Variation Of Coordination Number Research Articles

Local and global expansivity in water.

The supra-molecular structure of a liquid is strongly connected to its dynamics, which in turn control macroscopic properties such as viscosity. Consequently, detailed knowledge about how this structure changes with temperature is essential to understand the thermal evolution of the dynamics ranging from the liquid to the glass. Here, we combine infrared spectroscopy (IR) measurements of the hydrogen (H) bond stretching vibration of water with molecular dynamics simulations and employ a quantitative analysis to extract the inter-molecular H-bond length in a wide temperature range of the liquid. The extracted expansivity of this H-bond differs strongly from that of the average nearest neighbor distance of oxygen atoms obtained through a common conversion of mass density. However, both properties can be connected through a simple model based on a random loose packing of spheres with a variable coordination number, which demonstrates the relevance of supra-molecular arrangement. Furthermore, the exclusion of the expansivity of the inter-molecular H-bonds reveals that the most compact molecular arrangement is formed in the range of ∼316-331K (i.e., above the density maximum) close to the temperature of several pressure-related anomalies, which indicates a characteristic point in the supra-molecular arrangement. These results confirm our earlier approach to deduce inter-molecular H-bond lengths via IR in polyalcohols [Gabriel et al. J. Chem. Phys. 154, 024503 (2021)] quantitatively and open a new alley to investigate the role of inter-molecular expansion as a precursor of molecular fluctuations on a bond-specific level.

Analysis of the Effect of Coordination Number on Permeability of the Three-Dimensional (3D) Rock Model Using the Lattice Boltzmann Method (LBM)

Understanding the correlation between coordination number and permeability is crucial for predicting fluid flow behavior in hydrocarbon reservoirs. In this study, we investigated the influence of coordination number on permeability in three-dimensional (3D) rock models. The research methodology involved the creation of synthetic 3D rock models, incorporating pores networks with varying coordination numbers. Utilizing the Lattice Boltzmann Methods (LBM), a computational fluid dynamics approach, we simulated fluid flow through these synthetic rock models and quantified their permeability. Our findings demonstrated a strong dependency of permeability on the coordination number of synthetic rock models. The application of the Lattice Boltzmann Method (LBM) proved to be an effective tool for understanding fluid flow behavior in porous rock formations and can serve as a basis for further optimization of reservoir management strategies to maximize hydrocarbon exploitation.

NHC aluminum chemistry on the rise.

This perspective highlights recent developments of the use of N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (cAACs) in alane and aluminum organyl chemistry. Especially in the last few years this flourishing research field led to some remarkable discoveries including various substitution patterns at the central aluminum atom, different oxidation states, neutral and charged compounds with varying coordination numbers and unique reactivities. Thereby NHCs play a vital role in the stabilization of these otherwise highly reactive compounds, which would not be realizable without the use of this intriguing class of ligands. Nevertheless, main group hydrides and especially NHC ligated alanes also tend to undergo NHC decomposition reactions, which are part of ongoing research and provide important information for NHC research in general.

Rare earth oxide based electrocatalysts: synthesis, properties and applications.

As an important strategic resource, rare earths (REs) constitute 17 elements in the periodic table, namely 15 lanthanides (Ln) (La-Lu, atomic numbers from 57 to 71), scandium (Sc, atomic number 21) and yttrium (Y, atomic number 39). In the field of catalysis, the localization and incomplete filling of 4f electrons endow REs with unique physical and chemical properties, including rich electronic energy level structures, variable coordination numbers, etc., making them have great potential in electrocatalysis. Among various RE catalytic materials, rare earth oxide (REO)-based electrocatalysts exhibit excellent performances in electrocatalytic reactions due to their simple preparation process and strong structural variability. At the same time, the electronic orbital structure of REs exhibits excellent electron transfer ability, which can reduce the band gap and energy barrier values of rate-determining steps, further accelerating the electron transfer in the electrocatalytic reaction process; however, there is a lack of systematic review of recent advances in REO-based electrocatalysis. This review systematically summarizes the synthesis, properties and applications of REO-based nanocatalysts and discusses their applications in electrocatalysis in detail. It includes the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), methanol oxidation reaction (MOR), nitrogen reduction reaction (NRR) and other electrocatalytic reactions and further discusses the catalytic mechanism of REs in the above reactions. This review provides a timely and comprehensive summary of the current progress in the application of RE-based nanomaterials in electrocatalytic reactions and provides reasonable prospects for future electrocatalytic applications of REO-based materials.

Design of platinum nanoflower catalyst exhibiting near-ideal local coordination in a complex shape

Here, we investigate the oxygen reduction reaction activity of near-monocrystalline platinum nanoflowers presenting a low microstrain to decorrelate the contribution of (i) distorted lattice and (ii) high coordination active sites found within the platinum nanoflower pores to the specific activity enhancement. The results are discussed within the framework of previous investigations onto various catalytic nanostructures. We evidenced that the microstrain of the platinum nanoflowers stands out of the trends previously established for defective platinum-based nanostructures, where the activity is driven by lattice distortion. This indicates that (i) ‘defective’ structures with low lattice distortion, but high variation in coordination number, can be synthesized and (ii) that their specific activity improvement is comparable to the surface distortion approach in liquid electrolyte.

Hexanuclear and octanuclear metal complexes of octadecaazamacrocycle based on diaminocyclohexane and pyridine units

An extended macrocyclic amine L derived from the [6+6] condensation of trans-1,2-diaminocyclohexane and 2,6-diformylpyridine contains 18 nitrogen atoms in its main cycle. All of these atoms can be engaged in the formation of polynuclear complexes with transition metal ions and the macrocycle L is able to bind six Cu(II), Ni(II) or Zn(II) ions within its interior. X-ray crystal structures of these complexes indicate variable coordination numbers of metal ions as well as various conformations adopted by the macrocyclic ligand. In the hexanuclear Cu(II) complex with coordinated nitrate anions Cu(II) ions are hexacoordinate and exhibit highly distorted octahedral geometry, while in analogous complexes with coordinated sulphate anions Cu(II) ions are pentacoordinate of distorted square pyramid geometry. Both complexes are relatively symmetric having (approximate) S6 axis and adopt a circular doughnut shape with a central void not occupied by metal ions. The NMR spectrum of the zinc(II) derivative also reflects the symmetrical coordination. In contrast, the macrocycle L is squeezed and less symmetric in the octanuclear chloride derivative where two additional Cu(II) ions are linked to a hexanuclear macrocyclic unit via chloride bridges. The macrocycle is also squeezed in the hexanuclear Ni(II) complex containing metal ions of approximate octahedral geometry. In all three Cu(II) complexes the macrocycle provides N3 donor set for each metal ion, while in the Ni(II) complex alternating N3 and N4 environments of the metal ions are observed.

Exploring the solvation of water molecules around radioactive elements in nuclear waste water treatment

Abstract Nuclear waste water contains many actinides which coordinate with water molecules to form complexes. The hydration of water molecules with varying coordination numbers and modes makes it interesting and intriguing in understanding the extraction process of these radioactive ions. In order to separate these complexes from the nuclear waste water, many organic ligands are being used. However, prior knowledge on the nature of electronic environment of these hydration patterns will help us to understand the extraction mechanism. Therefore, a series of complexes such as [Np(H2O)9]4+, [Cm(H2O)9]3+, [Am(H2O)9]3+, [Pu(H2O)9]4+, [Pu(H2O)9]3+, [U(H2O)9]3+, [NpO2(H2O)5]+, [UO2(H2O)5]2+ and [PuO2(H2O)5]2+ have been calculated by means of relativistic DFT. Bond length analysis and energy decomposition analysis are executed with the intention to comprehend the bonding situation of these complexes. To account for the stabilizing interactions amid the radioactive ion and the water molecules, a detailed QTAIM investigation is done. It is seen that the metals having higher oxidation state readily complex with water molecules. Energy decomposition analysis throws light on the significant orbital interactions in the [M(H2O)9] n complexes, whereas in the metal oxide complexes significant contribution is resulted from electrostatic interactions. In summary, this investigation brings out the nuances of coordination modes of solvation in nuclear waste water which will help us to explore and design novel extraction techniques in near future.

Toward a Comprehensive Understanding of Amorphous Photocatalysts: Fundamental Hypotheses and Applications in CO2 Photoreduction.

In principle, photocatalytic activity can be precisely controlled with crystalline catalysts. However, an amorphous photocatalyst could be a viable candidate for CO2 photoreduction to form value-added products. The amorphous phase is currently part of the crystalline material in several ongoing CO2 photoreduction studies. Additionally, no study indicates the amorphous material required for overall CO2 photoreduction. This perspective review article highlights fundamental assumptions that are necessary to gain insights and understand the effectiveness of amorphous photocatalysts for CO2 photoreduction. We start with basic ideas and theories about these materials, including light harvesting, variable coordination number, and the interaction of CO2 molecules with the amorphous catalytic surface. To understand the prospects of the amorphous photocatalyst, we explore machine learning with EXAFS. Furthermore, we discuss product selectivity and regeneration of photocatalysts in detail. Finally, we briefly review the work in progress on amorphous materials and compare it to that on crystalline ones.

An effective method of applying octahedral Pt-Ni/C to membrane electrode assembly and related in-situ X-ray absorption fine structures study

To further promote the commercialization of fuel cells, it is essential to develop high-performance catalysts and apply them to the membrane electrode assembly (MEA). Here, the octahedral Pt-Ni/C is successfully used in MEA by a double-layer structure to obtain a higher performance than commercial Pt/C. The design of double-layer catalyst layer structure effectively hinders the diffusion of non-noble metals and improves the durability of MEA. Meanwhile, the catalytic and failure mechanism in proton-exchange membrane fuel cells (PEMFCs) is also directly reflected by the in-situ X-ray absorption fine structure (XAFS) for the first time. Compared to commercial Pt/C, the addition of Ni can effectively protect Pt by significantly reducing the valence change of Pt. Besides, the variation of coordination number of Pt in the Pt-Ni/C is smaller than that in Pt/C when the potential rises. This proves that the synergistic effect between different metals can improve the stability of Pt and reduce the adsorption capacity of oxygen on the surface of Pt atoms. It can provide a theoretical reference for putting the shape-controlled catalysts into practical application in the future.

Suppressing disorder-order phase transition of Gd2Zr2O7 pyrochlore by Dy3+ doping and their impact on luminescence

The functionality of the Gd2Zr2O7 matrix is dictated by ordering at the cationic lattice (pyrochlore or defective fluorite) and associated local manifestations. In this work, we have synthesized nano and bulk undoped Gd2Zr2O7 (GZO) and Gd2Zr2O7:Dy3+ (GZOD) by molten salt synthesis adopting a crystal engineering approach and investigated their impact on photoluminescence properties. DFT calculations along with the higher intensity of yellow emission in the bulk GZOD confirm that the presence of Dy3+ is energetically favorable in defect fluorite structure compared to ordered pyrochlore counterpart. Increase/adjustments in the Zr–O bond distances in Dy3+ surroundings facilitate the release of local strain/stress in defective fluorite structure, which is not feasible in the perfectly ordered pyrochlore structure. Overall, the preference of defective fluorite structure by the GZO NCs, GZOD NCs, and bulk GZOD has been attributed to inherent flexibility in local structure with varying coordination numbers and polyhedral arrangements that assist in reducing the strain arising due to smaller particle size and/or by Dy3+ doping. It is believed that the present work provides simplistic pathways to control the order-disorder transformation, and in turn, the functionality of the Gd2Zr2O7 matrix for better hosting of nuclear waste and phosphor centers.

The role of local heteropolyhedral substitutions in the stoichiometry, topological characteristics and ion-migration paths in the eudialyte-related structures: a quantitative analysis.

Topological analysis of the heteropolyhedral MT framework (where M and T are octahedral and tetrahedral cations, respectively) in the eudialyte-type structure and its derivatives was performed based on a natural tiling analysis of the 3D cation. To analyze the migration paths of sodium cations in these structures, the Voronoi method was used. The parental eudialyte-type MT framework is formed by isolated ZO6 octahedra, six-membered [M(1)6O24] rings of edge-sharing M(1)O6 octahedra, and two kinds of rings of tetrahedra, [Si3O9] and [Si9O27]. Different occupancies of M(2), M(3) and M(4) sites with variable coordination numbers by the additional Q, T* and M* cations, respectively, result in 12 different types of the MT framework. Based on the results of natural tilings calculations as well as theoretical analysis of migration paths, it is found that Na+ ions can migrate through six- and seven-membered rings, while all other rings are too small for the migration. In eight types of MT frameworks, Na+-ion migration and diffusion is possible at ambient temperature and pressure, while in four other types cages are connected by narrow windows and, as a result, the Na+ diffusion in them is complicated at ambient conditions because of the window diameter, but may be possible either at higher temperatures or under mild geological conditions for long periods of time.

The mean-field Bose glass in quasicrystalline systems

We confirm the presence of a mean-field Bose glass (BG) in 2D quasicrystalline Bose–Hubbard models. We focus on two models where the aperiodic component is present in different parts of the problem. First, we consider a 2D generalisation of the Aubry–André (AA) model, where the lattice geometry is that of a square with a quasiperiodic onsite potential. Second, we consider the randomly disordered vertex model, which takes aperiodic tilings with non-crystalline rotational symmetries, and forms lattices from the vertices and lengths of the tiles. For the disordered vertex models, the mean-field BG forms across large ranges of the chemical potential, and we observe no significant differences from the case of a square lattice with uniform random disorder. Small variations in the critical points in the presence of random disorder between quasicrystalline and crystalline lattice geometries can be accounted for by the varying coordination number and the different rotational symmetries present. In the 2D AA model, substantial differences are observed from the usual phase diagrams of crystalline disordered systems. We show that weak modulation lines can be predicted from the underlying potential and may stabilise or suppress the mean-field BG in certain regimes. This results in a lobe-like structure for the mean-field BG in the 2D AA model, which is significantly different from the case of random disorder. Together, the two quasicrystalline models studied in this work show that the mean-field BG phase is present, as expected for 2D quasiperiodic models. However, a quasicrystalline geometry is not sufficient to result in differences from crystalline realisations of the BG, whereas a quasiperiodic form of disorder can result in different physics, as we observe in the 2D AA model.

Vapochromic Luminescent π-Conjugated Systems with Reversible Coordination-Number Control of Hypervalent Tin(IV)-Fused Azobenzene Complexes.

The dynamic and reversible changes of coordination numbers between five and six in solution and solid states, based on hypervalent tin(IV)-fused azobenzene (TAz) complexes, are reported. It was found that the TAz complexes showed deep-red emission owing to the hypervalent bond composed of an electron-donating three-center four-electron (3c-4e) bond and an electron-accepting nitrogen-tin (N-Sn) coordination. Furthermore, hypsochromic shifts in optical spectra were observed in Lewis basic solvents because of alteration of the coordination number from five to six. In particular, vapochromic luminescence was induced by attachment of dimethyl sulfoxide (DMSO) vapor to the coordination point at the tin atom accompanied with a crystal-crystal phase transition. Additionally, the color-change mechanism and degree of binding constants were well explained by theoretical calculation. To the best of our knowledge, this is the first example of vapochromic luminescence by using stable and variable coordination numbers of hypervalent bonds.

Multiple investigations of aqueous Eu(III)-oxalate complexes: the reduction in coordination number and validation of spectral linear correlation.

Detailed information on the An(iii)/Ln(iii) complexation properties in solution is essential for separation chemistry and the prediction of their potential for radionuclide migration from nuclear waste repositories into natural aquifers. In the present study, to better reveal and confirm the structural information of [Eu(Ox)x (H2O)h-2x]3-2x (h = 8, 9; x = 0-3) aqueous species, especially the variable coordination number (CN), and explore the validity of the spectral linear correlation between the luminescence lifetime and the residual hydration number in the first coordination sphere of Eu(iii) compounds in solution, a comparison between the spectral results and the theoretical calculations in a wide parametric space in terms of the pH value and oxalate concentration was carried out by combining time-resolved luminescence spectroscopy (TRLS) with speciation modelling and density functional theory (DFT) calculations. We have found direct and clear evidence for the 9-fold to 8-fold coordination number reduction of Eu(iii) atoms upon coordination with more than one oxalate in an aqueous medium, and as well systematically validated the applicability of the spectral linear correlation in an aqueous system (otherwise solid state) involving multiple species with the support of relatively reliable and clear speciation modelling.

Effect of thermal annealing on the properties of ZnO thin films

This work reports the effect of thermal annealing ranging from 0 to 800 °C on the various properties of zinc oxide thin film grown using radio-frequency sputtering. X-ray diffraction studies reveal the relaxation of stress up to thermal annealing of 400 °C and induction of residual stress thereafter. Zn K-edge X-ray absorption spectroscopic measurements reveal the variation of Zn–O and Zn–Zn bond distances with annealing temperature. Variation of coordination number with annealing temperature shows onset of oxygen vacancies at lower annealing temperatures. These vacancies and defects are also supported by the photoluminescence measurements. The optical band-gap of these films exhibit a significant variation with annealing temperature, which is in-line with the variation of crystallite size. Further, the magnetic behavior of these films is observed to follow the behavior of O 2p states along with defects as investigated from the X-ray magnetic circular dichroism.

PH effect on hydrothermal synthesis of the coordination polymers containing pyrazine-2,3-dicarboxylate: Investigation of thermal stability, luminescence and electrical conductivity properties

Hydrothermal reactions of lanthanide(III) salt with 2,3-pyrazinedicarboxylic (2,3-pzdc) acid yielded the coordination polymers [La2(2,3-pzdc)3(H2O)]n.3nH2O (1) and [La2(2,3-pzdc)3(H2O)]n.2nH2O (2). Compounds were obtained in a three dimensional form with different pH values under subcritical water conditions. The structures had variable coordination numbers. In addition, pH values play an important role in the structural chemistry of these materials. Different characterization techniques (Elemental analysis, FT-IR, ICP-OES, TG/DTA, FESEM, PXRD, BET and single crystal X-ray) were carried out to confirm crystallinity, porosity, purity and chemical composition of the coordination polymers. Crystal structures of the polymers were examined in detail. Their thermal stability, luminescence and electrical conductivity properties were investigated in the solid state.

A stochastic approach for predicting tortuosity in porous media via pore network modeling

Abstract In this study, a novel procedure is introduced and employed to calculate the tortuosity of porous media using pore network modeling. For this purpose, a 3D pore network model for porous media is constructed using network elements (pores and throats) along with its geometrical and topological characteristics. A unique method is introduced for connecting each pore to neighbors in an arbitrary direction with a variable pore coordination number. Next, fluid flow is simulated and the pressure distribution is achieved. Then, a stochastic procedure is applied to determine the tortuosity of the porous media. Since the probability of each path is calculated based on fluid flow rates, the stochastic approach makes it possible to calculate the tortuosity for effective paths, and there is no need for total path determination. The tortuosity of Berea sandstone, as a case study, is computed to be 2.07 using the described procedure, which is in good agreement with the experimental data. Finally, the impact of the repetition number and network parameters, including network size, pore coordination number, throat radius distribution, pore radius distribution, and pore/throat shape on the tortuosity calculation is investigated in detail.

Understanding the luminescence properties of Cu(i) complexes: a quantum chemical perusal.

Electronic structures and excited-state properties of Cu(i) complexes with varying coordination numbers have been investigated by means of advanced quantum chemical methods. The computational protocol employs density functional-based methods for geometry optimizations and vibrational analyses including solvent effects through continuum models. Excitation energies, spin-orbit couplings and luminescence properties are evaluated using multireference configuration interaction methods. Rate constants of spin-allowed and spin-forbidden transitions have been determined according to the Fermi golden rule. The computational results for the 4-coordinate (DPEPhos)Cu(PyrTet), the 3-coordinate [IPr-Cu-Py2]+, and the linear CAACMe2-Cu-Cl complexes agree well with experimental absorption and emission wavelengths, intersystem crossing (ISC) time constants, and radiative lifetimes in liquid solution. Spectral shifts on the ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) transitions caused by the polarity of the environment are well represented by the continuum models whereas the shifts caused by pseudo-Jahn-Teller distortions in the MLCT states are too pronounced in comparison to solid-state data. Systematic variation of the ligands in linear Cu(i) carbene complexes shows that only those complexes with S1 and T1 states of LLCT character possess sufficiently small singlet-triplet energy gaps ΔEST to enable thermally activated delayed fluorescence (TADF). Complexes whose S1 and T1 wavefunctions are dominated by MLCT excitations tend to emit phosphorescence instead. Unlike the situation in metal-free TADF emitters, the presence of low-lying locally excited triplet states does not promote ISC. These states rather hold the danger of trapping the excitation with nonradiative deactivation being the major deactivation channel.

Wheel-like{Ln6}luminescent lanthanide complexes covering the visible and near-infrared domains

Luminescence properties of wheel-like lanthanide complexes.

Late first-row transition metal complexes of a 17-membered piperazine-based macrocyclic ligand: structures and magnetism.

A 17-membered piperazine-based macrocyclic ligand LdiProp (1,5,13,17,22-pentaazatricyclo[15.2.2.17,11]docosa-7,9,11(22)-triene) was resynthesized in high yield by using a linear pump. Its Mn(ii), Fe(ii), Co(ii) and Ni(ii) complexes of the general formula [MnLdiProp(ClO4)2] (1), [FeLdiProp(CH3CN)](ClO4)2 (2), [CoLdiProp(CH3CN)](ClO4)2 (3), [NiLdiProp](ClO4)2 (4) were prepared and thoroughly characterized. X-ray diffraction analysis confirmed that Mn(ii) complex 1 has capped trigonal prismatic geometry with a coordination number of seven, Fe(ii) and Co(ii) complexes 2 and 3 are trigonal prismatic with a coordination number of six and Ni(ii) complex 4 has square pyramidal geometry with a coordination number of five. The decrease of the coordination number is accompanied by a shortening of M-N distances and an increase of torsion of the piperazine ring from the equatorial plane. Magnetic measurement reveals moderate anisotropy for 4 and rather large magnetic anisotropy for 2 and 3 (axial zero-field splitting parameter D(Ni) = 9.0 cm-1, D(Fe) = -14.4 cm-1, D(Co) = -25.8 cm-1, together with rather high rhombicity). Co(ii) complex 3 behaves as a field-induced SMM with a combination of Raman and direct or Orbach and direct relaxation mechanisms. Obtained magnetic data were extensively supported by theoretical CASSCF calculations. The flexibility and rather large 17-membered macrocyclic cavity of ligand LdiProp could be responsible for the variation of coordination numbers and geometries for the investigated late-first row transition metals.