Density Functional Study of Size-Dependent Hydrogen Adsorption on Ag n Cr (n = 1-12) Clusters.

Increasing interest has been paid for hydrogen adsorption on atomically controlled nanoalloys due to their potential applications in catalytic processes and energy storage. In this work, we investigate the interaction of H2 with small-sized Ag n Cr (n = 1-12) using density functional theory calculations. It is found that the cluster structures are preserved during the adsorption of H2 either molecularly or dissociatively. Ag3Cr-H2, Ag6Cr-H2, and Ag9Cr-H2 clusters are identified to be relatively more stable from computed binding energies and second-order energy difference. The dissociation of adsorbed H2 on Ag2Cr, Ag3Cr, Ag6Cr, and Ag7Cr clusters is favored both thermodynamically and kinetically. The dissociative adsorption is unlikely to occur because of a considerable energy barrier before reaching the final state for Ag4Cr or due to energetic preferences for n = 1, 5, and 8-12 species. Comprehensive analysis shows that the geometric structure of clusters, the relative electronegativity, and the coordination number of the Cr impurity play a decisive role in determining the preferred adsorption configuration.

First-row transition metal doped germanium clusters Ge16M: some remarkable superhalogens.

A theoretical study of geometric and electronic structures, stability and magnetic properties of both neutral and anionic Ge16M0/- clusters with M being a first-row 3d transition metal atom, is performed using quantum chemical approaches. Both the isoelectronic Ge16Sc- anion and neutral Ge16Ti that have a perfect Frank-Kasper tetrahedral T d shape and an electron shell filled with 68 valence electrons, emerge as magic clusters with an enhanced thermodynamic stability. The latter can be rationalized by the simple Jellium model. Geometric distortions from the Frank-Kasper tetrahedron of Ge16M having more or less than 68 valence electrons can be understood by a Jahn-Teller effect. Remarkably, DFT calculations reveal that both neutral Ge16Sc and Ge16Cu can be considered as superhalogens as their electron affinities (≥3.6 eV) exceed the value of the halogen atoms and even that of icosahedral Al13. A detailed view of the magnetic behavior of Ge16M0/- clusters shows that the magnetic moments of the atomic metals remain large even when they are quenched upon doping. When M goes from Sc to Zn, the total spin magnetic moment of Ge16M0/- increases steadily and reaches the maximum value of 3 µ B with M = Mn before decreasing towards the end of the first-row 3d block metals. Furthermore, the IR spectra of some tetrahedral Ge16M are also predicted.

Systematic Investigation of the Structure, Stability, and Spin Magnetic Moment of CrM n Clusters (M = Cu, Ag, Au, and n = 2-20) by DFT Calculations.

Binary clusters of transition-metal and noble-metal elements have been gathering momentum for not only advanced fundamental understanding but also potential as elementary blocks of novel nanostructured materials. In this regard, the geometries, electronic structures, stability, and magnetic properties of Cr-doped Cu n , Ag n , and Au n clusters (n = 2-20) have been systematically studied by means of density functional theory calculations. It is found that the structural evolutions of CrCu n and CrAg n clusters are identical. The icosahedral CrCu12 and CrAg12 are crucial sizes for doped copper and silver species. Small CrAu n clusters prefer the planar geometries, while the larger ones appear as on the way to establish the tetrahedral CrAu19. Our results show that while each noble atom contributes one s valence electron to the cluster shell, the number of chromium delocalized electrons is strongly size-dependent. The localization and delocalization behavior of 3d orbitals of the chromium decide how they participate in metallic bonding, stabilize the cluster, and give rise to and eventually quench the spin magnetic moment. Moreover, molecular orbital analysis in combination with a qualitative interpretation using the phenomenological shell model is applied to reveal the complex interplay between geometric structure, electronic structure, and magnetic moment of clusters. The finding results are expected to provide greater insight into how a host material electronic structure influences the geometry, stability, and formation of spin magnetic moments in doped systems.

Correction: The binary aluminum scandium clusters Al x Sc y with x + y = 13: when is the icosahedron retained?

[This corrects the article DOI: 10.1039/D1RA06994B.].

The binary aluminum scandium clusters Al x Sc y with x + y = 13: when is the icosahedron retained?

Geometrical and electronic structures of the 13-atom clusters Al x Sc y with x + y = 13, as well as their thermodynamic stabilities were investigated using DFT calculations. Both anionic and neutral isomers of Al x Sc y were found to retain an icosahedral shape of both Al13 and Sc13 systems in which an Al atom occupies the endohedral central position of the icosahedral cage, irrespective of the number of Al atoms present. Such a phenomenon occurs to maximize the number of stronger Al-Al and Sc-Al bonds instead of the weaker Sc-Sc bonds. NBO analyses were applied to examine their electron configurations and rationalize the large number of open shells and thereby high multiplicities of the mixed clusters having more than three Sc atoms. The SOMOs are the molecular orbitals belonged to the irreducible representations of the symmetry point group of the clusters studied, rather than to the cluster electron shells. Evaluation of the average binding energies showed that the thermodynamic stability of Al x Sc y clusters is insignificantly altered as the number y goes from 0 to 7 and then steadily decreases when y attains the 7-13 range. Increase of the Sc atom number also reduces the electron affinities of the binary Al x Sc y clusters, and thus they gradually lose the superhalogen characteristics with respect to the pure Al13.

Whole-genome sequencing and comparative genomic analysis of Shewanella xiamenensis strains carrying blaOXA-48-like genes isolated from a water environment in Vietnam.

OBJECTIVES: The aim of this study was to understand the natural bacterial hosts of antimicrobial resistance genes (ARGs) and their impact on the processes of evolution, spread and positive selection of acquired ARGs. METHODS: Environmental carbapenem-resistant Gram-negative bacteria in Vietnam were screened for based on a One Health approach. Whole-genome sequencing (WGS) and comparative genomic analysis of the isolates were performed. WGS of three carbapenem-resistant Shewanella xiamenensis strains (SxND_W2_2018, SxND_W5_2018 and SxND_W9_2018) isolated from canals in Truc Ninh District and Nghia Hung District, Nam Dinh Province, Vietnam, in 2018 was performed using an Illumina MiniSeq system. ARGs in the draft genome sequences were detected using ResFinder, and comparison of genomic regions was performed using BLASTn and Easyfig. RESULTS: TheblaOXA-48-like carbapenem-hydrolysing class D ß-lactamase genes blaOXA-48, blaOXA-252 and blaOXA-547 were detected in chromosomal contigs of SxND_W2_2018, SxND_W5_2018 and SxND_W9_2018, respectively. Comparative analysis of the surrounding regions of the blaOXA-48-like genes, including both 10 kb upstream and 10 kb downstream of the genes, showed that the genomic regions were highly conserved in all three isolates. CONCLUSION: This study analysed the draft genome sequences of carbapenem-resistantS. xiamenensis strains isolated from a water environment in Vietnam. All of the strains carried blaOXA-48-like gene variants in their chromosomes. This information will contribute to highlight the evolution of blaOXA-48 family carbapenemase genes in nature and the importance of S. xiamenensis as a natural reservoir of important ARGs in the environment in Vietnam.

Structure, stability, absorption spectra and aromaticity of the singly and doubly silicon doped aluminum clusters Al n Si m 0/+ with n = 3-16 and m = 1, 2.

Structures of the binary Al n Si m clusters in both neutral and cationic states were investigated using DFT and TD-DFT (B3LYP/6-311+G(d)) and (U)CCSD(T)/cc-pvTZ calculations. Silicon-doped aluminum clusters are characterized by low spin ground states. For small sizes, the Si dopant prefers to be located at vertices having many edges. For larger sizes, the Si atom prefers to be endohedrally doped inside an Al n cage. Relative stability, adiabatic ionization energy and dissociation energies of each cluster size were evaluated. A characteristic of most Si doped Al clusters is the energetic degeneracy of two lowest-lying isomers. Calculated results confirm the high stability of the sizes Al4Si2, Al12Si and Al11Si2 + as "magic" clusters, that exhibit 20 or 40 shell electrons and are thermodynamically more stable as compared to their neighbors. Electronic absorption spectra of isoelectronic magic clusters Al13 -, Al12Si, and Al11Si2 + that have two pronounced bands corresponding to blue and violet lights, have been rationalized by using the electron shell model. The magnetically included ring current density (MICD) analyses suggest that they are also aromatic structures as a result of the "magic" 40 shell electrons.

Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites.

Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag-Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent-density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag4(H2O) x2+ (x = 2 and x = 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.

Au19M (M=Cr, Mn, and Fe) as magnetic copies of the golden pyramid.

An investigation on structure, stability, and magnetic properties of singly doped Au19M (M=Cr, Mn, and Fe) clusters is carried out by means of density functional theory calculations. The studied clusters prefer forming magnetic versions of the unique tetrahedral Au20. Stable sextet Au19Cr is identified as the least reactive species and can be qualified as a magnetic superatom. Analysis on cluster electronic structures shows that the competition between localized and delocalized electronic states governs the stability and magnetic properties of Au19M clusters.

Electronic properties of the polypyrrole-dopant anions ClO4- and MoO42-: a density functional theory study.

The conductive properties of polypyrrole chains doped with ClO4- or MoO42- anions and the existence of polarons and bipolarons in these doped polypyrrole chains were investigated by performing computational calculations based on density functional theory (DFT). Doping with these anions was found to decrease the band gap of the polypyrrole. Theoretical calculations revealed that changing the type of oxidative agent applied does not affect the conversion of polypyrrole into a conducting polymer, but the conductivity of the doped polypyrrole does depend on the ratio of oxidant to polypyrrole.

A Systematic Investigation on CrCun Clusters with n = 9-16: Noble Gas and Tunable Magnetic Property.

A systematic investigation on structure, dissociation behavior, chemical bonding, and magnetic property of Cr-doped Cun clusters (n = 9-16) is carried out using the mean of density functional theory calculations. It is found that CrCu12 is a crucial size, preferring an icosahedral Cu12 cage with the central Cr dopant. Smaller cluster sizes appear as on the way to form the CrCu12 icosahedron while larger ones are produced by attaching additional Cu atoms to the CrCu12 core. The presence of Cr dopant obviously enhances the stability of CrCun clusters in comparison to that of pure counterparts. Exceptionally stable CrCu12 has an 18-electron closed-shell electronic structure, mimicking a noble gas in the viewpoint of superatom concept. Analysis on cluster electronic structure shows that the interplay between 3d orbitals of Cr and 4s orbitals of Cu has a vital role on the magnetic properties of CrCun clusters.

Optical properties of the hydrated charged silver tetramer and silver hexamer encapsulated inside the sodalite cavity of an LTA-type zeolite.

The optical spectra in the UV-VIS region of the hydrated doubly charged tetramer Ag4(2+) and hydrated multiply charged hexamer Ag6(p+) silver clusters encapsulated inside the sodalite cavity of an LTA-type zeolite have been systematically predicted using DFT, TD-DFT and CASSCF/CASPT2 methods. The optical behaviour of the model hydrated clusters [Ag6(H2O)8(Si24H24O36)](p+) is very sensitive to their charge. Among the cations [Ag6(H2O)8(Si24H24O36)](p+), only the embedded hydrated quadruply charged silver hexamer [Ag6(H2O)8(Si24H24O36)](4+) shows a strong absorption band at ∼420 nm (blue light) and emits light in red color. The absorption spectrum of the hydrated doubly charged silver tetramer cluster [Ag4(H2O)m(Si24H24O36)](2+), which shifts slightly and steadily with the increasing amount of interacting water molecules to longer wavelengths, has a strong peak in the blue region. The water environment forces the silver tetramer to relocate into one side of the cavity instead of at its center as in the case of the non-hydrated [Ag4(Si24H24O36)](2+) cluster. Water molecules act as ligands significantly splitting the energy levels of excited states of the Ag4(2+) and Ag6(4+) clusters. This causes the absorption spectra of the clusters to broaden and the emission to shift to the green-yellow and red part of the visible region.

Theoretical modeling of optical properties of Ag8 and Ag14 silver clusters embedded in an LTA sodalite zeolite cavity.

Optical properties of silver Ag(n) nanoclusters are demonstrated to be dependent on their size, structure and charge state. It is found that when being contained in the sodalite cavity of LTA zeolite the tetradecanuclear hexacation silver cluster Ag14(6+) is stable. Its lower-lying states and optical spectrum are theoretically determined using the quantum chemical TD-DFT method. Its ground state possesses an outer-shell electron configuration of A1g(2)T2g(6) mimicking the s(2)p(6) valence of noble gas atoms. These frontier orbitals are constructed from 5s,5p(Ag)-AOs with contributions from framework oxygen atoms. Light absorption of Ag14(6+) embedded in the sodalite cage which is characterized by strong peaks centered at 331 and 476 nm (transitions 5s,p(Ag) → 5s,p(Ag)) leads to much longer wavelength emission. The sodalite cage, as a container, stabilizes the central Ag14(6+) cluster by electrostatic attraction. The absorption spectrum of the isovalent neutral Ag8 cluster embedded inside the same sodalite cavity is also simulated using TD-DFT and CASPT2 methods. This absorption spectrum which is similar to that of the Ag14(6+) cluster has two absorption bands in the near UV and visible regions.

Copper doping of small gold cluster cations: influence on geometric and electronic structure.

The effect of Cu doping on the properties of small gold cluster cations is investigated in a joint experimental and theoretical study. Temperature-dependent Ar tagging of the clusters serves as a structural probe and indicates no significant alteration of the geometry of Au(n) (+) (n = 1-16) upon Cu doping. Experimental cluster-argon bond dissociation energies are derived as a function of cluster size from equilibrium mass spectra and are in the 0.10-0.25 eV range. Near-UV and visible light photodissociation spectroscopy is employed in conjunction with time-dependent density functional theory calculations to study the electronic absorption spectra of Au(4-m)Cu(m) (+) (m = 0, 1, 2) and their Ar complexes in the 2.00-3.30 eV range and to assign their fragmentation pathways. The tetramers Au(4) (+), Au(4) (+)[middle dot]Ar, Au(3)Cu(+), and Au(3)Cu(+)[middle dot]Ar exhibit distinct optical absorption features revealing a pronounced shift of electronic excitations to larger photon energies upon substitution of Au by Cu atoms. The calculated electronic excitation spectra and an analysis of the character of the optical transitions provide detailed insight into the composition-dependent evolution of the electronic structure of the clusters.

Cholera outbreaks caused by an altered Vibrio cholerae O1 El Tor biotype strain producing classical cholera toxin B in Vietnam in 2007 to 2008.

Vibrio cholerae O1 isolates collected during cholera outbreaks occurring from late 2007 to early 2008 in northern Vietnam were revealed to represent an altered strain containing the RS1 element followed by a CTX prophage harboring El Tor type rstR and classical ctxB on the large chromosome.