Multicenter Bonds Research Articles

Theoretical study on structural evolution, photoelectron and vibrational spectra, and thermochemistry properties of neutral, anionic and di-anionic titanium-doped tin (TiSnn0/−/2− (n = 4–17)) nanoalloy clusters

The structural evolution, chemical stability, electronic and vibrational properties, as well as charge transfer and bonding character of TiSnn0/−/2− (n = 4–17) clusters have been performed with density functional theory calculations using ABCluster search technique. Structurally, it is found that the growth patterns prefer three kinds of absorbed stages from polygonal bipyramidal configuration for n = 4–6, to absorbing additional Sn on the adjacent surfaces of pentagonal bipyramid unit from n = 7–12, and finally to the TiSn130/−/2− cluster as the first foundational architectures, of which the encapsulated cage structure is formed when n = 11. The simulated PES spectra agree with available experiments. More interestingly, the neutral TiSn16 cluster not only possesses the high thermodynamic and relative stability but also preferable photochemical reactivity, that can be further explained by superatom features and delocalized multi-center bonds (AdNDP), while the strong p-d hybridization between Ti atom and Sn unit plays an important role in the stabilities of clusters, making it as the most suitable building units. In addition, the UV–Vis absorption spectra of TiSn16 are discussed, and the main transitions of crucial excited states are analyzed in detail. The Infrared and Raman vibrational characteristic peaks of all these neutral and charged species are properly assigned, of which the TiSnn0/−/2− (n = 10–17) clusters possess degenerating deformation mode of Ti atom wagging in Sn cage framework (Infrared active) and breathing mode of Sn cage framework (Raman active). All these findings will provide a further understanding for the nanoalloy cluster as the most suitable building block with further development as a potential optoelectronic material.

On the Electride Nature of Na-hP4.

Early quantum mechanical models suggested that pressure drives solids towards free-electron metal behavior where the ions are locked into simple close-packed structures. The prediction and subsequent discovery of high-pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na-hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na-pd hybrid orbitals point towards the center of an 11-membered penta-capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na-hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.

Role of π-Radical Localization on Thermally Stable Cross-Links Between Polycyclic Aromatic Hydrocarbons.

The thermal stability of cross-links between polycyclic aromatic hydrocarbons (PAHs) is critical for understanding the formation of soot pollutants, graphite, and carbon blacks. Recently, a variety of different π-radicals have been directly imaged and suggested to enable thermally stable bonding; however, a systematic study of reactivity has been lacking. In this work, we use density functional theory to study the reactivity of PAH π-radicals. The Mulliken spin densities are initially used to categorize the different classes of localization, and the bond energy is computed to determine the degree of localization required for thermal stability. The results showed that the bond energies of PAHs are strongly correlated with the calculated spin densities, but bond energies do not exist with the bond lengths due to significant rearrangement and steric effects during bond formation. A threshold for π-radical localization is suggested that will be stable in combustion and pyrolysis environments of ρMα ≥ 0.5. Finally, the formation of multicenter bonds between localized and delocalized π-radicals was investigated using the nudge elastic band (NEB) scan, and it was found that only delocalized π-radicals provided local energy minima. These results show that the localization of π-radicals is critical for the formation of thermally stable single-center bonds between aromatic radicals.

Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding

Actinide-ligand bonds with high multiplicities remain poorly understood. Decades ago, an effect known as 6p pushing from below (PFB) was proposed to enhance actinide covalency. A related effect—also poorly understood—is inverse trans influence (ITI). The present computational study of actinide-ligand covalent interactions with high bond multiplicities quantifies the energetic contributions from PFB and identifies a hitherto overlooked fourth bonding interaction for 2nd-row ligands in the studied organometallic systems. The latter are best described by a terminal O/N ligand exhibiting quadruple bonding interactions with the actinide. The 4th interaction may be characterized as a multi-center or charge-shift bond involving the trans ligand. It is shown in this work that the 4th bonding interaction is a manifestation of ITI, assisted by PFB, and provides a long-sought missing piece in the understanding of actinide chemistry.

Global Minimum Search and Bonding Analysis of Tl2 Hx and Tl3 Hy (x=0-6; y=0-5) Series.

A remarkable distinction between boron and carbon hydrides lies in their extremely different bonding patterns and chemical reactivity, resulting in diverse areas of application. Particularly, carbon, characterized by classical two-center - two-electron bonds, gives rise to organic chemistry. In contrast, boron forms numerous exotic and non-intuitive compounds collectively called non-classical structures. It is reasonable to anticipate that other elements of Group 13 exhibit their own unusual bonding patterns; however, our knowledge of the hydride chemistry for other elements in Group 13 is much more limited, especially for the heaviest stable element, thallium. In this work, we performed a conformational analysis of Tl2 Hx and Tl3 Hy (x=0-6, y=0-5) series via Coalescence Kick global minimum search algorithm, DFT, and ab initio quantum chemistry methods; we investigated the bonding pattern using the AdNDP algorithm, thermodynamic stability, and stability toward electron detachment. All found global minimum structures are classified as non-classical structures featuring at least one multi-center bond.

Extraction of C6 α-olefin against n-paraffin in coker naphtha with deep eutectic solvents: Quantum chemical calculations and experimental investigation

Extraction technology is often employed to separate olefin/paraffin mixtures due to the advantages of mature process, flexible operation, and large processing capacity. This work aims to investigate the effectiveness of novel deep eutectic solvents (DESs) for the extraction of 1-hexene/n-hexane model coker naphtha using a combination of simulations and experiments. The results showed that the aliphatic or aromatic groups in the hydrogen bond acceptors or hydrogen bond donors have a strong affinity for 1-hexene, and the components containing hydroxyl or aromatic groups possess the ability to provide protons, which is beneficial to dissolve 1-hexene. Combining the results of the melting point predicted by the group and group-interaction contribution model, three DESs, namely ethanolamine-sulfolane 1:5 (DES1), tetrabutylammonium bromide-bis[2-(2-hydroxyethoxy)ethyl] ether 1:4 (DES2), and methyltriphenylphosphonium bromide-Triethylene glycol 1:4 (DES3), were finally prepared. Then, quantum chemical calculations were used to investigate the separation mechanism between the DESs and 1-hexene/n-hexane, demonstrating single-center and multi-center hybrid weak hydrogen bonds dominated by the dispersion attraction between DESs and 1-hexene. Finally, liquid–liquid equilibrium experiments were used to examine the effects of DESs type, composition, and temperature on the separation performance, and the results showed that the DES1 has both high distribution coefficient of 1-hexene and selectivity of 1-hexene/n-hexane, as well as good recycle stability. This work demonstrates that new DESs can be used as alternative solvents to traditional organic solvents and ionic liquids, and provides ideas for the separation of olefins and paraffins in coker naphtha.

Two Electron Multicenter Bonding (Pancake Bonding) between Electron Donors and Acceptors in Charge-Transfer Compounds ofN,N,N′,N′-Tetramethyl-p-phenylenediamine and (Semi)quinoid Electron Acceptors

Five charge transfer complexes of electron donor N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD or Wurster’s blue) with quinoid electron acceptors tetrafluoro- (F4Q), tetrachloro- (Cl4Q), tetrabromoquinone (Br4Q), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) were prepared and studied by X-ray crystallography, impedance spectroscopy, and DFT computations. Mixed donor–acceptor π-stacks were observed in all compounds; in TMPD·F4TCNQ, the moieties formed 2D stacked arrays. All moieties had a partial radical character due to charge transfer. The presence of two-electron multicenter bonding (pancake bonding) was confirmed by quantum chemical computation. In TMPD·F4Q, alternating longer and shorter interplanar distances were consistent with formation of pancake-bonded dimers; in complexes of TMPD with Cl4Q, Br4Q, and TCNQ, the stacks were equidistant implying existence of pancake-bonded polymers.

The Myth of "Metavalency" in Phase-Change Materials.

Phase-change memory materials (PCMs) have unusual properties and important applications, and recent efforts to find improved materials have focused on their bonding mechanisms. "Metavalent bonding" or "metavalency," intermediate between "metallic" and "covalent" bonding and comprising single-electron bonds, has been proposed as a fundamentally new mechanism that is relevant both here and for halide perovskite materials. However, it is shown that PCMs, which violate the octet rule, have two types of covalent bond: two-center, two-electron (2c-2e) bonds, and electron-rich, multicenter bonds (3c-4e bonds, hyperbonds) involving lone-pair electrons. The latter have bond orders less than one and are examples of the century-old concept of "partial"bonds.

Exploring the Structure and Bonding of Group 13 Homogeneous and Heterogeneous Hydrides with the X2H4 Stoichiometry: A Theoretical Investigation

We investigated the structure evolution of the Group 13 hydrides on the example of X2H4 (X = B, Al, Ga, In, Tl) and BAlH4, AlGaH4, GaInH4, and InTlH4 stoichiometries via density functional theory (DFT) and ab initio quantum chemistry methods performing Coalescence Kick (CK) global minimum search and AdNDP chemical bonding analysis. We found that all global minimum structures possess multicenter electron bonds. The difference between the structures of X2H4 stoichiometry for boron and aluminum is much more significant than that between other pairs of Al-Ga, Ga-In, and In-Tl. The evolution of Group 13 hydride structure involves the gradual prevalence of classical 2c-2e bonds over multicenter bonds for heavier elements. The found structural features of the heterogeneous hydrides are in total agreement with those of homogeneous hydrides and with the trends common for the periodic table that allows us to investigate the structural evolution of Group 13 hydrides more completely.

Understanding the effects of building block rings of π electron-rich organic photocatalysts in CO2 transformation to amino acids

Understanding the effective factors in the performance of some Oligo (p-phenylenes) (OPPs) and Polycyclic Aromatic Hydrocarbons (PAHs), as efficient organocatalysts in photocatalytic CO2 transformations are the main goals of this investigation. The studies are based on density functional theory (DFT) calculations on the mechanistic aspects of C–C bond formation through a coupling reaction between CO2•– and amine radical. The reaction is performed through two successive single electron transfer steps. After careful kinetic investigations by Marcus’ theory rules, powerful descriptors are used to describe the behavior of observed barrier energies of electron transfer steps. The studied PAHs and OPPs consist of different numbers of rings. Thus, it can be considered different charge densities, afforded from π electrons, of PAHs and OPPs that cause distinguished efficiency in kinetic aspects of electron transfer steps. Electrostatic Surface Potential (ESP) analyses reveal a good relationship between the charge density of the studied organocatalysts in single electron transfer (SET) steps and the kinetic parameters of the steps. Moreover, the contribution of rings in the framework of PAHs and OPPs would be another effective factor in the barrier energies of SET steps. Aromatic properties of the rings, studied by the Anisotropy of the Current-Induced Density (ACID), Nucleus-Independent Chemical Shift (NICS), the multi-center bond order (MCBO), and AV1245 Indexes, are the other impressive factors in the role of rings in SET steps. The results show that the aromatic properties of the rings are not similar to each other. Higher aromaticity affords remarkable reluctance of the corresponding ring to participate in SET steps.

Source Data: Actinide Inverse Trans Influence: Cooperative `Pushing from Below' and Multi-Center Bonding

Source Data: Actinide Inverse Trans Influence: Cooperative `Pushing from Below' and Multi-Center Bonding

Far-infrared transparent conductors

The long-standing challenge in designing far-infrared transparent conductors (FIRTC) is the combination of high plasma absorption edge (λp) and high conductivity (σ). These competing requirements are commonly met by tuning carrier concentration or/and effective carrier mass in a metal oxide/oxonate with low optical dielectric constant (εopt = 2–7). However, despite the high σ, the transparent band is limited to mid-infrared (λp < 5 μm). In this paper, we break the trade-off between high σ and λp by increasing the “so-called constant” εopt that has been neglected, and successfully develop the material family of FIRTC with εopt > 15 and λp > 15 μm. These FIRTC crystals are mainly octahedrally-coordinated heavy-metal chalcogenides and their solid solutions with shallow-level defects. Their high εopt relies on the formation of electron-deficiency multicenter bonds resulting in the great electron-polarization effect. The new FIRTC enables us to develop the first “continuous film” type far-infrared electromagnetic shielder that is unattainable using traditional materials. Therefore, this study may inaugurate a new era in far-infrared optoelectronics.

NAl4X4 + (X = S, Se, Te): Clusters with a planar tetracoordinate nitrogen and significantly improved stability.

The design of clusters featuring non-classical planar hypercoordinate atoms (phAs) often depends on the delocalized multicenter bonds involving reactive electron-deficient elements, which both destabilize the clusters and lead to difficulty in achieving the phA arrangement for electronegative elements such as nitrogen due to their preference for localized bonds. In this work, we computationally designed a series of aluminum chalcogenide clusters NAl4X4 + (X = S, Se, Te) with a desired planar tetracoordinate nitrogen and meaningfully improved chemical stability, as evidenced by the wide gaps (6.51-7.23eV) between their highest occupied molecular orbitals and lowest unoccupied molecular orbitals, high molecular rigidity (dynamically stable up to 1500K), and exclusively low global energy minima nature (their isomers locate at least 51.2 kcal/mol higher). Remarkably, these clusters are stabilized by peripheral chalcogen atoms, which not only sterically protect the NAl4 core moiety but also electronically compensate for the electron-deficient aluminum atoms via X → Al π back bonds, meeting the description of our recently proposed "electron-compensation" strategy.

High-symmetry core-shell B12@Ca20C12: A nanocluster-based hydrogen storage material

Since the discovery of C60 fullerene, scientists devoted themselves to searching novel clusters. Among them, the high-symmetry clusters, especially clusters with Ih symmetry, has drawn increasing attention due to their special properties and stabilities. Here, a stable core-shell B12@Ca20C12 with Ih symmetry, which is featured as an icosahedral Ih-B12 centered on the dodecahedron Ca20C12, has been identified using first principles calculations. Its stability has been further confirmed by vibrational frequency analysis with the highest frequency of 834.9 cm−1 and the lowest frequency of 93.7 cm−1. Besides, molecular dynamics simulations indicate that its core-shell structure cannot be destroyed at 325 K under NVT ensembles with Nosé-Hoover chain thermostat. As for the electronic structure, 64 multi-center two-electron bonds were observed throughout the entire core-shell B12@Ca20C12 to keep the cluster maintain its core-shell configuration. More importantly, the core-shell B12@Ca20C12 can storage about 77 H2 molecules by closed-shell interactions with an average adsorption energy of −0.16 eV/H2, resulting in the gravimetric hydrogen density of 12.6 wt%. Based on these results we infer that core-shell B12@Ca20C12 is a potential high-capacity hydrogen storage material and might promote positive experimental efforts in designing excellent hydrogen storage materials.

Assessment of the performance of six indices in predicating the aromaticity of planar porphyrinoids.

Aromaticity is a fundamental chemical concept that has been widely used in explaining the reactivity, stability, structure, and magnetic properties of many molecules such as conjugated macrocycles, metal heterocyclic compounds, and certain metal clusters. Porphyrinoids (including porphyrin) are of particular interest in terms of diverse aromaticity. Various indices therefore have been used to predict the aromaticity of porphyrin-like macrocycles. However, the reliability of these indices for porphyinoids is always questionable. In order to assess the performance of the indices, we have selected six representative indices to predict the aromaticity of 35 porphyrinoids. The calculated values were then compared with the corresponding results obtained from experiments. Our studies suggest that the theoretical prediction by nucleus independent chemical shifts (NICS), topology of the induced magnetic field (TIMF), anisotropy of the induced current density (AICD), and gauge including magnetically induced current method (GIMIC) are essentially consistent with experimental evidence in all 35 cases and thus are preferred indices. Based on density functional theory, the performance of the NICS, TIMF, AICD, GIMIC, harmonic oscillator model of aromaticity (HOMA), and multicenter bond order (MCBO) indices were evaluated theoretically. Molecular geometries were optimized at the M06-2X/6-311G** level. NMR calculations using GIAO or CGST method were performed at the M06-2X/6-311G** level. The above calculations were carried out using Gaussian16 suite. The TIMF, GIMIC, HOMA, and MCBO indices were obtained using the Multiwfn program. The AICD outputs were visualized using the POV-Ray software.

Superconducting boron allotrope featuring pentagonal bipyramid at ambient pressure.

Elemental boron has evoked substantial interest owing to its chemical complexity in nature. It can form multicenter bonds due to its electron deficiency, which induces the formation of various stable and metastable allotropes. The search for allotropes is attractive for finding functional materials with fascinating properties. Using first-principles calculations with evolutionary structure search, we have explored boron-rich K-B binary compounds under pressure. A series of dynamically stable structures (Pmm2 KB5, Pmma KB7, Immm KB9, and Pmmm KB10) containing boron framework with open channels are predicted, which can possibly be synthesized under high pressure and high temperature conditions. After the removal of K atoms, we obtain four novel boron allotropes, o-B14, o-B15, o-B36, and o-B10, which exhibit dynamical, thermal, and mechanical stability at ambient pressure. Among them, o-B14 contains an unusual B7 pentagonal bipyramid and appears in a bonding combination of seven-center-two-electron (7c-2e) B-B π bonds, which is the first time to be identified in three-dimensional boron allotropes. Interestingly, our calculation reveals that o-B14 can act as a superconductor with a Tc value of 29.1 K under ambient conditions.

Theoretical Investigation of Geometries and Bonding of Indium Hydrides in the In2Hx and In3Hy (x = 0-4,6; y = 0-5) Series.

Boron hydrides have been an object of intensive theoretical and experimental investigation for many decades due to their unusual and somewhat unique bonding patterns. Despite boron being a neighboring element to carbon, boron hydrides almost always form non-classical structures with multi-center bonds. However, we expect indium to form its interesting molecules with non-classical patterns, though such molecules still need to be extensively studied theoretically. In this work, we investigated indium hydrides of In2Hx (x = 0-4,6) and In3Hy (y = 0-5) series via DFT and ab initio quantum chemistry methods, performing a global minimum search, chemical bonding analysis, and studies of their thermodynamical stability. We found that the bonding pattern of indium hydrides differs from the classical structures composed of 1c-2e lone pairs and 2c-2e bonds and the bonding pattern of earlier investigated boron hydrides of the BnHn+2 series. The studied stoichiometries are characterized by multi-center bonds, aromaticity, and the tendency for indium to preserve the 1c-2e lone pair.

Stabilization mechanism of molecular orbital crystals in IrTe2

Doped IrTe2 is considered a platform for topological superconductivity and therefore receives currently a lot of interest. In addition, the superconductivity in these materials exists in close vicinity to electronic order and the formation of molecular orbital crystals, which we explore here by means of high-pressure single crystal x-ray diffraction in combination with density functional theory. Our crystallographic refinements provide detailed information about the structural evolution as a function of applied pressure up to 42 GPa. Using this structural information for density functional theory calculations, we show that the local multicenter bonding in IrTe2 is driven by changes in the Ir-Te-Ir bond angle. When the electronic order sets in, this bond angle decreases drastically, leading to a stabilization of a multicenter molecular orbital bond. This unusual local mechanism of bond formation in an itinerant material provides a natural explanation for the different electronic orders in IrTe2. It further illustrates the strong coupling of the electrons with the lattice and is most likely relevant for the superconductivity in this material.

Au12@Au30 core-shell molecule:A stable Ih symmetry nanoparticle

A stable core-shell structure with Ih symmetry, Au12@Au30, has been proposed based on first-principles density functional theory. The stability of the core-shell Au42 structure is evaluated by vibrational frequency analysis and molecular dynamics simulations. The adaptive natural density partitioning (AdNDP) shows the existence of multi-center bonds between core and shell, which can be the reason of the structural stability. The analysis of AdNDP, Hirshfeld charges and Electron Localization Function (ELF) indicate the charges transfer from the core to the outer shell, which makes the surface of the nanoparticle more abundant in electrons. Besides, the visual highest occupied molecular orbital (HOMO) and the partial density of state (PDOS) indicate there are a large number of one-center valence lone electron pairs with the characteristics of d-like orbitals. These lone-pair electrons may lead to the application of Au12@Au30 in the fields of medicine and catalysis.

Structural Evolution of Carbon-Doped Aluminum Clusters AlnC- (n = 6-15): Anion Photoelectron Spectroscopy and Theoretical Calculations.

Carbon-doped aluminum cluster anions, AlnC- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of AlnC- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al6C- is a carbon endohedral triangular prism. The Al7C- anion is a magic cluster with high stability. The structures of Al7-9C- can be viewed as the additional aluminum atoms attached around the triangular prism Al6C-. Two isomers of Al10C- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al9C- with the carbon atom located in a pentagonal bipyramid. The Al11C- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al12-14C- can be viewed as evolving from Al11C- by adding aluminum atoms to interact with the carbon atom. In Al15C-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al13 unit can be identified as a part of the geometric structure of Al15C-.