First experimental photoelectron spectra of superhalogens and their theoretical interpretations

Cryogenic Photoelectron Spectroscopic and Theoretical Study of the Electronic and Geometric Structures of Undercoordinated Osmium Chloride Anions OsCln- (n = 3-5)

A series of anionic transition metal halides, OsCln- (n = 3-5), have been investigated using a newly developed, home-constructed, cryogenic anion cluster photoelectron spectroscopy. The target anionic species are generated through collision-induced dissociation in a two-stage ion funnel. The measured vertical detachment energies (VDEs) are 3.48, 4.54, and 4.81 eV for n = 3, 4, and 5, respectively. Density functional theory calculations at the B3LYP-D3(BJ)//aug-cc-pVTZ(-pp) level predict the lowest energy structures of the atomic form of OsCln- (n = 3-5) to be a quintet triangle, quartet square, and quintet square-based pyramid, respectively. The CCSD(T)-calculated VDEs and corresponding adiabatic detachment energies agree well with our experimental measurements. Analysis of the corresponding frontier molecular orbitals and charge density differences suggests that the d-orbitals of the transition metal Os play a primary role in the single-photon detachment processes, and the detached electrons originating from different molecular orbitals are distinguishable.

Achieving Accuracy and Economy for Calculating Vertical Detachment Energies of Molecular Anions: A Model Chemistry Composite Methods

The vertical detachment energy (VDE) is a vital factor for predicting the stability of anions that have important applications in the atom, molecule and cluster science. Due to the synthetic or characterization difficulty of anions, accurate and efficient predictions of VDE independent of laboratory data have always been an appealing task to remedy the experimental deficiencies. Unfortunately, the generally adopted CCSD(T) and electron propagator theory (EPT) methods have respectively been proven to be reliable but very cost-expensive, and cost-effective but sometimes problematic when Koopman's theorem is invalid. Here, we for the first time introduced and benchmarked a series of model chemistry composite methods (e. g., CBS-QB3, G4 and W1BD) on calculating VDE for 57 molecular anions. Notably, CBS-QB3 exceeds the accuracy of CCSD(T) while approaching the economy of EPT. Therefore, we highly recommend the composite method CBS-QB3 to compute VDEs for molecular anions in the attractive "killing two birds with one stone" manner.

Dual-Valence Characteristics of Be11: Tin/Lead-like Superatom

To enhance the superatom family, the new superatom analogue Be11 of group IVA elements has been developed. Be11 can exhibit multiple valence states (+2 and +4), similar to carbon-group elements, and is capable of forming stable ionic compounds with other atoms such as carbon, chalcogen, (super)halogen, and hydroxyl. This resembles how tin and lead atoms combine with these elements to form stable molecules. Their special stability can be rationalized from the perspective of a cluster shell model. Sn or Pb could be the nearest atomic analogue to Be11 in group IVA, as the +2 oxidation state is more stable than the +4 oxidation state. This comparative investigation highlights the resemblance between Be11 and carbon-group elements, which encourages additional exploration within the superatom family.

Design and Investigation of Superatoms for Redox Applications: First-Principles Studies

A superatom is a cluster of atoms that acts like a single atom. Two main groups of superatoms are superalkalis and superhalogens, which mimic the chemistry of alkali and halogen atoms, respectively. The ionization energies of superalkalis are smaller than those of alkalis (<3.89 eV for cesium atom), and the electron affinities of superhalogens are larger than that of halogens (>3.61 eV for chlorine atom). Exploring new superalkali/superhalogen aims to provide reliable data and predictions of the use of such compounds as redox agents in the reduction/oxidation of counterpart systems, as well as the role they can play more generally in materials science. The low ionization energies of superalkalis make them candidates for catalysts for CO2 conversion into renewable fuels and value-added chemicals. The large electron affinity of superhalogens makes them strong oxidizing agents for bonding and removing toxic molecules from the environment. By using the superatoms as building blocks of cluster-assembled materials, we can achieve the functional features of atom-based materials (like conductivity or catalytic potential) while having more flexibility to achieve higher performance. This feature paper covers the issues of designing such compounds and demonstrates how modifications of the superatoms (superhalogens and superalkalis) allow for the tuning of the electronic structure and might be used to create unique functional materials. The designed superatoms can form stable perovskites for solar cells, electrolytes for Li-ion batteries of electric vehicles, superatomic solids, and semiconducting materials. The designed superatoms and their redox potential evaluation could help experimentalists create new materials for use in fields such as energy storage and climate change.

Boronyl-Based Polycyclic Superhalogens

Superhalogenity refers to the tendency of radicals to have higher electron affinity (EA) than halogens or anions to possess higher vertical detachment energy (VDE) than do halides (3.64 eV). In Srivastava, A. K. J. Phys. Chem. A 2023, 127, 4867-4872, we demonstrated a simple strategy in which some polycyclic hydrocarbons (PHs) can be turned into polycyclic superhalogens (PSs) by substituting CN groups in the place of hydrogens. We also notice that the superhalogenity of cyanide-based PSs is related to their aromaticity. Boronyl (BO) is an isoelectronic and inorganic analog of the cyano (CN) group. Therefore, we consider the substitution of BO in PHs and compare them with CN-based PSs using the density functional theory. In the case of C5H5-, we notice that the B3LYP and CCSD(T) calculated VDEs of resulting C5H5-n(BO)n- anions increase with the increase in the number of BO substituents (n) such that they become superhalogens for n ≥ 3 like C5H5-n(CN)n- anions. However, their aromaticity does not correspond to the superhalogenity, unlike C5H5-n(CN)n-. Similarly, all BO-substituted PHs possess structures similar to their CN analogs. Although their aromaticity is reduced compared to CN-based anions, the VDE of all these BO-based polycyclic anions and the EA of their radicals exceed 5 eV than those of the corresponding CN-based PSs. Therefore, this study proposes a new class of boronyl-based polycyclic superhalogens. These superhalogen anions might attract synthetic chemists and experimentalists for further explorations.

Benchmark computational investigations for the basic model of the salt-water complex: NaCl(H2O) and its anion NaCl(H2O). Phys Chem Chem Phys 2023;25:27215-27229. [PMID: 37791409 DOI: 10.1039/d3cp03421f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The microsolvation of salts in water is a fundamental physicochemical process. In this work, the aqueous salt complex NaCl(H2O) and its anion NaCl(H2O)- were investigated using comprehensive calculations, including the costly and accurate CCSD(T)-F12a and focal point analysis (FPA) methods. For the neutral NaCl(H2O), three isomers exist, two of which are mirror-symmetric with almost identical structures and their corresponding anions are also mirror-symmetric. For the NaCl(H2O)- anion, there are four isomers. Several transition states are found for the first time. The structural rearrangements of neutral NaCl(H2O) and NaCl(H2O)- anions are mainly caused by breaking and forming of the hydrogen bonds and the enhancement and weakening of interactions between Na and O atoms. The distributions of the anion complexes from 15-300 K are computed and compared to recent experimental results. The analysis of the intermolecular weak interactions shows the weak van der Waals interactions between Na and O atoms, as well as hydrogen bonding between H and Cl. Moreover, the theoretically predicted anion photoelectron spectra are assigned and analyzed in detail, and they agree with experimental spectra satisfactorily. The Na-Cl stretching vibrational mode dominates the vibrational structure in both anion spectra with some minor contributions from the intermolecular motions between H2O and NaCl.

Insights into the catalytic activity of boron-doped thiazoles in the Diels-Alder reaction

The role of boron-doped thiazoles as a Lewis acid catalyst in [4+2] cycloaddition reaction between 1,3-butadiene and acrolein has been addressed. Three different organic heterocycles were designed to study their catalytic activity. It has been observed that these heterocycles efficiently work as catalysts than the well-known Lewis acid BF3. All the reactions follow the normal electron demand process and are exothermic. Different conceptual DFT-based reactivity descriptors and electronic structure principles such as maximum hardness and minimum electrophilicity lend additional support to the feasibility of the reaction mechanism. The reaction force (RF), reaction electronic flux (REF), and its different components exhibit a detailed electronic activity throughout the reaction.

Recent progress on the design and applications of superhalogens

The research on superhalogens has successfully completed four decades. After their prediction in 1981 and experimental verification in 1999, such species have attracted attention due to their unusual structures and intriguing applications. Superhalogens are species whose electron affinity exceeds that of halogen or whose anions possess a larger vertical detachment energy than that of halides. Initially, these species were designed using s and p block atoms having a central electropositive atom as the core with excess electronegative atoms as ligands such as F, Cl, O etc. The last decade has witnessed enormous progress in the field of superhalogens. The transition metal atoms have played the role of the central core and a variety of new ligands have been explored. Further, new classes of superhalogens such as polynuclear superhalogens, magnetic superhalogens, aromatic superhalogens, etc. have been reported. The first application of superhalogens as strong oxidizers appeared much before their conceptualization. In the last decade, however, their applications have spanned a variety of fields such as energy storage, superacids, organic superconductors, ionic liquids, liquid crystals, etc. This makes research in the field of superhalogens truly interdisciplinary. This article is intended to highlight the progress on the design and applications of superhalogens in the last decade.

Recent advances in in silico design and characterization of superalkali-based materials and their potential applications: A review

In the advancement of novel materials, chemistry plays a vital role in developing the realm where we survive. Superalkalis are a group of clusters/molecules having lower ionization potentials (IPs) than that of the cesium atom (3.89 eV) and thus, show excellent reducing properties. However, the chemical industry and material science both heavily rely on such reducing substances; an in silico approach-based design and characterization of superalkalis have been the focus of ongoing studies in this area along with their potential applications. However, although superalkalis have been substantially sophisticated materials over the past couple of decades, there is still room for enumeration of the recent progress going on in various interesting species using computational experiments. In this review, the recent developments in designing/modeling and characterization (theoretically) of a variety of superalkali-based materials have been summarized along with their potential applications. Theoretically acquired properties of some novel superalkali cations (Li3 +) and C6Li6 species, etc. for capturing and storing CO2/N2 molecules have been unveiled in this report. Additionally, this report unravels the first-order polarizability-based nonlinear optical (NLO) response features of numerous computationally designed novel superalkali-based materials, for instance, fullerene-like mixed-superalkali-doped B12N12 and B12P12 nanoclusters with good UV transparency and mixed-valent superalkali-based CaN3Ca (a high-sensitivity alkali-earth-based aromatic multi-state NLO molecular switch, and lead-founded halide perovskites designed by incorporating superalkalis, supersalts, and so on) which can indeed be used as a new kind of electronic nanodevice used in designing hi-tech NLO materials. Understanding the mere interactions of alkalides in the gas and liquid phases and the potential to influence how such systems can be extended and applied in the future are also highlighted in this survey. In addition to offering an overview of this research area, it is expected that this review will also provide new insights into the possibility of expanding both the experimental synthesis and the practical use of superalkalis and their related species. Superalkalis present the intriguing possibility of acting as cutting-edge construction blocks of nanomaterials with highly modifiable features that may be utilized for a wide-ranging prospective application.

Functionalized nona-silicide [Si9R3] Zintl clusters: a new class of superhalogens

Superatoms, due to their various applications in redox and materials chemistry, have been a major topic of study in cluster science. Superhalogens constitute a special class of superatoms that mimic the chemistry of halogens and serve as building blocks of novel materials such as super and hyper salts, perovskite-based solar cells, solid-state electrolytes, and ferroelectric materials. These applications have led to a constant search for new class of superhalogens. In this study, using density functional theory, we show that recently synthesized [Si₉{Si (^tBu)₂H}₃] and [Si₉{Si (TMS)₃}₃] Zintl clusters not only behave like halogens but also when functionalized with suitable ligands exhibit superhalogen characteristics. Frontier molecular orbital (FMO) analyses give insights into the electron-accepting nature of the Zintl clusters. Additional bonding techniques such as energy density at the bond critical point (BCP) and adaptive natural density partitioning (AdNDP) gives complementary information about the nature of bonding in Si₉-based Zintl clusters. The potential of these Zintl clusters in the synthesis of new electrolytes in Li-ion batteries is also investigated.

Design of a Novel Series of Hetero-Binuclear Superhalogen Anions MM′X4− (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br)

A series of hetero-binuclear superatom motifs involving chloride/bromide ligands, that is, MM′X₄⁻ (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) anions, have been characterized by using many-body perturbation theory calculations. Large vertical electron detachment energies (VDEs, 5.470–6.799 eV) confirm the superhalogen identity of these anions. A larger VDE value can be obtained by introducing small M or large M′ central atoms and small halogen ligand atoms. Thus, one isomer of LiCaCl₄⁻ possesses the largest VDE value. Besides, when the extra electron is shared by all ligand atoms or three bridging ligand atoms, the isomers have relatively larger VDE values.

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.

Assessment of XC functionals for the study of organic molecules with superhalogen substitution. A systematic comparison between DFT and CCSD(T)

A systematic density functional theory study, including 17 exchange-correlation functionals, was performed on 22 composite structures consisting of organic molecules, e.g., ethylene, ethane, and benzene, and superhalogen substitutions arising from [MgX₃]^- and [Mg₂X₅]^- (X = F, Cl). Range-separated hybrid functionals ωB97M-V, ωB97X-D3(BJ), ωB97XD, ωB97X, and CAM-B3LYP, as well as double-hybrid functionals B2PLYP and DSD-PBEP86-D3(BJ), are verified to provide reliable results with accuracy approaching that at the coupled-cluster single double triple [CCSD(T)] level. The basis set effect of density functional theory calculation is usually moderate, and triple-ξ quality, e.g., Def2-TZVP, is enough in most cases. In addition, the average values from HF and MP2 method, indicated as (MP2 + HF)/2, are also quite close to those of CCSD(T).

Superhalogen Anions Supported by the Systems Comprising Alternately Aligned Boron and Nitrogen Central Atoms

Using DFT/(B3LYP/wB97XD/B2PLYPD) and OVGF electronic structure methods with flexible atomic orbital basis sets, we examined the series of polynuclear superhalogen anions matching the (BF₃(BN) _n F_4n+1)^- formula (for n = 1-10,13,18-20) containing alternately aligned boron and nitrogen central atoms decorated with fluorine ligands. It was found that the equilibrium structures of these anions correspond to fully extended chains (with each B and N central atom surrounded by four substituents arranged in a tetrahedral manner) and thus mimic the globally stable fully extended (all-trans) conformations of higher n-alkanes. The vertical electron detachment energies of the (BF₃(BN) _n F_4n+1)^- anions were found to exceed 8 eV in all cases and gradually increase with the increasing number of n. The approximate limiting value of vertical electron binding energy that could be achieved for such polynuclear superhalogen anions was estimated as equal to ca. 10.7 eV.

Synergetic ligand and size effects of boron cage based electrolytes in Li-ion batteries

We explore the potential application of boron-based clusters as high-performance electrolytes in lithium-ion batteries using first-principles density functional theory. We use small and halogen-free ligands (such as CN, BO, NH₂, NO₂, and CH₃) to replace H in closo-boron cages with different sizes to investigate the ligand and size effects. According to their geometric and electronic stability, Li⁺ dissociation energy in the lithium salt form, and electrochemical stabilities, we screen nine candidate electrolyte anions potentially overcoming the currently used electrolytes in lithium-ion batteries. We show that, when CH₃ is used as a boron cage ligand, both the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels are high, ensuring their electrochemical stability against the oxidation (or reduction) reaction at the anode (or cathode). Solvent effects are also evaluated and high electrostatic screening was found to be favorable for practical usage.

High-Spin Superatom Stabilized by Dual Subshell Filling

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals. While these stable clusters are intrinsically diamagnetic, we use our understanding of their electronic structures to theoretically predict that a cluster with 107 valence electrons would uniquely combine high stability and high-spin magnetic moment, attained by filling a majority subshell of 57 electrons and a minority subshell of 50 electrons. We experimentally demonstrate this predicted stability, high-spin magnetic moment (S = 7/2), and fully delocalized electronic structure in a new cluster, [NEt₄]₅[Fe₆S₈(CN)₆]. This work presents the first computational and experimental demonstration of the importance of dual subshell filling in transition-metal chalcogenide clusters.

M(BO)k+1- Anions: Novel Superhalogens Based on Boronyl Ligands

Superhalogens have been a subject of continuous attraction in the last four decades due to their unusual structures and interesting applications. In the quest for new superhalogens based on boronyl (BO) ligands, we investigate M(BO)_k+1^- anions using a high-level ab initio CCSD(T) and B3LYP density functional method for M = Li, Na, K, Be, Mg, Ca, B, and Al. Their structures are linear for M = Li, Na, K; trigonal planar for M = Be, Mg, Ca; and tetrahedral for M = B, Al. These anions are energetically and thermodynamically stable against various fragmentations. Their superhalogen property has been established due to their higher vertical detachment energy (VDE) than halogen, being in the range 4.44-7.81 eV. For a given k, the stability and VDE of M(BO)_k+1^- anions decrease with an increase in the size of M, which is due to the decrease in charge delocalization on BO moieties. There exists a linear correlation between frontier orbitals energy gap and VDE with coefficient R² = 0.93888. It was also noticed that the BO ligand, due to the lower dimerization energy, can be preferably used as an inorganic analogue of CN. However, the structural relaxation in BO-based neutral species significantly affect the VDE and, hence, their superhalogen nature. We believe that these findings should be interesting for researchers working in superatomic chemistry as well as in boron chemistry.

SbCl4: An Exceptional Superhalogen as the Building Block of a Mixed Valence Supercrystal with Unconventional Ferroelectricity

Superhalogens are nanoclusters with high electron affinities, exhibiting behavior similar to that of halogens. Their dimerization yields nonpolar symmetrical clusters, akin to diatomic halogen molecules, and they are unstable in the condensed phase in the absence of charge-compensating cations. Herein, we provide ab initio evidence that SbCl₄ superhalogen is an exception: its dimerization yields a polar cluster that can be viewed as a quasi-bonded [SbCl₅]^δ- and [SbCl₃]^δ+ Lewis acid-base cluster. The symmetry breaking arises from the valence stratification of Sb into Sb⁵⁺ and Sb³⁺ as well as their lone pair electrons. When assembled, SbCl₄ clusters form a supercrystal that is thermodynamically stable up to 600 K, with the unique bonding feature of Sb₂Cl₈ prevailing in the bulk phase. Combination of mixed valence and lone pair electrons leads to electric polarizations along all directions, generating a type of unconventional multimode ferroelectricity in which three different modes of ferroelectricity with distinct magnitudes and Curie temperature are revealed.

High electron affinity triggered by lithium coordination: quasi-chalcogen properties of Li2Sn8Be

This work puts forward an unusual but rational strategy to design superatoms mimicking the properties of group VIA elements. A stable dianion with closo-configuration, namely Li2Sn8Be2−, has been obtained by...

Reactivity of the superhalogen/superalkali ion encapsulating C60 fullerenes

The Diels-Alder cycloaddition reaction between 1,3-cyclohexadiene and a series of C₆₀ fullerenes with encapsulated (super)alkali/(super)halogen species (Li⁺@C₆₀, Li₂F⁺@C₆₀, Cl^-@C₆₀, and LiF₂^-@C₆₀) was explored by means of DFT calculations. The reactivity of the ion encapsulating systems was compared to that of the parent C₆₀ fullerene. Significant enhancement in reactivity was found for cation-encapsulating Li⁺/Li₂F⁺@C₆₀ complexes. The cycloadduct formed by LiF₂^-@C₆₀ was found to be the most thermodynamically favorable among the studied ones. In contrast, encapsulation of Cl^- anions disfavors the cycloaddition reaction both kinetically and thermodynamically. Higher activation energy barrier and less stability of the reaction product in the case of Cl^-@C₆₀ were associated with the higher deformation energies of the fullerene cage and the lower interaction energy between the reactants in comparison with the other studied complexes.

Small lithium-chloride clusters: Superalkalis, superhalogens, supersalts and nanocrystals

In the present paper, the results of the theoretical investigation of the small lithium-chloride clusters are reported. The geometrical structures, electronic and thermodynamic stability of superalkalis, superhalogens, and their single and double charged ions are obtained using efficient and accurate quantum chemistry methods. Further, low-lying isomers of the Li_n Cl_n ( n = 2 - 5 ) clusters and their stability parameters are calculated. Two ways of formation of the Li_n Cl_n clusters, polymerization of LiCl fragments and combination of superalkalis and superhalogen clusters, are compared. By examination the lattice energy and the average Li-Cl bond length in rectangular Li_n Cl_n ( n ≤ 60 ) clusters, it was concluded that already 50 LiCl are enough to mostly resembles the structure and stability of the bulk LiCl.

Theory-Guided Discovery of Novel Materials

The traditional approach for materials discovery has been the domain of experimentalists, where elemental composition and synthesis conditions are often based on a trial-and-error method. Such processes are time-consuming and expensive. To minimize cost and to develop new materials at a faster pace, an alternate approach is to use theory to predict new materials with tailored properties and have experiments validate such predictions. The phenomenal increase in computing power, development of new first-principles methodologies, and a myriad of advanced computer codes in recent years have enabled researchers to predict novel materials that can be verified by later experiments. In this Perspective, we present advances in density functional theory-based methods and computational procedures that have made possible the discoveries of materials with varying size, composition, and dimensionalities. The challenges and opportunities in theory-guided discovery of materials, going forward, are also discussed.

Isomer-Specific Vibrational Spectroscopy of Microhydrated Lithium Dichloride Anions: Spectral Fingerprint of Solvent-Shared Ion Pairs

The vibrational spectroscopy of lithium dichloride anions microhydrated with one to three water molecules, [LiCl2 (H2 O)1-3 ]- , is studied in the OH stretching region (3800-2800 cm-1 ) using isomer-specific IR/IR double-resonance population labelling experiments. The spectroscopic fingerprints of individual isomers can only be unambiguously assigned after anharmonic effects are considered, but then yield molecular level insight into the onset of salt dissolution in these gas phase model systems. Based on the extent of the observed frequency shifts ΔνOH of the hydrogen-bonded OH stretching oscillators solvent-shared ion pair motifs (<3200 cm-1 ) can be distinguished from intact-core structures (>3200 cm-1 ). The characteristic fingerprint of a water molecule trapped directly in-between two ions of opposite charge provides an alternative route to evaluate the extent of ion pairing in aqueous electrolyte solutions.

Ionization of NO by superhalogens: DFT and QTAIM approaches

 Nitric oxide (NO) is a precursor to NO2, toxic gas, and a major air pollutant. Its ionization has been difficult due to its high ionization energy. We propose here the ionization of NO to NO+ using superhalogens. We study the interaction of NO with superhalogens (X = LiF2, BeF3 and, BF4) using DFT and QTAIM approaches, which lead to the formation of NO-X complexes. These complexes and their isomers are ionic with positively charged NO, which can be stabilized by two F–N bonds and F–O bonds, respectively. This reveals that NO can be ionized by electron transfer to superhalogens. The size of superhalogens has been noticed to play a crucial in the ionization of NO.

Exploring the Superhalogen Properties of Polynuclear Structures without Halogen Ligands: A Combined Ab Initio and DFT Study on Triple-Bridged [Mg2L5]-1 (L = -OCN, -SCN) Anions

Via combined ab initio and DFT calculations, 16 [Mg₂(OCN)₅]^-1 and [Mg₂(SCN)₅]^-1 isomers are studied to explore their potentials as superhalogens. The results of high-level OVGF calculations verify that these systems are superhalogen anions due to their high VDE values which lie within 6.74-7.04 eV and 5.73-6.27 eV for [Mg₂(OCN)₅]^-1 and [Mg₂(SCN)₅]^-1, respectively. The results of low-cost methods, either ab initio or DFT, are generally consistent with those of OVGF, and some of them demonstrate promising accuracy. The best performance of low-cost ab initio methods comes from (HF+MP2)/2, which represents the average value of the MP2 and HF results. In the aspect of DFT, five functionals (CAM-B3LYP, ωB97X-D3, M06, M06-2X, and B2PLYP) provide the most accurate results when compared to OVGF. Thus, these low-cost methods could be used to calculate the VDE value of the future systems of larger size. Useful information about the VDEs of [Mg₂(OCN)₅]^-1 and [Mg₂(SCN)₅]^-1 could be obtained from the analysis of the extra electron density and HOMO as well as spin density.

Zintl Lewis Superacids: Al(Ge9L3)3 (L = H, CH3, CHO, CN)

In quest of a Zintl ion-based Lewis superacids, Al(Ge₉L₃)₃ {L= H, CH₃, CHO and CN} compounds have been designed and their properties have been studied within the framework of conceptual density functional theory-based reactivity descriptors. Superacid property has been identified for these complexes as per the fluoride ion affinity (FIA) values. Studies reveal that Al[Ge₉(CN)₃]₃ and Al[Ge₉(CH₃)₃]₃ behave like superacids as their FIA exceeds the value of SbF₅, which is considered as the strongest Lewis acid. It has been observed that the ligand plays an important role in reactivity as well as in Lewis acidic property.

Superhalogens as Building Blocks of Ionic Liquids

Ionic liquids (ILs) are composed of large asymmetric organic cations with a wide range of anions. The simple anions, e.g., halogen, result in less stable ILs, and therefore, ILs generally consist of complex anions such as BF₄ and PF₆. These anions coincidently belong to a special class known as superhalogen. This prompted us to enquire whether the concept of superhalogen can be exploited to design new ILs. We study the complexes of 1-butyl-3-methylimidazolium (BMIM) cation and typical superhalogen (X) anions such as LiF₂, BeF₃, BO₂, NO₃, BF₄, and PF₆ including Cl using density functional theory and the quantum theory of atoms in molecule. Our ωB97XD/6-311++G(d,p) calculations suggest that the BMIM-X complexes are stable in which the charge transfer of 0.90-0.97 e takes place from BMIM to X. The charge-transferred tends to delocalize as the size of X increases. These complexes are stabilized by several ionic and/or covalent intramolecular interactions (H-bonds). The BMIM-X complexes prefer to dissociate into ionic fragments (BMIM⁺ + X^-) than neutral fragments (BMIM + X). The dissociation energy and energy gap of BMIM-X complexes are closely related to the electron affinity of superhalogens (X). These findings not only reveal the superhalogens as building blocks of ILs but also suggest the design of highly stable ILs by employing the superhalogens with higher electron affinities.

A family of ionic supersalts with covalent-like directionality and unconventional multiferroicity

Ionic crystals composed of elemental ions such as NaCl are non-polar due to directionless ionic bonding interactions. Here, we show that these can develop polarity by changing their building blocks from elemental ions to superalkali and superhalogen cluster-ions, which mimic the chemistry of alkali and halogen atoms, respectively. Due to the non-spherical geometries of these cluster ions, corresponding supersalts form anisotropic polar structures with ionic bonding, yet covalent-like directionality, akin to sp3 hybridized systems. Using density functional theory and extensive structure searches, we predict a series of stable ferroelectric/ferroelastic supersalts, PnH4MX4 (Pn = N, P; M = B, Al, Fe; X = Cl, Br) composed of superalkali PnH4 and superhalogen MX4 ions. Unlike traditional ferroelectric/ferroelastic materials, the cluster-ion based supersalts possess ultra-low switching barrier and can endure large ion displacements and reversible strain. In particular, PH4FeBr4 exhibits triferroic coupling of ferroelectricity, ferroelasticity, and antiferromagnetism with controllable spin directions via either ferroelastic or 90-degree ferroelectric switching.

Caralumane Superacids of Lewis and Brønsted Character

Carborane Brønsted superacids have proven to be useful reagents in a variety of organic and inorganic synthetic processes. In this work, analogs in which the icosahedral CB₁₁ carborane core is replaced by a CAl₁₁ core are studied using ab initio electronic structure tools. Each so-called caralumane Brønsted acid is formed by adding HF, HCl, or HH to a corresponding caralumane Lewis acid possessing a vacant Al-centered orbital that acts to accept an electron pair from the HF, HCl, or HH. The Lewis acid strengths of the species involved, as measured by their F^- ion affinities, are all found to exceed the threshold for labeling them Lewis superacids. Also, the deprotonation Gibbs free energies of the Brønsted acids are found to be small enough for them to be Brønsted superacids. When HF or HCl is bound to a caralumane Lewis acid to form the Brønsted acid, the HF or HCl is bound datively to a single Al atom, and hydrogen bonds can be formed between this molecule's H atom and nearby F or Cl atoms attached to other Al atoms. In contrast, when HH is bound to the Lewis acid to form the Brønsted acid, two novel low-energy structures arise, both of which are Brønsted superacids. One has an essentially intact HH molecule attached to a single Al atom in a η² fashion. In the other, the HH molecule is heterolytically cleaved to generate a hydride ion that attaches to a single Al atom and a proton that binds in a multicenter manner to other Al atoms. The structures and relative energies of a multitude of such caralumane Lewis and Brønsted superacids are provided and discussed.

Tripodal Podand Ligand with a Superhalogen Nature as an Effective Molecular Trap

Tris(2-methoxyethyl) fluoroborate anion (TMEFA), anovel tripodal ligand based on the BF4− superhalogen anion, is proposed and was investigated theoretically using ab initio MP2 (second-order Møller-Plesset perturbational method) and OVGF (outer valence Green function) methods. The studied molecule comprises three 2-methoxyethoxy groups (-O-CH2-CH2-O-CH3) connected to a central boron atom, which results in the C3-symmetry of the compound. The resulting anion was stable against fragmentation processes and its vertical electron detachment energy was found to be 5.72 eV. Due to its equilibrium structure resembling that of classical tripodal podands, the [F-B(O-CH2-CH2-O-CH3)3]− anion is capable of binding metal cations using its three arms, and thus may form strongly bound ionic complexes such as [F-B(O-CH2-CH2-O-CH3)3]−/Li+ and [F-B(O-CH2-CH2-O-CH3)3]−/Mg2+. The binding energies predicted for such compounds far exceed those of the similar neutral classical podand ligands, which likely makes the [F-B(O-CH2-CH2-O-CH3)3]− system a more effective molecular trap or steric shielding agent with respect to selected metal cations.

Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages

Theoretical investigation of the H(CHB₁₁ X₁₁ ) (X=H, F, Cl, CN), H(CHB₁₁ X_n Y_11-n ) (X,Y=F, Cl; n=1,5), and dimeric (H(CHB₁₁ X₁₁ ))₂ (X=F, Cl) carborane superacids performed at the B3LYP/6-311++G(d,p) theory level revealed the similarity of their equilibrium structures and the possibility of nearly barrierless hydrogen atom migration among the substituents attached to one side of the icosahedral CB₁₁ cage. The vertical electron detachment energies predicted at the OVGF/6-311++G(3df,2pd) theory level for the conjugate bases (CHB₁₁ X₁₁ )^- were found to span the 5.82-9.00 ev range. The acid strengths (manifested by the Gibbs free deprotonation energies spanning the 213-266 kcal/mol range) predicted for the icosahedral H(CHB₁₁ X₁₁ ) carborane systems confirm their superacidic properties which might be increased even further by the attachment of the second carborane H(CHB₁₁ X₁₁ ) unit that leads to a dimeric structure mimicking a part of an experimentally observed H-bridged polymeric chain. The Gibbs free deprotonation energy of the dimeric (H(CHB₁₁ Cl₁₁ ))₂ acid was predicted to be smaller by 17 kcal/mol than that of the corresponding monomeric H(CHB₁₁ Cl₁₁ ) acid.

Designing aromatic heterocyclic superacids in terms of Brønsted and Lewis perspectives

The unexplored area of organic superacids was investigated in terms of both Brønsted and Lewis concepts of acids and bases. The primary requirement of a superacid-high affinity for electron/fluoride ions was fulfilled using two strategies: (i) using the superhalogen-type heterocyclic framework and (ii) selecting systems that have an electron count one short of attaining (4n + 2) Hückel aromaticity. With these in mind, eleven systems were considered throughout the study, expected to cross the target of 100% H₂SO₄ acidity and/or the fluoride affinity of SbF₅. To enhance the pK_a and F^- affinity values of the considered systems, electron-withdrawing ligands F and CN were employed. The superhalogen and aromaticity properties were verified by vertical detachment energy (VDE) and nucleus independent chemical shift (NICS) calculations, respectively. Finally, the collective effect of the potential super Lewis acids was looked into using a BL₃ skeleton with them acting as ligands.

Combining proton and silaborane-based superhalogen anions - an effective route to new superacids as verified via systematic DFT calculations

Based on systematic DFT calculations, silaborane-based superhalogen anions, which obey the Wade-Mingos rule, are shown to be capable of giving rise to superacids via their combination with protons. Compared to previous carborane-based systems, the acidities of the composites here are stronger in both the gas phase and solution phase. Thus, the potential of candidates based on silaborane could be greater than those based on carborane in the search for ultra-strong acidic systems. Within a given group, a higher superhalogen anion vertical electron detachment energy (VDE) generally leads to stronger acidity. This consistency arises from the dominant role of the VDE, as established through the decomposition of the gas-phase acidity into different contributions. Thus, constructing superacids from superhalogens is a rational route whose future should be positive. Besides the VDE, other effects, i.e., the deformation energy (DE) and bond dissociation energy (BDE), could also be crucial, especially in terms of the differences between the acidities of composites belonging to different groups. A comparison between the results in the gas phase and solution phase indicates that complete calculations of both gas-phase ΔG_acid and solution-phase pK_a values are necessary to obtain an unbiased description of the acidity. The solvation free energies of the participants in the deprotonation process, especially the conjugate acid, are responsible for the discrepancies between gas phase and solution phase behavior.

Greatly Enhanced Electron Affinities of Au2n Cl Clusters (n = 1-4): Effects of Chlorine Doping

Au2n Cl- (n = 1-4) clusters are investigated by both laser ablation mass spectrometry and theoretical calculations. It is interesting to find that the electron affinities of neutral Au2n Cl (n = 1-4) clusters are much larger than those of corresponding pure Au2n clusters. Among them, the electron affinity of Au2Cl is 4.02 eV, which can be defined as a very unique superhalogen that is quite different from classical ones of M n X m (M = metal, X = halogen, and n < m). Natural bond orbital and highest occupied molecular orbital analyses indicate that the extra electron is predominantly delocalized over the positively charged metal moiety in these anionic Au2n Cl- clusters, which is the main reason for the large electron affinities of the corresponding neutral species.

Exploring the structure, bonding and stability of noble gas compounds promoted by superhalogens. A case study on HNgMX3 (Ng = Ar-Rn, M = Be-Ca, X = F-Br) via combined high-level ab initio and DFT calculations

A series of complexes (HNgMX₃), formed from superhalogen MX₃ (M = Be-Ca, X = F-Br) noble gas (Ar-Rn) and the hydrogen atom, were investigated via combined high-level ab initio and DFT calculations. The high vertical electron detachment energy (VDE) of the superhalogen part will lead to charge transfer from the noble gas hydride to it. This charge transfer gives rise to attractive ionic interaction between the two components and to the existence of these complexes as local minima on the potential energy surface eventually. However, the VDE value of the superhalogen part is not always monotonically correlated with the thermodynamic/kinetic stability of the whole complexes. Therefore the superhalogen itself might not be enough to provide information for the correct prediction of the properties of the whole composites. Although there are exothermic channels of dissociation, the existence of energy barrier might ensure the existence of these Ng hydrides under certain conditions. Our analysis indicates the existence of two important factors, functioning in opposite directions, for the energy barriers along the exothermic channel. To achieve a high energy barrier, the attractive interaction between superhalogen and the H atom in the TS, which lowers the barrier, needs to be suppressed effectively. An understanding of the superhalogen-based composites will provide valuable information on the functional properties and potential application of superhalogens. The details of the interaction between different parts of these composites should be one of the areas of focus in these studies.

Magnesium-Based Oxyfluoride Superatoms: Design, Structure, and Electronic Properties

The ability of mixed ligands to form stable dinuclear and trinuclear magnesium-based superatoms has been investigated theoretically. The Mg₂F_{5-2 m}O _m and Mg₃F_{7-2 m}O _m systems (where m = 1-3) were found able to form stable and strongly bound anionic clusters, and those assumptions were validated by (i) the analysis of the geometrical stability; (ii) the estimated Gibbs free energies for the most probable disproportion paths these clusters might be vulnerable to (which allows examining their thermodynamic stabilities); (iii) the localization of the electron density; and (iv) the adiabatic electron affinity (AEA), vertical electron detachment energy (VDE), and adiabatic electron detachment energy (ADE) values calculated for the studied systems. It is demonstrated that the stability of the anionic daughters of these clusters increases with the number of electronegative ligands, and Mg _nF_{2 n+1-2 m}O _m^- ( n = 2, 3; m = 1-3) clusters are stable against electron emission. The largest electron binding energy was found for the Mg₃F₅O^- anion (VDE = 6.826 eV). The strong VDE dependence on (i) the geometrical structure, (ii) the number of central atoms, (iii) ligand type, and (iv) bonding/antibonding character of the highest molecular orbital (HOMO) was also remarked upon and discussed.