Structural transitions in mixed ternary noble gas clusters

The properties of noble gas systems can be greatly extended by heterogeneous mixtures of elements. The geometrical structures and energies of mixed Ar–Kr–Xe clusters were investigated using ternary Lennard-Jones (TLJ) potential. For the Ar₁₉Kr _n Xe₁₉, Ar₁₉Kr₁₉Xe _n , and Ar _n Kr₁₉Xe₁₉ (n?=?0–17) clusters investigated, the results show that only two minimum energy configurations exist, i.e., polytetrahedron and six-fold pancake. The inner core of all these clusters is composed mainly of Ar atoms, and Kr and Xe atoms are distributed on the surface with well mixed pattern for polytetrahedral and segregate pattern for six-fold pancake configurations. The relative stability property of Ar–Kr–Xe clusters with a certain composition is discussed. Moreover, the role of heterogeneity on the strain was investigated, and reduced strain energies in Ar–Kr–Xe clusters were studied to find possible ways of reducing strain. The results showed that the strain energies were affected mainly by Ar–Ar, Ar–Kr, and Xe–Xe bonds.

A DFT study of the effect of NNN Al atom on strength of Brönsted acid sites of HY zeolite

A density functional theory study has been employed to investigate the effect of next-nearest-neighbours aluminium (NNN Al) atom on the acid strength of Brönsted acid site in Y zeolite. Additionally, the distribution of NNN Al atom was also studied. The obtained results show that thermodynamically, the favourable sites for location of NNN Al atom are the diagonal sites of the four-membered ring and meta-position of hexagon I. Furthermore, the increase of NNN Al atoms causes the nonlinear reduction of the strength of Brönsted acid sites. When the number of NNN Al atoms is greater than two, the increasing extent of acid strength is not obvious with the reduction in the number of NNN Al atoms. But the acid strength will increase linearly with the further reduce of the number of NNN Al atoms. Compared with deprotonation energy (E_D), ammonia adsorption energy (E_ads) could give a more reasonable measuring result for the acid strength.     

Density functional study of structural and electronic properties of small binary Be n Cu m (n + m = 2∼7) clusters

The geometrical structures, electronic properties and relative stabilities of small bimetallic Be _n Cu _m (n?+?m?=?2–7) clusters have been systematically investigated by using a density functional method at the B3PW91 level. In the most stable structures of Be _n Cu _m , the Be atoms tend to gather together and construct similar configurations to those of pure Be _n clusters. Meanwhile, there is a tendency for Cu atoms to segregate toward the Be _n cluster surface. The successive binding energies, cohesive energies, second difference of energies, the highest occupied-lowest unoccupied molecular orbital energy gaps and chemical hardness of Be _n Cu _m are also investigated. All of them demonstrate that the clusters with even number of copper atoms present relatively higher stabilities. The natural population analyses on the Be _n Cu _m clusters reveal that, the charge transfers from Be to Cu when the average coordination numbers (Nc) of Be atom is less than 3, whereas the charge-transferring direction reverses when Nc(Be) increases.     

Synthesis, characterization and X-ray crystal structures of dinuclear nickel(II) complexes of a novel potentially hexadentate but functionally tetradentate binucleating ligand

A binucleating potentially hexadentate chelating agent containing oxygen, nitrogen and sulfur as potential donor atoms (H₂ONNO) has been synthesized by condensing α,α-xylenebis(N-methyldithiocarbazate) with 2,4-pentanedione. An X-ray crystallographic structure determination shows that the Schiff base remains in its ketoimine tautomeric form with the protons attached to the imine nitrogen atoms. The reaction of the Schiff base with nickel(II) acetate in a 1:1 stoichiometry leads to the formation of a dinuclear nickel(II) complex [Ni(ONNO)]₂ (ONNO²⁻ = dianionic form of the Schiff base) containing N,O-chelated tetradentate ligands, the sulfur donors remaining uncoordinated. A single crystal X-ray structure determination of this dimer reveals that each ligand binds two low spin nickel(II) ions, bridged by a xylyl group. The nickel(II) atoms adopt a distorted square-planar geometry in a trans-N₂O₂ donor environment. Reaction of the Schiff base with nickel(II) acetate in the presence of excess pyridine leads to the formation of a similar dinuclear complex, [Ni(ONNO)(py)]₂, but in this case comprises five coordinate high-spin Ni(II) ions with pyridine ligands occupying the axial coordination sites as revealed by X-ray crystallographic analysis.     

NH3 on a BC3 nanotube: effect of doping and decoration of aluminum

The adsorption of the NH₃ molecule was investigated on pristine, Al-doped and Al-decorated BC₃ nanotubes (BC₃NT) using density functional theory calculations. It was found that NH₃ prefers to be adsorbed on a B atom of the tube wall, releasing energy of 1.02 eV. Al-doping increases the acidity of the tube surface and, therefore, its reactivity toward NH₃ so that the released energy in this case is about 1.62 eV, while decorating the BC₃NT with Al atom decreases the acidity and reactivity. Although Al-doping has no significant effect on the electronic properties of the BC₃NT, Al-decoration significantly reduces its HOMO/LUMO energy gap from 2.37 to 1.16 eV so that the tube becomes an n-type semiconductor. However, we believe that the acidity of the BC₃NTs may be controlled by doping or decoration of Al atoms.     

Fatty acid composition of membrane lipids and energy metabolism in mitochondria of pea seedlings under water deficit

The impacts of water deficit and melamine salt of bis(oximethyl)phosphonic acid (melaphen) on the fatty acid (FA) composition of membrane lipids and energy metabolism in mitochondria of 5-day-old pea (Pisum sativum L., cv. Flora-2) seedlings were studied. Insufficient watering resulted in the accumulation of saturated and a decrease in the content of unsaturated FAs with 18 and 20 carbon atoms. Seed treatment with 3 × 10^?10 M melaphen prevented these changes in the FA composition in the mitochondrial membrane lipids. Changes in the FA compositions of membrane lipids were correlated with changes in energy metabolism in mitochondria: the efficiency of oxidative phosphorylation and the rate of NAD-dependent substrate oxidation in the presence of ADP and FCCP (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone) were reduced. A close correlation was observed between changes in the highest rates of NAD-dependent substrate oxidation and the relative content of FAs with 18 (r = 0.76489) and 20 (r = 0.9637) carbon atoms. The regulatory role of C₁₈ and C₂₀ unsaturated FAs in the mitochondrial energy metabolism of pea seedlings is discussed.     

From pure C36 fullerene to cagelike nanocluster: a density functional study

The geometrical structures, energetics properties, and aromaticity of C_36-n Si _n (n?≤?18) fullerene-based clusters were studied using density functional theory calculations. The geometries of C_36-n Si _n clusters undergo strong structural deformation with the increase of Si substitution. For the most energy favorable structures of C_36-n Si _n , the silicon and carbon atoms form two distinct homogeneous segregations. Subsequently, the binding energy, HOMO–LUMO energy gap, vertical ionization potential, vertical electron affinity, and chemical hardness for the energetic favorable C_36-n Si _n geometries were computed and analyzed. In addition, the aromatic property of C_36-n Si _n cagelike clusters was investigated, and the result demonstrate that these C_36-n Si _n cagelike structures possess strong aromaticity.     

A first-principles study of Ni<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Pd<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (n = 1 − 5) clusters

A first-principle investigation of structures and properties of Ni _n Pd _n (n=1-5) clusters is presented. For this study, the linear combination of Gaussian-type orbitals auxiliary density functional theory (LCGTO-ADFT) method has been employed. In order to determine the lowest energy structures, several isomers in different spin multiplicities were studied, for each cluster size. Initial structures, for which successive geometry optimization was computed without any constrain, were taken along Born–Oppenheimer molecular dynamics (BOMD) trajectories. To discriminate between minima and transition state structures, harmonic frequency analyses were performed at the optimized structures. Ground state structures, bond lengths, harmonic frequencies, dissociation energy, ionization potential, electron affinity and spin density plots are presented. This work demonstrates, that the Pd atoms prefer to allocate on the surface of the cluster structures whose core is formed by the 3d TM atoms type. Moreover, it has been observed that the ground-state structure spin multiplicity increases as the system size grows. The results of this study contribute to gain insight into how structures and energy properties change with cluster size in bimetallic Pd-based alloys.     

Changes in organic phosphorus composition in boreal forest humus soils: the role of iron and aluminium

Organic phosphorus (P) is an important component of boreal forest humus soils, and its concentration has been found to be closely related to the concentration of iron (Fe) and aluminium (Al). We used solution and solid state ³¹P NMR spectroscopy on humus soils to characterize organic P along two groundwater recharge and discharge gradients in Fennoscandian boreal forest, which are also P sorption gradients due to differences in aluminium (Al) and iron (Fe) concentration in the humus. The composition of organic P changed sharply along the gradients. Phosphate diesters and their degradation products, as well as polyphosphates, were proportionally more abundant in low Al and Fe sites, whereas phosphate monoesters such as myo-, scyllo- and unknown inositol phosphates dominated in high Al and Fe soils. The concentration of inositol phosphates, but not that of diesters, was positively related to Al and Fe concentration in the humus soil. Overall, in high Al and Fe sites the composition of organic P seemed to be closely associated with stabilization processes, whereas in low Al and Fe sites it more closely reflected inputs of organic P, given the dominance of diesters which are generally assumed to constitute the bulk of organic P inputs to the soil. These gradients encompass the broad variation in soil properties detected in the wider Fennoscandian boreal forest landscape, as such our findings provide insight into the factors controlling P biogeochemistry in the region but should be of relevance to boreal forests elsewhere.     

Electronic structure and stabilities of Ni-doped germanium nanoclusters: a density functional modeling study

The present study reports the geometry, electronic structure, growth behavior and stability of neutral and ionized nickel encapsulated germanium clusters containing 1–20 germanium atoms within the framework of a linear combination of atomic orbital density functional theory (DFT) under a spin polarized generalized gradient approximation. In the growth pattern, Ni-capped Ge_n and Ni-encapsulated Ge_n clusters appear mostly as theoretical ground state at a particular size. To explain the relative stability of the ground state clusters, variation of different parameters, such as average binding energy per atom (BE), embedding energy (EE) and fragmentation energy (FE) of the clusters, were studied together with the size of the cluster. To explain the chemical stability of the clusters, different parameters, e.g., energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO gap), ionization energy (IP), electron affinity (EA), chemical potential (μ), chemical hardness (η), and polarizability etc. were calculated and are discussed. Finally, natural bond orbital (NBO) analysis was applied to understand the electron counting rule applied in the most stable Ge₁₀Ni cluster. The importance of the calculated results in the design of Ge-based superatoms is discussed.

Adsorption and decomposition mechanism of hexogen (RDX) on Al(111) surface by periodic DFT calculations

The adsorption of hexogen (RDX) molecule on the Al(111) surface was investigated by the generalized gradient approximation (GGA) of density functional theory (DFT). The calculations employ a supercell (4×4×3) slab model and three-dimensional periodic boundary conditions. The strong attractive forces between RDX molecule and aluminum atoms induce the N?O and N?N bond breaking of the RDX. Subsequently, the dissociated oxygen atoms, NO₂ group and radical fragment of RDX oxidize the Al surface. The largest adsorption energy is ?835.7 kJ mol^–1. We also investigated the adsorption and decomposition mechanism of RDX molecule on the Al(111) surface. The activation energy for the dissociation steps of V4 configuration is as large as 353.1 kJ mol^–1, while activation energies of other configurations are much smaller, in the range of 70.5–202.9 kJ mol^–1. The N?O is even easier than the N?NO₂ bond to decompose on the Al(111) surface.     

Changes in specific area and energy of root surface of cereal plants in Al-solution cultures. Water vapor adsorption studies

Surface areas and energetic properties of the shooting stage roots of rye (Secale L.), triticale (Triticale), barley (Hordeum L.) and four wheat (Triticum L.) varieties were estimated from experimental water vapor adsorption data. Roots stressed during 10 days at pH 4 with aluminium concentrations ranging from 0 to 40 mg dm^–3 were studied. Roots grown continuously at pH 7 were taken as controls. The surface properties of the roots grown at pH 4 without Al addition were apparently the same as those of the control roots. With the increase of the concentration of the aluminium treatment the surface area of the roots increased for all of the plants, beginning at 5 mg Al dm^–3 for barley, at 10 mg Al dm^–3for wheat and triticale, and at 40 mg Al dm^–3 for rye. The average water vapor adsorption energy of the root surface decreased in general with the increase of Al stress concentration for all plants but triticale, for which this increased. The sensitive cereal varieties seem to have greater amount of high energy adsorption centers (more polar surface) than the resistant ones (lower surface polarity), however more data is needed to justify this hypothesis. For Al-sensitive roots, fraction of high energy adsorption sites decreased and fraction of low energy sites increased under the Al stress. Smaller changes in adsorption energy sites were noted for roots of Al-resistant plants.     

Analytical studies on the incorporation of aluminium in the cell walls of the marine diatom Stephanopyxis turris

The eukaryotic diatoms are unicellular algae. They are well known for their filigree micro- and nanostructured cell walls which mainly consist of amorphous silica as well as various organic compounds. However, diatoms are also known to incorporate certain amounts of aluminium into their cell walls. Unexpectedly, enhanced Al concentrations in the Southern Yellow Sea were found to be correlated with a diatom spring bloom. Therefore, we have analyzed the influence of strongly enhanced Al concentrations in the culture medium upon the growth behaviour of the diatom Stephanopyxis turris (S. turris). The uptake and incorporation of Al into the cell walls was monitored. It turned out that S. turris survives aluminium concentrations up to 105.5 μM (2.85 mg/l) in the culture medium. Under the applied conditions, this corresponds to an Al/Si ratio of 1:1. These large amounts of Al had to be offered in the form of bis–tris-chelates in order to prevent uncontrolled precipitation. Under these conditions, the Al/Si ratio in the cell walls could be increased up to about 1:15 as determined by ICP-OES, the highest amount of aluminium found in diatom cell walls yet. Structural characterization of the biosilica by ATR-FTIR and solid-state ²⁷Al NMR spectroscopy revealed that an amorphous aluminosilicate phase is formed where the aluminium exists as four- and sixfold-coordinated species.     

The influence of aluminium availability on phosphate uptake in Phaseolus vulgaris L. and Phaseolus lunatus L.

Aluminium toxicity is one of the major limiting factors of crop productivity on acid soils. High levels of available aluminium in soil may induce phosphorus deficiency in plants. This study investigates the influence of Aluminium (Al) on the phosphate (P_i) uptake of two Phaseolus species, Phaseolus vulgaris L. var. Red Kidney and Phaseolus lunatus L. The two bean species were treated first with solutions of Al at different concentrations (0, 25, 50 and 100 μM, pH 4.50) and second with solutions of P_i (150 μM) at pH 4.50. The higher the Al concentration the higher the Al concentration sorbed but P. vulgaris L var. Red Kidney adsorbed significantly more Al than P. lunatus L. Both species released organic acids: P. vulgaris L var. Red Kidney released fumaric acid and P. lunatus L. fumaric and oxalic acids which could have hindered further Al uptake.The two bean species showed a sigmoid P_i uptake trend but with two different mechanisms. P. vulgaris L var. Red Kidney showed a starting point of 3 h whereas P. lunatus L. adsorbed P_i immediately within the first minutes. In addition, P. vulgaris L var. Red Kidney presented significantly higher P_i uptake (higher uptake rate ‘k’ and higher maximum adsorption ‘a’ of the kinetic uptake model). The Al treatments did not significantly influence P_i uptake. Results suggest that P. lunatus L. might adopt an external Al detoxification mechanism by the release of oxalic acid. P. vulgaris L var. Red Kidney on the other hand seemed to adopt an internal detoxification mechanism even if the Al sorbed is poorly translocated into the shoots. More detailed studies will be necessary to better define Al tolerance and/or resistance of Phaseolus spp.     

Aluminium-induced changes in the surface and micropore properties of wheat roots: a study using the water vapor adsorption-desorption technique

The geometric and energetic characteristics of root surfaces of two wheat (Triticum L.) varieties, Al tolerant (Inia 66/16) and Al sensitive (Henika), were estimated from experimental water vapor adsorption–desorption data. Roots stressed for around 1 week at pH 4 without and with a toxic aluminium level (0.741 mol m^–3) were studied at the tillering and shooting stages. Roots grown continuously at pH 7 were taken as control. The surface properties of the pH 4 stressed roots were apparently the same as those of the control roots whatever the root age. For the roots of both varieties, the surface area and total micropore volume increased markedly after aluminium treatment. The average micropore radius increased significantly for the sensitive wheat, whereas it increased only slightly for the resistant one. Under Al treatment the number of large pores increased while small pores were fewer for both plants, indicating a possible alteration of the build-up of root tissue. The root surface pores were fractal. The fractal dimension of the sensitive wheat roots decreased under Al treatment, whereas for the resistant wheat this remained apparently unchanged. The adsorption energy distribution functions had different shapes for the sensitive and the resistant wheat varieties: the sensitive variety had greater number of high energy adsorption centers, which implies that the root tolerance on Al stress may be connected with lower polarity of the surface.     

Theoretical investigation of ZnO and its doping clusters

Four clusters of ZnO, O–Zn–SR (–SR = ligand) and doping ZnO structures (with Cr, Cu, Al atoms) were investigated using density functional theory at theB3LYP/Lanl2dz level. The characteristics of Zn₃O₃ and Zn₄O₄ structures, which are the units of experimental wurtzite and zinc blende structures, were found to be similar to those of experimental ZnO nanocrystals. Moreover, the calculated Raman and IR spectra of ZnO clusters were almost consistent with experimental results. Raman spectra were observed to shift to higher frequencies with decreasing numbers of atoms. Both ligands and solvent make the wavelength of absorption peaks shift to blue. All transitions of absorption peaks for these pure clusters were from d to p orbitals. Finally, doping clusters and experimental doping nanocrystals were similar in character. The doping of metal changed the orbital of ZnO nanocrystals. The transitions in doping clusters (Cr–ZnO, Cu–ZnO) are from d to d orbitals, while Al–ZnO clusters have s–p transitions.     

Relationship between the weathering of clay minerals and the nitrification rate: a rapid tree species effect

We compared the properties of the clay mineral fraction and the composition of soil solutions in a Fagus sylvatica coppice (native forest) and four adjacent plantations of Pseudotsuga menziesii, Pinus nigra, Picea abies and Quercus sessiliflora planted in 1976. The results revealed changes of clay fraction properties due to tree species effect. Clay samples from Douglas fir and pine stands differ when compared to other species. Twenty-eight years after planting, we observed the following changes: a more pronounced swelling after citrate extraction and ethylene glycol solvation, a higher CEC and a smaller poorly crystallised aluminium content. All these changes affecting the clay fraction agreed well with soil solution analyses which revealed high NO₃ ^?, H⁺ and Al concentrations under Douglas fir and pine. These changes were explained by a strong net nitrification under Douglas fir and pine stands when compared with other tree species. The higher NO₃ ^? concentrations in soil solutions should be linked to the presence, type and activity of ammonia-oxiding bacteria which are likely influenced by tree species. The production of NO₃ ^? in excess of biological demand leads to a net production of hydrogen ion and enhances the dissolution of poorly crystallised Al-minerals. Secondary Al-bearing minerals constituted the principal acid-consuming system in these soils. As a consequence, the depletion of interlayer spaces of hydroxyinterlayered minerals increases the number of sites for exchangeable cation fixation and increases CEC of the clay fraction. The dissolution of Al oxy-hydroxides explain the increase in Al concentrations of soil solutions under Douglas fir and pine stands when compared to other species. Nitrate and dissolved aluminium were conjointly leached in the soil solutions. A change in environmental conditions, like an introduction of tree species, enough modifies soil processes to induce significant changes in the soil mineralogical composition even over a period of time as short as some tens of years. Generally, mineral weathering has been considered to be very slow and unlikely to change over tens of years, resulting in few studies capable of detecting changes in mineralogy. This study appears to have detected changes in clay mineralogy during a period of 28 years after the planting of forest species. Our study represents a single location with a limited block design, but causes us to conclude that the observed changes could be widely representative. Where available, archived samples should be utilized and long-term experiments set up so that similar changes can be tested for and detected using more robust designs. The plausible hypothesis we present to explain apparent changes in clay mineralogy has strong relevance to the sustainable management of land.     

Modeling of Platinum Clusters in H-Mordenite

The size, location and structure of Pt clusters in H-mordenite have been investigated by molecular mechanics energy minimization and molecular dynamics simulation techniques using the Catalysis software of Molecular Simulations (MSI). Lattice energy minimizations are performed to study the effects of the specific framework aluminum positions on the location and stability of monoatomic Pt sites in H-mordenite. The lattice energies relative to the siliceous platinum-aluminosilicate structure reveal that the stability of a single Pt atom in H-mordenite is remarkably influenced by the specific location of the Al atoms in the lattice. At the studied Si/Al ratio of two Al ions per unit cell, a stabilization of the H-mordenite lattice upon Pt deposition is obtained. Moreover, lattice energy calculations on Pt/aluminosilicate mordenites of different metal contents per unit cell have been performed. An optimum size for the aggregate confined to the 12-ring main channel that is almost independent of the Pt content per mordenite unit cell has been found. The structural features of the resulting clusters at the end of molecular dynamics simulations on Pt/alumina-mordenites reflect a strong metal-zeolite interaction. The present results are consistent with a previous molecular dynamics simulation study on the structure of platinum deposited on SiO₂ surfaces.     

Soil microbial community indices as predictors of soil solution chemistry and N leaching in Picea abies (L.) Karst. forests in S. Sweden

High rates of inorganic nitrogen (N) deposition or internal N turnover increases the risks of N loss from forests with negative effects on stream water quality. We hypothesized that soil fungi may be more important N sinks than bacteria, and thus examined the impact of soil microbial community composition on N leaching from forests. We studied 19 spruce stands to examine relationships between microbial community composition, stem growth, soil-, and lysimeter-collected soil solution characteristics, and N leaching. We used nitrate concentration in the soil solution below the rooting zone as an N leaching index and phospholipid fatty acid (PLFA) analysis for characterisation of microbial communities. Microbial community composition in the organic horizon and soil solution chemistry below the rooting zone was highly correlated. Stands with low concentrations of nitrate (NO₃ ^?) and aluminium (Al) had higher fungi: bacteria ratio compared with stands with higher concentrations of NO₃ ^? and Al. Stem growth and fungi: bacteria ratio explained 70 % of the variation in N and Al leaching. We identified three microbial predictors of variation in soil solution chemistry, of which the fungi: bacteria was the strongest. The other two were putative indicators of microbial C limitation, a condition known to stimulate N mineralisation and nitrification.