Strongly Bound Polynuclear Anions Comprising Scandium Fluoride Building Blocks

The stability of polynuclear anions composed of ScF3 building blocks was studied by using ab initio and density functional theory electronic structure methods and flexible basis sets. Thorough exploration of ground state potential energy surfaces of (Sc2F7)-, (Sc3F10)-, and (Sc4F13)- anions which may be viewed as comprising ScF3 fragments and the additional fluorine atom led to determining the isomeric structures thereof. It was found that the most stable isomers which are predicted to dominate at room temperature correspond to the compact structures enabling the formation of a large number of Sc-F-Sc bridging linkages rather than to the chain-like structures. The vertical electron detachment energies of the (ScnF3n+1)- anions were found to be very large (spanning the 10.85-12.29 eV range) and increasing with the increasing number of scandium atoms (n) and thus the ScF3 building blocks involved in the structure. Thermodynamic stability of (ScnF3n+1)- anions (i.e., their susceptibility to fragmentation) was also verified and discussed.

Electron Propagator Theory of Vertical Electron Detachment Energies of Anions: Benchmarks and Applications to Nucleotides

A new generation of ab initio electron-propagator self-energy approximations that are free of adjustable parameters is tested on a benchmark set of 55 vertical electron detachment energies of closed-shell anions. Comparisons with older self-energy approximations indicate that several new methods that make the diagonal self-energy approximation in the canonical Hartree-Fock orbital basis provide superior accuracy and computational efficiency. These methods and their acronyms, mean absolute errors (in eV), and arithmetic bottlenecks expressed in terms of occupied (O) and virtual (V) orbitals are the opposite-spin, non-Dyson, diagonal second-order method (os-nD-D2, 0.2, OV²), the approximately renormalized quasiparticle third-order method (Q3+, 0.15, O²V³) and the approximately renormalized, non-Dyson, linear, third-order method (nD-L3+, 0.1, OV⁴). The Brueckner doubles with triple field operators (BD-T1) nondiagonal electron-propagator method provides such close agreement with coupled-cluster single, double, and perturbative triple replacement total energy differences that it may be used as an alternative means of obtaining standard data. The new methods with diagonal self-energy matrices are the foundation of a composite procedure for estimating basis-set effects. This model produces accurate predictions and clear interpretations based on Dyson orbitals for the photoelectron spectra of the nucleotides found in DNA.

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.

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.

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.

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.

Noble gas insertion compounds of hydrogenated and lithiated hyperhalogens

Based on density functional theory (DFT) calculations, hydrogenated hyperhalogen HM(BO2)2, lithiated hyperhalogen LiM(BO2)2 (M = Cu, Ag, Au), and their compounds with xenon were studied. Different insertion sites of Xe resulted in various isomers. According to the natural population analysis, the Xe atom donated 0.12-0.77 electrons to HM(BO2)2 and 0.14-0.41 electrons to LiM(BO2)2 when they combined, leading to metastable charge-transfer compounds in most cases. The nature of bonding between xenon and HM(BO2)2/LiM(BO2)2 was found to be related to its location. Covalent bonds were formed when Xe bound with hydrogen atoms, as indicated by the large Wiberg bond indices of the Xe-H bonds. The same was true for most Xe-M bondings. When an Xe-O connection was formed, it was either an ionic or van der Waals force in nature depending on the specific structural feature of the isomer. A parallel study on hyperhalogen-supported Ar and Kr compounds indicated that they were not very stable and were less likely to exist at room temperature, which was in accordance with the high inertness of both Ar and Kr atoms.

Structures and Superhalogen Properties of Pt2Cl n (2 ≤ n ≤ 10) Clusters

Laser ablation mass spectrometry was applied to study the cluster anions of Pt₂Cl _n^-. Among them, Pt₂Cl₆^- and Pt₂Cl₉^- were observed with remarkable intensities in the mass spectrum. Collision-activated dissociation experiments were also performed for the two anions. A systematic study of the structures and electron affinities (EAs) of Pt_1-2Cl _n (2 ≤ n ≤ 10) clusters was performed on the level of B3LYP/Lanl2dz/6-311++G(2d, p). For both systems, their EAs do not increase monotonically with the increase of Cl atoms. All the clusters of PtCl _n (3 ≤ n ≤ 10) and Pt₂Cl _n (3 ≤ n ≤ 10) show EAs larger than that of the Cl atom. The highest EAs for the series of Pt₂Cl _n and PtCl _n are 6.55 and 5.82 eV, corresponding to those of Pt₂Cl₉ and PtCl₉, respectively. The results indicate that these clusters might be applied as effective units in constructing new "hypersalts".

Why do higher VDEs of superhalogen not ensure improved stabilities of the noble gas hydrides promoted by them? A high-level ab initio case study

A series of 20 composite structures, consisting of superhalogen and noble gas (Ng) hydrides, was explored via high-level coupled-cluster single, double and perturbative triple excitations calculations in this work. The existence of these composites, as local minima on the potential energy surface, arises from the charge transfer from the Ng hydride part to the superhalogen moiety. Clearly, this transfer could lead to stabilizing the interaction of the ionic type between the two components. The driving force of the charge transfer should be the high vertical electron detachment energy (VDE) of the superhalogen part leading to its enough capability of extracting the electron from the Ng hydride moiety. However, except triggering the ionic attractive interaction, there is nomonotonic correlation between the VDE value and the thermodynamic stability of the whole composite. This counter-intuitive result actually originates from the fact that, irrespective of various superhalogens, only two of their F ligands interact with the Ng atoms directly. Thus, although leading to higher VDE values, the increase in the number of electronegative ligands of the superhalogen moiety does not affect the stabilizing interaction of the composites here directly. In other words, with the necessary charge transfer generated, further increase of the VDE does not ensure the improvement of the thermodynamic stabilities of the whole composite. Moreover, in the transition state of the exothermic dissociation channel, more F atoms will give rise to higher probability of additional attractions between the F and H atoms which should lower the energy barrier. That is to say, increasing VDE, i.e., having more F atoms in many cases, will probably reduce the kinetic stability. Knowing the inevitable existence of the exothermic channel, kinetic stability is crucial to the ultimate goal of experimental observation of these Ng hydrides. Thus, in some cases, only the superhalogen itself may not provide enough information for the correct prediction on the properties of the whole composites. The understanding of the superhalogen-based composites will provide valuable information on the functional properties as well as the application potential of superhalogen clusters. Thus, the corresponding researches should focus on not only the superhalogen itself but also other related aspects, especially the details of the interaction between different parts.

Prediction of the Iron-Based Polynuclear Magnetic Superhalogens with Pseudohalogen CN as Ligands

To explore stable polynuclear magnetic superhalogens, we perform an unbiased structure search for polynuclear iron-based systems based on pseudohalogen ligand CN using the CALYPSO method in conjunction with density functional theory. The superhalogen properties, magnetic properties, and thermodynamic stabilities of neutral and anionic Fe₂(CN)₅ and Fe₃(CN)₇ clusters are investigated. The results show that both of the clusters have superhalogen properties due to their electron affinities (EAs) and that vertical detachment energies (VDEs) are significantly larger than those of the chlorine element and their ligand CN. The distribution of the extra electron analysis indicates that the extra electron is aggregated mainly into pseudohalogen ligand CN units in Fe₂(CN)₅^¯ and Fe₃(CN)₇^¯ cluster. These features contribute significantly to their high EA and VDE. Besides superhalogen properties, these two anionic clusters carry a large magnetic moment just like the Fe₂F₅^¯ cluster. Additionally, the thermodynamic stabilities are also discussed by calculating the energy required to fragment the cluster into various smaller stable clusters. It is found that Fe(CN)₂ is the most favorable fragmentation product for anionic Fe₂(CN)₅^¯ and Fe₃(CN)₇^¯ clusters, and both of the anions are less stable against ejection of Fe atoms than Fe(CN)_n-x.

Hyperhalogen properties of early-transition-metal borates

The hyperhalogen character of three series of early-transition-metal borates has been proposed.

Stability of Superhalogen Anions in the Aqueous Phase

The issue of stability of superhalogen anions in an aqueous solution is investigated on the basis of theoretical calculations carried out at the CCSD(T)/6-311++G(d,p)//MP2/6-311++G(d,p) level for two representative negatively charged systems (NaF₂^- and AlF₄^-) whose fragmentation products differ in polarity. The presence of a water solvent is simulated independently by employing the polarized continuum solvation model and by involving eight H₂O molecules explicitly to allow interactions at the molecular level. The best estimates of the Gibbs free energies characterizing the AlF₄^- and NaF₂^- fragmentation reactions in a water solvent are evaluated as equal to 33-34 and 12-14 kcal/mol, respectively (assuming the F^- and AlF₃/NaF products) or 14-15 and 26-28 kcal/mol, respectively (assuming the HF and AlF₃OH^-/NaFOH^- products). The corresponding fragmentation routes are suggested to be nonoperative at T = 298.15 K. The conclusion concerning the thermodynamic stability of the AlF₄^- and NaF₂^- superhalogen anions in the aqueous phase is formulated and discussed.

Superhalogen properties of BS2(-) and BSO(-): photoelectron spectroscopy and theoretical calculations

We investigate BS2(-) and BSO(-) clusters using photoelectron spectroscopy and theoretical calculations. The electron affinities of BS2 and BSO are measured to be 3.80 ± 0.03 and 3.88 ± 0.03 eV, respectively, higher than those of halogen atoms. Thus, BS2 and BSO can be considered as superhalogens. The comparison of experimental and theoretical results confirmed that the ground state structures of BS2(-), BSO(-), and their neutrals are all linear. Analyses of natural bond orbitals suggest that both BS2(-) and BSO(-) have dual 3c-4e π hyperbonds.

Theoretical characterization of a series of N5-based aromatic hyperhalogen anions

Hyperhalogens are a class of highly electronegative molecules whose electron affinities even exceed those of their superhalogen ligands. Such species can serve as new oxidizing agents, biocatalysts, and building blocks of unusual salts, and hence are important to the chemical industry. Utilizing stable N5(-) as the ligand, a series of aromatic hyperhalogen anions, namely mononuclear M(N5)(k+1)(-) (M = Li, Be, B) and dinuclear M2(N5)(2k+1)(-) (M = Li, Be), have been reported here for the first time. Calculation results based on the density functional theory revealed that all the N5(-) subunits preserve their structural and electronic integrity as well as aromatic characteristics in these anions. Especially, these anionic molecules exhibit larger vertical electron detachment energies (6.76-7.86 eV) than that of the superhalogen ligand N5(-), confirming their hyperhalogen nature. The stability of these studied anions is guaranteed by their large HOMO-LUMO gaps, and positive dissociation energies of predetermined fragmentation pathways. We hope this work will not only provide evidence of a new type of hyperhalogen molecule but also stimulate more research interest and efforts in the amazing superatom realm.

Is the regulation of the electronic properties of organic molecules by polynuclear superhalogens more effective than that by mononuclear superhalogens? A high-level ab initio case study

Polynuclear superhalogens are more effective in regulating the electronic properties of organic molecules based on a high-level ab initio study.

Identification of hyperhalogens in Agn(BO2)m (n = 1–3, m = 1–2) clusters: anion photoelectron spectroscopy and density functional calculations

We identified hyperhalogens in Ag_n(BO₂)_m (n = 1–3, m = 1–2) clusters by using anion photoelectron spectroscopy and density functional calculations.

Amino acids form strongly bound anions when substituted with superhalogen ligands

The properties of AA-Y(-) anions (where AA = cysteine, aspartic acid, lysine; Y = BF3, PF5) were investigated at the ab initio Outer Valence Green's Function (OVGF)∕6-311++G(3df,3pd)∕∕MP2∕6-311++G(d,p) level of theory. It is shown that introducing a superhalogen-like substituent to an amino acid (i.e., Cys, Asp, and Lys) results in obtaining molecules that bind an excess electron relatively strongly. The electronic stabilities of such resulting daughter anions are predicted to be substantial (5.3-6.9 eV).