Editorial: Superhalogens & superalkalis: Exploration of structure, properties and applications

Editorial on the Research Topic
Superhalogens & superalkalis: Exploration of structure, properties and applications

Atomic clusters (Srivastava et al., 2021), containing a few to a hundred of atoms, are intermediates between a molecule and bulk materials. Superatoms are a special class of atomic clusters, mimicking the properties of atom. Superhalogens are superalkalis are two examples of superatoms, which possess higher electron affinity than halogen and lower ionization energy than alkali atoms, respectively. The Research Topic “Superhalogens & Superalkalis: Exploration of Structure, Properties and Applications” provides a compendium of the recent research and development in this field. This Research Topic concludes two review articles and seven research articles.

Srivastava and Srivastava reviewed the application of superalkalis in the activation of carbon dioxide. Owing to low ionization energy, superalkalis become reducers and can easily transfer an electron to CO₂, which possesses no positive electron affinity. They have discussed the CO₂ reduction by various types of superalkalis such as typical superalkalis, binuclear superalkalis, special superalkalis, non-metallic superalkalis. These superalkalis differ in design based on different electron-counting rules. The authors also included the successive activation of six CO₂ molecules using hexalithiobenzene (C₆Li₆), a closed-shell molecule with low ionization energy. Superalkalis have been substantially explored over the past couple of decades. In the review of Pandey et al., the recent developments in the theoretical design and characterization of a variety of superalkali-based compounds and their potential applications have been enumerated. They unveiled the potential applications of some novel superalkalis for capturing and storing CO₂/N₂ and also analyzed the first-order hyperpolarizability-based nonlinear optical (NLO) responses features of fullerene-like superalkali-doped B₁₂N₁₂ and B₁₂P₁₂ nanoclusters with good ultraviolet (UV) transparency and superalkali-based CaN₃Ca system, a high sensitivity alkali-earth-based aromatic multi-state NLO molecular switch, and so on. They have also highlighted the interactions of superalkalis in gas and liquid phases in this review. The authors expected that their review will provide new insights on the possibility of expanding both the experimental synthesis and the practical use of superalkalis and related species.

Ye et al. designed the X@3⁶adz complexes by embedding X (=H, B, C, N, O, F, and Si) into the 3⁶adamanzane (3⁶adz) complexant. It is interesting to find the low adiabatic ionization energies (AIEs), 0.78–5.28 eV of these complexes. Although the IEs of X atoms are high, the AIEs of X@3⁶adz (X = H, B, C, N, and Si) are even lower than the IE of the Cs atom (3.89 eV), which identifies their non-metallic superalkali characteristics. Furthermore, the presence of diffuse excess electron in B@3⁶adz enables it not only to possess a fairly low AIE of 2.16 eV but also significantly high first hyperpolarizability (β₀) of 1.35 × 10⁶ au. This may suggest its potential to be a bifunctional molecule with both strong reducibility and larger NLO response.

Cyraniak et al. adopted the electronic transmutation concept for designing novel series of polynuclear superhalogen anions. They investigated the stability of (BF₃(BN)_nF_4n+1)⁻ anions for n = 1–10, 13, 18–20 having alternate boron and nitrogen central atoms in the form of the (BN)_n “core”, 4n+1 fluorine as well as one BF₃ ligand using density functional theory (DFT) and outer valence Green function (OVGF) methods with flexible atomic orbital basis sets. The newly proposed anions are reflecting the structures of chain-like C_nH_2n+2 molecules in which the (BN)_n cores are supposed to resemble the bonding features of the C_2n chain, which advocates the existence of 4n+2 substituents similar to those in typical saturated hydrocarbons. The authors confirmed that the equilibrium configurations of these anions are completely extended chains with each B and N core atom having four peripheral substituents organized in a tetrahedral pattern. Consequently, these anions mimic the universally stable completely extended conformational changes of higher n-alkanes. Moreover, the calculations revealed that the vertical electron detachment energies (VDEs) of the (BF₃(BN)_nF_4n+1)⁻ anions were observed to be greater than 8 eV and to rise steadily with the increase in n. Additionally, the authors predicted that the upper limit of VDE which could be achieved for such polynuclear superhalogen anions is about 10.7 eV.

Guo et al. reported a quantum chemical study on a binary B₅O₆⁻ cluster and found a global-minimum planar C_2v (¹A₁) structure having a B₃O₃ hexagonal ring in its center. They noticed that two boronyl (BO) groups and one O⁻ ligand terminates three unsaturated B sites and therefore, formulated this cluster as B₃O₃(BO)₂O⁻ unlike its predecessors, C_s B₅O₅⁻ and T_d B₅O₄⁻, containing a tetrahedral B at the center. According to them, the B₅O_n⁻ series undergoes a significant structural change after oxidation, showing an interesting rivalry among tetrahedral versus heterocyclic structures. Their investigations reveal that the B₅O₆⁻ cluster has a weak 6π aromaticity, making it a boronyl counterpart of the phenolate anion (C₆H₅O⁻) or boronyl boroxine. The authors estimated that the B₅O₆⁻ cluster has a VDE of 5.26 eV at PBE0 method, which is much higher than the electron affinities of typical halogens, limited to 3.61 eV for Cl, and consequently, declared it as a superhalogen anion.

Zhu et al. studied the structural evolution, bonding, stability, charge transfer, and nonlinear optical characteristics of AuMg_n (n = 2–12) nanoclusters using the DFT method. They unraveled a planar structure of the AuMg₃ and the symmetrically perfect cage-like structure of the AuMg₉. They analyzed the charge transfer from the Mg to Au atoms and identified the covalent Mg-Mg bonds in nanoclusters larger than that of AuMg₃. Their polarizability and hyperpolarizability estimations predict the strong NLO behavior of the AuMg₉ nanocluster. Based on their theoretical results, the authors also suggested that these nanoclusters are identifiable by spectroscopic experiments.

Tkachenko et al. examined ionization potentials (IPs) for a series of clusters of various cryptand such as [bpy.bpy.bpy]cryptand (bpy = bi-pyridine), [2.2.2]cryptand, and spherical cryptand with Na, K, NH₄, and H₃O with by using the DFT as well as ab initio methods. They noticed that the encapsulation of Rydberg molecules (NH₄ and H₃O) inside an organic cage leads to a decrease in their IPs, reaching the values of ∼1.5 eV and an even lower value of 1.3 eV. They also showed that the Rydberg molecules coated with the “organic skin” can increase their thermodynamic stability. Therefore, they suggested that these findings provide an opportunity to obtain such strong reducing agents in the experiment. This work was contributed by the research group of Prof. Alexander Boldyrev, one of the pioneers in the field of superatoms.

He et al. investigated and studied the substituent effect in M@Al₁₂N₁₁ and M@Al₁₁N₁₂ (M = Be, Mg, and Ca) nanocages through the replacement of one Al or N atom of aluminum nitride nanocage (Al₁₂N₁₂) with an alkaline-earth metal atom by DFT methods. Their calculations reveal that these nanocages are highly stable and have excess electron systems. They, further, suggested that these substituted nanocages exhibit larger first hyperpolarizabilities (β₀) than that of pure Al₁₂N₁₂ nanocages due to the presence of diffuse excess electrons. According to them, these modified cages possess the transparency to infrared light (IR) (>1,800 nm) as well as ultraviolet light (UV) (250 nm). The authors concluded that their studied highly stable excess electron compounds might be suitable candidates for novel UV and IR NLO systems.

Yang et al. proposed and systematically investigated a new group of hetero-binuclear superhalogen anion matching the MM′X₄^– (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) formula by using second-order Moller-Plesset perturbation theory (MP2) and OVGF methods and 6–311+G (3df) basis set. The calculations revealed that all of the studied anions possess vertical electron detachment energies (VDEs) larger than 5.4 eV and thus confirmed their superhalogen nature. Moreover, the authors also noticed the dependence of the VDE of the MM′X₄^– anions on the M and M′ atomic radius, and electronegativity of the ligand used. In particular, the inclusion of a smaller alkali metal atom M, a bigger alkaline earth metal atom M′, and a greater electronegative ligand atom X might result in larger VDE values for these heteronuclear superhalogen anions. Therefore, one isomer of LiCaCl₄^– was found to possess the largest VDE value (6.799 eV). In addition, the analysis of charge distribution of all studied anions shows that the isomers exhibit considerably greater electrical stability when the excess of electrons charge is shared across all of the ligand atoms as well as with the three bridging ligand atoms.

Thus, the Research Topic not only covers the articles based on the exploration of structures, properties and applications of superatoms but also includes a few articles on atomic clusters as well. We are of the strong opinion that the contents of this Research Topic offer readers an overview of the recent progress of this constantly expanding field.

Author contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Reference

Srivastava, A. K., Anusiewicz, I., Velickovic, S., Sun, W-M., and Misra, N. (2021). Editorial: Atomic clusters: Theory & experiments. Front. Chem. 9, 795113. doi:10.3389/fchem.2021.795113

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