Browsing by Subject "Ab-initio"

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  • Wang, Qian; Pyykkö, Jaakko; Dimitrova, Maria; Taubert, Stefan; Sundholm, Dage (2023)
    We have calculated the current density induced by an external magnetic field in a set of figure-eight-shaped expanded porphyrinoids. The studied octaphyrins can be divided into three classes (N2, N4, and N6) based on the number of the inner hydrogen atoms of the pyrrole rings. Using the Runge-Kutta method, the current density is split into diatropic and paratropic contributions that are analyzed separately. The calculations show that one common ring current consists of two rather independent pathways. Each of them follows the outer side of the molecular frame of one half of the molecule and passes to the inner side of the frame on the other half. The ring-current pathways are similar to the ones for [12]infinitene. However, the current density of the octaphyrins is more complex having many branching points and pathways. Vertical through-space current-density pathways pass in the middle of the molecules through a plane that is parallel to the figure-eight-shaped view of the molecules when the magnetic field is perpendicular to the plane. The isolectronic N2 and the N4 dication sustain a weak paratropic ring current inside the molecule, which is also observed in the H-1 NMR magnetic shielding constant of the inner hydrogen atoms. The diatropic current-density contribution dominates in the studied molecules. For the N4 and N6 molecules, the global current-density pathways are only diatropic and N6 sustains the strongest global diatropic current-density flux of 13.2 nA T-1.
  • Minwegen, Heiko; Döntgen, Malte; Hemken, Christian; Büttgen, Rene Daniel; Leonhard, Kai; Heufer, Karl Alexander (2019)
    Recently the possibility of hot beta-scission pathways gained attention. These reactions give a shortcut during the important fuel consumption phase in combustion processes leading from H-atom abstraction directly to the beta-scission products without fuel radical thermalization. Methyl formate (MF) was shown to be prone to hot beta-scission due to a low beta-scission barrier height. Furthermore, MF as smallest methyl ester can be considered as biodiesel surrogate and it is an important intermediate product during combustion of various ethers. In this work a predominantly ab-initio derived detailed kinetic model of MF combustion is developed including hot beta-scission pathways and compared to a sophisticated literature model based on classical estimation methods. For this, new stoichiometric MF in air ignition delay time measurements in a shock tube and a rapid compression machine over a wide temperature range (790 K-1250 K) and pressures of 10, 20 and 40 bar served as validation targets. The experimental ignition delay times (IDT) show Arrhenius type behavior in both facilities at all conditions. The newly developed quantum-based model catches the pressure dependency and low-temperature reactivity well although overpredicting the IDT at higher temperatures. It was found that hot beta-scission is the major depletion pathway of formate group-centered MF radicals. This, however, does not change the overall reactivity of MF combustion due to the low stability of the alkyl peroxide (RO2) at the formate group. For species with competing thermal beta-scission and RO2 formation, however, hot beta-scission may have a significant impact. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
  • Vurpillot, Francois; Parviainen, Stefan; Djurabekova, Flyura; Zanuttini, David; Gervais, Benoit (2018)
    The ideal picture of a near-perfect 3D microscope often presented regarding Atom Probe Tomography faces several issues. These issues degrade the metrological performance of the instrument and find their roots in the phenomena acting at the atomic to the mesoscopic level in the vicinity of the surface of a field emitter. From the field evaporation process at the atomic scale, to the macroscopic scale of the instrument, the path to model the imaging process and to develop more accurate and reliable reconstruction algorithms is not a single lane road. This paper focused on the numerical methods used to understand, treat, and potentially heal imaging issues commonly affecting the data in atom probe experiments. A lot of room for improvement exists in solving accuracy problems observed in complex materials by means of purely electrostatic models describing the image formation in a classical approach. Looking at the sample at the atomic scale, the phenomena perturbing the imaging process are subtle. An examination of atomic scale modifications of the sample surface in the presence of a high surface electric field is therefore mandatory. Atomic scale molecular dynamic models integrating the influence of the high surface electric are being developed with this aim. It is also demonstrated that the complex behavior of atoms and molecules in high fields, and consequences on collected data, can be understood through the use of accurate ab-initio models modified to include the impact of the extreme surface electric field.