Organic Superhalogens

Catalytic Potential of Supported Superatoms

The importance of catalysts in industrial products is a driving factor in the search of efficient and cost-effective catalysts, creating considerable interest in the past decade in single-atom catalysis. One of the first requirements of a good catalyst is that it should bind to the molecules with energies intermediate between physisorption and chemisorption while simultaneously activating them. Herein, it is shown that superatoms, which are atomic clusters with fixed size and composition, can meet this challenge even better than the atoms whose chemistry they mimic. The reactions of molecules such as H2, O2, N2, CO, NO, and CO2 with an atom (Li) and its corresponding superatom (Li3O) are confirmed through study. As these clusters need to be supported on a substrate for practical applications, the study focuses on the reaction of CO2 with Li and Li3O supported on graphene, Au(111), and Cu(111) substrates. Using density functional theory, it is shown that the Li3O superatom can activate CO2 far greater than the Li atom - stretching the CO bond from 1.16 Å to as large as 1.30 Å and bending the O─C─O bond angle from 180° to as low as 120°. Equally interesting, the results are not very sensitive to the substrate.

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.

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.

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.

Toward Ultracold Organic Chemistry: Prospects of Laser Cooling Large Organic Molecules

Ultracold organic chemistry enables studies of reaction dynamics and mechanisms in the quantum regime. Access to ultracold molecules hinges on the ability to efficiently scatter multiple photons via quasi-closed cycling transitions. Optical cycling in polyatomic molecules is challenging due to their complex electronic structure. Using equation-of-motion coupled-cluster calculations, we demonstrate that an alkaline earth metal attached to various aromatic ligands (such as benzene, phenol, cyclopentadienyl, and pyrrolide) offers nearly closed cycling transitions with only a few additional repump lasers. We also show that aromatic ligands such as benzene can accommodate multiple cycling centers in various geometrical arrangements, opening new avenues in quantum information science, precision measurements, and ultracold chemistry.

Constructing organic superacids from superhalogens is a rational route as verified by DFT calculations

The construction route of organic superacids from the combination of organic superhalogens and protons is verified to be a rational one based on a systematic theoretical study covering different planar conjugated backbones, e.g., [C5H5]- and [BC5H6]-, and electron-withdrawing substituents, e.g., -F, -CN and -NO2. In both the gas phase and the solution phase, the acidities of the composites here have a consistent strengthening with the increase of the vertical electron detachment energy of the superhalogen part. Decomposition of the acidity into different contributions further verifies the dominant role of the superhalogen part in the variation of the acidity. Thus, tuning of the acidity of systems of this type could be achieved via rational design of the constituent part of the superhalogen. That is to say, the design of a novel organic superacid with enhanced properties could be guided by the search for a new strong superhalogen of organic nature eventually. Having provided important contributions to the topic of superhalogens, theoretical calculation should be trusted to provide useful guidance for the research of organic superacids and could be expected to promote related experimental studies in the near future.

Rational Design of Stable Dianions by Functionalizing Polycyclic Aromatic Hydrocarbons

Using density functional theory, we have carried out a systematic study of the stability and electronic properties of neutral and multiply charged molecules B_n C_10-n X₈ (n=0, 1, 2; X=H, F, CN). Our main objective is to explore if the replacements of core C atoms and/or H atoms in naphthalene (C₁₀ H₈ ) can enhance the stability of their dianions. Indeed, we find that the dianions of B_n C_10-n (CN)₈ are more stable than their monoanions with energies of 0.61 eV, 0.57 eV, and 1.97 eV for n=0, 1, 2, respectively. In addition, polycyclic aromatic hydrocarbons become stable as dianions only when H atoms are substituted by more electronegative species. Thus, a rational design approach by tailoring composition and ligands can lead to a new class of organic molecules that are capable of carrying multiple charges.

Application of superhalogens in the design of organic superconductors

This study shows that the acceptors of super-electrons in organic superconductors belong to the class of superhalogens and proposes that a new series of salts, (TMTSF)₂X, can be realized where X is a superhalogen, which possess similar properties to those of the existing Bechgaard salts. Thus, the concept of superhalogens can be useful in designing potential candidates for organic superconductors.

Li@organic superhalogens: possible electrolytes in Li-ion batteries

First principles calculations on Li salts of organic heterocyclic superhalogens confirm that they can be used as potential electrolytes in Li-ion batteries.

Super/hyperhalogen aromatic heterocyclic compounds

Aromatic heterocyclic molecules with negative electron affinity values can be transformed to highly oxidizing super/hyperhalogens based on a systematic in silico approach.

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.

Unusual stability of multiply charged organo-metallic complexes

The addition of electron(s), capable of simultaneously satisfying three electron counting rules, renders unusual stability to the charged organometallic complexes.