Density functional study on size-dependent structures, stabilities, electronic and magnetic properties of Au n M (M = Al and Si, n?=?1–9) clusters: comparison with pure gold clusters

The density functional theory (DFT) method has been employed to systematically investigate the geometrical structures, relative stabilities, and electronic and magnetic properties of Au(n)M (M = Al and Si, n = 1-9) clusters for clarifying the effect of Al(Si) modulation on the gold nanostructures. Of all the clusters studied, the most stable configurations adopt a three-dimensional structure for Au(n)Al at n = 4-8 and Au(n)Si at n = 3-9, while for pure gold systems, no three-dimensional lowest energy structures are obtained. Through a careful analysis of the fragmentation energy, second-order difference of energy, HOMO-LUMO energy gap, and magnetic moment as a function of cluster size, an odd-even alternative phenomenon has been observed. The results show that the clusters with even-number valence electrons have a higher relative stability, but lower magnetic moments. Furthermore, Al(Si) doping is found to enhance the stabilities of gold frameworks. In addition, the charge analysis has been given to understand the different effects of individual doped atom on electronic properties and compared further.     

An insight into the structures,stabilities, and bond character of BnPt (n=1∼6) clusters

We perform a systematical investigation on the geometry, thermodynamic/kinetic stability, and bonding nature of low-lying isomers of B_nPt (n=1-6) at the CCSD(T)/[6-311+G(d)/LanL2DZ]//B3LYP/[6-311+G(d)/LanL2DZ] level. The most stable isomers of B_nPt (n=1-6) adopt planar or quasi-planar structure. B_nPt (n=2-5) clusters can be generated by capping a Pt atom on the B-B edge of pure boron clusters. However, For B₆Pt with non-planar structure, a single doped Pt atom significantly affects the shape of the host boron cluster. The dopant of the Pt atom can improve the stability of pure boron clusters. The valence molecular orbital (VMO), electron localization function (ELF), and Mayer bond order (MBO) are applied to gain insight into the bonding nature of B_nPt (n=2-6) isomers. The aromaticity for some isomers of B_nPt (n=2-6) is analyzed and discussed in terms of VMO, ELF, adaptive natural density partitioning (AdNDP), and nucleus-independent chemical shift (NICS) analyses. Results obtained from the energy and cluster decomposition analyses demonstrate that B₂Pt and B₄Pt exhibits as highly stable. Importantly, some isomers of B_nPt (n=2-5) are stable both thermodynamically and kinetically, which are observable in future experiment.     

Large gallanes and the PSEPT theory: a theoretical study of Ga n H n+2 clusters (n?=?7–9)

Do alanes Al(n)H(n+2) and gallanes Ga(n)H(n+2) satisfy the polyhedral skeletal electron pair theory (PSEPT)? Taking into account previous work on this question, this paper provides a convincing answer and proposes the reformulation of the (n + 1) electron pairs rule of Wade and Mingos (W-M) for such systems. Following recent studies of tetra-, penta-, hexa-, hepta-, octa-, and nonaalanes as well as valence-isoelectronic/related gallanes, in this paper we present an analysis of the hydrides of aluminum and gallium A(n)H(n+2) (A = Al, Ga and n = 7-9). The aim is still to examine the applicability of PSEPT, especially the W-M rule, to these clusters. Exploration of the total potential energy surfaces (PESs) of hepta-, octa-, and nonagallanes shows that the absolute minima have a nido-like polyhedron arrangement. Unlike the smaller Ga(n)H(n+2) (n = 4, 5, 6), it seems that the size of the cluster largely dictates its preferred geometry. Although none of them have closed (totally triangular) cages, these clusters exhibit significant compactness, comparable to borane double anions, B(n)H(n) (2-), which are the archetypes for the PSEPT theory.     

Ab initio investigations on planar (MgO)n clusters (n = 1–5) and their hydrogen adsorption behaviour

Ab initio calculations (B3LYP and PBE-D3) of the structures, stabilities, vibrational, electronic and hydrogen adsorption behaviour of (MgO)_n clusters are performed using 6-311+ + G(d,p) basis set. The planar (MgO)_n clusters are found to be global minima for n ≤ 3 and local minima for n = 4 and 5. In addition, we have also analysed global minimum structures of (MgO)₄ and (MgO)₅. The binding energies suggest that their stabilities increase successively. Vibrational frequencies and IR intensities further support the enhanced stability with an increase in the size of (MgO)_n clusters. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) surfaces are used to explain and discuss the electronic properties. Finally, we have demonstrated hydrogen storage capacity of (MgO)_n clusters, considering hydrogen adsorption on planar as well as global minimum (MgO)₄ and (MgO)₅ clusters. We have noticed that four and five H₂ molecules can be easily adsorbed by (MgO)₄ and (MgO)₅ clusters having adsorption energy of 0.13–0.14 eV with mass ratio of 4.76%. Thus, the present study is expected to motivate further the applications of small clusters for efficient hydrogen energy storage.     

Shared‐storage clusters

Clusters represent a collection of interconnected computers that collaborate on executing an application and present themselves as one unified computing resource. They are becoming an important segment in the computer industry. The two main flavors of cluster architectures are sharedstorage and sharednothing. This article presents host and I/O implementation details, and performance tradeoffs that need to be enforced due to sharing data in sharedstorage clusters. Sharing data requires the need for global concurrency and coherency protocols to maintain consistency of the database, and enforce data consistency in the local nodes buffers, respectively. Various sharedstorage architectures will be investigated, including the Virtual Shared and SharedIntermediate Memory models. This article also presents few selected sharedstorage clusters, including the DEC VAXCluster, IBM parallel Sysplex and Compaq/Tandem ServerNet.     

Structure,ecology and physiology of root clusters – a review

Hairy rootlets, aggregated in longitudinal rows to form distinct clusters, are a major part of the root system in some species. These root clusters are almost universal (1600 species) in the family Proteaceae (proteoid roots), with fewer species in another seven families. There may be 10–1000 rootlets per cm length of parent root in 2–7 rows. Proteoid roots may increase the surface area by over 140× and soil volume explored by 300× that per length of an equivalent non-proteoid root. This greatly enhances exudation of carboxylates, phenolics and water, solubilisation of mineral and organic nutrients and uptake of inorganic nutrients, amino acids and water per unit root mass. Root cluster production peaks at soil nutrient levels (P, N, Fe) suboptimal for growth of the rest of the root system, and may cease when shoot mass peaks. As with other root types, root cluster production is controlled by the interplay between external and internal nutrient levels, and mediated by auxin and other hormones to which the process is particularly sensitive. Proteoid roots are concentrated in the humus-rich surface soil horizons, by 800× in Banksia scrub-heath. Compared with an equal mass of the B horizon, the A₁ horizon has much higher levels of N, P, K and Ca in soils where species with proteoid root clusters are prominent, and the concentration of root clusters in that region ensures that uptake is optimal where supply is maximal. Both proteoid and non-proteoid root growth are promoted wherever the humus-rich layer is located in the soil profile, with 4× more proteoid roots per root length in Hakea laurina. Proteoid root production near the soil surface is favoured among hakeas, even in uniform soil, but to a lesser extent, while addition of dilute N or P solutions in split-root system studies promotes non-proteoid, but inhibits proteoid, root production. Local or seasonal applications of water to hakeas initiate non-proteoid, then proteoid, root production, while waterlogging inhibits non-proteoid, but promotes proteoid, root production near the soil surface. A chemical stimulus, probably of bacterial origin, may be associated with root cluster initiation, but most experiments have alternative interpretations. It is possible that the bacterial component of soil pockets rich in organic matter, rather than their nutrient component, could be responsible for the proliferation of proteoid roots there, but much more research on root cluster microbiology is needed.     

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.     

Structures and stabilities of ScBn (n = 1–12) clusters: an ab initio investigation

The geometries, stabilities, and electronic properties of ScB_n (n?=?1–12) clusters have been systematically investigated by using density functional theory B3LYP method and coupled–cluster theory CCSD(T) method. It is found that the ground state isomers of ScB_n have planar or quasi–planar structure when n?≤?6, which can be viewed as a B atom of the corresponding B_n+1 cluster is substituted by a Sc atom. From n?≥?7, the ground state isomers favor nest–like structure, in which the Sc atom sits on a nest–like B_n cluster. The calculated second–order differences of energies manifest that the magic numbers of stability are n?=?3, 7, 8, 9 and 11 for the ScB _n clusters. Further analysis indicates that the ScB₇ cluster with C _6v symmetry represents the outstanding stable ScB_n cluster, as confirmed by its electronic structure and molecular orbitals.     

First-principles study on the structures and electronic properties of graphene-supported Nin (n = 1–6) clusters

ABSTRACT

The structural and electronic properties, such as adsorption energy, magnetic property, and charge-transferring process of Ni_n (n?=?1–6) clusters interacting with pristine, strained and defective graphene were investigated by using the density functional theory calculations with the Perdew–Burke–Ernzerhof exchange-correlation energy functional. By introducing strain and defects, the stability of the cluster-graphene system was improved significantly. The magnetic moments increased monotonically for Ni_n clusters on pristine and strained graphene while exhibited an oscillating behaviour for defective graphene. On the other hand, more charges being transferred from Ni_n clusters to defective graphene were observed compared with pristine and strained graphene.     

Complexes of DNA bases and Watson–Crick base pairs interaction with neutral silver Agn (n = 8, 10, 12) clusters: a DFT and TDDFT study

We study the binding of the neutral Ag_n (n = 8, 10, 12) to the DNA base-adenine (A), guanine (G) and Watson–Crick –adenine-thymine, guanine-cytosine pairs. Geometries of complexes were optimized at the DFT level using the hybrid B3LYP functional. LANL2DZ effective core potential was used for silver and 6–31 + G^** was used for all other atoms. NBO charges were analyzed using the Natural population analysis. The absorption properties of Ag_n–A,G/WC complexes were also studied using time-dependent density functional theory. The absorption spectra for these complexes show wavelength in the visible region. It was revealed that silver clusters interact more strongly with WC pairs than with isolated DNA complexes. Furthermore, it was found that the electronic charge transferred from silver to isolated DNA clusters are less than the electronic charge transferred from silver to the Ag_n–WC complexes. The vertical ionization potential, vertical electron affinity, hardness, and electrophilicity index of Ag_n–DNA/WC complexes have also been discussed.     

Iron-sulfur clusters: why iron?

This communication addresses a simple question by means of density functional calculations: Why is iron used as the metal in iron-sulfur clusters? While there may be several answers to this question, it is shown here that one feature - the well-defined inner-sphere reorganization energy of self-exchange electron transfer - is very much favored in iron-sulfur clusters as opposed to metal substituted analogues of Mn, Co, Ni, and Cu. Furthermore, the conclusion holds for both 1Fe and 2Fe type iron-sulfur clusters. The results show that only iron provides a small inner-sphere reorganization energy of 21 kJ/mol in 1Fe (rubredoxin) and 46 kJ/mol in 2Fe (ferredoxin) models, whereas other metal ions exhibit values in the range 57-135 kJ/mol (1Fe) and 94-140 kJ/mol (2Fe). This simple result provides an important, although partial, explanation why iron alone is used in this type of clusters. The results can be explained by simple orbital rules of electron transfer, which state that the occupation of anti-bonding orbitals should not change during the redox reactions. This rule immediately suggests good and poor electron carriers.     

How icosahedral are icosahedral clusters?

Icosahedral symmetry, not contemplated within the crystallographic space groups, is nevertheless present to a high degree of perfection in a variety of clusters, in molecular, ionic, covalent or metallic structures. The application of continuous shape measures to those structures allows for a quantification of the degree of icosahedral symmetry present in each case, a first necessary step for a deeper discussion of the factors that favor the adoption of the icosahedron as a stable structural motif. Examples analyzed include boranes and carbaboranes, main group, rare earth or transition metal clusters, ligand-bridged polynuclear complexes and sets of donor atoms in mono- or polynuclear complexes. Specific examples are found of structures that appear along the minimal distortion pathways from the icosahedron to the cuboctahedron or the anticuboctahedron.     

Phylogenomic analyses of KCNA gene clusters in vertebrates: why do gene clusters stay intact?

Background  

Gene clusters are of interest for the understanding of genome evolution since they provide insight in large-scale duplications events as well as patterns of individual gene losses. Vertebrates tend to have multiple copies of gene clusters that typically are only single clusters or are not present at all in genomes of invertebrates. We investigated the genomic architecture and conserved non-coding sequences of vertebrate KCNA gene clusters. KCNA genes encode shaker-related voltage-gated potassium channels and are arranged in two three-gene clusters in tetrapods. Teleost fish are found to possess four clusters. The two tetrapod KNCA clusters are of approximately the same age as the Hox gene clusters that arose through duplications early in vertebrate evolution. For some genes, their conserved retention and arrangement in clusters are thought to be related to regulatory elements in the intergenic regions, which might prevent rearrangements and gene loss. Interestingly, this hypothesis does not appear to apply to the KCNA clusters, as too few conserved putative regulatory elements are retained.     

用2n作母本,用4n作父本也能产生3n西瓜种子吗?

高中《生物》课本 P150上,谈到三倍体西瓜的培育过程。其中有这样一段话:“……用四倍体植株作母本,用二倍体植株作父本,进行交配,就能在四倍体植株上结出三倍体西瓜的种子。”对此,不少师生提出另一种设想:如果反过来,用二倍体(2n)植株作母本,用四倍体     

AmBisome,tres retos: infección por Candida auris,infección del sistema nervioso central e infección asociada a biopelículas

The treatment of invasive fungal infections remains a challenge, both for the diagnosis and for the need of providing the appropriate antifungal therapy. Candida auris is a pathogenic yeast that is responsible for hospital outbreaks, especially in intensive care units; it is characterized by a high resistance to the antifungal agents and can become multidrug-resistant. At present, the recommended antifungal agents for the invasive infections with this pathogen are echinocandins, always after carrying out an antifungal susceptibility testing. In case of no clinical response or persistent candidemia, the addition of liposomal amphotericin B or isavuconazole may be considered. Both fungal infection of the central nervous system and that associated with biomedical devices remain rare entities affecting mainly immunocompromised patients. However, an increase in their incidence in recent years, along with high morbidity and mortality, has been shown. The treatment of these infections is conditioned by the limited knowledge of the pharmacokinetic properties of antifungals. A better understanding of the pharmacokinetic and pharmacodynamic parameters of the different antifungals is essential to determine the efficacy of the antifungal agents in the treatment of these infections.