Small Crown-Ether Complexes as Molecular Models for Dihydrogen Adsorption in Undercoordinated Extraframework Cations in Zeolites

1:1 metal complexes of small crown ethers are structurally similar to extraframework sites in metal-exchanged zeolites. Using ab initio calculations, we show that adsorbed molecular hydrogen follows the same trends in adsorption energies and vibrational frequencies at both types of metal sites. Unlike zeolites, crown ethers can be characterized in the gas phase, which opens new possibilities for understanding the bonding of dihydrogen at undercoordinated metal sites to help guide the rational design of porous materials for hydrogen isotope separation. Because more strongly binding adsorbates affect the geometry of the hosts, the similarity of crown ethers and zeolites with regard to the vibrational spectra of the adsorbed molecule seems to be limited to H2

Magnetic Coupling Control in Triangulene Dimers

Metal-free magnetism remains an enigmatic field, offering prospects for unconventional magnetic and electronic devices. In the pursuit of such magnetism, triangulenes, endowed with inherent spin polarization, are promising candidates to serve as monomers to construct extended structures. However, controlling and enhancing the magnetic interactions between the monomers persist as a significant challenge in molecular spintronics, as so far only weak antiferromagnetic coupling through the linkage has been realized, hindering their room temperature utilization. Herein, we investigate 24 triangulene dimers using first-principles calculations and demonstrate their tunable magnetic coupling (J), achieving unprecedented strong J values of up to −144 meV in a non-Kekulé dimer. We further establish a positive correlation between bandgap, electronic coupling, and antiferromagnetic interaction, thereby providing molecular-level insights into enhancing magnetic interactions. By twisting the molecular fragments, we demonstrate an effective and feasible approach to control both the sign and strength of J by tuning the balance between potential and kinetic exchanges. We discover that J can be substantially boosted at planar configurations up to −198 meV. We realize ferromagnetic coupling in nitrogen-doped triangulene dimers at both planar and largely twisted configurations, representing the first example of ferromagnetic triangulene dimers that cannot be predicted by the Ovchinnikov rule. This work thus provides a practical strategy for augmenting magnetic coupling and open up new avenues for metal-free ferromagnetism

Small Crown-Ether Complexes as Molecular Models for Dihydrogen Adsorption in Undercoordinated Extraframework Cations in Zeolites

1:1 metal complexes of small crown ethers are structurally similar to extraframework sites in metal-exchanged zeolites. Using ab initio calculations, we show that adsorbed molecular hydrogen follows the same trends in adsorption energies and vibrational frequencies at both types of metal sites. Unlike zeolites, crown ethers can be characterized in the gas phase, which opens new possibilities for understanding the bonding of dihydrogen at undercoordinated metal sites to help guide the rational design of porous materials for hydrogen isotope separation. Because more strongly binding adsorbates affect the geometry of the hosts, the similarity of crown ethers and zeolites with regard to the vibrational spectra of the adsorbed molecule seems to be limited to H2

DFTB Parameters for the Periodic Table: Part III, Spin-Orbit Coupling

Spin-orbit coupling (SOC) is crucially important for the correct description of the electronic structure and transport properties of inorganic semiconductors, and for assessing topological properties as in topological insulators. We present a consistent set of SOC parameters for the density-functional based tight-binding (DFTB) method covering the elements throughout the periodic table. The parameters are based on atomic SOC data calculated at the level of density-functional theory (DFT). We tested these parameters for representative systems with significant SOC, including transition metal dichalcogenide two-dimensional crystals, III-V bulk semiconductors, and topological insulators. Our parameterization opens the door for DFTB-based electronic structure and transport calculations of very large systems, such as twisted van der Waals heterostructures

Statistical Representation of Stacking Disorder in Layered Covalent Organic Frameworks

Covalent organic frameworks (COFs) are among the fastest-growing classes of materials with an almost unlimited number of achievable structures, topologies, and functionalities. The exact structure of layered COFs is, however, hard to determine due to an often significant mismatch between experimental powder X-ray diffraction (PXRD) pattern and predicted geometries. We attribute these discrepancies to an inherent disorder in the stacking of layered COFs, invalidating standard theoretical three-dimensional (3D) models. We have represented the structures of COF-1, COF-5, and ZnPc-pz by stacking layers following the Maxwell–Boltzmann energy distribution of their stacking modes. The simulated PXRD patterns of the statistical COF models are close to the experimental ones, featuring an unprecedented agreement in peak intensity, width, and asymmetry. The rarely considered ABC stacking mode proved to be important in layered COFs, as well as including solvent molecules. Our model also shows several general features in PXRD originating from the stacking disorder

Structure and Electron Delocalization in Al₄^2- and Al₄^4-^†

Structure, dynamics, and electron delocalization of Al42- and Al44- based clusters are investigated. Gradient-corrected Density-Functional Born−Oppenheimer Molecular Dynamics simulations indicate that Al42- based clusters have a rigid planar Al framework, while the Al44- based moieties show large distortions from planarity. The induced magnetic field analysis of these species indicates that both systems have diatropic σ-systems, while the π-system is diatropic for Al42- and paratropic for Al44-. The total magnetic response is diatropic for Al42-, while Al44- is “bitropic”:  it has typical antiaromatic long-range cones, while the magnetic field in the Al44- ring plane is similar to that of aromatic annulenes

Interaction of Small Gases with the Unsaturated Metal Centers of the HKUST‑1 Metal Organic Framework

The interactions of CO, CO₂, OCS, SO₂, NO, NO₂, N₂O, NH₃, PH₃, and other small molecules with the undercoordinated metal centers of the HKUST-1 metal organic framework are studied by means of density functional theory. These molecules are potentially harmful for humans and the environment and are widely studied because of their spectroscopic properties. In this work, the energetic and vibrational characteristics of the adsorbed species are calculated. Adsorption energies on the Cu²⁺ sites of the paddlewheel have been calculated, and the order is: NH₃ > H₂O > PH₃ > H₂S > SO₂ > CO ∼ OCS ∼ CO₂ ∼ N_<i>y</i>O_<i>x</i> > N₂ > O₂. The results show that the interactions can be classified into three categories: (1) weak physisorption, (2) polarization and electrostatics, and (3) strong acid–base. Moreover, interesting vibrational properties are calculated especially for carbonyl sulfide and dinitrogen monoxide, which can be bound via two different configurations on the metal atoms. The vibrational modes are shifting in different directions depending on the binding way of the molecule; e.g., the symmetric stretching of OCS is shifted by +17 or −16 cm^–1 when bound via the oxygen or the sulfur atom, respectively

ADS2 lesson02 conceptual design of structures

We investigated computationally the α-, γ-, and β-isomeric structures, relative stabilities, and the electronic and basicity properties of magnetic [V^IV₁₄E₈O₅₀]^12– (hereafter referred to as <b>{<b>V₁₄E₈</b>}</b>) heteropolyoxovanadates (heteroPOVs) and their heavier chalcogenide-substituted [V^IV₁₄E₈O₄₂X₈]^12– (<b>{<b>V₁₄E₈X₈</b>}</b>) derivatives for E = Si^IV, Ge^IV, and Sn^IV and X = S, Se, and Te. We used density functional theory (DFT) with scalar relativistic corrections in combination with the conductor-like screening model of solvation. The main purpose of this investigation is to introduce the structure–property relations in heteroPOVs as well as to assist the synthesis and molecular deposition of these molecular vanadium-oxide spin clusters on surfaces. “Fully-reduced” polyoxoanions <b>{<b>V₁₄E₈</b>}</b> and <b>{<b>V₁₄E₈X₈</b>}</b> are virtually comprised of [V^IV₁₄O₃₈]^20– {V₁₄} skeletons of different symmetries, that is, <i>D</i>_2<i>d</i> for α-, <i>D</i>₂ for γ-, and <i>D</i>_4<i>h</i> for β-isomers, which are stabilized by the four {E₂O₃}²⁺ and four {E₂OX₂}²⁺ moieties, respectively. Our DFT calculations reveal stability trends α > γ > β for polyoxoanions <b>{<b>V₁₄E₈</b>}</b> and <b>{<b>V₁₄E₈X₈</b>}</b>, based on relative energies and HOMO–LUMO energy gaps. The α-isomeric polyoxoanions <b>{<b>V₁₄E₈</b>}</b> and <b>{<b>V₁₄E₈X₈</b>}</b> with the high negative net charges may easily pick up protons at the terminal E–O_t and E–X_t sites, respectively, which is evidenced by strongly negative enthalpies of monoprotonation. Energetically favorable sites on polyoxoanions α-<b>{<b>V₁₄E₈</b>}</b> and α-<b>{<b>V₁₄E₈X₈</b>}</b> for electrostatic pairing with countercations were also determined. Among β and γ isomers, the hitherto unknown γ-[V₁₄Sn₈O₅₀]^12– and γ-[V₁₄Sn₈O₄₂S₈]^12– seem to be the most viable targets for isolation. Furthermore, these Sn-substituted polyoxoanions are of high interest for electrochemical studies because of their capability to act as two-electron redox catalysts

Interaction of Small Gases with the Unsaturated Metal Centers of the HKUST‑1 Metal Organic Framework

The interactions of CO, CO₂, OCS, SO₂, NO, NO₂, N₂O, NH₃, PH₃, and other small molecules with the undercoordinated metal centers of the HKUST-1 metal organic framework are studied by means of density functional theory. These molecules are potentially harmful for humans and the environment and are widely studied because of their spectroscopic properties. In this work, the energetic and vibrational characteristics of the adsorbed species are calculated. Adsorption energies on the Cu²⁺ sites of the paddlewheel have been calculated, and the order is: NH₃ > H₂O > PH₃ > H₂S > SO₂ > CO ∼ OCS ∼ CO₂ ∼ N_<i>y</i>O_<i>x</i> > N₂ > O₂. The results show that the interactions can be classified into three categories: (1) weak physisorption, (2) polarization and electrostatics, and (3) strong acid–base. Moreover, interesting vibrational properties are calculated especially for carbonyl sulfide and dinitrogen monoxide, which can be bound via two different configurations on the metal atoms. The vibrational modes are shifting in different directions depending on the binding way of the molecule; e.g., the symmetric stretching of OCS is shifted by +17 or −16 cm^–1 when bound via the oxygen or the sulfur atom, respectively