bluuebunny/crossref_metadata_2025_split huggingface dataset:
Double and Triple Helices of Nucleic Acid Polymers, Studied by UV‐Resonance Raman Spectroscopy
Authors: Gfrörer A. , Schnetter M. E. , Wolfrum J. , Greulich K. O. 
Published: July 2012

AbstractUY‐resonance Raman (RR) spectra with laser excitation at 260 nm and 220 nm were taken of the triple helix poly (U) · poly(A) · poly(U), the double helix poly(A) · poly(U) and a random copolymer poly(AU). The experiments were carried out both in H2O and in D2O as solvent. It is shown that by appropriate choice of excitation wavelength, structure‐spectra correlations can be established that allow these different conformations to be clearly distinguished. Whereas the formation of the double helix poly(A) · poly(U) results in both frequency and intensity changes, only minor frequency shifts are discernible when we compare the spectra of the double helix and the triple helix poly(U) · poly(A) · poly(U). As far as the uracil modes are concerned, no significant differences between the spectra of the triple and the double helices can be discerned. Therefore the RR‐spectra indicate a very similar base conformation of the two poly(U) strands in poly(U) · poly(A) · poly(U) and that in poly(U) · poly(A). However, there are considerable changes in the relative intensities of strong adenine modes with 260 nm excitation. For example, the intensity ratio of bands around 1480 cm−1 and 1338 cm−1 increases from 0.8 in poly(A), and 0.95 in poly(A) · poly(U) to 1.4 in poly(U) · poly(A) · poly(U). The spectra suggest that both the poly(U) strands in the triple helix are of quite similar structure, whereas the conformation of the poly(A) strands changes when triple helix is formed. With 220 nm excitation, no prominent marker bands or intensity patterns are found that would allow a distinction between double and triple helical conformations. The experiments demonstrate that UV‐Raman spectroscopy, with its possibility to choose an appropriate excitation wavelength adds a valuable degree of freedom in identifying different nucleic acid structures on the basis of intensity criteria.
On the Temperature Dependence of the Second Refractivity Virial Coefficient
Authors: Artym R. , Preiss U. 
Published: July 2012

AbstractIt has been shown that the second refractivity virial coefficient bR vanishes at the Boyle temperature TB. A theory has been developed with account of change of sign of bR at TB, and analytical formulas have been derived. For the first time the temperature dependence of bR has been calculated for He at temperatures over the range 5 K to 800 K using the Woolley potential.
Formation of Metal Clusters in Zeolites Studied by Differential X‐Ray Scattering
Authors: Vogel W. , Sachtler W. M. H. , Zhang Z. 
Published: July 2012

AbstractPd/NaY catalysts, ex situ calcined at 500°C and in situ reduced at 200°C, contain mainly small palladium clusters which are identified by XRD as Pd13 icosahedra. A fraction of 20 wt% are larger than the supercage dimension and produce lattice damage of the NaY support. Admission of CO at room temperature partly transforms the particle structure to fcc. Depending on temperature and time of CO exposure, crystallites in the order of 100 Å in size are formed. Methanol exposure at 224°C for 75 min decreases the X‐ray dispersion from 84% to 54%.
Infrared Study of Supported Rhodium Catalysts Exposed to CO/H2 Mixtures at High Pressure and Temperature
Authors: Mcquire Mandy W. , Rochester Colin H. 
Published: July 2012

AbstractInfrared spectra have been recorded of Rh/SiO2 catalyst exposed to CO/H2 mixtures at various temperatures (473–553 K), pressures (3–50 atm) and CO:H2 ratios (1:14 to 14:1). Information was gained about surface hydroxyl groups on the support, the formation of gaseous methane, adsorbed carbon monoxide on rhodium and surface species containing alkyl groups. The dominant observed products of reaction were methoxy and ethoxy groups on the silica support, methane and higher hydrocarbons consisting predominantly of methylene chains. Depending on reaction conditions selectivities could be widely varied to give product distributions including 100% methoxy/methanol, a high proportion of ethoxy/ethanol or 100% higher hydrocarbon. Methane became more favoured at higher temperature, methanol rather than ethanol was favoured at low temperature and high pressure and higher hydrocarbons were favoured by CO‐rich feedstock. — Reaction of CO/H2 over Rh/Al2O3 and Rh/TiO2 gave higher selectivity to methane than Rh/SiO2 and also led to adsorbed carboxylate species on the oxide supports. The products were in part formed by catalysis involving the oxide components of the catalysts.
Reaction of CO2/H2 on K‐Cu(100): RAIRS Studies at Ultrahigh Vacuum and Higher Pressures
Authors: Pritchard J. , Chadwick D. , Zheng K. 
Published: July 2012

AbstractThe reaction of CO2/H2 on clean and potassium dosed Cu(100) was investigated by in situ RAIRS using a high pressure gas cell. For clean Cu(100), exposure to 120 torr of CO2/H2 mixture in the RAIRS cell from 25°C to 200°C resulted in no clearly resolvable RAIRS bands. The result indicates that under the conditions used, the coverage of formate must be very low. A K predosed Cu(100) surface was exposed to CO2/H2 in the RAIRS cell at 120 torr. At temperatures lower than 70°C, weak RAIRS features were observed, the most prominent being at ∼1600 cm−1. At temperatures above 70°C, a RAIRS band was observed at ∼1550 cm−1 with the band at 1600 cm−1 disappearing. CO2 adsorption on N2O oxidized K/Cu(100) surfaces also produced a RAIRS band at ∼1545–1550 cm−1, suggesting that the surface was oxidized in CO2/H2 at elevated temperatures and reacted with CO2 to form a carbonate species.
Variation of the Binding Energy due to High Electrostatic Fields (NO on Rh(111))
Authors: Madenach R. P. , Abend G. , Kreuzer H. J. , Block J. H. 
Published: July 2012

AbstractThe influence of high electrostatic field strengths (V/nm to V/A) on the binding energy has been studied by pulsed field desorption mass spectrometry (PFDMS). For this purpose the equilibrium between adsorption and thermal desorption of NO on Rh(111) has been observed as a function of temperature and field strength. — We obtained 102.6 kJ/mol for the binding energy of NO on Rh(111) which is in good agreement with TPD measurements of Bugyi and Solymosi [1]. With a negative field strength of 3 V/nm the binding energy increases about 10%, and a decrease of about the same amount was observed for a positive field of 3 V/nm. — In an attempt to understand the effect of high field strengths on binding energy, calculations of the change in electron density due to various field strengths have been performed. These calculations were carried out within density functional theory using a nonlocal spin density approximation. The change of the electron density involved in the bonding will be shown.
Work Function Investigations on Nickel Surfaces in Dynamic Equilibrium with Water Vapor Covering the Pressure Range 10−3 — 10 Pa
Authors: Oertel M. , Schulze M. , Bradke M. V. , Schnurnberger W. 
Published: July 2012

AbstractThe interaction of water vapor with clean and oxidized surfaces of polycrystalline Nickel was studied at temperatures from 300 K to 430 K. — The changes of the work function Δθ were recorded as a function of the interaction time of water with these surfaces. Immediately after starting water exposure a steep decrease of the work function was observed at all temperatures studied. The amount of this initial decrease depends on the H2O partial pressure. The subsequently observed increase of the work function was found to be mainly temperature dependent with a typical time constant of a few minutes. — Determining the relative stability of the H2O molecules and their dissociation products the influence of the substrate temperature must be emphasized. Size and sign of Δθ in the equilibrium state change due to variation of the substrate temperature from 320 K to 435 K. — The observed time dependence of the work function was attributed to a two step dissociation process. Its slight increase was related to the dissociation of OHad and to the beginning of oxide layer formation. This work function increase is a thermally activated process with an activation energy of 61.8 kJ/mol and preexponential factor of 6.3 · 106 s−1.
Molecular Structure and Dynamics within Self‐Assembled Hexakisalkoxy‐triphenylene Monolayers and Alkane Wetting Films
Authors: Askadskaya L. , Boeffel C. , Rabe J. P. 
Published: July 2012

AbstractScanning tunneling microscopy (STM) has been employed to investigate two classes of ultrathin molecular layers on the basal plane of highly oriented pyrolytic graphite (HOPG). First, molecules with three‐fold symmetry were studied at solid‐fluid interfaces. Structure and dynamics within the first monolayer of hexa‐substituted triphenylenes with two different substitutions, —OC7H15 and —OC16H33, could be readily observed, since the molecules exhibit a strong contrast between the triphenylene cores and the alkyl side chains. While the dynamic disorder prohibited atomic‐scale resolution imaging, sub‐molecular resolution images were suitable to determine packing patterns together with a number of characteristic packing defects. No ordered monolayers could be observed from unsymmetrically substituted triphenylenes like triphenylene dimers, which in bulk materials do only exhibit glassy but no crystalline phases. Secondly, long chain alkanes have been observed, for the first time, in the vicinity of the edge of a solidified alkane‐melt drop. The data indicate frozen disordered lamellae at the drop‐edge, where the alkane phase is still many monolayers thick, while a columnar phase is found in a thin precursor layer some millimeter away from the drop‐edge.
A Computational Evaluation of Populations of Complexes under Atmospheric Conditions: The (H2O)2 and (CO2)2 Cases
Authors: Slanina ZdenĚK 
Published: July 2012

AbstractVarious van‐der‐Waals or hydrogen‐bonded complexes have been understood as significant components of the Earth's or other planetary atmospheres. The species can be characterized by computational‐chemistry techniques. Present possibilities are illustrated with the water dimer described by flexible potentials. It is shown that the dimer mole fraction in equilibrium with the monomer should be about 0.1 ‐0.2% close to the Earth's surface under saturation conditions. For sample atmospheric conditions about 3 km above the surface the mole fraction decreases to some 0.004‐0.005%. Another example is supplied with the carbon dioxide dimer in the Venus atmosphere, using the Stogryn‐Hirschfelder treatment within a Lennard‐Jones scheme and calculations based on recent quantum chemical data. The dimer mole fraction in equilibrium with carbon dioxide can be higher than 4% close to the Venus surface and the fraction should decrease with distance from the surface.
Extension of the Group‐Contribution Equation of State Parameter Matrix for the Prediction of Phase Equilibria Containing Argon, Ammonia, Propene and Other Alkenes
Authors: Pusch Jens , Schmelzer JüRgen 
Published: July 2012

AbstractThe Group‐Contribution Equation of State (GC‐EOS) by Skjold‐Jørgensen [1, 2] for predicting phase equilibria has been extended by groups (components) Ar and NH3 and olefin‐groups C3H6, CH2 = CH, CH2 = C, CH = CH and CH = C. It was necessary to determine parameters for pure groups and to adjust binary interaction parameters. By comparison calculations were made using the Schmidt‐Wenzel‐EOS [3] and the hole model of Smirnova and Viktorov [4].
Dielectric Studies of the Association of Strongly Polar Molecules
Authors: Kȩdziora P. , Jadž J. , Bonnet P. 
Published: July 2012

AbstractThe dipole‐dipole association of compounds of different shape with the CN terminal group has been studied by the static dielectric polarization method. The apparent dipole moment behavior of nematogen molecules (n‐pentylcyanobiphenyl) with comparison to non‐nematogen molecules (benzonitrile) has been discussed.
Behaviour of the Electroclinic Effect and the Soft Mode Rotational Viscosity in the Smectic A Phase of Ferroelectric Liquid Crystalline Polymer/Monomer Mixtures
Authors: Bennemann Dirk , Bömelburg Jan , Heppke Gerd 
Published: July 2012

AbstractMeasurements of the electroclinic effect in the smectic A phase of a chiral side‐chain polyacrylate, of a low molecular mass liquid crystal with very similar structure and of mixtures between these two compounds are reported. The ratio of the induced tilt angle and the electric field strength is found to be independent of the concentration, whereas the coupling constant between induced tilt and induced polarization decreases with increasing amount of polymer. This behaviour of the electroclinic effect implies a considerably lower tilt controlling susceptibility coefficient for the chiral polyacrylate than for the low molecular mass compound. The values of the soft mode rotational viscosity of the polyacrylate, which are obtained from the electrooptical measurements in combination with dielectric spectroscopy, are found to be three orders of magnitude larger than for the monomer.
Ultrasonic Properties of a Nematic Sidechain Polysiloxane in the 100 MHz—1 GHz Range
Authors: Deeg F. W. , Diercksen K. , Bräuchle C. 
Published: July 2012

AbstractLiquid crystalline polymers exhibit a number of properties, e.g. the viscoelastic behaviour in the ultrasonic range, which are neither observed in low molecular weight liquid crystals nor in isotropic polymers. Acoustic waves in the 100 MHz‐1 GHz range have been generated in a nematic liquid single crystal sidechain polysiloxane elastomer through impulsive stimulated thermal scattering and their propagation characteristics have been studied. The experiments reveal a dispersion step for the speed of sound and a strong anisotropy for the acoustic attenuation constant in the investigated frequency range consistent with a recently proposed hydrodynamic model of a coupling of the elasticity to the nematic order parameter in this class of substances.
Columnare und nematische Phasen neuer makrodiscotischer Materialien [1]
Authors: Praefcke K. , Singer D. , Gündogan B. , Gutbier K. , Langner M. 
Published: July 2012

AbstractThe first radially multi(biphenylethinyl) substituted arenes possessing benzene or naphthalene cycles as molecular centers as well as Palladium atoms containing macrocyclic complexes derived from hexaalkoxy substituted aldobisimines by cyclopalladation have been synthesized and investigated by physical methods. Most of these flat compounds with molecular core diameters ≧24 Å and molecular weights up to 4000 are novel thermotropic liquid crystals, in part highly unsaturated or metallomesogens in structure, respectively, exhibiting columnar and/or nematic phases. Sterical influences on the mesophase formation of these spacious molecules are discussed as well as interesting changes of their properties by variation of their self‐organization occurring in binary systems. Thus, addition of an electron acceptor, for example 2,4,7‐trinitrofluorenone, leads to the formation of charge transfer complexes, mesophase stabilizations or even to inductions of other mesophases including nematic ones of the ND or NC type. Dissolving such thermotropic columnar mesogens in various organic solvents gives rise to lyomesomorphism; in case of at least one of the palladomesogens a transition between two different lyotropic nematic phases (N1 and N2) has been observed.
Conformational Order of Amphiphilic Chain Molecules in Lyotropic Liquid‐Crystalline Phases
Authors: Schnepp WErner , Schmidt Claudia 
Published: July 2012

AbstractThe nonionic surfactant hexa(ethyleneglycol) n‐dodecyl methylether in aqueous liquid‐crystalline phases is studied by deuterium NMR spectroscopy. From the quadrupole splittings of the surfactant, which is deuterated either in the hydrophobic alkyl chain or in the hydrophilic oxyethylene chain, order profiles of both the alkyl and oxyethylene moiety are obtained. Characteristic features of the order profiles are related to the conformations of the surfactant molecules. The observed temperature dependence of the quadrupole splittings is explained by changes of the conformational equilibrium.
On the Identification of Oxiranes Formed in the Reactions of NO3 Radicals with Alkenes
Authors: Wille U. , Schindler R. N. 
Published: July 2012

AbstractProducts of the reaction of the NO3 radical with monoalkenes, i.e. 2,3‐dimethyl‐2‐butene, 2‐methyl‐2‐butene, cis‐2‐butene, isobutene and propene were identified using a fast flow system with mass spectrometric detection. The reaction mixtures were analyzed in situ without applying enrichment procedures. Oxiranes were found to be the predominant hydrocarbon compound produced at low pressures with yields of (90±10)% for 2,3‐dimethyl‐2‐butene and (80±10)% for cis‐2‐butene. For the unsubstituted butenes and for propene formation of products containing a carbonyl group was also suggested.
Ultrasensitive Chemical Analysis. Ultrasensitive Laser Spectroscopy of Trace Elements
Authors: Letokhov V. S. 
Published: July 2012

AbstractThe application of the ultrasensitive method of laser spectroscopy – resonance ionization spectroscopy (RIS) – is briefly reviewed. It is used for trace analysis of elements in different samples: high‐purity materials, sea water, ores, etc.
Planar Liquid and Gas Visualization
Authors: Melton Lynn A. 
Published: July 2012

AbstractPlanar Laser Induced Fluorescence (PLIF) diagnostics based on the fluorescent intensity, lifetime, and/or quenching properties of organic molecules have been developed for a variety of applications: (1) Exciplex‐Based Vapor/Liquid Visualization systems provide spectrally‐separated emissions from the vapor and liquid phases and thus allow the separate visualization of the vapor and liquid phases of an evaporating fuel spray; (2) Exciplex Fluorescence Thermometry systems allow the measurement of liquid phase temperatures over the range 20–400 °C. Such systems have been used to measure the temperature of hydrocarbon droplets falling through heated nitrogen; (3) Streamlines by Oxygen Quenching, in combination with droplet‐slicing‐imaging, has allowed the visualization of internal circulation patterns within sub‐millimeter droplets hydrocarbon droplets; and (4) Fluorescence Lifetime Imaging, a technique which uses two PLIF images, one obtained a few nanoseconds after the other, has been applied to the imaging of the fuel/oxygen equivalence ratio in isothermal methane jets and to the imaging of temperature in oxygen‐free jets. This paper reviews the photophysical and diagnostic principles upon which the current systems are based and addresses their applications and limitations.
Optical Characterization of Soot
Authors: D'Alessio A. , Minutolo P. , Gambi G. , D'Anna A. 
Published: July 2012

AbstractThe chemistry of fuel‐rich hydrocarbon flames is extremely complex because different types of high molecular mass structures are produced. In fact, Polycyclic Aromatic Hydrocarbons (PAH) and submicronic turbostratic graphitic‐like particles (soot) are not the only products present but also partially aromatic polymers, gums, fullerenes and diamond‐like particles may be formed during pyrolysis and rich combustion. In addition, the heavy fractions of liquid fuels like the asphaltenes or the large polar‐aromatic compounds pyrolyze to specific forms of solid carbon. The tight requirement of a high space‐time resolution of the measurements during the high temperature combustion privileges the optical and spectroscopic diagnostics. This contribution will shortly review the main optical effects employed in this field namely absorption and fluorescence spectroscopy and elastic light scattering.
Fiber‐Optic Quasielastic Light Scattering in Concentrated Dispersions: The On‐Line Process Control of Carotenoid Micronization
Authors: Wiese H. , Horn D. 
Published: July 2012

AbstractThe article reviews the application of fiber‐optic quasielastic light scattering (FOQELS) for the on‐line and in‐situ characterization of concentrated dispersions. In the typical configuration a single optical fiber is directly immersed in the system and is used both for illuminating the particles by laser light and recollecting part of the scattered light. Specifically, when a single‐mode telecommunication fiber is used, only singly scattered light is intercepted even from highly turbid dispersions. In this case the autocorrelation function of the scattered light can be analyzed directly in terms of the diffusion coefficient of the particles which is strongly related to their size. The application range of the single‐fiber approach in terms of concentration, particle size and dispersant viscosity is complemented by two‐fiber techniques, which, e.g., can be used to measure the light multiply scattered from the dispersions (fiber‐optic diffusing‐wave spectroscopy, FODWS). As an example for the application of the FOQELS method, the on‐line control of the carotenoid micronization process is described.
Tomographic Measurements of Carbon Monoxide Temperature and Concentration in a Bunsen Flame Using Diode Laser Absorption
Authors: Nguyen Q. V. , W. Dibble R. , Hofmann D. , Kampmann S. 
Published: July 2012

AbstractThe temperature and concentration profiles of carbon monoxide (CO) in a laminar, axisymmetric, premixed methane‐air Bunsen flame are measured using line‐of‐sight diode laser absorption spectroscopy in conjunction with computer tomographic (CT) reconstruction. The absorption spectra for the P(20) (v = 2 ← 1) and P(27) (v = 1 ← 0) vibrotational transitions of CO were measured at 21 evenly spaced positions over a 1.33 cm span for a 1.3 cm radius flame. The CT reconstruction algorithm was based on the Fourier convolution technique. The tomographically reconstructed normalized transmission profiles derived from the absorption spectra, in conjunction with a quantum mechanical model for the vibrotational behavior of CO, yielded both the temperature and concentration profiles. The Bunsen flame investigated had 3 distinct zones: (1) an inner rich‐premixed flame zone, (2) an outer non‐premixed flame zone and (3) an unburnt core region. The reconstructed temperature profile showed that the core region temperature was close to ambient and rapidly increased to the adiabatic flame temperature at the inner flame zone. The temperature was observed to reach a local minima of approximately 200 K less than the adiabatic flame temperature. At the outer flame zone, the temperature increased to the adiabatic flame temperature at the outer flame zone. The reconstructed concentration profile was reasonable, low in the core region, with a maximum at the approximate location of the inner fuel‐rich flame zone and subsequently decreased to a low level towards the outer flame zone where the CO concentration goes towards zero as expected for a non‐premixed flame zone. An advantage of the tomographic technique is that the knowledge of the laser pathlength across the actual flame is not required for quantitative measurements of the concentration.In an attempt to measure the low concentrations of CO as it occurs at the edge of a Bunsen flame, we switched from direct absorption to the second harmonic technique. With this improved sensitivity, we have discovered a large spectral shift of the CO P(38) (v = 1 ← 0) transition depending on whether the post flame combustion products are fuel‐lean (blue shift +0.009 cm−1) or fuel‐rich (no shift).