The Baylis-Hillman approach to quinoline derivatives

Baylis-Hillman reactions of 2-nitrobenzaldehydes with various activated alkenes afford adducts that undergo reductive cyclisation to quinoline derivatives. The chemo- and regioselectivity of cyclisation appears to be influenced by the choice of both the substrate and the reagent system, and competing reactions have been observed.     

Oligoindenopyrenes: a new class of polycyclic aromatics

A new class of polycyclic aromatic hydrocarbons--oligoindenopyrenes--has been synthesized featuring a Pd-catalyzed Suzuki-Heck coupling cascade. The oligoindenopyrenes are robust, highly colored substructures of C(70) and have properties that might prove useful in new organic materials or devices. After excitation, the tetraindenopyrene derivative 3d undergoes efficient deactivation (99%) by internal conversion to the ground state. The small fluorescence quantum yield (0.004) is in accordance with the short (0.6 ns) fluorescence decay time.     

Structure of the naphthalene dimer from rare gas tagging

Excitation spectra of naphthalene dimer-argonn (n = 1-3) clusters are obtained by resonance enhanced multiphoton ionization time-of-flight mass spectroscopy. The spectra are generally independent of the number of attached argon atoms and reveal sharp structures which are fitted by superimposing independent monomer spectra. It is concluded that the rare-gas tagging technique reveals the presence of a T-shaped naphthalene dimer chromophore in the molecular beam.     

Experimental analysis of the flow field over a novel owl based airfoil

The aerodynamics of a newly constructed wing model the geometry of which is related to the wing of a barn owl is experimentally investigated. Several barn owl wings are scanned to obtain three-dimensional surface models of natural wings. A rectangular wing model with the general geometry of the barn owl but without any owl-specific structure being the reference case for all subsequent measurements is investigated using pressure tabs, oil flow pattern technique, and particle-image velocimetry. The main flow feature of the clean wing is a transitional separation bubble on the suction side. The size of the bubble depends on the Reynolds number and the angle of attack, whereas the location is mainly influenced by the angle of attack. Next, a second model with a modified surface is considered and its influence on the flow field is analyzed. Applying a velvet onto the suction side drastically reduces the size of this separation at moderate angles of attack and higher Reynolds numbers.     

Silica-Polymers for Processing Gas Separation Membranes: High Temperature Growth of Fractal Structure

A study is performed to investigate the structure and shape of silicate molecules, grown from TEOS (tetra-ethyl-ortho-silicate) at 65°C, as building bricks for the preparation of gas separation membranes. The study is mainly carried out with in situ Small Angle X-ray Scattering (SAXS). The structure of the resulting silicate polymers could be interpreted in terms of fractals with a fractal dimension Df and a radius of gyration Rg.During the first minutes of reaction of TEOS with water in the presence of protons as catalysts fast hydrolysis take place with a heat effect of about 15–30 kJ/mol. Condensation (growth) of hydrolyzed species follows according to a diffusion-limited cluster-cluster aggregation (DLCCA). Growth is faster with higher amount of water and acid but a plateau-value is reached for both Rg and Df on a 60-hour timescale when a moderate amount of water or protons are present. These plateau-values are 15 nm and 1.8 for Rg and Df, respectively.The values of Rg and Df have large effects on gas separation performance of the resulting membranes made from these sols. With relatively small deviations in the values of Rg and Df a 2-decade difference in the permeation of helium through the membrane was observed.     

Viscosity dependence of intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane in alkane solvents as a function of temperature

Intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane, (1PC(5)1PC) is studied as a function of temperature in a series of alkane solvents and in toluene, covering a wide range of solvent viscosities η, from 0.2 to 125 cP. The rate constant k(a) of the monomer → excimer reaction is determined from the effectively single exponential monomer fluorescence decays. For the viscosity dependence of k(a) in n-alkanes, the Stokes-Einstein relation k(a) ～ η(-1.0) does not hold. Instead, k(a) is proportional to η(-α), with α increasing upon cooling, from 0.56 at 85 °C to 0.86 at -30 °C. The activation energy E(a) of excimer formation with 1PC(5)1PC, always larger than the activation energy E(T/η) of solvent viscous flow, decreases when the solvent viscosity becomes smaller, from 20.7 kJ/mol in n-hexadecane to 11.8 kJ/mol in n-butane, approaching a value of 11-12 kJ/mol for the low viscosity solvents. As the excimer formation process depends on the restricted diffusion of the 1PC end groups as well as on the C-O and C-C rotations in the -O(CH(2))(5)O- chain, the limiting barrier of 11-12 kJ/mol is attributed to the activation energy E(c) of the multiple bond rotations. This fractional viscosity dependence (α < 1.0) is caused by the multidimensional character of the barrier crossing in the excimer formation process. This multidimensional character should also be taken into account in investigations of polymers and biological media employing excimer formation.     

Cycle resolved multi-planar flow measurements in a four-valve combustion engine

The non-reacting flow in a one-cylinder four-valve combustion engine is measured via cycle resolved two-component/two-dimensional (2C/2D) particle-image velocimetry (PIV). The three-dimensional structure of the velocity field is analyzed based on the flow field measured in eight planar planes within the cylinder for several crank angles during the intake and compression phase. Using the mean and statistical values of the single planes quasi three-dimensional flow fields are reconstructed for crank angles of 80°, 160°, and 240° atdc. This enables the detailed analysis of the spatial distribution of the large and small scale flow structures, e.g., by visualizing large vortical structures and the distribution of the turbulent kinetic energy. It was found that two ring vortices evolving beneath the inlet valves are the dominant large scale structures that seem to be of major concern for the mixing process in the cylinder of a four-valve combustion engine operated at 1500 rpm. Furthermore, the temporal evolution of the flow field within the symmetry plane of the cylinder, measured for crank angles between 40° and 320° atdc in steps of 20°, is discussed. The results give new insight into the complex three-dimensional flow in the combustion chamber of a one-cylinder four-valve combustion engine. That is, the tumble vortex only seems to be of secondary importance for the flow concerning the mixing process at 1500 rpm. This is an essential result for future work considering the fluid mechanics of fuel-air-interaction processes and mixing principles in combustion engines.     

Dipole moments of molecules solvated in helium nanodroplets

Stark spectra are reported for hydrogen cyanide and cyanoacetylene solvated in helium nanodroplets. The goal of this study is to understand the influence of the helium solvent on measurements of the permanent electric dipole moment of a molecule. We find that the dipole moments of the helium solvated molecules, calculated assuming the electric field is the same as in vacuum, are slightly smaller than the well-known gas-phase dipole moments of HCN and HCCCN. A simple elliptical cavity model quantitatively accounts for this difference, which arises from the dipole-induced polarization of the helium.     

Molecular dynamics computer simulation of the hydration of two simple organic solutes

The hydration of two simple organic solutes has been studied using the molecular dynamics (MD) computer simulation method. Results of the simulations of a single 1,4-dioxane or 1,3-dioxane molecule dissolved in 122 water molecules are compared with those of a MD simulation of an empty cavity of corresponding size in 216 water molecules. This yields the opportunity to trace the specific effects of the polar and dispersion solute-solvent interactions on the properties of the water molecules in the hydration shell of the solute.

The hydration shell properties of 1,4-dioxane (μ) = 0·14 D) are very similar to those of the corresponding cavity, whereas those of 1,3-dioxane (μ) = 1·91 D) show significant deviations. Earlier conclusions that water structure-making and water structure-breaking properties of 1,4-dioxane are about equally balanced, while 1,3-dioxane is definitely structure-breaking, are confirmed. Moreover, it is shown that a slower self-diffusion and reorientation of water molecules upon addition of a cosolvent does not necessarily point at structure-making properties, additional to those that are already induced by the cavity formation. The introduction of an empty cavity also slows down self-diffusion and molecular reorientation in the hydration shell.