Superalkalis with Hydrogen as Central Electronegative Atom and their Possible Applications: Ab Initio and DFT Study

Superalkalis are unusual species having ionization energies lower than that of the alkali metals. These species with various applications are of great importance in chemistry due to their low ionization energies and strong reducing property. A typical superalkali contains a central electronegative core decorated with excess metal ligands. In the quest for novel superalkalis, we have designed the superalkalis HLi2, HLiNa and HNa2 using hydrogen as central electronegative atom for the first time employing high level ab initio (CCSD(T), MP2) and density functional theory (ωB97X-D) methods. The superalkalis exhibit very low ionization energies, even lower than that of cesium. Stability of these species is verified from binding energy and dissociation energy values. The superalkalis are capable of reducing SO2, NO, CO2, CO and N2 molecules by forming stable ionic complexes and therefore can be used as catalysts for the reduction or activation of systems possessing very low electron affinities. The superalkalis form stable supersalts with tailored properties when interact with a superhalogen. They also show remarkably high non-linear optical responses, hence could have industrial applications. It is hoped that this work will enrich the superalkali family and spur further theoretical and experimental research in this direction.

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.

Superalkali Coated Rydberg Molecules

A series of complexes of Na, K, NH4, and H3O with [bpy.bpy.bpy]cryptand, [2.2.2]cryptand, and spherical cryptand were investigated via DFT and ab initio methods. We found that by coating Rydberg molecules with the "organic skin" one could further decrease their ionization potential energy, reaching the values of ∼1.5 eV and a new low record of 1.3 eV. The neutral cryptand complexes in this sense possess a weakly bounded electron and may be considered as very strong reducing agents. Moreover, the presence of an organic cage increases the thermodynamic stability of Rydberg molecules making them stable toward the proton detachment.

Designing Special Nonmetallic Superalkalis Based on a Cage-like Adamanzane Complexant

In this study, to examine the possibility of using cage-like complexants to design nonmetallic superalkalis, a series of X@3⁶adz (X = H, B, C, N, O, F, and Si) complexes have been constructed and investigated by embedding nonmetallic atoms into the 3⁶adamanzane (3⁶adz) complexant. Although X atoms possess very high ionization energies, these resulting X@3⁶adz complexes possess low adiabatic ionization energies (AIEs) of 0.78–5.28 eV. In particular, the adiabatic ionization energies (AIEs) of X@3⁶adz (X = H, B, C, N, and Si) are even lower than the ionization energy (3.89 eV) of Cs atoms, and thus, can be classified as novel nonmetallic superalkalis. Moreover, due to the existence of diffuse excess electrons in B@3⁶adz, this complex not only possesses pretty low AIE of 2.16 eV but also exhibits a remarkably large first hyperpolarizability (β₀) of 1.35 × 10⁶ au, indicating that it can also be considered as a new kind of nonlinear optical molecule. As a result, this study provides an effective approach to achieve new metal-free species with an excellent reducing capability by utilizing the cage-like organic complexants as building blocks.

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.

Novel and Polynuclear K- and Na-Based Superalkali Hydroxides as Superbases Better Than Li-Related Species and Their Enhanced Properties: An Ab Initio Exploration

Hydroxides of superalkalis (particularly, K- and Na-related species) are shown for the first time to function as superbases. A new small series of hydroxides (XM n+1OH) is designed based on superalkali species (XM n+1) where M (K and Na) is alkali metal atoms, n is the maximal formal valence of the central atom X (F, O, and N), and n ≥ 1. To probe whether such fascinating polynuclear superalkali hydroxides (SAHs), especially the K- and Na-associated moieties are as basic as the representative alkali metal hydroxides (KOH, NaOH, and LiOH) as well as similar Li-based SAHs, a comprehensive computational exploration (in the gas phase) has been reported using the framework of an ab initio method. The ab initio calculations reveal that both the K- and Na-related SAHs consisting of larger gas-phase proton affinity (PA) and gas-phase basicity (GB) values demonstrate stronger basic character compared to the LiOH and Li-based SAHs. However, the available SAHs act as strong bases as well as superbases; among the proposed K- and Na-based SAHs, remarkably, the OK3OH moiety having the highest PA (1168.4 kJ/mol) and GB (1146.9 kJ/mol) values shows the evidence of the strongest basicity (i.e., superbase/hyperbase), which exceed enough (ΔPA: 142.1 kJ/mol and ΔGB: 146.9 kJ/mol) the IUPAC-defined superbasicity threshold values (PA: 1026.3 kJ/mol and GB: 1000 kJ/mol) of 1,8-bis(dimethylamino)naphthalene (DMAN). Furthermore, theoretical signatures have been predicted via the electronic structure calculation approach in probing the dissociation energy, ionization potential, electron affinity, HOMO-LUMO gap, and chemical hardness as well as the NCI plot and QTAIM tools are used for the bonding feature analysis and such parameters are well linked with the basicity analyzing parameters. In this study, the ab initio-based computational experiments provide some new insights into the basicity features and understanding of the structural and electronic features of a small series of designed K- and Na-related SAHs. Design and synthesis of such theoretically examined SAHs may pave alternative routes for the experimentally rewarding applications.

CO2 Activation Within a Superalkali-Doped Fullerene

With the aim of finding a suitable synthesizable superalkali species, using the B3LYP/6-31G* density functional level of theory we provide results for the interaction between the buckminsterfullerene C60 and the superalkali Li3F2. We show that this endofullerene is stable and provides a closed environment in which the superalkali can exist and interact with CO2. It is worthwhile to mention that the optimized Li3F2 structure inside C60 is not the most stable C2v isomer found for the "free" superalkali but the D3h geometry. The binding energy at 0 K between C60 and Li3F2 (D3h) is computed to be 119 kJ mol-1. Once CO2 is introduced in the endofullerene, it is activated, and theO C O ^ angle is bent to 132°. This activation does not follow the previously studied CO2 reduction by an electron transfer process from the superalkali, but it is rather an actual reaction where a F (from Li3F2) atom is bonded to the CO2. From a thermodynamic analysis, both CO2 and the encapsulated [Li3F2⋅CO2] are destabilized in C60 with solvation energies at 0 K of 147 and < -965 kJ mol-1, respectively.

Record Low Ionization Potentials of Alkali Metal Complexes with Crown Ethers and Cryptands

Electronic properties of series of alkali metals complexes with crown ethers and cryptands were studied via DFT hybrid functionals. For [M([2.2.2]crypt)] (M=Li, Na, K) extremely low (1.70-1.52 eV) adiabatic ionization potentials were found. Such low values of ionization energies are significantly lower than those of alkali metal atoms. Thus, the investigated complexes can be defined as superalkalis. As a result, our investigation opens up new directions in the designing of chemical species with record low ionization potentials and extends the explanation of the ability of the cryptates and alkali crown ether complexes to stabilize multiple charged Zintl ions.

Recent Progress on the Design, Characterization, and Application of Superalkalis

Superalkalis are clusters or molecules featuring lower ionization energies (IEs) than that of cesium atoms, and thus exhibit excellent reducing properties. Such special species have great potential to be used in the synthesis of unusual charge-transfer salts and cluster-assembled nanomaterials with tailored properties, in the reduction of carbon dioxide, or as hydrogen storage materials and noble-gas-trapping agents, etc. In this regard, ongoing efforts have been devoted to designing and characterizing superalkalis of new types. The recent progress on the study of superalkalis in terms of theoretical design, characterization, and potential application is summarized in this minireview. We hope this review will not only provide a broad overview of this research field, but also highlight the prospect of further extending the experimental synthesis and practical application of superalkalis.

Design of the NnH3n+1+ series of “non-metallic” superalkali cations

A new series of non-metallic superalkali cations, N_nH_3n+1⁺ by using ammonium (NH₄⁺) cations, possessing vertical electron affinity (EA_v), 4.39 eV for n = 1 to 2.39 eV for n = 5 has been proposed. This series can be continued for obtaining new superalkali cations, for instance N₉H₂₈⁺ with an EA_v of 1.84 eV. The EA_v of N_nH_3n+1⁺ cations is governed by the electron localization on the central N-atom. The EA_v of N_nH_3n+1⁺ cations decays exponentially with an increase in n.

Computational investigation of LiF containing hypersalts

This study explores the design of possible hypersalts starting from the hyperhalogen Li₃F₄ plus a Li atom and the hyperalkali Li₄F₃ plus a F atom.

The effect of hydration on the electronic structure and stability of the superalkali cation Li3+

Insights into the interaction between the superalkali cation Li₃⁺ and water molecules and the stability of the resulting hydrates.

Ab initio analysis on potential superbases of several hyperlithiated species: Li3F2O and Li3F2OHn (n = 1, 2)

A systematic investigation on the potential basicity of the novel hyperlithiated species Li₃F₂O and Li₃F₂(OH)_n (n = 1, 2) based upon the superalkali cluster Li₃F₂ was conducted using high-level ab initio techniques.

Structure and stability of small lithium-chloride LinClm(0,1+) (n ≥ m, n = 1–6, m = 1–3) clusters

Detailed theoretical investigations along with the experimental observations of new small chlorine-doped lithium clusters are presented.

Reduction of carbon dioxide with a superalkali

The ability of the superalkali Li₃F₂ to reduce CO₂ and N₂ is investigated using the CBS-QB3 composite method and intriguing results are presented.

Organo-Zintl Clusters [P7R4]: A New Class of Superalkalis

Zintl ions composed of Group 13, 14, and 15 elements are multiply charged cluster anions that form the building blocks of the Zintl phase. Superalkalis, on the other hand, are cationic clusters that mimic the chemistry of the alkali atoms. It is, therefore, counterintuitive to expect that Zintl anions can be used as a core to construct superalkalis. In this paper, using density functional theory, we show that this is indeed possible. The results are compared with calculations at the MP2 level of theory. A systematic study of a P7(3-) Zintl core decorated with organic ligands [R = Me, CH2Me, CH(Me)2 and C(Me)3] shows that the ionization energies of some of the P7R4 species are smaller than those of the alkali atoms and hence can be classified as superalkalis. This opens the door to the design and synthesis of a new class of superalkali moieties apart from the traditional ones composed of only inorganic elements.

Organic heterocyclic molecules become superalkalis

A new strategy has been developed to create superalkali molecules from aromatic heterocycles. C₃N₂(CH₃)₅ has the ionization energy close to 3.0 eV.

Remarkable NLO responses of hyperalkalized species: the size effect and atomic number dependence

The large first order static hyperpolarizabilities (β_o) of hyperalkalized species establish their strong NLO responses.

Nonlinear optical behavior of Li n F (n = 2-5) superalkali clusters

Hyperlithiated clusters are known for their unusual bonding characteristics and lower ionization potentials. This study reports nonlinear optical (NLO) properties of a series of hyperlithiated clusters, Li n F (n = 2-5) for the first time. The structures of small Li n F (n = 2-5) clusters, obtained using second order Møller-Plesset perturbative method, are found to be stable against eliminations of F, F‾, and LiF. These Li n F species are stabilized by both ionic as well as covalent interactions. Our study shows that Li n F species can be thought of as superalkali-halogen (Li n  - F) clusters but belong to the class of superalkalies themselves. These clusters may also possess alkalide and/or electride characteristics due to excess electrons. The dipole moment, mean polarizability, and hyperpolarizability suggest their significant NLO responses which are explained using the highest occupied molecular orbital surfaces and TD-DFT analysis. The exceptionally large hyperpolarizability of Li2F (~10(5) a.u.) and its electride characteristics are particularly highlighted. This study may guide the researchers in the design of novel materials with significant NLO responses useful for electro-optical applications. Graphical Abstract Li2F superalkali resemble an electride in which the excess electron is pushed out by Li2 (+) moiety, leading to its high hyperpolarizability of order of 10(5) a.u.

Superalkali-hydroxides as strong bases and superbases

The gas-phase basicity values of superalkali hydroxides are closely related to the ionization potentials of the superalkalies and their HOMO–LUMO gaps.

A novel class of compounds—superalkalides: M+(en)3M3′O− (M, M′ = Li, Na, and K; en = ethylenediamine)—with excellent nonlinear optical properties and high stabilities

A novel class of inorganic salts wherein the superalkali occupies the anionic site, termed superalkalides, M⁺(en)₃M₃′O⁻ (M, M′ = Li, Na, and K) have been designed and predicted to be candidates for NLO materials.

Ab initio investigations on the gas phase basicity and nonlinear optical properties of FLinOH species (n = 2–5)

The superalkali hydroxide (FLi₅OH) possesses alkalide characteristics which is responsible for its remarkable mean hyperpolarizability i.e. NLO properties.

Stability and bonding of new superalkali phosphide species

New superalkali phosphide species (F₂Li₃P, F₂Li₃P₂, and F₄Li₆P) are investigated using CBS-QB3 composite method and intriguing structural features are presented.

Trivalent acid radical-centered YLi4+(Y = PO4, AsO4, VO4) cations: new polynuclear species designed to enrich the superalkali family

A new series of polynuclear superalkali cations YLi₄⁺(Y = PO4, AsO4, VO4) has been designed and characterized to enrich the superalkali family.