J(77Se,1H) and J(77Se,13C) couplings of seleno‐carbohydrates obtained by 77Se satellite 1D 13C spectroscopy and 77Se selective HR‐HMBC spectroscopy

Seleno‐carbohydrates are those in which the oxygen of the glycosidic bond or the hydroxyl group is artificially replaced with selenium. This substitution changes ¹H and ¹³C chemical shifts and produces spin coupling constants involving ⁷⁷Se. Coupling constants, such as ^2‐3J(⁷⁷Se, ¹H), are likely to be useful for conformational analyses of glycans because such couplings are never observed in natural glycans. Several papers have discussed the relationship between ^2‐3J(⁷⁷Se, ¹H) and conformation; however, only few reports describe ^1‐3J(⁷⁷Se, ¹³C), which could also be useful. Here, we obtain ⁷⁷Se coupling constants of seleno‐carbohydrates from ⁷⁷Se‐selective HR‐HMBC and ⁷⁷Se satellites in 1D ¹³C spectra and examine their conformations using the Newman projection scheme.     

Melting of gold microclusters

The transition from solid-like to liquid-like behavior in Au _n ,n=6, 7, 13, clusters is studied using molecular dynamics simulations. A Gupta-type potential with all-neighbour interactions is employed to incorporaten-body effects. The melting-like transition is described in terms of short-time averages of the kinetic energy per particle, root-mean-square bond length fluctuations and mean square displacements. A comparison between melting temperatures of Au _n and Ni _n clusters is presented.     

Structure and spectroscopy of water-fluoride microclusters: a combined genetic algorithm and DFT-based study

In this article, we explore the efficiency of using a coupled genetic algorithm (GA) and density functional theory (DFT) based strategy to evaluate probable structures of (H(2) O)(n) F(-) micro-clusters, with n = 1 - 6. We use the stochastic optimization technique of GA to arrive at structures of the cluster systems and once the structures are obtained, do a DFT calculation with the optimized coordinates from the GA calculation as input to get the infra-red spectrum of all the systems. The results of our work closely resembles the pure quantum chemical results obtained by Baik et al. (J Chem Phys 1999, 110, 9116-9127).     

Optical spectra of sodium microclusters

Photoabsorption spectra have been measured for free neutral sodium clusters containing fromN=3 to 40 atoms. In the size range ofN≈3 to 5, a transition occurs from molecule-like absorption to collective excitations of the valence electrons. ForN≈6 to 12, the data are well described by an ellipsoidal shell model. In open-shell clusters, the multiple surface plasma resonances expected for spheroidal or ellipsoidal shapes are observed. The experimental resonance positions provide a sensitive measurement of the cluster distortions. ForN?13, the per atom strength of these collective resonances is reduced; this may be due to peak fragmentation caused by interaction between the surface plasmon and nearby single-electron resonances. In three distinct wavelength regions, one of which corresponds to the position of the Na atom “D-lines”, additional absorption is seen in the spectra of all investigated clusters.     

Electronic states of Mn microclusters

Electronic states of Mn _n ⁺ microclusters forn=4～7 are calculated by the spin-polarized DV-Xα-LCAO method. The energy levels of valence electrons are composed of two parts, the dense levels which contain much 3d electron components and the sparse levels which contain much 4s electron components. The distribution of the 4s electron levels roughly corresponds to that of one-electron orbitals in the shell model. The highest occupied level of the Mn ₅ ⁺ microcluster is located at a 4s electron level, which is filled with electrons, whereas those of the other microclusters are located at partially filled 4s levels or at dense 3d levels. These results explain the origin of the magic numbers observed in Mn cation microclusters.     

Lifetime-broadening-suppressed selective XAFS spectroscopy.

Lifetime-broadening-suppressed (LBS) state-selective XAFS spectra can be deduced by analyzing resonant inelastic x-ray scattering (RIXS) in terms of formulae derived from the Kramers-Heisenberg equation. By a combination of third-generation synchrotron sources and spectrometers equipped with a large acceptance as well as high-resolution analyzers, high-quality RIXS data to extract LBS selective XAFS can be collected. In this review, basic aspects of this novel technique are described, and its potentiality as an analytical tool to determine the local electronic and magnetic structures of metal atoms is demonstrated with several recent examples.     

Vibrational properties of sodium microclusters

The structural and vibrational properties of Na_n (n=2–8) microclusters at very low temperature (T≈0.1 K) are analyzed by tight-binding molecular dynamics simulations. By using the Fourier transform of the velocity autocorrelation function from the trajectories of tight-binding molecular dynamics, the present model is successful in predicting some of the experimental, far infrared spectra and agrees very well with a standard finite difference harmonic method and some other more sophisticated calculations.     

Atomic and electronic structure of microclusters

The atomic and electronic structure of alkali metal, noble metal and Si_N (N=6,10) clusters examined theoretically are reviewed. On metal clusters, validity of the spherical jellium model is discussed by examining stabilization by multiplets formation and crystalline fields. In connection with the fragmentation of metal clusters, the shell correction of a largely deformed metal clusters is discussed by referring to Strutinsky's theory of nuclear fission. On the Si₆ and Si₁₀ clusters a large reconstruction theoretically found is discussed in terms of the interaction among dangling bonds. Finally, recent work on dynamical properties of clusters are briefly reviewed.     

Thermodynamic properties of germanium/carbon microclusters

Theoretical studies on the Ge(n)C(m) (n=1,2; m=1-3) microclusters have been performed using the state of the art calculations. Several alternative structures of these clusters were studied to locate the lowest-energy isomers. It is observed that the structures of the complexes result from the competition between ionic Ge-C, conjugated covalent C-C, and metallic Ge-Ge bonds. The ionization of the molecules enhances the ionic character of the Ge-C bond and has significant structural consequences. Using theoretically determined partition functions, thermodynamic data are computed and experimental enthalpies are enhanced. The ab initio atomization energies of germanium carbides compare well with corrected experimental functions. The experimental appearance potentials are well reproduced by the theoretical ionization potentials.     

Complex colloidal microclusters from aerosol droplets

In this Article, we report on a packing scheme of monodisperse colloidal nanospheres by aerosol-assisted clustering. When an aqueous suspension of colloidal nanospheres was sprayed into aerosol droplets by an ultrasonic nebulizer, the nanospheres were encapsulated in the aerosol droplets and the evaporation of water from the droplets at high temperatures led to the packings of nanospheres. The configurations of the colloidal nanospheres minimized the interparticle potential energy or the second moment of mass distribution depending on the number of the constituting nanospheres. Other types of nonspherical microparticles or hollow microclusters were also produced by self-organizing organic-inorganic binary colloids of different sizes in an aerosol spray pyrolysis reactor. The aerosol-assisted fabrication of colloidal clusters was very effective as compared to the method based on oil-in-water or water-in-oil emulsions, in which the removal of residual oil was not easy and time-consuming.     

Size selective self-sorting in coordination-driven self-assembly of finite ensembles

Size selective self-sorting in the coordination-driven self-assembly of two-dimensional (2-D) polygons and three-dimensional (3-D) cages is presented. Two types of polygons (rectangular and triangular) of different size are formed spontaneously from within mixtures of a molecular "clip" or a 60 degrees organoplatinum acceptor with dipyridyl linkers of different lengths via self-sorting. Furthermore, two different sized 3-D supramolecular cages are formed upon mixing one ditopic organoplatinum acceptor and two different sized tritopic donors from the self-sorting process. The formation of these polygons and cages is characterized using NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). In all cases, the self-sorting process is directed by the size of the donor building blocks and a dynamic, thermodynamically driven self-correction process resulting in the formation of discrete products from complex mixtures.     

Isomer selective infrared spectroscopy of neutral metal clusters

We report experimental infrared spectra of neutral metal clusters in the gas phase. Multiple photon dissociation of the argon complexes of niobium clusters is used to obtain vibrational spectra in the 80-400 cm(-1) region. The observed spectra for Nb(9)Ar(n) (n=1-4) are different for different values of n. This is explained by the presence of two isomers of Nb(9) that have different affinities towards Ar and the isomer specific infrared spectra are obtained. The structures of the isomers are determined by comparing the observed spectra with the outcome of density-functional theory calculations.     

Size of neutral argon clusters from core-level photoelectron spectroscopy

Theoretical models of lineshapes in Ar2p photoionization spectra have been calculated for free, neutral argon clusters of different sizes. The lineshape models are fitted to experimental spectra and used to estimate the mean cluster size realized in the experiment. The results indicate that size estimators working from stagnation conditions [R. Karnbach, M. Joppien, J. Stapelfeldt, J. W?rmer and T.M?ller, Rev. Sci. Instrum., 1993, 64, 2838] may underestimate the mean cluster size.     

Evolution of bulk-like properties in mercury microclusters

The transition from non-metallic to metallic behavior of mercury microclusters, Hg _n , withn ranging from 2 to 79, is investigated using first principles tight-binding linear muffin-tin orbital method. Then dependence of the ionization potential, the cohesive energy, the energy gap, and the nature of the bonding indicates metallic behavior for Hg _n withn≧80. The average bond length of Hg clusters is found to be larger than that of the bulk. Our results are in good agreement with experiments.     

Structural and dynamical properties of magnesium microclusters

We present systematic Density Functional Theory-Local Density Approximation computations for neutral Magnesium clusters Mg _n withn≤13. For the smaller sizes the ground state structure is optimized starting from selected symmetries and allowing for relaxation, Jahn-Teller distorsion and spin polarization. For the larger sizes we perform a simulated annealing based on the ab-initio Molecular Dynamics. By the same method, we study the thermal and dynamical properties of Mg₁₀ and Mg₁₆. The general picture emerging from these computations shows that already atn ≈10 these clusters have acquired many characteristic features of metallic Magnesium.     

Surface-enhanced Raman scattering on mercaptopyridine-capped CdS microclusters

We obtained the high-quality Raman spectra of 4-mercaptopyridine (4-Mpy) adsorbed on CdS microclusters. The Raman signals were enhanced relative to the same molecules in solution. We compared the Raman spectra of 4-Mpy molecules adsorbed on CdS microclusters and Ag substrate. The difference of 4-Mpy molecules adsorbed on semiconductor and metal substrate was revealed. The results demonstrated that adsorbed species on semiconductor CdS can be detected by SERS spectroscopy.     

Size fractionation of graphene oxide sheets by pH-assisted selective sedimentation

Graphene oxide (GO) sheets prepared by Hummers' method have been separated into two portions with large (f1) or small (f2) lateral dimensions from their aqueous dispersion. This method is based on the selective precipitation of GO sheets with lateral dimensions mostly (>90%) larger than 40 μm(2) at a pH value of 4.0 because of their larger hydrophobic planes and fewer hydrophilic oxygenated groups. The hydrazine reduced Langmuir-Blodgett (LB) films of f1 showed much higher conductivities than those of f2. Furthermore, the thin film of f1 prepared by filtration exhibited a smaller d-space and much higher tensile strength and modulus than those of f2 films. The one-step size fractionation method reported here is simple, cheap, efficient, and environmentally friendly, which can be used for the size fractionation of GO sheets in large scale.     

Phase and structure transformations in water microclusters

The behavior of 12- and 14-molecule water microclusters with ST2 potential in the range of 50–325 K is studied by the molecular dynamics method. Structure and phase transitions, including evaporation of molecules from the clusters and transition through the percolation threshold via hydrogen bonds, were found. Topological, temporal, and energy characteristics of the bond network are analyzed based on the dynamic criterion of hydrogen bonding. Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences. Translated fromZhurmal Strukturnoi Khimii, Vol. 35, No. 1, pp. 64–70, January–February, 1994. Translated by L. Chernomorskaya     

Multipotential model for atoms in alkali microclusters

An alkali-atom shell model for alkali microclusters is introduced, where the quantum constituent is the neutral atom itself considered as a heavy fermion in a central potential created by all atoms in each shell of the cluster. Thus, here the magic numbers are due to atoms and not to electrons as in jellium models. The model is applicable under conditions (of production, temperature, size, etc.) not favoring delocalization of valence electrons. For conditions favoring electron delocalization the jellium model is applicable. New experiments are suggested via nuclear methods.     

Site selective spectroscopy and structural analysis of yttria-doped zirconias

Yttria-stabilized zirconias Zr_1?xY_xO_2?0.5x (tetragonal, cubic fluorite, and cubic CLn₂O₃-type forms) are investigated by spectroscopic methods. Optically active europium ions that partially substitute yttrium ions act as a structural microprobe. Both conventional ultraviolet and dye laser (site-selective) excitation are used. The yttria-stabilized zirconias may be well described by a cationic sublattice in which tetravalent and trivalent cations are statistically distributed. The anionic sublattice accommodates vacancies at random on oxygen positions. These vacancies in turn cause complex oxygen displacements giving rise to what may be described as a “glass of anions.”