Alkali metals in ethylenediamine: a computational study of the optical absorption spectra and NMR parameters of [M(en)3(δ+)·M(δ-)] ion pairs

Ground and excited electronic structures of electride and alkalide units: The cases of Metal-Tren, -Azacryptand, and -TriPip222 complexes

A systematic electronic structure analysis was conducted for M(L)n molecular electrides and their corresponding alkalide units M(L)n @M' (M/M' = Na, K; L = Tren, Azacryptand, TriPip222; n = 1, 2). All complexes belong to the "superalkali" category due to their low ionization potentials. The saturated molecular electrides display M+ (L)n - form with a greatly diffuse quasispherical electron cloud. They were identified as "superatoms" considering the contours of populating atomic-type molecular orbitals. The observed superatomic Aufbau order of M(Tren)2 is 1S, 1P, 1D, 1F, 2S, 2P, and 1G and it is consistent with those of M(Azacryptand) and M(TriPip222) up to the analyzed 1F level. Their excitation energies decrease gradually moving from M(Tren)2 to M(Azacryptand) and to M(TriPip222). The studied alkalide complexes carry [M(L)n ]+ @M'- ionic structure and their dissociation energies vary in the sequence of K(L)n @Na > Na(L)n @Na > K(L)n @K > Na(L)n @K. Similar to molecular electrides, the anions of alkalide units occupy electrons in diffuse Rydberg-like orbitals. In this work, excited states of [M(L)n @M']0/+/- and their trends are also analyzed.

Converting the Bulk Transition Metal Dichalcogenides Crystal into Stacked Monolayers via Ethylenediamine Intercalation

We report the synthesis of ethylenediamine-intercalated NbSe2 and Li-ethylenediamine-intercalated MoSe2 single crystals with increased interlayer distances and their electronic structures measured by means of angle-resolved photoemission spectroscopy (ARPES). X-ray diffraction patterns and transmission electron microscopy images confirm the successful intercalation and an increase in the interlayer distance. ARPES measurement reveals that intercalated NbSe2 shows an electronic structure almost identical to that of monolayer NbSe2. Intercalated MoSe2 also returns the characteristic feature of the monolayer electronic structure, a direct band gap, which generates sizable photoluminescence even in the bulk form. Our results demonstrate that the properties and phenomena of the monolayer transition metal dichalcogenides can be achieved with large-scale bulk samples by blocking the interlayer interaction through intercalation.

Intercalated Iron Chalcogenides: Phase Separation Phenomena and Superconducting Properties

Organic molecule-intercalated layered iron-based monochalcogenides are presently the subject of intense research studies due to the linkage of their fascinating magnetic and superconducting properties to the chemical nature of guests present in the structure. Iron chalcogenides have the ability to host various organic species (i.e., solvates of alkali metals and the selected Lewis bases or long-chain alkylammonium cations) between the weakly bound inorganic layers, which opens up the possibility for fine tuning the magnetic and electrical properties of the intercalated phases by controlling both the doping level and the type/shape and orientation of the organic molecules. In recent years, significant progress has been made in the field of intercalation chemistry, expanding the gallery of intercalated superconductors with new hybrid inorganic–organic phases characterized by transition temperatures to a superconducting state as high as 46 K. A typical synthetic approach involves the low-temperature intercalation of layered precursors in the presence of liquid amines, and other methods, such as electrochemical intercalation, intercalant or ion exchange, and direct solvothermal growths from anhydrous amine-based media, are also being developed. Large organic guests, while entering a layered structure on intercalation, push off the inorganic slabs and modify the geometry of their internal building blocks (edge-sharing iron chalcogenide tetrahedrons) through chemical pressure. The chemical nature and orientation of organic molecules between the inorganic layers play an important role in structural modification and may serve as a tool for the alteration of the superconducting properties. A variety of donor species well-matched with the selected alkali metals enables the adjustment of electron doping in a host structure offering a broad range of new materials with tunable electric and magnetic properties. In this review, the main aspects of intercalation chemistry are discussed, involving the influence of the chemical and electrochemical nature of intercalating species on the crystal structure and critical issues related to the superconducting properties of the hybrid inorganic–organic phases. Mutual relations between the host and organic guests lead to a specific ordering of molecular species between the host layers, and their effect on the electronic structure of the host will be also argued. A brief description of a critical assessment of the association of the most effective chemical and electrochemical methods, which lead to the preparation of nanosized/microsized powders and single crystals of molecularly intercalated phases, with the ease of preparation of phase pure materials, crystal sizes, and the morphology of final products is given together with a discussion of the stability of the intercalated materials connected with the volatility of organic solvents and a possible degradation of host materials.

Superalkali-Alkalide Interactions and Ion Pairing in Low-Polarity Solvents

The nature of anionic alkali metals in solution is traditionally thought to be "gaslike" and unperturbed. In contrast to this noninteracting picture, we present experimental and computational data herein that support ion pairing in alkalide solutions. Concentration dependent ionic conductivity, dielectric spectroscopy, and neutron scattering results are consistent with the presence of superalkali-alkalide ion pairs in solution, whose stability and properties have been further investigated by DFT calculations. Our temperature dependent alkali metal NMR measurements reveal that the dynamics of the alkalide species is both reversible and thermally activated suggesting a complicated exchange process for the ion paired species. The results of this study go beyond a picture of alkalides being a "gaslike" anion in solution and highlight the significance of the interaction of the alkalide with its complex countercation (superalkali).

Ab initio molecular dynamics study of sodium NMR chemical shifts in the methylamine solution of [Na+ [2.2.2]cryptand Na-]

The sodium anion (Na-) was once thought to behave like a 'genuine' anion, with both the [Ne] core and the 3s valence shell interacting very weakly with their environments. In the present work, following a recent study of the surprisingly small quadrupolar line widths of Na-, NMR shielding calculations were carried out for the Na-/Na+ [2.2.2]cryptand system solvated in methylamine, based on ab initio molecular dynamics simulations, followed by detailed analyses of the shielding constants. The results confirm that Na- does not act like a quasi-free ion that interacts only weakly with its surroundings. Rather, the filled 3s shell of Na- interacts strongly with its chemical environment, but only weakly with the ion's own core and the nucleus, and it isolates the core from the chemical environment. As a consequence, the Na- ion appears in NMR experiments like a free ion.

The Sodium Anion Is Strongly Perturbed in the Condensed Phase Even Though It Appears Like a Free Ion in Nuclear Magnetic Resonance Experiments

Solvated sodium anions (Na^-) were thought to behave essentially like isolated gas-phase ions that interact only weakly with their environments. For example, ²³Na NMR signals for solvated Na^- are very sharp, despite the potential for strong quadrupolar broadening. The sharp NMR signals appear to indicate a nearly spherical electron density of the ion. For the present study, ab initio molecular dynamics simulations and quadrupolar relaxation rate calculations were carried out for the Na^-/Na⁺ [2.2.2]cryptand system solvated in methylamine, followed by detailed analyses of the electric field gradient at the sodium nuclei. It is found that Na^- does not behave like a quasi-free ion interacting only weakly with its environment. Rather, the filled 3s shell of Na^- interacts weakly with the ion's own core and the nucleus, causing Na^- to appear in NMR experiments like a free ion.

Solvation of Na? in the Sodide Solution, LiNa?10MeNH2

Alkalides, the alkali metals in their ?1 oxidation state, represent some of the largest and most polarizable atomic species in condensed phases. This study determines the solvation environment around the sodide anion, Na^?, in a system of co-solvated Li⁺. We present isotopically varied total neutron scattering experiments alongside empirical potential structure refinement and ab initio molecular dynamics simulations for the alkali?alkalide system, LiNa?10MeNH₂. Both local coordination modes and the intermediate range liquid structure are determined, which demonstrate that distinct structural correlations between cation and anion in the liquid phase extend beyond 8.6 ?. Indeed, the local solvation around Na^? is surprisingly well defined with strong solvent orientational order, in contrast to the classical description of alkalide anions not interacting with their environment. The ion-paired Li(MeNH₂)₄⁺?Na^? species appears to be the dominant alkali?alkalide environment in these liquids, whereby Li⁺ and Na^? share a MeNH₂ molecule through the amine group in their primary solvation spheres.

Theoretical study of substituent effects on electride characteristics and the nonlinear optical properties of Li@calix[4]pyrrole

A relationship between the electride characteristics and the NLO properties is found: the more delocalization the excess electron of the electride experiences, the larger the β₀ value is.

Superconducting selenides intercalated with organic molecules: synthesis, crystal structure, electric and magnetic properties, superconducting properties, and phase separation in iron based-chalcogenides and hybrid organic-inorganic superconductors

Layered iron-based superconducting chalcogenides intercalated with molecular species are the subject of intensive studies, especially in the field of solid state chemistry and condensed matter physics, because of their intriguing chemistry and tunable electric and magnetic properties. Considerable progress in the research, revealing superconducting inorganic-organic hybrid materials with transition temperatures to superconducting state, T _c, up to 46 K, has been brought in recent years. These novel materials are synthesized by low-temperature intercalation of molecular species, such as solvates of alkali metals and nitrogen-containing donor compounds, into layered FeSe-type structure. Both the chemical nature as well as orientation of organic molecules between the layers of inorganic host, play an important role in structural modifications and may be used for fine tuning of superconducting properties. Furthermore, a variety of donor species compatible with alkali metals, as well as the possibility of doping also in the host structure (either on Fe or Se sites), makes this system quite flexible and gives a vast array of new materials with tunable electric and magnetic properties. In this review, the main aspects of intercalation chemistry are discussed with a particular attention paid to the influence of the unique nature of intercalating species on the crystal structure and physical properties of the hybrid inorganic-organic materials. To get a full picture of these materials, a comprehensive description of the most effective chemical and electrochemical methods, utilized for synthesis of intercalated species, with critical evaluation of their strong and weak points, related to feasibility of synthesis, phase purity, crystal size and morphology of final products, is included as well.

Does the endohedral borospherene supersalt FLi2@B39maintain the “super” properties of its subunits?

The behavior of the entirely unique system represented by superalkaline species incorporated into a superhalogen cage has been studied using density functional theory. The calculations revealed that superhalogen and superalkaline properties inherent in the separated fragments are lost in FLi₂@B₃₉complexes.

On the Potential of Using the Al7 Superatom as an Excess Electron Acceptor To Construct Materials with Excellent Nonlinear Optical Properties

With the aid of density functional theory (DFT) calculations, we found that, when alkali metal approaches the Al7 superatom, its outermost s-value electron can be trapped by Al7 to give the superatom compound MAl7 (M = Li, Na, K) with an excess electron. Different analyses including natural bond orbital (NBO), electron localization function (ELF), and energy decomposition analysis (EDA) show that the resulting M-Al bond is strong and has a polar covalent character. The optimizations of self-assemblies (MAl7)n (n = 2, 3) have been performed to explore the stability of MAl7 in the solid state. The results reveal that only NaAl7 can keep its structural integrity as a building block upon self-assembling, while serious aggregations between Al7 clusters occur in the dimers and trimers of LiAl7 and KAl7, despite the fact that the Li-Al7 and K-Al7 bond energies are comparable to that of Na-Al7. Born-Oppenheimer molecular dynamics (BOMD) simulations for (NaAl7)n (n = 2, 3) indicate that these species are stable toward fragmentation at 300 K. The β0 values of (NaAl7)n (n = 1, 2, and 3) predicted at the CAM-B3LYP/6-311+G(3df) level of theory are in the range of 1.6 × 10(4)a.u. to 7.5 × 10(4) a.u.. This theoretical study implies that NaAl7 is a promising candidate for nolinear optical (NLO) materials. We provide theoretical evidence for the possibility of using the Al7 superatom as an excess electron acceptor to construct materials with excellent NLO properties. Further experimental research is invited.

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.