Perturbative treatment of quasi-resonant proton neutralization at alkali-halide surfaces

A theoretical investigation of proton neutralization by proton scattering from several alkali-halide surfaces is presented. These systems are suitable for a perturbative treatment since no hydrogenic atomic shell is embedded in the valence band of the solid where the neutralizing electron originates, which is a necessary condition for fast convergence of the perturbative expansion for the neutralization probability. The perturbative interaction is modeled by a Fano-Anderson effective potential, and the dependence of the results on the properties of the systems (namely, the width of the valence band of the solid and its position relative to the discrete atomic level) and on the dynamics of the process (determined here by a single parameter which controls the duration of the interaction, i.e., the collision energy) are critically discussed.     

Rhenium complexes of tris(pyrazolyl)methanes and sulfonate derivative

The trioxo [ReO(3){SO(3)C(pz)(3)}] (1) (pz = pyrazolyl) and oxo [ReOCl{SO(3)C(pz)(3)}(PPh(3))]Cl (2) compounds with tris(pyrazolyl)methanesulfonate were obtained by treatment of Re(2)O(7) or [ReOCl(3)(PPh(3))(2)], respectively, with Li[SO(3)C(pz)(3)], whereas [ReCl(3){HC(pz)(3)}] (3), [ReCl(3){HC(3,5-Me(2)pz)(3)}] (4) and [ReCl(4){eta(2)-HC(pz)(3)}] (5) were prepared by reaction of [ReOCl(3)(PPh(3))(2)] (3,4) or [ReCl(4)(NCMe)(2)] (5) with hydrotris(pyrazolyl)methane HC(pz)(3) (3,5) or hydrotris(3,5-dimethyl-1-pyrazolyl)methane HC(3,5-Me(2)pz)(3) (4). [ReO{SO(3)C(pz)(3)}{OC(CH(3))(2)pz}][ReO(4)] 6, with a chelated pyrazolyl-alkoxide, was derived from an unprecedented ketone-pyrazolyl coupling on reaction of crude 1 with acetone. The compounds have been characterized by elemental analyses, IR and NMR spectroscopies, FAB-MS spectrometry and cyclic voltammetry and, in the case of 5 and 6, also by single-crystal X-ray diffraction. The electrochemical E(L) Lever parameter has been estimated, for the first time, for the SO(3)C(pz)(3)(-) and oxo ligands allowing the measurement of their electron-donor character and comparison with other ligands. Compounds 1, 2 and 6 appear to be the first tris(pyrazolyl)methanesulfonate complexes of rhenium to be reported.     

Physicochemical aspects of novel surfactantless, self-templated mesoporous SnO2 thin films

A novel method of synthesis consisting of the production of ordered arrangements of tubular pores distributed inside SnO2 annealed thin films, which are prepared from a rotating disk process carried out at 2000-3500 rpm, is herein described. The main novelty is that no surfactant molecules are required in order to create these ordered pore structures; the templating entities are supramolecular assemblies of oligomeric chains formed during the extra-long aging allowed to the sol-gel processing of tin(IV) tetra-tert-amiloxide, Sn(OAm(t))4, chelated with acetylacetone molecules. Low angle X-ray diffraction peaks of SnO2 thin films calcined at 500 degrees C clearly certify the existence of ordered mesostructures when employing the right H2O/Sn(OAm(t))4 molar ratio during the SnO2 sol-gel synthesis. The final SnO2 ordered mesostructures are reminiscent of those linked to MCM-41 and SBA-15 substrates. Pore-size distribution analyses proceeding from N2 sorption isotherms at 76 K on the SnO2 thin films calcined at 500 degrees C unequivocally confirm the presence of tubular mesopores (mode pore sizes ranging from 5 to 7 nm). The thicknesses of the SnO2 films range from 80 to 150 nm after performing a drying process at 100 degrees C and from 70 to 125 nm after calcining in air at 500 degrees C; these film thicknesses show, in general, decreasing trends when either the spinning rate or the H2O/(Sn(OAm(t))4 ratio is increased.     

The small peptide-catalyzed direct asymmetric aldol reaction in water

Simple modular di- and tripeptides with a primary amine at the N-terminus catalyze the aqueous asymmetric aldol reaction between unmodified ketones and aldehydes to furnish the corresponding beta-hydroxy ketones with up to 86% ee in water and 99% ee in aqueous media.     

Isopropylcyclopropane + OH gas phase reaction: a quantum chemistry + CVT/SCT approach

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from isopropylcyclopropane (IPCP) is presented. Optimum geometries, frequencies and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all stationary points, as well as for additional points along the minimum energy path (MEP). Energies have been improved by single-point calculations at the above geometries using CCSD(T)/6-311++G(d,p) to produce the potential energy surface. The rate coefficients are calculated for the temperature range 260-350 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel, for all the modeled paths. The reactant complexes are examined in detail, because they exhibit alkene-like structure. The excellent agreement between the overall calculated and experimental rate coefficients at 298 K supports the reliability of the parameters obtained for the temperature dependence and branching ratios of the IPCP + OH reaction, proposed here for the fist time. The expression that best describes the studied reaction is k(overall) = 6.15 x 10(-13)e1747/RT cm3 x molecule(-1) x s(-1). The predicted activation energy is -0.89 kcal/mol.     

Experimental evidence of guided-resonances in photonic crystals with aperiodically ordered supercells

We report on the first experimental evidence of guided resonances (GRs) in photonic crystal slabs based on aperiodically ordered supercells. Using Ammann-Beenker (quasiperiodic, eightfold symmetric) tiling geometry, we present our study on the fabrication, experimental characterization, and full-wave numerical simulation of two representative structures (with different filling parameters) operating at near-IR wavelengths (1300-1600 nm). Our results show a fairly good agreement between measurements and numerical predictions and pave the way for the development of new strategies (based on, e.g., the lattice symmetry breaking) for GR engineering.     

Dynamic region visit routing problem for vehicles with minimum turning radius

In this paper we address the problem of planning optimized routes among dynamically selected target regions for vehicles with a turning radius motion constraint, hereinafter called dynamic Dubins traveling salesman problem with neighborhoods (DDTSPN). Initially, we present a heuristic to solve a simpler version of this problem, called off-line step, where only previously given targets are concerned. We further extend this approach for the more complex case of dynamic scenarios, called on-line step, addressing the inclusion of new targets during the execution of the initial route, whilst minimizing the impact on the total traveled distance. Formal analyzes of our techniques are provided, presenting upper bounds for the total length of the final tour. Results with statistical investigation over a large number of trials in a simulated environment are also provided. Finally, to demonstrate the applicability of our technique in solving the DDTSPN at real-world scenarios, we also report on results of an experiment performed with a real car-like robot.     

Complete flat surfaces with two isolated singularities in hyperbolic 3-space

We construct examples of flat surfaces in H³ which are graphs over a two-punctured horosphere and classify complete embedded flat surfaces in H³ with only one end and at most two isolated singularities.     

Statistical properties of subgroups of free groups

The usual way to investigate the statistical properties of finitely generated subgroups of free groups, and of finite presentations of groups, is based on the so‐called word‐based distribution: subgroups are generated (finite presentations are determined) by randomly chosen k ‐tuples of reduced words, whose maximal length is allowed to tend to infinity. In this paper we adopt a different, though equally natural point of view: we investigate the statistical properties of the same objects, but with respect to the so‐called graph‐based distribution, recently introduced by Bassino, Nicaud and Weil. Here, subgroups (and finite presentations) are determined by randomly chosen Stallings graphs whose number of vertices tends to infinity. Our results show that these two distributions behave quite differently from each other, shedding a new light on which properties of finitely generated subgroups can be considered frequent or rare. For example, we show that malnormal subgroups of a free group are negligible in the graph‐based distribution, while they are exponentially generic in the word‐based distribution. Quite surprisingly, a random finite presentation generically presents the trivial group in this new distribution, while in the classical one it is known to generically present an infinite hyperbolic group. © 2012 Wiley Periodicals, Inc. Random Struct. Alg., 2013