Photochemical reaction of N,N-dimethyl-4-chloroaniline with dienes: new synthetic paths via a phenyl cation

The irradiation of N,N-dimethyl-4-chloroaniline in the presence of open-chain dienes in acetonitrile leads to addition of the aminophenyl and chloro groups across one of the double bonds; transannular cyclization takes place with cyclic dienes, leading to an arylnortricyclene from norbornadiene and to 1-arylbicyclo[3.3.0]octanes from 1,5-cyclooctadiene. The reaction proceeds by photoheterolysis of the chloroaniline to yield the 4-aminophenyl cation and addition to a CdoublebondC . The chemistry of the adduct cation depends on structure and medium, involving ion pairs in MeCN and solvated ions in CF(3)CH(2)OH. In the latter solvent, formation of ethers from open-chain alkenes is accompanied by Wagner-Meerwein hydride shift. In acetonitrile, the cation from cyclooctadiene partitions between deprotonation and Ritter addition, while the one from norbornadiene is reduced; both cations undergo nucleophilic addition in trifluoroethanol. The relevance of these cationic reactions under unusually mild conditions is discussed.     

Quantum mechanical study of stereoselectivity in the oxazaborolidine-catalyzed reduction of acetophenone

Chiral oxazaborolidines, known as CBS catalysts after the work of Corey, Bakshi and Shibata, are used for the stereoselective reduction of prochiral ketones to secondary chiral alcohols. Due to their relative low cost, ease of use, and high selectivity, their popularity has remarkably grown in the last 15 years. Oxazaborolidine-catalyzed reductions have been much studied, both experimentally and computationally, by means of semiempirical methods. Though, a more accurate high level quantum mechanical study on the complete system, capable of elucidating reliably the origins of stereoselectivity, is still lacking. Therefore, the acetophenone (PhMK) reduction with Corey's oxazaborolidine has been modeled for the first time with ab initio and DFT-B3LYP calculations on the complete system as well as with AM1. Calculations on the complexation of BH(3) to CBS, which can occur only in a cis fashion with respect to the hydrogen on the stereogenic C-4 carbon atom, have allowed us to confirm the great rigidity of Corey's catalyst, possibly determining its excellent enantioselectivity. Acetophenone-CBS-BH(3) complexes were characterized at various levels of theory, and it was found that the picture obtained depends heavily on the method adopted. A computational strategy for identifying the hydride transfer transition states of the competing pathways was developed and tested, using a model system for which the transition state geometry was already known. The application of the TS search method to the reduction of acetophenone allowed the characterization of the TS's for the competing pathways in this reaction, making it possible to predict with good quantitative accuracy the stereochemical outcome of the reaction at all the levels of theory adopted. The characterization of the intermediate oxazadiboretane products confirmed that the highly exothermic hydride transfer provides the thermodynamical drive for the reaction.     

First comparative emission assay of single-wall carbon nanotubes--solutions and dispersions

A variety of single-wall carbon nanotube samples were tested in steady-state and time-resolved emission experiments, where it was found that chemical functionalization and different degrees of oxidation affect the emission behavior.     

Local coordination geometry around Cu and Cu ions in silicate glasses: an X-ray absorption near edge structure investigation

We present an X-ray absorption near edge structure (XANES) study on Cu⁺ and Cu²⁺ ions in silicate glasses at the Cu K-edge, aimed to determine the geometry of the local structure around the metal. This study is based on the comparison between experimental data and theoretical calculations made in the framework of multiple scattering theory. The XANES signals relative to several clusters are simulated on the basis of known crystalline structures involving Cu⁺ and Cu²⁺ ions in silicate matrices. Concerning the Cu²⁺ in glass, the simulations suggest the presence of a square coordination of oxygen atoms around the absorber, with a possible presence of metal ions in the second shell. As for the Cu⁺ ions, the metal clustering is excluded and a linear O-Cu-O coordination is evidenced. Received 30 April 1999     

Electrochemically Driven Swinging of a Nitrobenzyl Pendant Arm in a Nickel Scorpionand Complex

A radical anion −NO₂^.− is formed upon an electrochemically reversible one-electron reduction of the square-planar Ni^II complex of N-nitrobenzylcyclam. The −NO₂^.− group goes to occupy an axial position of the metal ion, thus establishing a significant electronic interaction with the metal center. In particular, the ESR spectrum supports the occurrence of an electron transfer from −NO₂^.− to the metal, which therefore presents a significant Ni^I character. On re-oxidation, the nitrobenzyl side chain detaches and the Ni^II complex is restored, providing an example of a fully reversible redox driven intramolecular motion.     

Influence of Pore Size in Benzoin Condensation of Furfural Using Heterogenized Benzimidazole Organocatalysts

A designed N-heterocyclic carbene (NHC) catalyst was covalently anchored on a range of mesoporous and hierarchical supports, to study the influence of pore size in the benzoin condensation of furfural. The structural and spectroscopic characteristics of the anchored catalysts were investigated, also with the help of molecular dynamics simulations, in order to rationalize the degree of stability and recyclability of the heterogenized organocatalysts. Quantitative yields (99 %) and complete recyclability were maintained after several cycles, vindicating the design rationale.     

On the incompressible flow in axial turbomachinery with several closely packed blade rows

Summary The velocity field in an axial multistage turbomachinery is easily investigated, ignoring the fluid viscosity and compressibility, by means of the actuator disc theory. By developing this theory, general expressions of fluid velocity in such a machine are obtained, these in turn yield the approximate expressions usually given in the literature.

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Sommario Lo studio del campo di moto in una turbomacchina a più stadi puramente assiale per fluido inviscido ed incomprimibile viene sviluppato in modo agevole mediante la teoria del disco attuatore. Nella presente nota vengono ricavate, sviluppando tale teoria, le espressioni generali delle velocità del fluido in una tale macchina e da queste vengono derivate successivamente le espressioni approssimate comunemente riportate nella bibliografia a proposito.

     

Influence of gravity and lift on particle velocity statistics and transfer rates in turbulent vertical channel flow

The present study reports detailed statistics for velocity and transfer rates of heavy particles dispersed in turbulent boundary layers. Statistics have been extracted from a homogeneous source of data covering a large target parameter space and are used here to analyze the effects of gravity and lift on particle dispersion and deposition in a systematic way. Datasets were obtained performing Direct Numerical Simulation (DNS) of particle-laden turbulent upward/downward flow in a vertical channel. Six values for the particle timescale (the particle Stokes number, St) ranging three orders of magnitude were considered to analyze the deposition process as the controlling mechanism was shifting from turbulent diffusion to inertia-moderated crossing trajectories. For the particle timescales examined, gravity and lift do not influence the qualitative behavior of particles even though velocity profiles and deposition coefficients are modified in a non-monotonic fashion, reaching an optimum for St ? 15. Physical mechanisms for the different behavior are discussed. Raw data and statistics obtained from the present DNS are made available at http://cfd.cineca.it (mirror site: http://158.110.32.35/download/database) and can be used to test simple models and closure equations for multiphase RANS and Large Eddy simulations.     

On a Korteweg–de Vries-like equation with higher degree non-linearity

We deal with a non-linear partial differential equation which has been widely investigated owing to its applications in quantum field theory, as well as plasma and solid-state physics. It is the matter of a third order KdV-like equation with higher degree non-linearity in the coefficient of the transport term; it can be derived from a Lagrangian or an Hamiltonian density. In the current literature specific attention has been devoted to the search for traveling-wave solutions, depending upon a positive parameter v, which assesses the speed of the solitary wave. The velocity v is always assumed to be constant, as its dependence on the wave-amplitude is neglected in the mathematical model. In this context, Coffey [On series expansions giving closed-form solutions of Korteweg–de Vries-like equations, SIAM J. Appl. Math. 50 (6) (1990) 1580–1592] exploits an algebraic recursive technique to obtain these solutions in closed form for particular values of v. The aim of this paper is to extend these results by showing that closed-form solutions are achievable for every value of v: to this purpose we supply a proper mathematical framework for these issues by taking into account a suitable special function, namely an elliptic function in the sense of Weierstraß. Furthermore we obtain two classes of the so-called kink solutions, see [M.W. Coffey, On series expansions giving closed-form solutions of Korteweg–de Vries-like equations, SIAM J. Appl. Math. 50 (6) (1990) 1580–1592; B. Dey, Domain wall solutions of KdV-like equations with higher order non-linearity, J. Phys. A 19 (1) (1986) L9–L12], as well as an exponential development of the general solution, for which we prove the convergence. Eventually we show how to implement the resulting functions by means of a symbolic manipulation program.     

Reflection of bounded acoustic beams on a fluid-solid interface

The reflection of an acoustic beam onto a fluid-solid interface is studied under the assumption that the solid medium is viscoelastic. The incident beam is represented as a superposition of plane monochromatic homogeneous waves and its profile is assumed to be Gaussian shaped. The outcoming wave field at the interface, and away from it in the fluid, is numerically calculated for different values of the frequency and of the beam width.

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Sommario Si studia la riflessione di un fascio di onde acustiche sulla superficie di separazione fra un liquido ed un solido nell'ipotesi che il mezzo solido sia viscoelastico. Il fascio incidente ha un profilo gaussiano e viene rappresentato mediante la sovrapposizione di onde piane monocromatiche omogenee. Si calcola numericamente il profilo del fascio riflesso sia sull'interfaccia, sia lontano da essa nel fluido, per diversi valori della frequenza e della larghezza del fascio.