas emissions and creating clean energy solutions. One of the major aspects of the CCS technology is designing energy-efficient adsorbent materials for carbon dioxide capture. In this research, using a combination of first-principles theory, synthesis, and property measurements, we explore the CO2 gas adsorption capacity of MoS2 sheets via doping with iron, cobalt, and nickel. We show that substitutional dopants act as active sites for CO2 adsorption. The adsorption performance is determined to be dependent on the type of dopant species as well as its concentration. Nickel-doped MoS2 is found to be the best adsorbent for carbon capture with a relatively high gas adsorption capacity compared to pure MoS2 and iron- and cobalt-doped MoS2. Specifically, Brunauer-Emmett-Teller (BET) measurements show that 8 atom % Ni-MoS2 has the highest surface area (51 m2/g), indicating the highest CO2 uptake relative to the other concentrations and other dopants. Furthermore, we report that doping could lead to different magnetic solutions with changing electronic structures where narrow band gaps and the semimetallic tendency of the substrate are observed and can have an influence on the CO2 adsorption ability of MoS2. Our results provide a key strategy to the characteristic tendencies for designing highly active and optimized MoS2-based adsorbent materials utilizing the least volume of catalysts for CO2 capture and conversion.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective.

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

Trade-off effect of 3d transition metal doped boron nitride on anchoring polysulfides towards application in lithium-sulfur battery.

Sulfur cathodes in lithium-sulfur batteries (LSBs) suffer from the notorious "shuttle effect", low sulfur use ratio, and tardy transformation of lithium polysulfides (LiPSs), while using two-dimensional (2D) polar anchoring materials combined with single-atom catalysis is one of the promising methods to address these issues. Herein, the 3d transition metal (TM) doped 2D boron nitrides (BN), labeled as TM-BN, are studied for the anchoring and redox kinetics of LiPSs using first principles calculations. From the simulated results, the TM atom and adjacent N atoms are active adsorption sites for binding S atoms in LiPSs/S8 and Li atoms in LiPSs, respectively. A negative d-band center closer to the Fermi level of TM-BN is key for enhancing the binding strength of TM-S and lowering the Li2S decomposition energy barrier, while it deteriorates the activity of adjacent N atoms. Fortunately, the electrolyte environment has little effect on the superiority of the TM-BN for binding polysulfides/S8, guaranteeing the sturdy anchor of polysulfides/S8 in realistic conditions. The trade-off effect on the activities of TM and adjacent N atom sites in TM-BN for binding LiPSs highlights the excellence of Ti/V/Cr-BN as modification materials for LSB.

Metal-N4@Graphene as Multifunctional Anchoring Materials for Na-S Batteries: First-Principles Study.

Developing highly efficient anchoring materials to suppress sodium polysulfides (NaPSs) shuttling is vital for the practical applications of sodium sulfur (Na-S) batteries. Herein, we systematically investigated pristine graphene and metal-N4@graphene (metal = Fe, Co, and Mn) as host materials for sulfur cathode to adsorb NaPSs via first-principles theory calculations. The computing results reveal that Fe-N4@graphene is a fairly promising anchoring material, in which the formed chemical bonds of Fe-S and N-Na ensure the stable adsorption of NaPSs. Furthermore, the doped transition metal iron could not only dramatically enhance the electronic conductivity and the adsorption strength of soluble NaPSs, but also significantly lower the decomposition energies of Na2S and Na2S2 on the surface of Fe-N4@graphene, which could effectively promote the full discharge of Na-S batteries. Our research provides a deep insight into the mechanism of anchoring and electrocatalytic effect of Fe-N4@graphene in sulfur cathode, which would be beneficial for the development of high-performance Na-S batteries.

First-principles study of pristine and Li-doped borophene as a candidate to detect and scavenge SO2gas.

In this research, the potential application of borophene as gas sensor device is explored. The first-principles theory is employed to investigate the sensing performance of pristine and Li-doped borophene for SO2and five main atmospheric gases (including CH4, CO2, N2, CO and H2). All gases are found to be adsorbed weakly on pristine borophene, which shows weak physical interaction between the pristine borophene and gases. The gas adsorption performance of borophene is improved by the doping of Li atom. The results of adsorption energy suggest that Li-borophene exhibits high selectivity to SO2molecule. Moreover, analyses of the charge transfer, density of states and work function also confirm the introduction of Li adatom on borophene significantly enhances the selectivity and sensitivity to SO2. In addition, desorption time of gas from pristine and Li doped borophene indicates the Li-borophene has good desorption characteristics for SO2molecule at high temperatures. This research would be helpful for understanding the influence of Li doping on borophene and presents the potential application of Li-borophene as a SO2gas sensor or scavenger.

Cage-like La4B24 and Core-Shell La4B290/+/- : perfect spherically aromatic tetrahedral metallo-borospherenes.

Cage-like and core-shell metallo-borospherenes exhibit interesting structures and bonding. Based on extensive global searches and first-principles theory calculations, we predict herein the perfect tetrahedral cage-like Td La4B24 (1) and core-shell Td La4B29 (2), Td La4B29+ (3), and Td La4B29- (4) which all possess the same geometrical symmetry as their carbon fullerene counterpart Td C28, with four equivalent interconnected B6 triangles on the cage surface and four nona-coordinate La centers in four conjoined η9-B9 rings. In these tetra-La-doped boron complexes, La4[B@B4@B24]0/+/- (2/3/4) in the structural motif of 1 + 4 + 28 contain a B-centered tetrahedral Td B@B4 core in a La-decorated tetrahedral La4B24 shell, with the negatively charged tetra-coordinate B- at the center being the boron analog of tetrahedral C in Td CH4 (B- ~ C). Detailed orbital and bonding analyses indicate that these Td lanthanide boride complexes are spherically aromatic in nature with a universal La--B9 (d-p) σ and (d-p) δ coordination bonding pattern. The IR, Raman, and UV-Vis or photoelectron spectra of these novel metallo-borospherenes are computationally simulated to facilitate their spectral characterizations. Graphical abstract.

Rh-doped MoTe2 Monolayer as a Promising Candidate for Sensing and Scavenging SF6 Decomposed Species: a DFT Study.

In this work, the adsorption and sensing behaviors of Rh-doped MoTe2 (Rh-MoTe2) monolayer upon SO2, SOF2, and SO2F2 are investigated using first-principles theory, wherein the Rh doping behavior on the pure MoTe2 surface is included as well. Results indicate that TMo is the preferred Rh doping site with Eb of - 2.69 eV, and on the Rh-MoTe2 surface, SO2 and SO2F2 are identified as chemisorption with Ead of - 2.12 and - 1.65 eV, respectively, while SOF2 is physically adsorbed with Ead of - 0.46 eV. The DOS analysis verifies the adsorption performance and illustrates the electronic behavior of Rh doping on gas adsorption. Band structure and frontier molecular orbital analysis provide the basic sensing mechanism of Rh-MoTe2 monolayer as a resistance-type sensor. The recovery behavior supports the potential of Rh-doped surface as a reusable SO2 sensor and suggests its exploration as a gas scavenger for removal of SO2F2 in SF6 insulation devices. The dielectric function manifests that Rh-MoTe2 monolayer is a promising optical sensor for selective detection of three gases. This work is beneficial to explore Rh-MoTe2 monolayer as a sensing material or a gas adsorbent to guarantee the safe operation of SF6 insulation devices in an easy and high-efficiency manner.

Ferromagnetic Half-Metal Cyanamides Cr(NCN)2 Predicted from First Principles Investigation.

The stability, physical properties, and electronic structures of Cr(NCN)2 were studied using density functional theory with explicit electronic correlation (GGA+U). The calculated results indicate that Cr(NCN)2 is a ferromagnetic and half-metal, both thermodynamically and elastically stable. A comparative study on the electronic structures of Cr(NCN)2 and CrO2 shows that the Cr atoms in both compounds are in one crystallographically equivalent site, with an ideal 4+ valence state. In CrO2, the Cr atoms at the corner and center sites have different magnetic moments and orbital occupancies, moreover, there is a large difference between the intra- (12.1 meV) and inter-chain (31.2 meV) magnetic couplings, which is significantly weakened by C atoms in Cr(NCN)2.

First-principles study of structures, electronic and elastic properties of LaNi5-xFex(x: 0.25-1.25).

The structural, electronic and elastic properties of LaNi5-xFex (x = 0.25, 0.5, 0.75, 1, 1.25) have been investigated employing the density functional theory with the generalized gradient approximation (GGA). The optimized results indicate that Fe prefers to substitute Ni atom in the 3 g site, and Fe replaces 2c site of Ni atom up to x = 1.25. The radius of 6 m interstitial site gradually increases as x goes from 0 to 1.25 due to volume expansion, which is one of the reasons for the increase of hydrogen storage. Combined with elastic constants and elastic modulus, the anti-pulverization ability of the La-Ni-Fe system enhances, and LaNi4.25Fe0.75 has the best anti-pulverization capability among six crystal structures. According to the density of states, the new peak appears at near -5 eV which is dominated by Fe-3d States. Based on the analysis of charge distributions and charge density differences, the sequence of interaction between atoms is Ni2c-Fe3g > Ni2c-Ni3g > Ni3g-Ni3g > Ni3g-Fe3g > La-Ni2c > La-Fe2c, while that of the interactions with H atom is as follows: Fe > Ni3g > Ni2c > La. The electron hybridization of H and La, Ni, Fe atoms form covalent bonds through electron transfer.

Fluxional bonds in quasi-planar B 18 2 - and half-sandwich MB 18 - (M = K, Rb, and Cs).

Detailed molecular orbital and bonding analyses reveal the existence of both fluxional σ- and π-bonds in the global minima Cs B 18 2 - (1) and Cs MB18 (3) and transition states Cs B 18 2 - (2) and Cs MB 18 - (4) of B 18 2 - dianion and MB 18 - monoanions (M = K, Rb, and Cs). It is the fluxional bonds that facilitate the fluxional behaviors of the quasi-planar B 18 2 - and half-sandwich MB 18 - which possess energy barriers smaller than the difference of the corresponding zero-point corrections. © 2019 Wiley Periodicals, Inc.

Fluxional Bonds in Planar B19 - , Tubular Ta@B20 - , and Cage-Like B39.

Based on detailed bonding analyses on the fluxional behaviors of planar B19 - , tubular Ta@B20 - , and cage-like B39 - , we propose the concept of fluxional bonds in boron nanoclusters as an extension of the classical localized bonds and delocalized bonds in chemistry. © 2018 Wiley Periodicals, Inc.

Cage-like B40C30, B40C40, and B40C50: high-symmetry heterofullerenes isovalent with C60, C70, and C80.

The recent discovery of the cage-like borospherenes B40-/0, composed of interwoven double chains of boron, presents the possibility of forming BmCn heterofullerenes as hybrids of borospherenes and carbon fullerenes in dual spaces. Based on extensive first-principles theory calculations, we predict herein the possible existence of the high-symmetry BmCn heterofullerenes S10 B40C30 (1), C5 B40C40 (2), and S10 B40C50 (3), which are isovalent with C60, C70, and C80, respectively. These beautiful borafullerenes with boron aggregations feature one B30 boron double-chain nanoring at the equator, two bowl-shaped C15 or C25 caps at the top and bottom, and ten quasi-planar tetracoordinate peripheral C atoms in ten B-centered B6C hexagonal pyramids that are evenly distributed around the waist in a seamless "patched" structural motif. Detailed orbital and bonding analyses indicate that, as they are isovalent with C60, C70, and C80, respectively, B40C30 (1), B40C40 (2), and B40C50 (3) possess 30, 35, and 40 π bonds, respectively, of which 20 are 5c-2e π bonds delocalized over ten hexagonal pyramids that are evenly distributed around the waist. Such structural and bonding patterns confer high stability to these B-C heterofullerenes, which may be synthesized in experiments.

The Effects of Graphene Stacking on the Performance of Methane Sensor: A First-Principles Study on the Adsorption, Band Gap and Doping of Graphene.

The effects of graphene stacking are investigated by comparing the results of methane adsorption energy, electronic performance, and the doping feasibility of five dopants (i.e., B, N, Al, Si, and P) via first-principles theory. Both zigzag and armchair graphenes are considered. It is found that the zigzag graphene with Bernal stacking has the largest adsorption energy on methane, while the armchair graphene with Order stacking is opposite. In addition, both the Order and Bernal stacked graphenes possess a positive linear relationship between adsorption energy and layer number. Furthermore, they always have larger adsorption energy in zigzag graphene. For electronic properties, the results show that the stacking effects on band gap are significant, but it does not cause big changes to band structure and density of states. In the comparison of distance, the average interlamellar spacing of the Order stacked graphene is the largest. Moreover, the adsorption effect is the result of the interactions between graphene and methane combined with the change of graphene's structure. Lastly, the armchair graphene with Order stacking possesses the lowest formation energy in these five dopants. It could be the best choice for doping to improve the methane adsorption.

A First-Principle Theoretical Study of Mechanical and Electronic Properties in Graphene Single-Walled Carbon Nanotube Junctions.

The new three-dimensional structure that the graphene connected with SWCNTs (G-CNTs, Graphene Single-Walled Carbon Nanotubes) can solve graphene and CNTs' problems. A comprehensive study of the mechanical and electrical performance of the junctions was performed by first-principles theory. There were eight types of junctions that were constituted by armchair and zigzag graphene and (3,3), (4,0), (4,4), and (6,0) CNTs. First, the junction strength was investigated. Generally, the binding energy of armchair G-CNTs was stronger than that of zigzag G-CNTs, and it was the biggest in the armchair G-CNTs (6,0). Likewise, the electrical performance of armchair G-CNTs was better than that of zigzag G-CNTs. Charge density distribution of G-CNTs (6,0) was the most homogeneous. Next, the impact factors of the electronic properties of armchair G-CNTs were investigated. We suggest that the band gap is increased with the length of CNTs, and its value should be dependent on the combined effect of both the graphene's width and the CNTs' length. Last, the relationship between voltage and current (U/I) were studied. The U/I curve of armchair G-CNTs (6,0) possessed a good linearity and symmetry. These discoveries will contribute to the design and production of G-CNT-based devices.

Electronic structure of aqueous solutions: Bridging the gap between theory and experiments.

Predicting the electronic properties of aqueous liquids has been a long-standing challenge for quantum mechanical methods. However, it is a crucial step in understanding and predicting the key role played by aqueous solutions and electrolytes in a wide variety of emerging energy and environmental technologies, including battery and photoelectrochemical cell design. We propose an efficient and accurate approach to predict the electronic properties of aqueous solutions, on the basis of the combination of first-principles methods and experimental validation using state-of-the-art spectroscopic measurements. We present results of the photoelectron spectra of a broad range of solvated ions, showing that first-principles molecular dynamics simulations and electronic structure calculations using dielectric hybrid functionals provide a quantitative description of the electronic properties of the solvent and solutes, including excitation energies. The proposed computational framework is general and applicable to other liquids, thereby offering great promise in understanding and engineering solutions and liquid electrolytes for a variety of important energy technologies.