Elastic and reactive collisions of atoms and molecules with neutral alkali clusters

We have measured absolute integral cross sections for low-energy collisions of atoms and molecules with neutral sodium clusters over a wide cluster size range (n=2–40). The cross sections are exceptionally large, reaching values of thousands of square angstroms. Consequently, the scattering involves long-range interactions. The van der Waals force, acting either alone (Na_n+N₂) or in concert with the inelastic charge-transfer “harpooning” channel (Na_n+Cl₂, Na_n+O₂) can describe the measurements. Using interaction parameters taken from spectroscopic studies of alkali clusters, we find very good agreement with the data. This provides a point of contact between beam scattering experiments and studies of cluster electromagnetic response properties.     

DFT study of small aluminum and boron hydrides: isomeric composition and physical properties

The structure, energetics, and physical properties, including rotational constants, characteristic vibrational temperatures, dipole moment, static polarizability, HOMO-LUMO gap, formation enthalpy and collision diameter of different isomeric forms of atomic Al _n H _m and B _n H _m clusters with n = 1..4 and all feasible m numbers are studied within the density functional theory framework. The search of isomer structures has been accomplished using multistep hierarchical algorithm. Temperature dependences of thermodynamic functions (enthalpy, entropy and specific heat capacity) have been calculated both for the individual isomers and for the ensemble of isomers with equilibrium composition for each class of clusters, taking into account the anharmonicity of cluster vibrations and the contribution of excited electronic states. The prospects of the application of small atomic Al _n H _m and B _n H _m clusters as the components of energetic and hydrogen storage materials are also discussed.     

Computational Cogitation of Cn@Al12 Clusters

A variety of novel C_nAl₁₂ core–shell nanoclusters have been investigated using density functional calculations. A series of C_n cores (n=1–4) have been encapsulated by icosahedral Al₁₂, with characteristic physical properties (energetics and stabilities, ionisation energies, electron affinities) calculated for each cluster. Other isomers, with the C_n moiety bound externally to the Al₁₂ shell, have also been studied. For both series, a peak in stability was found for n(C)=2, a characteristic that appears to be inextricably linked with the relaxation of the constituent parts upon dissociation. Analysis of trends for ionisation energies and electron affinities includes evaluation of contributions from the carbon and aluminium components, which highlights the effects of composition and morphology on cluster properties.     

Electronic structure and bonding in clusters: Theoretical studies

The electronic structures of small Al _n ,n=5, 9, 13, clusters with bulk geometry are studied using the ab initio Hartree-Fock-LCAO method. The cluster ground states have always multiplicity higher than the lowest possible value. However, the energy difference between ground and lowest low spin state decreases with increasing cluster size. The energy range of the Al _n cluster valence levels is comparable with the width of the occupied part of the 3sp band in bulk Al. The different binding mechanisms that arise when a CO molecule interacts with Al _n clusters in different coordination sites are analyzed in detail with the constrained space orbital variation (CSOV) method. Electrostatic and polarization contributions to the interaction are found to be important. Among charge transfer (donation) contributions π electron transfer from Al _n to CO corresponding to π backbonding is energetically more important than σ electron transfer from CO to Al _n characterizing the σ bond.     

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium–silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O₂ background. Reactions of the heteronuclear oxide cations with methane in a fast‐flow reactor were studied with a time‐of‐flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V₂O₅)_n(SiO₂)_m]⁺ (n=1, m=1–4; n=2, m=1), and the estimated first‐order rate constants for the reactions were close to that of the homonuclear oxide cluster V₄O₁₀⁺ with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal‐oxygen‐centered radicals (O_t ^. ) were found in all of the stable isomers. These O_t ^. radicals are active sites of the clusters in reaction with CH₄. The O_t ^. radicals in [V₂O₅(SiO₂)_1–4]⁺ clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH₄ activation on silica‐supported vanadium oxide catalysts.     

Geometrical and Electronic Properties of Neutral and Anionic AlnBm (n?+?m?=?13) Clusters

Successively substituted Al₁₃ cluster by B atom both neutral and anionic Al _n B _m  (n + m = 13) clusters have been investigated by the density functional theory (DFT) at B3LYP/6-31G (d) level, the aim is to understand the evolution of the structural and electronic properties as a function of composition. The results clearly show Al₁₃ cluster as well as Al rich Al _n B _m clusters prefer the icosahedral geometries while increasing boron contents promote quasi-planar configurations. The geometrical structures of the optimized anionic Al _n B _m ⁻ clusters are very close to those of the neutral clusters with smaller structural modifications. Overall, the vertical ionization potential (VIP), the adiabatic electron affinity (AEA), and the energy gaps (E _g ) of Al _n B _m clusters decrease with increasing of substitution. The largest values of second-order energy differences ( \Updelta₂E), VIP\Updelta_{2}E), VIP, and E _g of Al₁₂B cluster indicate it possesses the most stability among all the investigated clusters, which accords to the experimental results. The simulated photoelectron spectroscopies (PES) of Al _n B _m ⁻ clusters have also been discussed in this article.     

Calculated properties of neutral and charged Al<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript> clusters by density functional theory

Density functional calculations are performed to study the structures and electronic properties of Al _n Co _m clusters with n = 1–7 and m = 1–2. Frequency analysis is also performed after structural optimization to make sure that the calculated ground states are real minima. The corresponding total and binding energies, adiabatic electron affinities and ionization potentials are presented and discussed to aid the identification of our calculations. The BSSE correction is also considered in our calculation. Among Al _n Co _m , Al _n Co _m ⁻, and Al _n Co _m ⁼ clusters (n = 1–7 and m = 1–2), Al₄Co⁻, Al₆Co⁻, Al₂Co₂, and Al₆Co₂ are predicted to be more stable. Our results are consistent with the available experimental data.     

Theoretical Study of Electronic Structure and Stability of Mixed AlmBn−mH n 2− and CmBn−mH n 2−m (n = 6, 10, 12 and m = 1, 2) Clusters

Density functional theory (DFT) method with B3LYP functional and 6-311++G(d,p) basis set has been used to predict the geometries, relative stabilities, electronic structures and bonding analysis of Mixed Al_mB_n?mH _n ^2? and C_mB_n?mH _n ^2?m (n = 6, 10, 12 and m = 1, 2) clusters; being compared to the B_nH _n ^2? ones. Therefore, the DFT results suggest that the replacing of boron by aluminium or carbon is governed by Natural net charges following Gimar’s and Williams’s rules. The Al_mB_n?mH _n ^2? structures are relatively distorted compared to those of B_nH _n ^2? and C_mB_n?mH _n ^2?m . In Al_mB_n?mH _n ^2? structures Al atoms prefer the adjacent sites, however for the C₂B_n?2H_n cluster cages, the carbon atoms are positioned at diametrically opposed sites. The large HOMO–LUMO gaps show that the predicted clusters have chemical stabilities, principally, those of Al_mB_n?mH _n ^2? ones, which are not experimentally isolated. The optimized geometries obtained through boron substitution by Al and C lead to compactness and to contracted structures, respectively, where B–B bonds are the shortest in mono- and di-carbaboranes.     

Binding of ammonia to small copper and silver clusters

We report equilibrium geometries, harmonic frequencies, and thermochemical data for the metal cluster–ammonia complexes Ag_n(NH₃) and Cu_n(NH₃) (n=1,2,3,4), Ag₄(NH₃)₂, and Cu₄(NH₃)₂ calculated by a density functional method. The calculated shifts in ammonia umbrella mode frequency correlate with the observed shifts and the calculated enthalpies of complexation. The preferred site for NH₃ adsorption and the calculated bond enthalpies can be rationalized by considering atomic charges obtained from a natural population analysis and polarization of the metal electron density.     

Cyclam as a Structure-Directing Agent in the Crystallization of Aluminophosphate Open Framework Materials from Fluoride Media

Two aluminofluorophosphates have been synthesized using 1,4,8,11-tetraazacyclotetradecane (cyclam) as a structure-directing agent (SDA). The two materials were synthesized hydrothermally and their structures solved by microcrystal diffraction using synchrotron radiation. In both cases, the SDA has been located crystallographically. The first material, [F, Cyclam]-AlPO-CHA, has the molecular formula Al₆P₆O₂₄F₂.C₁₀N₄H₂₆ and has a framework structure related to the mineral chabazite, with the cyclam molecules occluded within chabazite-like cages (space group P-1, a=9.0993(4) Å, b=9.2232(5) Å, c=9.3929(4) Å, α=77.881(2)^o, β=87.205(1)^o, γ=87.777(1)^o, Z=1, wR(F²)=0.1354, R(F)=0.0487). The second material, [F, Cu-Cyclam]-AlPO-SAS, has the molecular formula Al₈P₈O₃₂F₂.[CuC₁₀N₄H₂₄.2H₂O] and has a framework structure closely related to STA-6 (SAS) zeolite structure type, although the usual tetragonal symmetry has been reduced to monoclinic by the presence of the fluoride ions (space group P2₁/n, a=10.3738(4) Å, b=14.8060(5) Å, c=13.4494(5) Å, β=90.275(1)^o, Z=2, wR(F²)=0.1484, R(F)=0.0524). The cyclam is occluded as a copper complex ordered within the cages of the structure.     

A density functional study on the aggregation of alumina clusters

A calculation has been performed to explore the mechanism of aggregation reaction between two small molecular clusters [(Al₂O₃) _n1 and (Al₂O₃) _n2] by the density functional theory method. Five pathways of aggregation reactions between two different (Al₂O₃)₁ clusters isomers were identified. The detailed process of (Al₂O₃)₁ interaction with (Al₂O₃)₂ were also obtained. All the products of the aggregation reactions between two cage structures are cage-dimer structures. We calculated the thermodynamic properties of all the aggregate clusters. The Gibbs free energy changes of aggregation reactions in 0–1000 K were negative and increased with the temperature increase. The energy changes of enthalpy and entropy were also obtained.     

Order parameter studies from effective order geometry in 4O.Om and 7O.Om liquid crystalline compounds

The effective geometry parameter, α_g = n _o /n _e, is used to evaluate the orientational order parameter, S, in the case of N-(p-n-butyloxybenzylidene)-p-n-alkoxy anilines, 4O.Om and N-(p-n-heptyloxybenzylidene)-p-n-alkoxy anilines, 7O.Om compounds with m?=?3–7 and 9 in the former case and m?=?3, 5–7 and 9 in the later materials. The results obtained are compared with those calculated using the standard techniques of molecular polarisability and birefringence. The effective geometry parameter's influence on the deflection of light by the liquid crystal compounds is also studied. The variation of temperature gradient of the ordinary refractive index, dn _o /dT, and extraordinary refractive index, dn _e /dT, of the liquid crystals is also studied.     

A computational investigation of the electronic properties of Octahedral Al n N n and Al n P n cages (n = 12, 16, 28, 36, and 48)

Density functionla theory (DFT) calculations are performed to characterize geometric and electronic features of the octahedral Al _n N _n and Al _n P _n cages (n = 12, 16, 28, 32, and 48). Toward this aim, ¹⁵N, ²⁷Al, and ³¹P chemical shielding (CS) tensors as well as natural charge analyses are calculated for the optimized structures. CS parameters detect three distinct electronic environments for atoms within the Al _n N _n and Al _n P _n cages. The chemical shifts of N2 sites belonging to a hexagon and surrounded by three hexagons and a square obtained are different from those of N3 sites belonging to a hexagon that is surrounded only by hexagons—due to different curvatures exerted at the sites with different local structures. In addition, there is an increasing tendency in the Δσ values of the three local structures, Δσ (N1) > Δσ (N2) > Δσ (N3), N1 sites belonging to four-membered rings. The chemical shieldings of those Al and P sites belonging to a hexagon that is surrounded only by hexagons in the cages (360.7–366.7 and 496.5–514.7 ppm) are close to those previously reported for AlP nanotubes. Three distinct electrostatic environments around the N, Al, and P nuclei are also confirmed by the calculated natural charges. It should be noted that the positively charged Al atoms on the cages turn out to be the available sites for adsorption of H2 molecules.     

Spin-glass state in Ni5(OH)6(CnH2n−4O4)2 metal-organic frameworks (n = 6, 8, 10, 12)

We present an extension of a previously published work (J. Solid State Chem. 181 (2008) 3229) concerning Metal-Organic Frameworks (MOFs) of general formula Ni₅(OH)₆(C_nH_2n−4O₄)₂. A modified synthesis procedure comprising a room-temperature step prior to the hydrothermal treatment was employed. This preliminary step made use of peristaltic pumps allowing slow mixing of the reactants at a constant pH value. Samples of better purity and crystallinity were consequently obtained. In particular, the better crystallinity allowed us to work on two other members of the series, n = 10 and n = 12, which were characterized using synchrotron powder X-ray powder diffraction. These two compounds are isoreticular with the n = 6 and n = 8 compounds previously reported. The crystal structure incorporates the long alkane dioic acid molecules as pillars between complex inorganic layers. Samples of better purity for n = 6 and 8, as well as those of the new compounds with n = 10 and 12, gave us the opportunity to revise the magnetic properties of these MOFs. We found similar magnetic behaviors, independently of the interlayer spacing. We show that, below 19 K, these materials most probably enter a spin-glass or cluster spin-glass state rather than a three-dimensionally long-range ordered state. We link this behavior to the complex topology of the magnetic exchange interactions within the inorganic layers which is very likely to be source of magnetic frustration.     

Optical properties of green-blue-emitting Ca-α-Sialon:Ce,Li phosphors for white light-emitting diodes (LEDs)

Ce³⁺,Li⁺-codoped Ca-α-Sialon phosphors with the formula [Ca_(1−2x)Ce_xLi_x]_m/2Si_12−(m+n)Al_m+nO_nN_16−n (0≤x≤0.25, 0.5≤m≤3.5, and 0.16≤n≤2.0) have been synthesized by gas pressure sintering (GPS). The effects of the activator concentration and the overall composition of host lattice on the phase evolution, morphology, and optical properties were investigated. The single-phase Ca-α-Sialon:Ce³⁺,Li⁺ can be synthesized at x<0.1, 1.0≤m≤2.5, and n≤1.2. The synthesized powders exhibit a relatively dispersive and uniform morphology. Under the near UV excitation, the bright green-blue emission centered at 500-518 nm is observed. The photoluminescence can be tailored by controlling Ce³⁺ concentration and the overall composition of the α-Sialon host lattice. With increasing the Ce concentration and m value both excitation and emission bands show a red shift, which perfectly matches with the near-UV LEDs in the range of 360-410 nm. The strongest luminescence is achieved at x=0.08-0.1, m=2.0-2.5, and n=1.0. Simultaneously, the highest quantum efficiency and better thermal stability are also present.     

Synthesis and characterisation of Co(II), Ni(II), Zn(II) and Cd(II) complexes with 5-bromo-N,N′-bis-(salicylidene)-o-tolidine

New complexes of type [M(HL)(CH₃COO)(OH₂)_m]·nH₂O (where M:Co, m = 2, n = 2; M:Ni, m = 2, n = 1.5; M:Zn, m = 0, n = 2.5 and M:Cd, m = 0, n = 0; H₂L:5-bromo-N,N′-bis-(salicylidene)-o-tolidine) have been synthesized and characterized by microanalytical, IR, UV–Vis-NIR and magnetic data. Electronic spectra of Co(II) and Ni(II) complexes are characteristic for an octahedral stereochemistry. The IR spectra indicate a chelate coordination mode for mono-deprotonated Schiff base and a bidentate one for acetate ion. The thermal transformations are complex according to TG and DTA curves including dehydration, acetate decomposition and oxidative degradation of the Schiff base. The final product of decomposition is the most stable metallic oxide.     

Study of water clusters in the n = 2–34 size regime, based on the ABEEM/MM model

Various properties of water clusters in the n = 2–34 size regime with the change of cluster size have been systemically explored based on the newly developed flexible-body and charge-fluctuating ABEEM/MM water potential model. The ABEEM/MM water model is to take ABEEM charges of all atoms, bonds, and lone-pairs of water molecules into the intermolecular electrostatic interaction term in molecular mechanics. The computed correlating properties characterizing water clusters (H₂O) _n (n = 2–34) include optimal structures, structural parameters, ABEEM charge distributions, binding energies, hydrogen bonds, dipole moments, and so on. The study of optimal structures shows that the ABEEM/MM model can correctly predict the following important structural features, such as the transition from two-dimensional (from dimer to pentamer) to three-dimensional (for clusters larger than the hexamer) structures at hexamer region, the transition from cubes to cages at dodecamer (H₂O)₁₂, the transition from all-surface (all water molecules on the surface of the cluster) to one water-centered (one water molecule at the center of the cluster, fully solvated) structures at (H₂O)₁₇, the transition from one to two internal molecules in the cage at (H₂O)₃₃, and so on. The first three structural transitions are in good agreement with those obtained from previous work, while the fourth transition is different from that identified by Hartke. Subsequently, a systematic investigation of structural parameters, ABEEM charges, energetic properties, and dipole moments of water clusters with increasing cluster size can provide important reference for describing the objective trait of hydrogen bonds in water cluster system, and also provide a strong impetus toward understanding how the water clusters approach the bulk limit.     

Structures,stabilities, and electronic properties of Al n Au (n = 1–15) clusters: A density functional study

We present density functional calculations of Al _n Au clusters for n = 1–15. The growth pattern for Al _n Au (n = 1–7, 12, 14, 15) clusters is the Au atom occupying a peripheral position of Al _n clusters, and the growth pattern for Al _n Au (n = 8, 10 and 13) clusters is Au-substituted Al _n+1 clusters. It is found that the Au atom replaces the surface atom of an Al _n+1 cluster and occupies a peripheral position. In addition, the ground state structures of Al _n Au clusters are more stable than pure Aln clusters. It is found that the Al₁₃Au cluster exhibits high stability.     

Stable shell structures in hydrogen-bonded complexes

Neutral clusters (NH₃) _n ((CH₃)₃N) _m and (H₂O) _n ((CH₃)₃N) _m , prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time-of-flight mass spectrometry technique. The observed mixed cluster ions display a maximum intensity atm=2(n+1) whenn ≦ 5 for (NH₃) _n ·((CH₃)₃N) _m H⁺ andm=n+2 whenn ≦ 4 for (H₂O) _n ((CH₃)₃N) _m H⁺ indicating that the cluster ions with these combinations have a stable closed shell structure. However, the pattern begins to break down whenn>5 for ammonia system andn>4 for water system. Thereupon, the most intense peaks are reached with one molecule less than the pattern required, i.e.m=2(n+1)?1 whenn=6 for (NH₃) _n ((CH₃)₃N) _m H⁺ andm=(n+2)?1 whenn=5 for (H₂O) _n ((CH₃)₃N) _m H⁺. These findings strongly suggest the onset of hydrogen-bonded ring structures from chain-like ones at critical cluster sizes. This is also supported by the studies of the metastable decomposition.