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Effect of anharmonicity on adsorption thermodynamics
Full text linkThe effect of anharmonic corrections to the vibrational energies of extended systems is explored. Particular attention is paid to the thermodynamics of adsorption of small molecules on catalytically relevant systems typically affected by anharmonicity. The implemented scheme obtains one-dimensional anharmonic model potentials by distorting the equilibrium structure along the normal modes using both rectilinear (Cartesian) or curvilinear (internal) representations. Only in the latter case, the modes are decoupled also at higher order of the potential and the thermodynamic functions change in the expected directions. The method is applied to calculate ab initio enthalpies, entropies, and Gibbs free energies for the adsorption of methane in acidic chabazite (H-CHA) and on MgO(001) surface. The values obtained for the adsorption of methane in H-CHA (273.15 K, 0.1 MPa, θ = 0.5) are ΔH = -19.3, -TΔS = 11.9, and ΔG = -7.5 kJ/mol. For methane on the MgO(001) (47 K, 1.3 × 10-14 MPa, θ = 1) ΔH = -14.4, -TΔS = 16.6, and ΔG = 2.1 kJ/mol are obtained. The calculated desorption temperature is 44 K, and the desorption prefactor is 4.26 × 10 12 s-1. All calculated results agree within chemical accuracy limits with experimental data. © 2014 American Chemical Society
ERRATUM TO: Mass-selective vibrational spectroscopy of vanadium oxide cluster ions [Mass Spect. Rev. 26, 542-562 (2007)]
Full text linkOn p. 558, right column, line 10 from bottom, the reference “(Santambrogio et al., 2007)” should be replaced by “(Santambrogio et al., 2006).
On p. 561, the reference
“Santambrogio G, Brümmer M, Wöste L, Döbler J, Sierka M, Sauer J, Meijer
G, Asmis KR. 2007. Gas Phase Infrared Spectroscopy of Mass-Selected
Vanadium Oxide Cluster Anions. Submitted to J Chem Phys.“
Should be replaced by
“Santambrogio G, Brümmer M, Wöste L, Döbler J, Sierka M, Sauer J, Meijer
G, Asmis KR. 2006. Gas Phase Vibrational Spectroscopy of Mass-Selected Vanadium Oxide Cluster Anions. In preparation.
Computergestützte Wissensakquisition.
No full textDie Wissensakquisition umfasst zentrale Prozesse beim Aufbau eines wissensbasierten Systems. Wenn auch der Begriff der Wissensakquisition nicht einheitlich verwendet wird, so werden im allgemeinen doch ähnliche Hauptphasen der Wissensakquisition dargestellt: die Wissenserhebung, die Wissensinterpretation bzw. -analyse sowie die Umsetzung des Wissens in eine operationale Wissensbasis
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ab initio study of methanol and ethanol adsorption on Brønsted sites in zeolite H-MFI
No full textWe examine the interaction of methanol and ethanol with a bridging OH group of H-MFI (Al12-O20(H)-Si3 site). The computational standard approach for molecule-surface interaction uses density functional theory with inclusion of dispersion for energies and harmonic vibrational frequencies for entropies and finite temperature effects for enthalpies. At 300 K, this yields -117 and -135 kJ mol-1 for adsorption enthalpies of methanol and ethanol, respectively, and 59 and 61 kJ mol-1, respectively for their entropy terms -T·ΔS. To reach chemical accuracy (±4 kJ mol-1) we go beyond this approach. The energies are calculated using a hybrid QM:QM scheme (QM-quantum mechanics) which combines plane-wave density functional theory accounting for the periodicity of the system with wave function-based methods (Møller-Plesset perturbation and Coupled Cluster theories). Finite temperature and entropy contributions are calculated from anharmonic vibrational partition functions. This yields as final predictions for methanol and ethanol -84 and -104 kJ mol-1, respectively, for the enthalpies of adsorption, 56 and 48 kJ mol-1, respectively, for the -T·ΔS term, and -28 and -56 kJ mol-1, respectively, for the Gibbs free energies at 300 K
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Ab-Initio Calculation of Rate Constants for Molecule-Surface Reactions with Chemical Accuracy
Full text linkThe ab initio prediction of reaction rate constants for systems with hundreds of atoms with an accuracy that is comparable to experiment is a challenge for computational quantum chemistry. We present a divide-and-conquer strategy that departs from the potential energy surfaces obtained by standard density functional theory with inclusion of dispersion. The energies of the reactant and transition structures are refined by wavefunction-type calculations for the reaction site. Thermal effects and entropies are calculated from vibrational partition functions, and the anharmonic frequencies are calculated separately for each vibrational mode. This method is applied to a key reaction of an industrially relevant catalytic process, the methylation of small alkenes over zeolites. The calculated reaction rate constants (free energies), pre-exponential factors (entropies), and enthalpy barriers show that our computational strategy yields results that agree with experiment within chemical accuracy limits (less than one order of magnitude). A new strategy enables accurate quantum-mechanical ab initio predictions for the methylation of small alkenes over zeolite catalysts. The calculated reaction rate constants (free energies), pre-exponential factors (entropies), and enthalpy barriers show that this computational strategy yields results that agree with experiment within chemical accuracy limits
Quantum Chemical Free Energies: Structure Optimization and Vibrational Frequencies in Normal Modes
No full textA computational protocol is presented that uses normal mode coordinates for structure optimization and for obtaining harmonic frequencies by numerical differentiation. It reduces numerical accuracy problems encountered when density functional theory with plane wave basis sets is applied to systems with flat potential energy surfaces. The approach is applied to calculate Gibbs free energies for adsorption of methane, ethane, and propane on the Brønsted acidic sites of zeolite H-CHA. The values obtained (273.15 K, 0.1 MPa,), -0.25, -5.95, and -16.76 kJ/mol, respectively, follow the trend of the experimental values, which is not the case for results obtained with the standard approach (Cartesian optimization, frequencies from Cartesian distortions). Anharmonicity effects have been approximately taken into account by solving one-dimensional Schrödinger equations along each normal mode. This leads to a systematic increase of the Gibbs free energy of adsorption of 4.5, 5.0, and 3.1 kJ/mol for methane, ethane, and propane, respectively, making adsorption at a given pressure and temperature less likely. This is due to an increase of low vibrational frequencies associated with hindered translations and rotations of the adsorbed molecules and the floppy modes of the zeolite framework. © 2013 American Chemical Society
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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