569 research outputs found
Analytical calculation of pressure for confined atomic and molecular systems using the eXtreme-Pressure Polarizable Continuum Model
No full textWe show that the pressure acting on atoms and molecular systems within the compression cavity of the eXtreme-Pressure Polarizable Continuum method can be expressed in terms of the electron density of the systems and of the Pauli-repulsion confining potential. The analytical expression holds for spherical cavities as well as for cavities constructed from van der Waals spheres of the constituting atoms of the molecular systems. \ua9 2018 Wiley Periodicals, Inc
Non-Bonded Radii of the Atoms Under Compression
No full textAbstract: We present quantum mechanical estimates for non-bonded, van der Waals-like, radii of 93 atoms in a pressure range from 0 to 300 gigapascal. Trends in radii are largely maintained under pressure, but atoms also change place in their relative size ordering. Multiple isobaric contractions of radii are predicted and are explained by pressure-induced changes to the electronic ground state configurations of the atoms. The presented radii are predictive of drastically different chemistry under high pressure and permit an extension of chemical thinking to different thermodynamic regimes. For example, they can aid in assignment of bonded and non-bonded contacts, for distinguishing molecular entities, and for estimating available space inside compressed materials. All data has been made available in an interactive web application
Relating atomic energy, radius and electronegativity through compression
No full textTrends in atomic properties are well-established tools for guiding the analysis and discovery of materials. Here, we show how compression can reveal a long sought-after connection between two central chemical concepts - van-der-Waals (vdW) radii and electronegativity - and how these relate to the driving forces behind chemical and physical transformations
Professionalität im Umgang mit Spannungsfeldern der Pädagogik : [der vorliegende Sammelband ist Prof. Dr. Sibylle Rahm gewidmet]
No full textSqueezing All Elements in the Periodic Table: Electron Configuration and Electronegativity of the Atoms under Compression
No full textWe present a quantum mechanical model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment. Studies of 93 atoms predict drastic changes to ground-state electronic configurations and electronegativity in the pressure range of 0-300 GPa. This extension of atomic reference data assists in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts. For example, we can speculate on the existence of pressure-induced polarity (red-ox) inversions in various alloys. Our study confirms that the filling of energy levels in compressed atoms more closely follows the hydrogenic aufbau principle, where the ordering is determined by the principal quantum number. In contrast, the Madelung energy ordering rule is not predictive for atoms under compression. Magnetism may increase or decrease with pressure, depending on which atom is considered. However, Hund's rule is never violated for single atoms in the considered pressure range. Important (and understandable) electron shifts, s→p, s→d, s→f, and d→f are essential chemical and physical consequences of compression. Among the specific intriguing changes predicted are an increase in the range between the most and least electronegative elements with compression; a rearrangement of electronegativities of the alkali metals with pressure, with Na becoming the most electropositive s1 element (while Li becomes a p group element and K and heavier become transition metals); phase transitions in Ca, Sr, and Ba correlating well with s→d transitions; spin-reduction in all d-block atoms for which the valence d-shell occupation is d n (4 ≤ n ≤ 8); d→f transitions in Ce, Dy, and Cm causing Ce to become the most electropositive element of the f-block; f→d transitions in Ho, Dy, and Tb and a s→f transition in Pu. At high pressure Sc and Ti become the most electropositive elements, while Ne, He, and F remain the most electronegative ones
Zur methodischen Funktion von Dilemmainterviews als Erhebungsverfahren in der Schulentwicklungsforschung am Beispiel einer Untersuchung zur Autonomiefrage in der Schulprogrammarbeit
No full textHeinrich M. Zur methodischen Funktion von Dilemmainterviews als Erhebungsverfahren in der Schulentwicklungsforschung am Beispiel einer Untersuchung zur Autonomiefrage in der Schulprogrammarbeit. In: Rahm S, Mammes I, Schratz M, eds. Schulpädagogische Forschung Bd. 2: Organisations- und Bildungsprozessforschung - Perspektiven innovativer Ansätze. Innsbruck: Studien-Verl.; 2006: 83-95
Latent Heat Flux Measurements over Complex Terrain by Airborne Water Vapour and Wind Lidars
No full textVertical profiles of the latent heat flux in a convective boundary layer (CBL) are obtained for the first time over complex terrain with airborne water vapour differential absorption lidar and Doppler wind lidar. During the Convective and Orographically-induced Precipitation Study (COPS) over the Black Forest Mountains in south-western Germany both lidars were installed nadir-viewing onboard the Falcon research aircraft of the Deutsches Zentrum f�¼r Luft- und Raumfahrt (DLR). On 30 July 2007, additional in-situ measurements by the Karlsruhe Institute of Technology (KIT) were performed with a Dornier-128 aircraft that flew below the Falcon. This unique instrument configuration allows to validate the lidar-derived fluxes and to assess lidar-specific issues such as instrument noise and data gaps that impinge on the results. The cospectra of in-situ humidity and vertical velocity peak at wavelengths between 1 - 3 km and reveal that the dominant scales of turbulent transport are larger than 700 m in space. Consequently the airborne lidarsâ�� horizontal and vertical resolution of ~200 m is sufficient to seize most of the flux. The lidar and in-situ fluxes of five collocated 45-km flight legs agree within �±20 %, the average difference over the total distance of 225 km is 3 %. A flux comparison with ground-based water vapour Raman and wind lidars shows agreement within the instrumentsâ�� accuracies under low-wind conditions. All latent heat fluxes vary between 100 - 500 W/m�² in the CBL and have small vertical divergences. Vertical velocity spectra in the mid-CBL enable to estimate the dissipation rate of turbulent kinetic energy that amounts to 5â�¢10-4 m2 s-3 in the Rhine Valley and 10-3 m2 s-3 over the Black Forest Mountains. This new airborne lidar instrumentation proves to be a valuable tool for the study of CBL processes and variability, particularly over complex terrain
Airborne water vapour and wind lidar measurements of latent heat fluxes during COPS 2007
No full textDuring the Convective and Orographically-induced Precipitation Study (COPS) in July 2007 over the Black Forest Mountains in south-western Germany, tropospheric profiles of water vapour and wind were measured with a differential absorption lidar (DIAL) and a heterodyne detection 2-μm Doppler wind lidar collocated on board the DLR Falcon research aircraft. The DIAL “WALES” is a newly developed four-wavelength system operating on three water vapour absorption lines of different strengths, one offline wavelength at 935 nm (each 50 Hz, 40 mJ), and 532 and 1064 nm for aerosol profiling. It is designed as an airborne demonstrator for a possible future spaceborne water vapour lidar mission.
For the study of summertime convection initiation over complex terrain, latent heat flux missions were flown where both lidars were pointed nadir-viewing. Using eddy-correlation of the wind and water vapor fluctuations beneath the aircraft, an area-representative water vapor or latent heat flux profile can be obtained by a single over-flight of the convective boundary layer in complex terrain. The lidars’ horizontal and vertical resolution is about 200 m which resolves the dominant circulation patterns and flux contributions. This novel instrumentation allows obtaining profiles of the latent heat flux beneath the aircraft from one single over-flight of the area of interest
Targeted observations with an airborne wind lidar
No full textThis study investigates the possibilities and limitations of airborne Doppler lidar for adaptive observations over the Atlantic Ocean. For the first time, a scanning 2-µm Doppler lidar was applied for targeted measurements during the Atlantic The Observing System Research and Predictability Experiment (THORPEX) Regional Campaign (A-TReC) in November and December 2003. The DLR lidar system was operated for 28.5 flight hours, and measured 1612 vertical profiles of wind direction and wind speed with a horizontal and vertical resolution of 5–10 km and 100 m, respectively. On average, there were 25 reliable
wind values on every profile, which cover 2500 m in the vertical (about one-third of the mean vertical extent of the profiles). A statistical comparison of 33 dropsondes and collocated lidar winds profiles allowed individual estimates of the standard deviation to be assigned to every wind value and to determine threshold values for an objective quality control of the data. The standard deviation of the difference between dropsonde and lidar winds was correlated with the derived quality indices of the lidar data and was within a range of 0.6–1.8 m/s. Comparisons of the lidar data to the operational analysis revealed differences of up to +/-15 m/s. This emphasizes the need for more representative and higher resolved wind measurements in data-sparse regions above the Atlantic Ocean. The study constitutes the basis for the assimilation of the lidar data and impact studies at the European Center for Medium Range Weather Forecasts (ECMWF)
Electronegativity at the Shock Front
No full textIn this work, a scale for pressure-adapted atomic electronegativity is used to make general predictions of bond polarity in H-, C-, N- and O-based compounds experiencing shock conditions. The qualitative picture that emerges is one of increasing polarity of several bonds common in energetic materials. The general predictions made are compared to, and found to support, claims of ionic decomposition routes in compressed nitromethane and nitrate esters at high pressure. Changing electronegativity is also suggested as a factor driving the ionic disproportionation of various molecular phases with compression. Calculations using the eXtreme-Polarizable Continuum Model (XP-PCM) predict increasing energy differences between ground and excited states in non-bonded H, C, N, and O atoms as a function of pressure. This data enables for a discussion on the reliability of electronegativity-based rationales at more extreme thermodynamic conditions
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