Chemical bonding in phase-change chalcogenides

Almost all phase-change memory materials (PCM) contain chalcogen atoms, and their chemical bonds have been denoted both as 'electron-deficient' [sometimes referred to as 'metavalent'] and 'electron-rich' ['hypervalent', multicentre]. The latter involve lone-pair electrons. We have performed calculations that can discriminate unambiguously between these two classes of bond and have shown that PCM have electron-rich, 3c-4e ('hypervalent') bonds. Plots of charge transferred between (ET) and shared with (ES) neighbouring atoms cannot on their own distinguish between 'metavalent' and 'hypervalent' bonds, both of which involve single-electron bonds. PCM do not exhibit 'metavalent' bonding and are not electron-deficient; the bonding is electron-rich of the 'hypervalent' or multicentre type.

Exploring the Efficiency of Algerian Kaolinite Clay in the Adsorption of Cr(III) from Aqueous Solutions: Experimental and Computational Insights

The current study comprehensively investigates the adsorption behavior of chromium (Cr(III)) in wastewater using Algerian kaolinite clay. The structural and textural properties of the kaolinite clay are extensively characterized through a range of analytical methods, including XRD, FTIR, SEM-EDS, XPS, laser granulometry, N2 adsorption isotherm, and TGA-DTA. The point of zero charge and zeta potential are also assessed. Chromium adsorption reached equilibrium within five minutes, achieving a maximum removal rate of 99% at pH 5. Adsorption equilibrium is modeled using the Langmuir, Freundlich, Temkin, Elovich, and Dubinin-Radushkevitch equations, with the Langmuir isotherm accurately describing the adsorption process and yielding a maximum adsorption capacity of 8.422 mg/g for Cr(III). Thermodynamic parameters suggest the spontaneous and endothermic nature of Cr(III) sorption, with an activation energy of 26.665 kJ/mol, indicating the importance of diffusion in the sorption process. Furthermore, advanced DFT computations, including COSMO-RS, molecular orbitals, IGM, RDG, and QTAIM analyses, are conducted to elucidate the nature of adsorption, revealing strong binding interactions between Cr(III) ions and the kaolinite surface. The integration of theoretical and experimental data not only enhances the understanding of Cr(III) removal using kaolinite but also demonstrates the effectiveness of this clay adsorbent for wastewater treatment. Furthermore, this study highlights the synergistic application of empirical research and computational modeling in elucidating complex adsorption processes.

Origin of structure and stability of M@C18 (M = Cu, Ag, and Au) complexes with D9h point group

Theoretical predictions and recent experimental studies lead to the discovery of an exciting new member of the carbon allotrope family polyynic cyclo[18]carbon (C₁₈ ). Present investigation aims to probe the structure, stability, and properties of coinage metal (M)@C₁₈ complexes using density functional theory (DFT) calculations. The DFT results unequivocally show that even Cu@C₁₈ , Ag@C₁₈ , and Au@C₁₈ complexes substantially preserve the ground state polyynic structure of C₁₈ . It is also worth to mention that only Au@C₁₈ is a stable D_9h structure, however the symmetry is distorted in the case of Cu@C₁₈ and Ag@C₁₈ . Due to computational limitations, in this investigation the M@C₁₈ complexes were scrutinized using the C_2v sub abelian group of D_9h . The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of D_9h conformers are a singlet a₁ and two same value singlets a₁  ⊕ b₁ generated from doublet e, respectively. The non-covalent interaction index (NCI), quantum theory of atoms in molecule (QTAIM), and energy decomposition analysis (EDA) vividly explains the interaction between a coinage metal atom and C₁₈ ring. It is found from the results that the stability of Cu@C₁₈ Ag@C₁₈ , and Au@C₁₈ is governed by the attractive electrostatic, orbital and dispersion interaction.

A fast approximate extension of the interacting quantum atoms energy decomposition to excited states

An approach is developed for the fast calculation of the interacting quantum atoms energy decomposition (IQA) from the information contained in the first order reduced density matrix only. The proposed methodology utilizes an approximate exchange-correlation density from Density Matrix Functional Theory without the need to evaluate the correlation-exchange contribution directly. Instead, weight factors are estimated to decompose the exact V_xc into atomic and pairwise contributions. In this way, the sum of the IQA contributions recovers the energy obtained from the electronic structure calculation. This method can, hence, be applied to obtain atomic contributions in excited states on the same footing as in their ground states using any method that delivers the reduced first-order density matrix. In this way, one can locate chromophores from first principles quantum chemical calculations. Test calculations on the ground and excited states of a set of small molecules indicate that the scaled atomic contributions reproduce vertical electronic transition energies calculated exactly. This approach may be useful to extend the applicability of the IQA approach in the study of large photochemical systems especially when the calculations of the second order reduced density matrices is prohibitive or not possible.

A new look at two polymorphic crystal structures of dibenzoylmethane: relationship between the crystal packing and the hydrogen atom position revealed by quantum chemistry and quantum crystallography methods

Chalcones, including dibenzoylmethane, are an important subgroup of natural polyphenolic compounds that exhibit a wide spectrum of pharmacological and industrial applications. Dibenzoylmethane was isolated from Hottonia palustris L. (Primulaceae). The compound was crystallized in two polymorphic forms: in monoclinic space group P2₁/c and orthorhombic space group Pbca. Crystal structures of the polymorphs were solved and refined against diffraction data measured at 100 and 293 K. In both crystal structures, the chalcone occurs in its keto-enol tautomeric form with the hydroxyl H atom mutually bound by two oxygen atoms rather than covalently attached to a particular oxygen atom. To explain this phenomenon in more detail, density functional theory and quantum theory of atoms in molecules based quantum chemistry calculations were applied. Additionally, high-resolution experimental data of very high quality measured for the monoclinic and orthorhombic crystals at 100 K allowed the engagement of the quantum crystallography method, based on Hirshfeld atom refinement, to determine the position of each individual H atom. It is suggested that the presence of the particular tautomeric form of dibenzoylmethane with a centred H atom position results from the π-stacking interaction between the phenyl ring and the malondialdehyde quasi-ring causes delocalization of the electron density in the latter.

Bonding and metastability for Group 12 dications

Electronic structure and bonding properties of the Group 12 dications M₂ ²⁺ (M = Zn, Cd, Hg) are investigated and electron density-derived quantities are used to characterize the metastability of these species. Of particular interest are the complementary descriptions afforded by the Laplacian of the electron density ∇² ρ(r) and the one-electron Bohm quantum potential (Q =  ∇ 2 ρ r / 2 ρ r ) along the bond path. Further, properties derived from the pair density including the localization-delocalization matrices (LDMs) and the interacting quantum atoms (IQA) energies are analyzed within the framework of the quantum theory of atoms in molecules (QTAIM). From the crossing points of the singlet (ground) and triplet (excited) potential energy curves, the barriers for dissociation (BFD) are estimated to be 25.2 kcal/mol (1.09 eV) for Zn₂ ²⁺ , 22.8 kcal/mol (0.99 eV) for Cd₂ ²⁺ , and 26.4 kcal/mol (1.14 eV) for Hg₂ ²⁺ . For comparison and benchmarking purposes, the case of N₂ ²⁺ is considered as a texbook example of metastability. At the equilibrium geometries, LDMs, which are used here as an electronic fingerprinting tool, discriminate and group together Group 12 M₂ ²⁺ from its isoelectronic Group 11 M₂ . While "classical" bonding indices are inconclusive in establishing regions of metastability in the bonding, it is shown that the one-electron Bohm quantum potential is promising in this regard.

The X-ray constrained wavefunction of the [Mn(CO)4{(C6H5)2P-S-C(Br2)-P(C6H5)2}]Br complex: a theoretical and experimental study of dihalogen bonds and other noncovalent interactions

The synthesis and X-ray structure determination of the [Mn(CO)₄{(C₆H₅)₂P-S-C(Br₂)-P(C₆H₅)₂}]Br complex (1) are described. The C-Br...Br dihalogen bond present in 1 has been characterized by means of topological studies of the electron density. Both the quantum theory of atoms in molecules and the electron localization function approaches have been applied to several theoretically calculated wavefunctions as well as to an X-ray constrained wavefunction. In addition, a number of theoretical techniques, such as the source function, the reduced density gradient method and the interacting quantum atoms approach, among others, have been used to analyse the dihalogen bond as well as several intramolecular interactions of the type C-H...Br-C which have also been detected in 1. The results show clearly that while bonding in the latter interactions are dominated by electrostatic components, the former has a high degree of covalency.

The Na-O bond in sodium fenamate

The one-dimensional polymeric structure of sodium diaquafenamate-water (1/1) was studied by X-ray diffraction. The sodium cation is coordinated to one oxygen atom of the carboxylate group and to four water oxygen atoms. To characterize the Na-O bonds, the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) approaches have been used. Both methods confirmed that the Na-O bonds are very weak, comparable with the weak N-H...O intramolecular hydrogen bond. The polymeric structure is stabilized by the interaction of the sodium cation with the surrounding water molecules.

Similarities and differences in the crystal packing of halogen-substituted indole derivatives

The role of different intermolecular interactions in the crystal structures of halogen-substituted indoles which are fused with six-membered or seven-membered cyclic rings is investigated here. Several crystal structures show isostructural characteristics due to the presence of similar supramolecular motifs. In the absence of any strong hydrogen bonds, the molecular packing of reported structures is primarily stabilized by the presence of non-classical N-H...π and C-H...π interactions in addition to C-H...X (X = F/Cl/Br) interactions. The nature and energetics of primary and secondary dimeric motifs are partitioned into the electrostatics, polarization, dispersion and exchange-repulsion components using the PIXEL method. Short and directional N-H...π interactions are further explored by a topological analysis of the electron density based on quantum theory of atoms in molecules.

Quantitative characterization of new supramolecular synthons involving fluorine atoms in the crystal structures of di- and tetrafluorinated benzamides

Strong hydrogen bonds play a significant role in crystal packing. In particular, the involvement of interactions involving fluorine in controlling the crystal packing requires appropriate attention, especially in the presence of other strong hydrogen bonds. In the present study, a detailed quantitative assessment has been performed of the nature, energetics and topological properties derived from the electron density in model compounds based on fluorinated benzamides (a total of 46 fluorine-substituted benzamides containing multiple fluorine atoms) in the solid state. The primary motivation in the design of such molecules is to enhance the acidity of the interacting H atoms in the presence of an increasing number of F atoms on the molecular scaffold, resulting in increased propensity towards the formation of intermolecular interactions involving organic fluorine. This exercise has resulted in the identification of new and frequently occurring supramolecular synthons involving F atoms in the packing of molecules in the solid state. The energetics associated with short and directional intermolecular Csp²-H...F-Csp² interactions with significantly high electrostatic contributions is noteworthy, and the topological analysis reveals the bonding character of these ubiquitous interactions in crystal packing in addition to the presence of Csp²-F...F-Csp² contacts.

On atom-atom 'short contact' bonding interactions in crystals

Professor Dunitz questions the usefulness of ascribing crystalline structural stability to individual atom-atom intermolecular interactions viewed as bonding (hence stabilizing) whenever linked by a bond path. An alternative view is expressed in the present essay that articulates the validity and usefulness of the bond path concept in a crystallographic and crystal engineering context.