Imaging of the reaction zone in a 100 kW oil-burning furnace by use of a broad-band excimer laser

The reaction zone in the hostile combustion environment of a 100 kW oil-burning furnace has been imaged by laser-induced fluorescence using a broad-band XeCl-excimer laser. Upon excitation, the averaged images obtained by using an interference filter around 320 nm (FWHM of 10 nm) show three distinct areas along the direction of the gas flow. An intense emission spreads around the spray axis and is attributed to the fluorescence of large hydrocarbons in the unburned fuel. Approximately 12 cm downstream of the nozzle, a narrow dark region is displayed suggesting the preheat zone of the combustion process where large hydrocarbons are considerably degraded. The third distinct region is characterized by a strong onset of the fluorescence intensity localized downstream of the dark region. This feature is strongly suppressed by replacing the interference filter by a broad-band transmission filter passing light from 350 to 500 nm. Since OH strongly absorbs at the laser wavelength and its fluorescence is significantly lower above 345 nm, the findings imply that the major contribution to the observed intensity in this region originates from the OH radical. This molecule reaches its maximum concentration immediately downstream of the flame front. However, a contribution from other flame species fluorescing around 320 nm cannot be ruled out. Nevertheless, the combined spatial and spectral information obtained imply that the reaction zone of the combustion process can be localized accurately. The results are compared with simultaneously performed numerical simulations of the burner and are in reasonable agreement.     

Gas-phase diagnostics by laser-induced gratings II. Experiments

In this article we review the results achieved in the past ten years at the Paul Scherrer Institute on the topic of diagnostics in the gas phase by laser-induced gratings (LIGs). The technique has been applied for thermometry in air and in flames at different pressures, for flow velocimetry, for concentration measurements, and for imaging purposes. The influence of collisional energy-transfer and relaxation processes in molecules on the temporal evolution of the LIG signals has also been investigated. It has been demonstrated that, for molecules with a low fluorescence quantum yield, excitation of laser-induced thermal gratings can be used as a sensitive spectroscopic tool. For the quantitative interpretation of the experiments shown in this work, the findings presented in the companion paper [1] have been used.     

Gas phase diagnostics by laser-induced gratings I. theory

Electrostriction and collisional thermalization of absorbed laser energy are the two dominant mechanisms leading to the formation of laser-induced gratings (LIGs) in the gas phase. In this article the results of the theoretical investigations that have been achieved in the past ten years at the Paul Scherrer Institute on this issue are summarized and yield a comprehensive understanding of the underlying physical concepts. Furthermore, a study of the influence of various parameters, such as the alignment and the spatial intensity profile of the beams on the generated electrostrictive and thermal signal is presented for the first time to the authors’ knowledge. The variations of the refractive index responsible for the appearance of laser-induced gratings have been theoretically described by solving the linearized hydrodynamic equations. The contributions from electrostriction, as well as from instantaneous and slow relaxation of the absorbed radiation energy into heat is obtained. These expressions are employed for analysis of experimental data presented in the companion paper [1] which is devoted to the application of the technique for diagnostic purposes in the gas phase. Much effort has been undertaken in order to allow a straightforward physical interpretation of the experimental findings of the expressions presented here.     

Tuning urea-phenanthridinium conjugates for DNA/RNA and base pair recognition

A series of novel urea-phenanthridine conjugates was prepared. The variation of linker length connecting two urea-phenanthridinium conjugates regulated their binding mode toward double stranded polynucleotides, consequently controlling selectivity of compounds toward ds-RNA over ds-DNA stabilization as well as selective fluorescence response toward addition of G-C base pair and A-U(T) base pair containing polynucleotides.     

Multicomponent Synthesis of Purines and Pyrimidines: From the Origin of Life to New Sustainable Approaches for Drug-Discovery Applications

It is widely accepted that purines and pyrimidines, the building blocks that gave origin to life on our planet, were created through multicomponent reactions (MCRs) on early abiotic Earth. These heterocyclic scaffolds gradually evolved into a wide range of biologically relevant molecules regulating many different physiological processes and thus becoming widely exploited as templates for the development of new drugs. Accordingly, over the years, the synthetic community has dedicated many efforts in the attempt to hypothesize and replicate the original abiotic synthesis of purines and pyrimidines, thus developing a number of multicomponent synthesis to access these scaffolds. The following evolution of synthetic chemistry towards green approaches for the production of new molecules and the recent interest in pharmaceutical sustainability underlines the importance of multicomponent synthesis of new heterocycles. This review article provides an overview of the most important multicomponent approaches for the synthesis of purine and pyrimidine derivatives for potential pharmacological applications.     

Dynamic steady‐state crack propagation in quasi‐crystals

The steady propagation of planar cracks in quasi‐crystalline bodies with velocity lower than the one of bulk elastic macroscopic waves is under scrutiny. Closed‐form solutions to the balance laws are provided. Unusual Mach number limits are determined. Numerical experiments describing peculiar aspects of the crack propagation in quasi‐crystals are performed by varying parametrically the coupling coefficient between macroscopic deformation and substructural events. In this way, classes of quasi‐crystals are then compared. Copyright © 2010 John Wiley & Sons, Ltd.     

A device to achieve low Reynolds numbers in an open surface water channel

When investigating flow structures, and especially flow transitions, research projects often seek to increase insight using complementary numerical and physical experiments. Obtaining exact Reynolds number correspondence can frequently be difficult in experiments, particularly when relatively low values are required. Often, available test facilities were designed and optimised for a specific velocity range, meaning they have restrictions on the minimum flow velocity. This study describes a device to reduce the flow velocity locally in an open surface water channel. The underlying idea is to divert a controlled fraction of the incoming flow from the working section by increasing the pressure there, resulting in reduced velocity. This idea is realised using a ‘sub-channel’ that can be inserted into the main test chamber, with a variable porosity perforated screen at its downstream end. This study assesses and optimises the flow quality inside this structure, such as usable test section length, uniformity of the velocity profiles and turbulence intensity. The results demonstrate that the device creates high quality low Reynolds number flows, which is exemplified with the canonical circular cylinder in cross-flow.     

Basicity of neutral organic superbases with vinamidine structure in gas phase and acetonitrile: a density functional theory study

The gas‐phase proton affinities (PAs) for a set of molecules with vinamidine structure are considered and their basicities in acetonitrile. It is shown that introducing double bonds to the imidazoline rings at the proton attachment site resulted in decrease in PA of the parent vinamidine. The increase in PA can be obtained by inducing modifications to the imidazoline ring at the junction of two diazepinylium rings. Placing methyl and dimethylamino substituents on the perimeter of the molecule further increased their gas‐phase PAs. Studied vinamidine molecules are superbases, which possess PA values in the range between 261.0 and 284.2 kcal mol^–1 in the gas phase and pK_a values of 24.6–31.9 units in acetonitrile. Dismembering proton attachment site by opening the two diazepinylium and imidazoline rings resulted in the largest drop in PA values, indicating its importance in constraining the positions of imino nitrogen atoms in the neutral form of the molecule. Vinamidine molecules studied here present important pieces of the ladder of highly basic organic compounds for they possess accessible vinamidine molecular framework. Copyright © 2012 John Wiley & Sons, Ltd.