Analysis Of Electron Density Research Articles

Electronic-structure and thermodynamic properties of ZnS1−xSex ternary alloys from the first-principles calculations

In this paper, the composition-dependent electronic-structure and thermodynamic properties for hexagonal wurtzite (WZ) and face-centered cubic zincblende (ZB) ZnS1−xSex ternary alloys have been studied utilizing first-principles pseudopotential plane-wave self-consistent field calculations. Special quasi-random structure method was applied to generate disordered structures of 16-atom supercell for x = 0.25, 0.5 and 0.75. We obtained a direct band-gap at Г point and a shrinking tendency of the band-gap with increasing Se content in both phases. The reasons for the band-gap shrinkage have been discussed based on the analysis of electronic density of states and band offsets. Δ-sol method was applied to correct the band gaps calculated by density functional theory. The Δ-sol corrected band-gaps are in good agreement with experimental band-gaps. The thermodynamic properties were calculated by the cluster expansion method. It turns out that, the stretching forces decrease considerably and linearly with the bond length, while the bending forces are dependent insignificantly on the bond length. The phase diagrams reveal that the lattice vibration effects can influence the critical temperature Tc and the solubility of S- and Se-rich ZnS1−xSex alloys, but the influence is not obvious for ZB-ZnS1−xSex. Based on the calculated phase diagrams without lattice vibration effects, the Tc for WZ- and ZB-ZnS1−xSex are 559 K and 421 K, respectively. When the vibration enthalpy is included, the Tc values of both phases are lowered to 543 K and 418 K, respectively. And if the vibration entropy is also taken into account, the Tc values of both phases are further lowered to 529 K and 415 K, respectively. With inclusion of lattice vibration effects, the ZnS1−xSex alloy starts a phase transition from ZB to WZ at a temperature of 1600 K, and the solid solubility of Se increases with the temperature rise.

Exploration of selected electronic characteristics of half-sandwich organoruthenium(II) β-diketonate complexes.

Based on experimental work, 12 half-sandwich organoruthenium(II) complexes with p-cymene and various substituted β-diketonates (acac) modified by several functional groups were explored. These complexes were optimized at the B3PW91/6-31 + G(d)/PCM/UFF computational level with the Ru atom described by Stuttgart pseudopotentials. The electron density analysis was performed using the B3LYP/ 6-311++G(2df,2pd)/DPCM/scaled-UAKS model. Electrostatic and averaged local ionization potential were explored and extremes on 0.001 e/a.u.3 isodensity surfaces discussed. Natural population analysis partial charges and electron densities in bond critical point of the key Ru(II) coordination bonds were determined. There was a clear correlation between the results obtained and experimentally known anticancer descriptors. Graphical abstract Top Average local ionization potential (ALIP) of half-sandwich organoruthenium(II) β-diketonate complex, bottom IC 50 of b-series for ovarian cancer and Ru-P distances (in Å).

Quantum‐mechanical condensed matter simulations with CRYSTAL

The latest release of theCrystalprogram for solid‐state quantum‐mechanical ab initio simulations is presented. The program adopts atom‐centered Gaussian‐type functions as a basis set, which makes it possible to perform all‐electron as well as pseudopotential calculations. Systems of any periodicity can be treated at the same level of accuracy (from 0D molecules, clusters and nanocrystals, to 1D polymers, helices, nanorods, and nanotubes, to 2D monolayers and slab models for surfaces, to actual 3D bulk crystals), without any artificial repetition along nonperiodic directions for 0–2D systems. Density functional theory calculations can be performed with a variety of functionals belonging to several classes: local‐density (LDA), generalized‐gradient (GGA), meta‐GGA, global hybrid, range‐separated hybrid, and self‐consistent system‐specific hybrid. In particular, hybrid functionals can be used at a modest computational cost, comparable to that of pure LDA and GGA formulations, because of the efficient implementation of exact nonlocal Fock exchange. Both translational and point‐symmetry features are fully exploited at all steps of the calculation, thus drastically reducing the corresponding computational cost. The various properties computed encompass electronic structure (including magnetic spin‐polarized open‐shell systems, electron density analysis), geometry (including full or constrained optimization, transition‐state search), vibrational properties (frequencies, infrared and Raman intensities, phonon density of states), thermal properties (quasi‐harmonic approximation), linear and nonlinear optical properties (static and dynamic [hyper]polarizabilities), strain properties (elasticity, piezoelectricity, photoelasticity), electron transport properties (Boltzmann, transport across nanojunctions), as well as X‐ray and inelastic neutron spectra. The program is distributed in serial, parallel, and massively parallel versions. In this paper, the original developments that have been devised and implemented in the last 4 years (since the distribution of the previous public version,Crystal14, occurred in December 2013) are described.This article is categorized under:Software > Quantum ChemistryStructure and Mechanism > Computational Materials ScienceElectronic Structure Theory > Density Functional Theory

Fixation of nitrous oxide (N2O) by 1, 4, 2, 5‐diazadiborinine: A DFT study

AbstractThe role of nitrous oxide (N2O) in stratospheric ozone depletion and as a greenhouse gas has inspired the scientific community across the globe in understanding the capture, activation, and decomposition of it. Very recently people have started fixing N2O using frustrated Lewis pairs. In this study, we have tried to analyze the fixation of N2O by 1,4,2,5‐diazadiborinine by applying various computational tools and techniques associated with density functional theory. 1,4,2,5‐Diazadiborinine is taken as the fixing agent because of the ambiphilic nature of the two boron centers within it as reported by Wang et al. There are three possible ways of binding of N2O within it as observed in this study. A free energy surface is also generated for the three possible paths representing their thermochemical as well as kinetic stability. The fixation of N2O may become possible using this species as demonstrated by the current results. The nature of bonding between them is also explored through NBO, EDA, and electron density analyses.

The Mysticism of Pericyclic Reactions: A Contemporary Rationalisation of Organic Reactivity Based on Electron Density Analysis

The molecular mechanism of the five most representative “pericyclic reactions” has been studied by applying molecular electron density theory (MEDT). The different phases into which the reaction paths are topologically divided can be regrouped into four well‐characterised steps: 1) the rupture of the C–C double bonds, 2) the formation of pseudoradical centres at the interacting carbons, 3) the formation of new C–C single bonds and 4) the formation of new C–C double bonds in the final products. Consequently, the bonding changes in these reactions are neither concerted nor cyclic. As the transition‐state structures are located at the end of the large first group of phases or in the narrow second group, the high activation enthalpies found in these reactions are mainly associated with the rupture of C–C double bonds. The present MEDT study makes it possible to rule out the definition of “pericyclic reactions”, made by Woodward and Hoffmann in 1969, in which “all first order changes in bonding relationships take place in concert on a closed curve”.

Electron-density critical points analysis and catastrophe theory to forecast structure instability in periodic solids.

The critical points analysis of electron density, i.e. ρ(x), from ab initio calculations is used in combination with the catastrophe theory to show a correlation between ρ(x) topology and the appearance of instability that may lead to transformations of crystal structures, as a function of pressure/temperature. In particular, this study focuses on the evolution of coalescing non-degenerate critical points, i.e. such that ∇ρ(xc) = 0 and λ1, λ2, λ3 ≠ 0 [λ being the eigenvalues of the Hessian of ρ(x) at xc], towards degenerate critical points, i.e. ∇ρ(xc) = 0 and at least one λ equal to zero. The catastrophe theory formalism provides a mathematical tool to model ρ(x) in the neighbourhood of xc and allows one to rationalize the occurrence of instability in terms of electron-density topology and Gibbs energy. The phase/state transitions that TiO2 (rutile structure), MgO (periclase structure) and Al2O3 (corundum structure) undergo because of pressure and/or temperature are here discussed. An agreement of 3-5% is observed between the theoretical model and experimental pressure/temperature of transformation.

Study of Confinement and Catalysis Effects of the Reaction of Methylation of Benzene by Methanol in H-Beta and H-ZSM-5 Zeolites by Topological Analysis of Electron Density

In this work we studied the host–guest interactions between confined molecules and zeolites and their relationship with the energies involved in the reaction of methylation of benzene by methanol in H-ZSM-5 and H-Beta zeolites employing density functional theory (DFT) methods and the quantum theory of atoms in molecules. Results show that the strength of the interactions related to adsorption and coadsorption processes is higher in the catalyst with the larger cavity; however, the confinement effects are higher in the smaller zeolite, explaining, from an electronic viewpoint, the reason why the stabilization energy is higher in H-ZSM-5 than in H-Beta. The confinement effects of the catalyst on the confined species for methanol adsorption, benzene coadsorption, and the formed intermediates dominate this stabilization. For the transition state (TS), the stability of the TS is achieved due to the stabilizing effect of the surrounding zeolite framework on the formed carbocationic species (CH3+) which is highe...

Experimental and theoretical investigations on magneto-structural correlation in trinuclear copper(II) hydroxido propellers

The trinuclear copper(II) compounds [Cu3(μ3-OH)(GL1)3](ClO4)2 (1–4) and [Cu3(μ3-OH)(GL2)3](ClO4)2 (5–8) with tridentate NNO Schiff base ligands GL1− and GL2− derived from 5-G-substituted salicylaldehydes (G = NO2, Br, H, Me) and the diamines 1,2-ethanediamine and 1,3-propanediamine, respectively, were investigated aiming at shedding light on possible magneto-structural correlation in this class of complexes. All derivatives contain [Cu3(μ3-OH)(L)3]2+ cations with partial cubane Cu3O4 cores, and the metal ions are linked together in a pyramidal fashion by a triple-bridging hydroxido group, giving rise to propellers with three [Cu(L)]+ blades. In these spin-frustrated magnetic systems, the three copper(II) ions within a cluster communicate anti-ferromagnetically (−2JŜi·Ŝj convention) through the bridging OH group with coupling constants J ranging from −4.5(1) for 4 (G = Me) to −10.1(1) cm−1 for 1 (G = NO2), and stabilization of the doublet S = 1/2 ground state. The structural features of the complexes reveal very minimal deviations upon variation of G or the diamine flexibility along the whole series of compounds, preserving almost constant magnetic cores. Similar conclusions are also drawn by DFT gas-phase geometry optimizations of the [Cu3(μ3-OH)(L)3]2+ cations. Therefore, confident of excluding structural influences on the magnetic super-exchange path, the modulating factor of J in our derivatives can be sought after the different electronic demand of G. Atomic NBO charges support this point, revealing small but systematic variations in the electron density flow along the blades and the positive charge on copper(II) ions with the electronic nature of G, with the most remarkable effect given by the nitro group. Topological analysis of electron density according to the Quantum Theory of Atoms In Molecules further support the distinguishing role of this group with respect to the other substituents taken into consideration, besides providing indirect information about the super-exchange path.

Stable NCNgNSi (Ng=Kr, Xe, Rn) Compounds with Covalently Bound C-Ng-N Unit: Possible Isomerization of NCNSi through the Release of the Noble Gas Atom.

Although the noble gas (Ng) compounds with either Ng-C or Ng-N bonds have been reported in the literature, compounds containing both bonds are not known. The first set of systems having a C-Ng-N bonding unit is predicted herein through the analysis of stability and bonding in the NCNgNSi (Ng=Kr-Rn) family. While the Xe and Rn inserted analogues are thermochemically stable with respect to all dissociation channels, but for the one producing CNSiN and free Ng, NCKrNSi has another additional three-body dissociation channel, NCKrNSi→CN+Kr+NSi, which is exergonic by -9.8 kcal mol-1 at 298 K. This latter dissociation can be hindered by lowering the temperature. Moreover, the NCNgNSi→Ng+CNSiN dissociation is also kinetically prohibited by a quite high free energy barrier ranging from 25.2 to 39.3 kcal mol-1 , with a gradual increase in going from Kr to Rn. Therefore, these compounds are appropriate candidates for experimental realization. A detailed bonding analysis by employing natural bond orbital, electron density, energy decomposition, and adaptive natural density partitioning analyses indicates that both Ng-N and C-Ng bonds in the title compounds are covalent in nature. In fact, the latter analysis indicates the presence of delocalized 3c-3e σ-bond within the C-Ng-N moiety and a totally delocalized 5c-2e σ-bond in these compounds. This is an unprecedented bonding characteristic in the sense that the bonding pattern in Ng inserted compounds is generally represented as the presence of covalent bond in one side of Ng, and the ionic interaction in the other side. Further, the dissociation of Ng from NCNgNSi facilitates the formation of a higher energy isomer of NCNSi, CNSiN, which cannot be formed from bare NCNSi as such, because of the very high free energy barrier associated with the isomeric transformation. Therefore, in the presence of Ng atoms it might be possible to detect the high energy isomer.

First-principles study on ZnV2O6 and Zn2V2O7: Two new photoanode candidates for photoelectrochemical water oxidation

We used first principles calculations based on density functional theory with generalized gradient approximation to investigate and compare the structural, electronic and optical properties of two photoanode materials, ZnV2O6 and Zn2V2O7, for use in photocatalytic water splitting. After geometry optimization, the calculated structural parameters evince a satisfactory agreement with the reported experimental results indicating that the used method and conditions are suitable. The electronic structures demonstrate that both photocatalysts possess favorable band gaps (2.31 and 2.52eV) and appropriate band edge positions for oxygen evolution reaction under solar radiation. The relatively light effective masses at the valence band maximum and conduction band minimum are expected to result in enhanced photocatalytic activity due to lower recombination probability of the photogenerated electrons and holes. The analysis of electronic density of states reveal that the higher coordination number of vanadium in ZnV2O6 with respect to Zn2V2O7 causes more delocalisation of bands owning to lower V-V and O-O distances in conduction and valence bands, respectively. Moreover, the origins of features that appear in solar energy harvesting characteristics (dielectric function and optical absorption coefficient) have been discussed for solar water splitting in detail.

Interactions of Biodegradable Ionic Liquids with a Model Naphthenic Acid

Density functional theory models are used to examine five biodegradable ionic liquids (ILs) each one consisting of a substitutional group (-OH, -NH2, -COOH, -COOCH3, and -OCH3) incorporated into the cation of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The results reveal that hydrogen atoms in -NH2, -COOH, and -COOCH3 form intramolecular hydrogen bonds with fluorine atoms in [BF4]−, whereas hydrogen atoms in -OH and -OCH3 do not form hydrogen bonds with [BF4]−. Further analysis of electron density at bond critical points and noncovalent interactions suggest that [BMIM][BF4] with -COOH has stronger intramolecular hydrogen bonds than other ILs. The extraction mechanism for a model naphthenic acid is hydrogen bonding, with F···H being the strongest hydrogen bond and O···H ranking second. More intermolecular hydrogen bonds occur when model naphthenic acid is adsorbed by [BMIM][BF4] with -COOH and -COOCH3. The interaction energy between model naphthenic acid and ILs with -COOH and -COOCH3 is higher than that with -OH, -NH2, and -OCH3.

Correlating Itinerant Magnetism in RCo2Pn2 Pnictides (R = La, Ce, Pr, Nd, Eu, Ca; Pn = P, As) to Their Crystal and Electronic Structures.

Rare-earth cobalt pnictides, RCo2Pn2 (Pn = P, As), belong to the ThCr2Si2 structure type, which is ubiquitous among intermetallic compounds. The structural and magnetic properties of simple ternary RCo2P2 phosphides, which combine partially delocalized (itinerant) 3d magnetic moments of cobalt and localized 4f magnetic moments of lanthanides, were investigated extensively in 1980-1990s, predominantly by the Jeitschko group. Those studies established that LaCo2P2 shows ferromagnetic (FM) ordering of Co moments, while the other members of the series, with R = Ce, Pr, Nd, or Sm, exhibit antiferromagnetic (AFM) ordering in both R and Co magnetic sublattices. This observation also correlated with the larger separation between the [Co2P2] layers in the crystal structure of LaCo2P2 as compared to the decreased interlayer distances in the other structures of the RCo2P2 series. Our work over the past decade has focused on unraveling the rich magnetic behavior that can be observed in these systems when internal chemical and external physical factors are used to perturb their crystal and electronic structures. We began our foray into these materials by demonstrating that the preservation of FM ordering of Co 3d moments in the mixed La1-xR'xCo2P2 phases also forces the R 4f moments to adopt FM arrangement, although antiparallel to the Co moments. As an example, in La0.75Pr0.25Co2P2 such mutual influence of the 3d and 4f moments leads to a cascade of magnetic phase transitions. All these changes were traced back to the modification of the crystal structure and, consequently, the electronic band structure of these materials. The substitution of smaller R3+ ions for the La3+ ions leads to structural compression along the tetragonal c axis, perpendicular to the [Co2P2] layers, and an increase in the Co-Co distances within the layer. This structural effect is translated into more localized Co magnetic moments, stronger magnetic exchange between Co sites, and higher ordering temperatures. A more dramatic change in properties is observed in EuCo2Pn2, which exhibit AFM ordering of the localized 4f moments of Eu2+ ions and only paramagnetic behavior in the Co sublattice. Under applied pressure, these compounds undergo structural collapse, which causes a dramatic decrease in the separation between the [Co2Pn2] layers, an increase in the oxidation state of Eu, and magnetic ordering of Co moments. We further demonstrated that similar effects can be stimulated by chemical compression, which is achieved by doping Eu into the more constrained lattice sites, for example, in PrCo2P2 or CaCo2As2. In both cases, the induced mixed valence of Eu results in the change from AFM to FM ordering in the Co sublattice. A series of solid solutions Ca1-xEuxCo2As2 shows a fascinating evolution of magnetic behavior from AFM ordering of Co 3d moments to simultaneous FM ordering of Co 3d and Eu 4f moments to AFM ordering of Eu 4f moments as one proceeds from CaCo2As2 to EuCo2As2. Importantly, all these changes in magnetic properties are well justified by the analysis of electronic density of states and crystal orbital Hamilton population, providing the understanding of how chemical factors can be leveraged, in general, to modify properties of itinerant magnets.

Method for hydrophytic plant sample preparation for light and electron microscopy (studies on Phragmites australis Cav.)

Nowadays there are no well-established, standard methods in electron microscopy despite its 50-year history. An excessive variety of research objects prompt researchers to modify and improve methodological approaches to sample preparation. One of the difficult objects to study by electron microscopy is hydrophytic plants, for example, Phragmites australis Cav. Traditional approaches to fixation and sample preparation do not give satisfactory results due to the peculiarities in structure and physiology of hydrophytic plants. The purpose of present research is modification description of the widespread method developed for double fixation of hydrophytic plant tissue for transmission electron microscopy. Suggested approach takes into account the features of hydrophyte plants.•The developed method allows improving the quality of plant samples by additional fixatives imbibition and removing of air bubbles from aerenchyma tissue using a vacuum.•The new step of sample preparation consisting in the layer-by-layer sample mixing in a special inclined mixer is applied for the embedding media penetrate sufficiently into the sample tissue.•The process of samples inclusion in polymeric resins is carried out in the flat-bottom capsules. Compare to standard conical capsules, flat-bottom capsules allow strictly defined orientation sample pieces, that is permit to produce a semi-thin and ultra-thin slices of perpendicular to the longitudinal structures of the plant. This is especially important to conduct an adequate comparative analysis of dimensions, shape, and electron density of fragments and parts of the studying samples.

Topological Evidence of Previously Overlooked Ni+1-H···Ni H-Bonds and Their Contribution to Protein Structure and Stability.

Hydrogen-bonds (H-bonds) between backbone N-H donors and CO acceptors are central to our understanding of protein structure and stability. However, while interactions between backbone N atoms and the N-H of the following residue are also common, they have been ignored as potential H-bonds due to their bent geometry and the assumption that the amide N is a poor H-bond acceptor. Recently, we reported indirect experimental evidence that these interactions constitute functional H-bonds. We now report a combined atoms in molecules and noncovalent interaction theoretical analysis of electron density that unambiguously supports the characterization of these interactions as H-bonds. The calculations further suggest that the Ni+1-H···Ni H-bonds are largely electrostatic in nature and, importantly, that they make a significant contribution to stability. Thus, given their apparently frequent occurrence, Ni+1-H···Ni H-bonds likely make critical, but previously unrecognized, contributions to protein structure and function.

DFT Studies and Topological Analyses of Electron Density on Acetophenone and Propiophenone Thiosemicarbazone Derivatives as Covalent Inhibitors of Falcipain-2, a Major Plasmodium Falciparum Cysteine Protease

Thiosemicarbazones (TSCs) possess significant antimalarial properties believed to be linked to the inhibition of major cysteine proteases, such as falcipain-2, in Plasmodium falciparum. However, the binding modes of TSCs to the active site of these enzymes are not clear. As a result of this, the nature of the bonding interactions between the active site of falcipain-2 and different derivatives of acetophenone thiosemicarbazone (APTSC) and propiophenone thiosemicarbazone (PPTSC) acting as inhibitors, have been studied herein via topological analyses of electron density. Derivatives of APTSC and PPTSC are well known to possess inhibitory effects against Plasmodium falciparum. Equilibrium geometries of the inhibitor–active site complexes in the aqueous phase have been obtained via dispersion-corrected density functional theory (DFT-D3) calculations. In-depth analyses of the covalent and noncovalent interactions in these complexes have been performed using the quantum theory of atoms in molecules (QTAIM) and the noncovalent interaction (NCI) index. Results have revealed a covalent interaction between the thiocarbonyl carbon of the TSC moiety and the thiolate sulfur of the active site cysteine residue of falcipain-2. Moreover, hydrogen bonding, dispersive-like van der Waals and π-stacking interactions have been elucidated between the TSC moiety of each inhibitor and the histidine residue of the enzyme’s active site. The synergy of these interactions can enable the TSCs studied herein to specifically but transiently bind to the active site residues of falcipain-2. Based on our results, APTSC and PPTSC derivatives are potential reversible covalent inhibitors of falcipain-2, and are therefore promising precursors for the manufacture of antimalarial drugs.

HNgBeF3 (Ng = Ar‐Rn): Superhalogen‐supported noble gas insertion compounds

AbstractAb initio and density functional theory‐based calculations are performed to study the structure, stability, and nature of bonding of superhalogen‐supported noble gas (Ng) compounds of the type HNgY where (Ng = Ar‐Rn; Y = BeF3). Here, BeF3 acts as the superhalogen. Calculations show that the HNgBeF3 spontaneously dissociates into product following the dissociation channels: HNgBeF3 → HBeF3 + Ng and HNgBeF3 → Ng + HF + BeF2. The transition states are optimized and the energy barriers are computed to show the metastable behavior of HNgBeF3. HNgBeF3 molecules are kinetically stable with respect to the first dissociation process having energy barriers of 1.0, 5.0, 10.6, and 13.9 kcal/mol for Ar, Kr, Xe, and Rn analogues, respectively, at CCSD(T)/Aug‐cc‐pVTZ level. These calculations suggest that the HXeBeF3 and HRnBeF3 can be shown to be stable up to ∼100 K temperature with a half‐life of ∼102 seconds. The nature of HNg and two different types of NgF bonds in HNgBeF3 molecules is explored through the natural bond orbital and electron density analyses. The large Wiberg bond index (WBI) values for the HNg bond indicate the formation of almost a single bond in between H‐atoms and Ng‐atoms, whereas small WBI values for the two NgF bonds indicate a noncovalent interaction in between them. The electron density analysis further supports the covalency of the HNg bond and noncovalent interaction in the two NgF bonds in HNgBeF3.

Insight into the π‐hole···π‐electrons tetrel bonds between F2ZO (Z = C, Si, Ge) and unsaturated hydrocarbons

AbstractThe intermolecular π‐hole···π‐electrons interactions between F2ZO (Z = C, Si, Ge) molecules and unsaturated hydrocarbons including acetylene, ethylene, 1,3‐butadiene and benzene were constructed to reveal the differences of tetrel bonds forming by carbon and heavier tetrel atoms. The ab initio computation in association with topological analysis of electron density, natural bond orbital, and energy decomposition analysis demonstrate that the strength of Si···π and Ge···π tetrel bonds is much stronger than that of C···π tetrel bonds. The Si···π and Ge···π tetrel bonds exhibit covalent or partially covalent interaction nature, while the weak C···π tetrel bonds display the hallmarks of noncovalent interaction, the electrostatic interaction is the primary influencing factor. The Si···π and Ge···π interactions are determined by both the σ‐ and π‐electron densities, while the C···π interactions are dominated mainly by the π‐electron densities. The π‐hole···π‐electrons tetrel bonds are dominated by electrostatic interaction, and polarization has a comparable contribution in the Si···π and Ge···π tetrel bonds.

Gold(I)-assisted catalysis – a comprehensive view on the [3,3]-sigmatropic rearrangement of allyl acetate

ABSTRACTThe unified reaction valley approach (URVA) combined with the local mode, ring puckering and electron density analysis is applied to elucidate the mechanistic differences of the non-catalysed and the Au[I]-N-heterocyclic carbene (NHC)-catalysed [3,3]-sigmatropic rearrangement of allyl acetate. Using a dual-level approach (DFT and DLPNO-CCSD(T)), the influence of solvation, counter-ions, bulky and electron withdrawing/donating substituents as well as the exchange of the Au[I]-NHC with a Au[I]-phosphine catalyst is investigated. The catalyst breaks up the rearrangement into two steps by switching between Au[I]–π and Au[I]–σ complexation, thus avoiding the energy-consuming CO cleavage in the first step. Based on local stretching force constants ka(C=C), we derive for the first time a quantitative measure of the π-acidity of the Au[I] catalyst; in all catalysed reactions, the bond order n(C=C) drops from 2 to 1.65. The ring puckering analysis clarifies that all reactions start and end via a six-membered ring with a boat form. All Au[I]–σ-complex intermediates show a considerable admixture of the chair form. The non-catalysed [3,3]-sigmatropic rearrangement goes through a maximum of charge separation between the allyl and acetate units at the transition state, while all catalysed reactions proceed via a minimum of charge separation reached in the region of the Au[I]–σ-complex.

Structural, mechanical, electronic and thermal properties of the newly predicted NB2 from ab initio calculations

The density functional theory (DFT) is used to investigate the structural, elastic, electronic and thermal properties of the newly predicted superhard material tetragonal NB2 (t-NB2). The estimated values of the elastic constants of this material satisfy its mechanical stability criteria. The calculated large values of bulk modulus (B), shear modulus (G), Young's modulus (E), Vickers hardness (Hv), and small Pugh's modulus and Poisson's ratio identify this compound as a potential candidate for superhard materials with high brittleness. The bulk modulus, Debye temperature, specific heats, and volume thermal expansion coefficient are also obtained as a function of temperature and pressure for the first time through the quasiharmonic Debye model with phononic effects. Large values of Debye and melting temperatures suggest the strong microhardness of t-NB2. The analysis of electronic density of states and Mulliken populations emphasizes the strong covalent bonding of B-B and B-N atoms which can be attributed to the mechanism of superhardness of t-NB2.

Successive Dimensional Transition in (TMTTF)_{2}PF_{6} Revealed by Synchrotron X-ray Diffraction.

A quasi-one-dimensional organic charge-transfer salt (TMTTF)_{2}PF_{6} undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer δ_{CO}=0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.