Design of superhalogens using a core-shell structure model

Rational Design of Endohedral Superhalogens without Using Metal Cations and Electron Counting Rules

Superhalogens, predicted 40 years ago, have attracted considerable attention due to their potential as building blocks of novel materials with various applications. While a large number of superhalogen clusters have been theoretically predicted and experimentally synthesized, they either require the use of a metal cation or electron counting rules. In particular, very rare endohedral cage clusters in defiance of the above requirements have been found to be superhalogens. In this work, motivated by recent experimental advances in endohedral cage clusters, we present a rational design principle for creating a new class of such superhalogens. Focusing on the chemical formula of A@Si₂₀X₂₀ (A = F, Cl, Br, I, BH₄, BF₄; X = H, F, Cl, Br, I, BO, CN, SCN, CH₃), we use first-principles calculations to study 54 different clusters and show that these clusters possess electron affinities as high as 8.5 eV. Some of these clusters with X = BO and CN can even be stable as dianions, with large second electron affinity ∼2 eV. Similarly, Cl@C₆₀ is found to be a superhalogen. This class of superhalogens is different from the conventional ones with chemical formula MX_k+1, where X is a halogen and M is a cation with a formal +k oxidation state. Interestingly, the electron affinities of A@Si₂₀X₂₀ are almost independent of the central A moiety, but are guided by the functional group X. The potential of these endohedral superhalogens as electrolytes in Li-ion batteries is discussed.

On the Precise and Continuous Regulation of the Superatomic and Spectroscopic Behaviors of the Quasi-Cubic W4C4 Cluster by the Oriented External Electric Field

Designing and realizing novel superatoms with controllable and tunable electronic properties is vital for their potential applications in cluster-assembly nanomaterials. Here, we investigated the effect of the oriented external electric field (OEEF) on the geometric and electronic structures as well as the spectroscopic properties of the quasi-cubic W₄C₄ cluster by utilizing the density functional theory (DFT) calculations. Compared with traditional models, the OEEF was observed to hold the special capability in continuously and precisely modulating the electronic properties of W₄C₄, that is, remarkably increasing its electron affinity (EA) (1.58 eV) to 5.61 eV under the 0.040 au OEEF (larger than any halogen atoms in the periodic table), which possesses the superhalogen behavior. Furthermore, the downward movement of the lowest unoccupied molecular orbital level of the cluster accompanied by the enhancement of the OEEF intensity was demonstrated to be the origin of the EA increment. Additionally, the photoelectron spectra (PES) of W₄C₄^- were also simulated under different OEEF intensities, where the PES peaks move to a higher energy area following the enhancement of the OEEF strength, exhibiting the blue-shift behavior. These findings observed here open a new avenue in conveniently and precisely adjusting the electronic properties of clusters, which will be beneficial for the rational design of superatoms or superatom-assembled nanomaterials under the external field.

Revealing the effect of the oriented external electronic field on the superatom-polymeric Zr3O3 cluster: Superhalogen modulation and spectroscopic characteristics

Seeking novel strategies for designing superatoms is of significance for the potential applications in cluster-assembled nanomaterials. Herein, by employing the density functional theory (DFT) calculations, the effect of the oriented external electronic field (OEEF) on the electronic and photoelectron spectroscopic properties of the superatom-polymeric Zr₃O₃ cluster was explored. We present the evidence that the increment of the OEEF along all directions results in the remarkable enhancement of the electron affinity (EA) of Zr₃O₃, which turns it into superhalogen with an EA value of 4.02 eV under 0.020 au OEEF along +y direction. Strikingly, this EA value is larger than that of any halogen atoms in the periodic table. The downward shift of the electronic spectrum induced by the OEEF was confirmed to be the origin of the observed EA enhancement. Furthermore, the investigation of the OEEF's effect on the molecular orbitals (MOs) and photoelectron spectra (PES) of the cluster reveals that the OEEF could alter the electron distribution as well as promoting the blue shift of the PES without changing the spacings between different energetic levels. The OEEF highlighted here provides a new strategy in designing superatoms together with tuning their electronic and spectroscopic properties conveniently and precisely. We wish this finding could stimulate more efforts in designing novel superatoms or superatom-assembled materials from both theory and experiments.

Magnesium-Based Clusters as Building Blocks of Electrolytes in Lithium-Ion Batteries

Superhalogens, owing to their large electron affinity (EA, exceeding those of any halogen atom), play an essential role in physical chemistry as well as new material design. They have applications in hydrogen storage and lithium-ion batteries. Owing to the unique geometries and electronic features of magnesium-based clusters, their potential to form a new class of lithium salts has been investigated here theoretically. The idea is assessed by conducting ab initio computations on Li⁺ /Mg_n F_2n+1-2m O_m ^- compounds (n=2, 3; m=0-3) and analyzing their performance as potential Li-ion battery electrolytes. The Mg₃ F₇ ^- cluster, with large electron binding energy (EA of 7.93 eV), has been proven to serve as a building block for lithium salts. It is shown that, apart from high electronic stability, the new superhalogen-based electrolytes exhibit a set of desirable properties, including a large band gap, high electrolyte stability window, easy mobility of the Li⁺ , and favorable insensitivity to water.

Superhalogens as Building Blocks of Super Lewis Acids

Lewis acids play an important role in synthetic chemistry. Using first-principle calculations on some newly designed molecules containing boron and organic heterocyclic superhalogen ligands, we show that the acid strength depends on the charge of the central atom as well as on the ligands attached to it. In particular, the strength of the Lewis acid increases with increasing electron withdrawing power of the ligand. With this insight, we highlight the importance of superhalogen-based ligands in the design of strong Lewis acids. Calculated fluoride ion affinity (FIA) values of B[C₂ BNO(CN)₃ ]₃ and B[C₂ BNS(CN)₃ ]₃ show that these are super Lewis acids.

Evidence for the Superatom-Superatom Bonding from Bond Energies

Metal clusters with specific number of valence electrons are described as superatoms. Super valence bond (SVB) model points out that superatoms could form the superatomic molecules through SVBs by sharing nucleus and electrons. The existence of superatom-superatom bonding was verified by the shape of their orbitals in former studies. In this paper, another important evidence-bond energy is studied as the criterion for the SVBs using the density functional theory method. In order to get the reliable values of bond energies, the series of Zn-Cu and Mg-Li superatomic molecules composed of two tetrahedral superatoms which do not share their nucleus are designed. Considering the number of the valence electrons in one tetrahedral superatomic unit, (Zn₄)₂/(Mg₄)₂, (Zn₃Cu)₂/(Mg₃Li)₂, (Zn₂Cu₂)₂/(Mg₂Li₂)₂, and (ZnCu₃)₂/(MgLi₃)₂ clusters are 8e-8e, 7e-7e, 6e-6e, and 5e-5e binary superatomic molecules with super nonbond, single bond, double bond, and triple bond, respectively, which are verified by chemical bonding analysis depending on the SVB model. Further calculations reveal that the bond energies increase and the bond lengths decrease along with the bond orders in Zn-Cu and Mg-Li systems which is in accordance with the classical nonbond, single bond, double bond, and triple bond in C-H systems. Thus, these values of bond energies confirm the existence of the SVBs. Moreover, electron localization function analysis is also carried on to describe the similarity between the superatomic bonds and atomic bonds in simple molecules directly. This study reveals the new evidence for the existence of the superatom-superatom bonding depending on the bond energies, which gives the new insight for the further investigation of the superatomic clusters.