Molecular dynamics study of phase transition and nucleation in supercooled clusters of potassium iodide

In a series of molecular dynamics (MD) runs on (KI)₁₀₈ clusters, the Born–Mayer–Huggins potential function is employed to study structural, energetic, and kinetic aspects of phase change and the homogeneous nucleation of KI clusters. Melting and freezing are reproducible when clusters are heated and cooled. The melted clusters are not spherical in shape no matter the starting cluster is cubic or spherical. Quenching a melted (KI)₁₀₈ cluster from 960 K in a bath with temperature range 200–400 K for a time period of 80 ps both nucleation and crystallization are observed. Nucleation rates exceeding 10³⁶ critical nuclei m⁻³ s⁻¹ are determined at 200, 250, 300, 350, and 400 K. Results are interpreted in terms of the classical theory of nucleation of Turnbull and Fisher and of Buckle. Interfacial free energies of the liquid–solid phase derived from the nucleation rates are 7–10 mJ m⁻². This quantity is 0.19 of the heat of transition per unit area from solid to liquid, or about two-thirds of the corresponding ratio which Turnbull proposed for freezing transition. The temperature dependence of σ_sl(T) of (KI)₁₀₈ clusters can be expressed as σ_sl(T)∝T^0.34.     

Four‐Shell Polyoxometalates Featuring High‐Nuclearity Ln26 Clusters: Structural Transformations of Nanoclusters into Frameworks Triggered by Transition‐Metal Ions

A series of polyoxometalates (POMs) that incorporate the highest‐nuclearity Ln clusters that have been observed in such structures to date (Ln₂₆ , Ln=La and Ce) are described, which exhibit giant multishell configurations (Ln⊂W₆⊂Ln₂₆⊂W₁₀₀). Their structures are remarkably different from known giant POMs that feature multiple Ln ions. In particular, the incorporated Ln–O clusters with a nuclearity of 26 are significantly larger than known high‐nuclearity (≤10) Ln–O clusters in POM chemistry. Furthermore, they also contain the largest number of La and Ce centers for any POM reported to date and represent a new kind of rare giant POMs with more than 100 W atoms. Interestingly, the La₂₆‐containing POM can undergo a single‐crystal to single‐crystal structural transformation in the presence of various transition‐metal ions, such as Cu²⁺, Co²⁺, and Ni²⁺, from an inorganic molecular nanocluster into an inorganic–organic hybrid extended framework that is built from POM building blocks with even higher‐nuclearity La₂₈ clusters bridged by transition‐metal complexes.     

Synthesis,Structural Characterization,Electronic Structure,and Magnetic Properties of the Zintl Phase Eu10Cd6Bi12

A new transition‐metal‐containing Zintl phase, Eu₁₀Cd₆Bi₁₂, was synthesized by combining the elements in excess molten Cd. Single‐crystal X‐ray diffraction studies indicated that this compound crystallizes in the orthorhombic space group Cmmm (No. 65) with a=7.840(2), b=24.060(7), and c=4.7809(14) Å. The crystal structure of Eu₁₀Cd₆Bi₁₂ can be viewed as a stacking of a series of [Cd₆Bi₁₂] double layers, which are arranged alternately along the b axial direction. The layers are composed of corner‐ and edge‐shared CdBi₄ tetrahedra, a common feature in the crystal chemistry of many transition‐metal Zintl phases. Electronic‐band‐structure calculations confirm the closed‐shell configuration of all constituent elements and corroborate the electron count inferred by the Zintl formalism, that is, [Eu²⁺]₁₀[Cd²⁺]₆[Bi^3?]₈[Bi^2?]₄. Magnetic‐susceptibility measurements confirm the divalency of europium and show the existence of a long‐range antiferromagnetic order of the Eu spins below 12.3 K.     

Theory of cluster self-organization for crystal-forming systems: A model of transition from disodered to ordered systems

The current state of ideas concerning the self-organization of crystal-forming systems where long-range order spontaneously appears in the arrangement of nanolevel structural units of any nature (micro- and macromolecules or atomic clusters) that initially existed in a dynamic state as a chaotic mixture is considered. Three partially overlapping stages of self-organization of a system accepted in physical models of “order-disorder” kinetic transitions are matched to those used in supramolecular chemistry. An algorithmically constructed model of transition from disordered to hierarchically ordered systems is considered. The geometrical and topological modeling of density fluctuations of n-atomic species (clusters) An in a crystal-forming medium is carried out. A specific set of An clusters with block-diagonal connectivity matrices is recognized. These types of clusters (S ₃⁰), having “sectional” or “hierarchic” partition, are defined as precursors of crystal structures that are capable of evolving most rapidly to give rise to a long-range order in structures. For an S ₃⁰ ring cluster shaped as a triangle, geometrical and topological modeling is carried out for all of the eight topologically and symmetrically possible types of S ₃¹ primary chains built of S ₃⁰ using theory of one-dimensional symmetry groups. Thirty three structural variants of morphologically and topologically different types of S ₃² micronets described by two-dimensional groups of symmetry are considered. Algorithms are presented for combinatorial and topological analysis to search for precursor clusters and restore a three-dimensional net of covalent and noncovalent bonds in a crystal structure by the matrix (cluster) self-assembly mechanism. The model advanced is universal. Examples of self-assembly of a series of cluster-assembled structures of AB₂ alloys of the unique Friauf-Laves family (which counts in 1400 of binary and ternary compounds) are given: for MgCu₂ (cF24) (with its superstructures of ZrCu ₅ and MgSnCu₄ types), MgZn₂ (hP12), and MgNi₂ (hP24) (from AB₂ or A₂B + B₃ three-atom clusters); for ZrZn₂₂ icosahedral structures (from a suprapolyhedral cluster built of a ZrZn₁₆ Friauf polyhedron and two ZnZn₁₂ icosahedra); NaCd₂ (from one A cluster with 61 atoms and two B clusters with 63 atoms); and for a bimolecular compound C₇₈H₃₀ (which is formed of fullerene C₆₀ and a C₂₈H₃₀ molecule). The scenario of formation of self-curving nets with icosahedral symmetry is considered: to form a B₁₂ icosahedron from two isomers with n = 3, a C₂₀ dodecahedron from two isomers with n = 5, and C₆₀ fullerene from pentagonal clusters with n = 5.     

二元钠硅酸盐的精细结构和 29Si化学位移的从头计算研究

本文应用量子化学从头计算方法研究二元钠硅酸盐的精细结构，对典型的系列二元钠硅酸盐团簇模型采用闭壳层Hartree-Fock(RHF)方法和基组6-31G(d)优化构型，并计算了硅酸盐团簇中 ²⁹Si核磁共振化学位移δ_iso^Si，用硅氧四面体应力指数这一精细结构参数分析了计算得到的δ_iso^Si数值。同时对 ²⁹Si核磁共振化学位移δ_iso^Si的计算值和实验值进行比较，并讨论了两者间在数值上存在的差异。研究表明， ²⁹Si核磁共振化学位移与硅酸盐的精细结构密切相关，这说明硅酸盐的宏观物理和化学性质可能主要依赖于其精细结构而非初级结构单元-硅氧四面体。     

La-O小团簇上超氧物种与过氧物种间的连接途径

采用密度泛函理论方法考察了La-O团簇上超氧物种与过氧物种间转化的连接途径. 单重态下, 团簇上单个超氧物种可通过一系列臭氧物种转化为过氧物种, 且转化能垒较高;三重态下, 单个超氧物种则并无与过氧物种间连接的途径. 然而, La-O团簇上两超氧物种间的相互作用及其转化也具单重态和三重态两条途径. 三重态下, 超氧物种可很容易地转化为过氧物种(O₂^- + O₂^-↔O₂^2- + O₂), 超氧物种与过氧物种处于快速的交换状态之中;单重态下, 超氧物种转化为过氧物种则需较高的活化能垒, 表明在单重态下这些氧物种具有较高的稳定性.     

PathOpt—A global transition state search approach: Outline of algorithm

We propose a new algorithm to determine reaction paths and test its capability for Ar₁₂ and Ar₁₃ clusters. Its main ingredient is a search for the local minima on a (n?1) dimensional hyperplane (n = dimension of the complete system in Cartesian coordinates) lying perpendicular to the straight line connection between initial and final states. These minima are part of possible reaction paths and are, hence, used as starting points for an uphill search to the next transition state. First, path fragments are obtained from subsequent relaxations starting from these transition states. They can be combined with information from the straight line connection procedure to obtain complete paths. Our test computations for Ar₁₂ and Ar₁₃ clusters prove that PathOpt delivers several reaction paths in one round. © 2013 Wiley Periodicals, Inc.     

Low‐Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ‐S) Complexes

Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L₃Fe)₂(μ‐S)] complexes that were isolated and characterized in the low‐valent oxidation states Fe^II? S? Fe^II, Fe^II? S? Fe^I, and Fe^I? S? Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe₂(μ‐S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low‐spin S=1/2 states upon reduction from Fe^II to Fe^I. The possibility of accessing low‐spin, pseudotetrahedral Fe^I sites compatible with S^2? as a ligand was previously unknown.     

High Magnetic Moments in Manganese‐Doped Silicon Clusters

We report on the structural, electronic, and magnetic properties of manganese‐doped silicon clusters cations, Si_nMn⁺ with n=6–10, 12–14, and 16, using mass spectrometry and infrared spectroscopy in combination with density functional theory computations. This combined experimental and theoretical study allows several structures to be identified. All the exohedral Si_nMn⁺ (n=6–10) clusters are found to be substitutive derivatives of the bare Si_n+1⁺ cations, while the endohedral Si_nMn⁺ (n=12–14 and 16) clusters adopt fullerene‐like structures. The hybrid B3P86 functional is shown to be appropriate in predicting the ground electronic states of the clusters and in reproducing their infrared spectra. The clusters turn out to have high magnetic moments localized on Mn. In particular the Mn atoms in the exohedral Si_nMn⁺ (n=6–10) clusters have local magnetic moments of 4 μ_B or 6 μ_B and can be considered as magnetic copies of the silicon atoms. Opposed to other 3d transition‐metal dopants, the local magnetic moment of the Mn atom is not completely quenched when encapsulated in a silicon cage.     

The Geometric Structure of Silver‐Doped Silicon Clusters

Cationic silver‐doped silicon clusters, Si_nAg⁺ (n=6–15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin‐hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low‐energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition‐metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si_nAg⁺ (n=7–9) has a pentagonal bipyramidal building block, whereas the larger Si_nAg⁺ (n=10–12, 14, 15) clusters have trigonal prism‐based structures. On comparing the structures of Si_nAg⁺ with those of Si_nCu⁺ (for n=6–11) it is found that both Cu and Ag adsorb on a surface site of bare Si_n⁺ clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si_n⁺ framework than the Cu dopant atom.     

Reactivity of Oxygen Radical Anions Bound to Scandia Nanoparticles in the Gas Phase: CH Bond Activation

The activation of C?H bonds in alkanes is currently a hot research topic in chemistry. The atomic oxygen radical anion (O^?.) is an important species in C?H activation. The mechanistic details of C?H activation by O^?. radicals can be well understood by studying the reactions between O^?. containing transition metal oxide clusters and alkanes. Here the reactivity of scandium oxide cluster anions toward n‐butane was studied by using a high‐resolution time‐of‐flight mass spectrometer coupled with a fast flow reactor. Hydrogen atom abstraction (HAA) from n‐butane by (Sc₂O₃)_NO^? (N=1–18) clusters was observed. The reactivity of (Sc₂O₃)_NO^? (N=1–18) clusters is significantly sizedependent and the highest reactivity was observed for N=4 (Sc₈O₁₃^?) and 12 (Sc₂₄O₃₇^?). Larger (Sc₂O₃)_NO^? clusters generally have higher reactivity than the smaller ones. Density functional theory calculations were performed to interpret the reactivity of (Sc₂O₃)_NO^? (N=1–5) clusters, which were found to contain the O^?. radicals as the active sites. The local charge environment around the O^?. radicals was demonstrated to control the experimentally observed size‐dependent reactivity. This work is among the first to report HAA reactivity of cluster anions with dimensions up to nanosize toward alkane molecules. The anionic O^?. containing scandium oxide clusters are found to be more reactive than the corresponding cationic ones in the C?H bond activation.     

Hexagonal Bipyramidal [Ta2B6]−/0 Clusters: B6 Rings as Structural Motifs

It has been a long‐sought goal in cluster science to discover stable atomic clusters as building blocks for cluster‐assembled nanomaterials, as exemplified by the fullerenes and their subsequent bulk syntheses. ¹ , ² Clusters have also been considered as models to understand bulk properties, providing a bridge between molecular and solid‐state chemistry. ³ Because of its electron deficiency, boron is an interesting element with unusual polymorphism. While bulk boron is known to be dominated by the three‐dimensional (3D) B₁₂ icosahedral motifs, ⁴ new forms of elemental boron are continuing to be discovered. ⁵ In contrast to the 3D cages commonly found in bulk boron, in the gas phase two‐dimensional (2D) boron clusters are prevalent. ⁶ – ⁸ The unusual planar boron clusters have been suggested as potential new bulking blocks or ligands in chemistry. ^6a Herein we report a joint experimental and theoretical study on the [Ta₂B₆]⁻ and [Ta₂B₆] clusters. We found that the most stable structures of both the neutral and anion are D_6h bipyramidal, similar to the recently discovered MB₆M structural motif in the Ti₇Rh₄Ir₂B₈ solid compound. ⁹      

Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3− (A=As,Sb, Bi) Complexes

Heteronuclear transition‐metal–main‐group‐element carbonyl complexes of AsFe(CO)₃⁻, SbFe(CO)₃⁻, and BiFe(CO)₃⁻ were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass‐selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃⁻. Chemical bonding analyses on the whole series of AFe(CO)₃⁻ (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp–p) type of transition‐metal–main‐group‐element multiple bonding. The σ‐type three‐orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃⁻ (A=As, Sb, Bi) complexes.     

Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3− (A=As,Sb, Bi) Complexes

Heteronuclear transition‐metal–main‐group‐element carbonyl complexes of AsFe(CO)₃^?, SbFe(CO)₃^?, and BiFe(CO)₃^? were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass‐selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃^?. Chemical bonding analyses on the whole series of AFe(CO)₃^? (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp–p) type of transition‐metal–main‐group‐element multiple bonding. The σ‐type three‐orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃^? (A=As, Sb, Bi) complexes.     

Activating Bi p-orbitals in Dispersed Clusters of Amorphous BiOx for Electrocatalytic Nitrogen Reduction

Catalytic strategies based on main group metals are significantly less advanced than those of transition metal catalysis, leaving untapped areas of potentially fruitful research. We here demonstrate an effective approach for the modulation of Bi 6p energy levels during the construction of atomically dispersed clusters of amorphous BiO_x. Bi oxidation state is proposed to strongly affects the nitrogen fixation activity, with the half-occupied p_z orbitals of the Bi²⁺ ions being highly efficient toward electron injection into the inert N₂ molecule. With sufficient catalytic sites to adsorb and activate N₂, the bonding between N₂ and catalyst is able to be in situ identified. The catalyst shows an outstanding Faraday efficiency (≈30 %) and high yield (≈113 μg h⁻¹ mg⁻¹_cat) in NH₃ production, outperforming most of the existing catalysts in aqueous solution. These results lay the basis for developing the potential of p-block elements for catalysis of multi-electron reactions.     

Electronic Structures of Perovskite Oxides of Transition Metals of the Type LaMO3 (M = Ti-Ni) as Revealed by MSXα Investigations

Systematic investigations of the electronic structures of MO^9-₆ (M = Ti-Ni) clusters, as in the LaMO₃ type perovskite oxides, have been carried out by employing the multiple-scattering Xα (MSXα) method. The crystal-field splitting of the metal d level is found to increase, while the oxygen-to-metal charge-transfer energy decreases across the transition metal series. Systematic trends are also seen in the mixing (covalency) between the metal d and the oxygen 2p orbitals in the series.     

Structural and Magnetic Properties of CoGen− (n=2–11) Clusters: Photoelectron Spectroscopy and Density Functional Calculations

A series of cobalt‐doped germanium clusters, CoGe_n^?/0 (n=2–11), are investigated by using anion photoelectron spectroscopy combined with density functional theory calculations. For both anionic and neutral CoGe_n (n=2–11) clusters, the critical size of the transition from exo‐ to endohedral structures is n=9. Natural population analysis shows that there is electron transfer from the Ge_n framework to the Co atom at n=7–11 for both anionic and neutral CoGe_n clusters. The magnetic moments of the anionic and neutral CoGe_n clusters decrease to the lowest values at n=10 and 11. The transfer of electrons from the Ge_n framework to the Co atom and the minimization of the magnetic moments are related to the evolution of CoGe_n structures from exo‐ to endohedral.     

Tautomerism and proton transfer in photoionized acetaldehyde and acetaldehyde–water clusters

Understanding the gas‐phase chemistry of acetaldehyde can be challenging because the molecule can assume several tautomeric forms, namely keto, enol and carbene. The two last forms are the most stable ionic forms. Here, insight into the gas‐phase cluster ion chemistry of homogeneous acetaldehyde and mixed water–acetaldehyde clusters is provided by mass spectrometry/vacuum ultraviolet photoionization combined with density functional theory calculations. (AA)_nH⁺ clusters (AA = acetaldehyde) and mixed (AA)_nH₃O⁺ clusters were detected using tunable vacuum ultraviolet photoionization. Barrierless proton transfers were observed during the geometry optimization of the most stable dimer structures and helped to explain the cluster ion chemistry induced by photoionization, namely the formation of deprotonated tautomers and protonated keto tautomers. Water was found to catalyze the keto–enol and keto–carbene isomerizations and facilitate the proton transfer from the ionized enol or carbene part of the cluster to the neutral keto part, resulting in protonated keto structures. The production of protonated keto structures was identified to be the main fragmentation channel following ionization of the homogeneous acetaldehyde cluster and a channel for ionized mixed clusters as well. These findings are significant for a broad range of fields, including current atmospheric models, because acetaldehyde is one of the most prominent organic species in the troposphere and ions play a crucial role in aerosol formation. Copyright © 2014 John Wiley & Sons, Ltd.