Ab initio kinetics predictions for H-atom abstraction from diethoxymethane by hydrogen, methyl, and ethyl radicals and the subsequent unimolecular reactions

Diethoxymethane (DEM) is a promising oxygenated fuel and fuel additive, which has similar positive combustion characteristics as dimethoxymethane. DEM contains C-C bonds and can form ethylene via beta-scission, which potentially increases its sooting tendency. Since DEM is rarely studied, however, kinetic modeling attempts are forced to rely on rate constant analogies. Therefore, we employ high level CCSD(T)/aug-cc-pV(T+D)Z//B2PLYPD3BJ/6-311++(d,p) theory along with transition state theory to predict reaction rate constants for H-abstraction by H and CH3 and the subsequent unimolecular reactions. We further prove that the DLPNO approximation to CCSD(T) leads to a deviation of less than 0.25 k/mol in barrier heights for the presently studied open-shell electronic structures and use it for the prediction of reaction rate constants for H-abstraction by C2H5 radicals. We find that H-abstraction by ethyl radicals might denote a significant pathway, which should not be neglected in kinetic modeling studies of DEM. It is also shown that reaction pathways leading to ethylene formation are of minor importance and give thereby a first insight into the fate of the C-C bonds. To the best of our knowledge, this study represents the first high-level ab-initio study of DEM, which makes the reaction kinetics and thermochemistry data provided by this study vital for future comprehensive kinetic modeling of DEM. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

SIMS-Untersuchungen an modernen Halbleitern : GaN und VO2

Für einen industriellen Einsatz der technologisch hochinteressanten Halbleiter Galliumnitrid (GaN) und Vanadiumdioxid (VO2) ist die Beherrschung der Herstellung und das Verständnis der Materialeigenschaften unerläßlich. Das erfordert zwangsläufig eine intensive Analytik an den Materialien und deren Bauelementen. In der vorliegenden Arbeit wird demonstriert, daß die Sekundärionenmassenspektrometrie (SIMS) eine dafür hervorragend geeignete Methode ist. Dabei wird das gesamte Spektrum der Meßmöglichkeiten von SIMS eingesetzt. Im Rahmen der vorliegenden Arbeit wurden Empfindlichkeitsfaktoren für verschiedene Elemente in GaN und VO2 bestimmt. Darüber hinaus wurde eine Methode entwickelt, die die Kontrolle der Stöchiometrie von VO2 erlaubt. Es sind damit in weiten Bereichen quantitative Analysen der Materialzusammensetzung möglich. Neben diesen Vorarbeiten für zukünftige SIMS-Untersuchungen und einer Vielzahl wertvoller Informationen für die Schichthersteller wurden SIMS-Analysen erfolgreich eingesetzt, um wissenschaftliche Fragen der aktuellen Forschung zu klären. Die Untersuchung der Magnesiumdotierung von GaN mit SIMS bildet einen Schwerpunkt der Arbeit. Bearbeitet wurden Fragen zur Einbaugrenze von Mg in c-GaN, zum Zusammenhang von Mg-Konzentration und Ladungsträgerkonzentration und zu den Gitterkonstanten Mg-dotierten GaN\u27s. Mit Hilfe der Ergebnisse des letzten genannten Punktes kann man den Zusammenhang von Mg-Konzentration und den g-Faktoren des Mg-Akzeptors in GaN erklären. Daneben konzentrieren sich die Messungen auf GaN/InGaN Mehrschichtstrukturen. Mit einer Tiefenauflösung von bis zu 2 nm wurden tiefliegende Probenbereiche gezielt hinsichtlich ihrer chemischen Zusammensetzung untersucht. Die SIMS-Untersuchungen dieser Arbeit an VO2 umfassen die Stöchiometriekontrolle, für die eine neuartige Methode auf der Basis des Valenzmodells der Sekundärionenemission entwickelt wurde, und die Dotierung der Proben. Daraus hervor gingen eine Reihe von Optimierungsmöglichkeiten die Herstellungstemperatur, die Methoden der Dotierung und die Handhabung betreffend. Desweiteren wurden VO2-Schichten auch eingehend optisch charakterisiert, um ihr Potential für den Einsatz als Architekturglasbeschichtung zu prüfen. Besonderer Wert wurde dabei auf die Optimierung der Schichteigenschaften hinsichtlich dieser Anwendung durch Fluor- und/oder Wolframdotierung und Antireflexschichten gelegt. Für Vanadiumdioxid wurde gezeigt, welches Potential es als Beschichtungsmaterial für intelligente Fensterverglasungen hat. Es zeichnet sich durch den einfachen Aufbau, das Fehlen von externen Regelungen und Versorgungen und den Erhalt der klaren Durchsicht gegenüber alternativen schaltenden und schaltbaren Architekturgläsern aus. Durch die Kombination der in dieser Arbeit aufgezeigten Optimierungsansätze wie Titandotierung und Antireflexschichten können VO2-beschichtete Fenster auch hinsichtlich der bautechnischen Kenngrößen zu einem marktfähigen Produkt entwickelt werden

The furan microsolvation blind challenge for quantum chemical methods: First steps

© 2018 Author(s). Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH-O motif in the predominant species. From the presence of another band, which can be attributed to an OH-π interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication

The first microsolvation step for furans : new experiments and benchmarking strategies

The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight

A reactive molecular dynamics study of chlorinated organic compounds, part I : force field development

This work presents a novel parametrization for the ReaxFF formalism as a means to investigate reaction processes of chlorinated organic compounds. Force field parameters cover the chemical elements C, H, O, Cl and were obtained using a novel optimization approach involving relaxed potential energy surface scans as training targets. The resulting ReaxFF parametrization shows good transferability, as demonstrated on two independent ab initio validation sets. While this first part of our two-paper series focuses on force field parametrization, we apply our parameters to the simulation of chlorinated dibenzofuran formation and decomposition processes in Part II

Hydrogen Abstraction from <i>n</i>‑Butyl Formate by H^• and HO₂^•

The combustion chemistry of esters has been elucidated in the past through the study of smaller formates and acetates. Hydrogen abstraction from the fuel as an initiation step is mostly modeled based on estimations for similar abstractions from nonoxygenated hydrocarbons. This study reports computed ab initio rates for abstractions by H^• and HO₂^• radicals from the recently proposed biofuel candidate <i>n</i>-butyl formate. The energies are evaluated with a double hybrid density functional that performs especially well for barrier heights (B2KPLYP/aug-cc-pvtz). Hindered rotation of HO₂^• with respect to <i>n</i>-butyl formate is treated using accurate eigenvalue summation and shows large influence on the rates. Transition states at the γ and δ positions are still influenced by the formate group. The abstraction from the γ carbon by HO₂^• is slowest, although proceeding over the lowest barriers, due to the important influence of transition state entropies. A comparison with smaller esters and <i>n</i>-butanol shows that estimated rates deviate within 1 order of magnitude from the ab initio computations for similar groups in <i>n</i>-butyl formate