Singly and doubly lithium doped silicon clusters: geometrical and electronic structures and ionization energies

The geometric structures of neutral and cationic Si(n)Li(m)(0/+) clusters with n = 2-11 and m = 1, 2 are investigated using combined experimental and computational methods. The adiabatic ionization energy and vertical ionization energy (VIE) of Si(n)Li(m) clusters are determined using quantum chemical methods (B3LYP/6-311+G(d), G3B3, and CCSD(T)/aug-cc-pVxZ with x = D,T), whereas experimental values are derived from threshold photoionization experiments in the 4.68-6.24 eV range. Among the investigated cluster sizes, only Si(6)Li(2), Si(7)Li, Si(10)Li, and Si(11)Li have ionization thresholds below 6.24 eV and could be measured accurately. The ionization threshold and VIE obtained from the experimental photoionization efficiency curves agree well with the computed values. The growth mechanism of the lithium doped silicon clusters follows some simple rules: (1) neutral singly doped Si(n)Li clusters favor the Li atom addition on an edge or a face of the structure of the corresponding Si(n)(-) anion, while the cationic Si(n)Li(+) binds with one Si atom of the bare Si(n) cluster or adds on one of its edges, and (2) for doubly doped Si(n)Li(2)(0/+) clusters, the neutrals have the shape of the Si(n+1) counterparts with an additional Li atom added on an edge or a face of it, while the cations have both Li atoms added on edges or faces of the Si(n)(-) clusters.     

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Pumping Schemes Impacts on SNR of Discreted Raman Amplifications

We compare different discreted DCF Raman amplifier configurations, including single-stage and dual-stage. The optimum design with respect to SNR degradation, compromise linear and nonlinear impairments.     

Structural and dielectric properties of Pb(Li1/4Dy1/4W1/2)O3

The polycrystalline samples of Pb(Li_1/4Dy_1/4W_1/2)O₃ have been synthesized by high-temperature solid-state reaction techniques. Room temperature X-ray diffraction (XRD) studies of the compound provided preliminary structural data, and hence formation of a single phase desired material was confirmed. Detailed studies of dielectric constant (ε) and loss (tanδ) as a function of frequency (100 Hz to 10 kHz) at room temperature (298 K) and also as a function of temperature (liquid nitrogen to 403 K) at 10 kHz suggest that the compound undergoes a ferroelectric phase transition of diffuse type.     

On the self-induced motion of vortex sheets

An accurate numerical scheme has been devised to study the self-induced motion of an infinitely thin, free vortex sheet of finite span in an unbounded, inviscid, incompressible fluid. The new numerical scheme has been tested against two vortex sheet problems for which exact solutions have also been obtained. The agreement between the numerical and exact solutions is excellent. The scheme has been further tested against two more examples for which analytical solutions for small times were available. Here too the agreement is excellent.     

Blurring the Lines between Host and Guest: A Chimeric Receptor Derived from Cucurbituril and Triptycene

We report the synthesis and X‐ray crystal structure of a cucurbituril–triptycene chimeric receptor ( 1 ). Host 1 binds to guests typical of CB[6]–CB[8], but also binds to larger guests such as blue box ( 20 ) and the Fujita square ( 22 ). Intriguingly, the geometries of the 1 ? 20 and 1 ? 22 complexes blur the lines between host and guest in that both components fulfill both roles within each complex. The fluorescence output of 1 is fully quenched by the formation of complexes with pyridinium‐derived guests.     

Bimetallic nanoparticles: a single step synthesis, stabilization, and characterization of Au-Ag, Au-Pd, and Au-Pt in sol-gel derived silicates

Nanobimetallic particles consisting of Au-Pd, Au-Ag, and Au-Pt have been synthesized in a single step by a sol-gel process and stabilized in liquid and solid matrices. Organically modified silicates (Ormosils) that play a dual role of a matrix and of a stabilizer have been used to obtain very stable dispersions in the form of sols, gels, and monoliths. The simultaneous reduction of metal ions leads to either a surface enriched with one component or an alloy type of structure depending on the bimetal combination. The nanometallic dispersions are characterized by absorbance, TEM, XRD, IR, XPS, and CO adsorption studies. The stabilized nanoparticles are found to be good electrocatalysts and the preliminary results on the electrochemical reduction of oxygen are reported.     

Anion template effect on the self-assembly and interconversion of metallacyclophanes

Reactions of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) with solvated first-row transition metals M(II) (M(II) = Ni, Zn, Mn, Fe, Cu) have been explored with emphasis on the factors that influence the identity of the resulting cyclic products for Ni(II) and Zn(II). The relatively small anions, namely [ClO4]- and [BF4]-, lead to the formation of molecular squares [{M4(bptz)4(CH3CN)8} subsetX][X]7, (M = Zn(II), Ni(II); X = [BF4]-, [ClO4]-), whereas the larger anion [SbF6]- favors the molecular pentagon [{Ni5(bptz)5-(CH3CN)10} subsetSbF6][SbF6]9. The molecular pentagon easily converts to the square in the presence of excess [BF4]-, [ClO4]-, and [I]- anions, whereas the Ni(II) square can be partially converted to the less stable pentagon under more forcing conditions in the presence of excess [SbF6]- ions. No evidence for the molecular square being in equilibrium with the pentagon was observed in the ESI-MS spectra of the individual square and pentagon samples. Anion-exchange reactions of the encapsulated ion in [{Ni4(bptz)4(CH3CN)8} subsetClO4][ClO4]7 reveal that a larger anion such as [IO4]- cannot replace [ClO4]- inside the cavity, but that the linear [Br3]- anion is capable of doing so. ESI-MS studies of the reaction between [Ni(CH3CN)6][NO3]2 and bptz indicate that the product is trinuclear. Mass spectral studies of the bptz reactions with Mn(II), Fe(II), and Cu(II), in the presence of [ClO4]- anions, support the presence of molecular squares. The formation of the various metallacyclophanes is discussed in light of the factors that influence these self-assembly reactions, such as choice of metal ion, anion, and solvent.