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Item Open Access The reactivity of pyridine in cold interstellar environments : the reaction of pyridine with the CN radical(2022) Heitkämper, Juliane; Suchaneck, Sarah; García de la Concepción, Juan; Kästner, Johannes; Molpeceres, GermánThe recent detection of cyclic species in cold interstellar environments is an exciting discovery with yet many unknowns to be solved. Among them, the presence of aromatic heterocycles in space would act as an indirect evidence of the presence of precursors of nucleotides. The seeming absence of these species in the observations poses a fascinating conundrum that can be tackled with computational insights. Whilst many arguments can be given to explain the absence of heterocycles in space, one of the possible scenarios involves fast chemical conversion and formation of new species to be detected. We have tested this hypothesis for the reaction of pyridine with the CN radical to find possible scenarios in which the detectability of pyridine, as an archetypical heterocycle, could be enhanced or diminished via chemical conversions. Using a combination of ab-initio characterization of the reactive potential energy surface and kinetic and chemical simulations, we have established that pyridine does react very fast with CN radicals, estimating that the studied reactions is between 2.5-4.5 times faster in pyridine than in benzene, with a total loss rate constant of 1.33 × 10-9 cm3s-1 at 30 K, with an almost null temperature dependence in the (30-150) K range. Addition reactions forming 1,2,3-cyanopyridine are favored over abstraction reactions or the formation of isocyanides. Besides, for 1 and 2-cyanopyridine there is an increase in the total dipole moment with respect to pyridine, which can help in their detection. However, the reaction is not site specific, and equal amounts of 1,2,3-cyanopyridine are formed during the reaction, diluting the abundance of all the individual pyridine derivatives.Item Open Access Use of time domain nuclear magnetic resonance relaxometry to monitor the effect of magnetic field on the copper corrosion rate in real time(2022) Igreja Nascimento Mitre, Cirlei; Ferreira Gomes, Bruna; Paris, Elaine; Silva Lobo, Carlos Manuel; Roth, Christina; Colnago, Luiz AlbertoThe corrosion of metals is a major problem of modern societies, demanding new technologies and studies to understand and minimize it. Here we evaluated the effect of a magnetic field (B) on the corrosion of copper in aqueous HCl solution under open circuit potential. The corrosion product, Cu2+, is a paramagnetic ion and its concentration in the solution was determined in real time in the corrosion cell by time-domain NMR relaxometry. The results show that the magnetic field (B = 0.23 T) of the time-domain NMR instrument reduces the corrosion rate by almost 50%, in comparison to when the corrosion reaction is performed in the absence of B. Atomic force microscopy and X-ray diffraction results of the analysis of the corroded surfaces reveal a detectable CuCl phase and an altered morphology when B is present. The protective effect of B was explained by magnetic forces that maintain the Cu2+ in the solution/metal interface for a longer time, hindering the arrival of the new corrosive agents, and leading to the formation of a CuCl phase, which may contribute to the rougher surface. The time-domain NMR method proved to be useful to study the effect of B in the corrosion of other metals or other corrosive liquid media when the reactions produce or consume paramagnetic ions.Item Open Access Quantum chemical rovibrational analysis of aminoborane and its isotopologues(2022) Schneider, Moritz; Rauhut, GuntramAminoborane, H2NBH2 and its isotopologues, H2N10BH2, D2NBD2, and D2N10BD2, have been studied by high‐level ab initio methods. All calculations rely on multidimensional potential energy surfaces and dipole moment surfaces including high‐order mode coupling terms, which have been obtained from electronic structure calculations at the level of explicitly correlated coupled‐cluster theory, CCSD(T)‐F12, or the distinguishable cluster approximation, DCSD. Subsequent vibrational structure calculations based on second‐order vibrational perturbation theory, VPT2, and vibrational configuration interaction theory, VCI, were used to determine rotational constants, centrifugal distortion constants, vibrationally averaged geometrical parameters and (an)harmonic vibrational frequencies. The impact of core‐correlation effects is discussed in detail. Rovibrational VCI calculations were used to simulate the gas phase spectra of these species and an in‐depth analysis of the ν7 band of aminoborane is provided. Color‐coding is used to reveal the identity of the individual progressions of the rovibrational transitions for this particular mode.Item Open Access Fortgeschrittene Konzepte zur Identifizierung und effizienten Berechnung von Schwingungszuständen innerhalb des VCI-Verfahrens(2022) Mathea, Tina; Rauhut, Guntram (apl. Prof. Dr.)Die vorliegende Arbeit beschäftigt sich mit der präzisen und zugleich effizienten Berechnung anharmonischer Schwingungsspektren kleinerer molekularer Systeme mithilfe der Schwingungskonfigurationswechselwirkungstheorie (VCI-Verfahren), für welche quantenmechanische Konzepte die Grundlage liefern. Dabei birgt die Identifikation von Zustandsidentitäten und die Berechnung von Spektren größerer Systeme Herausforderungen, welche Gegenstand dieser Arbeit sind. Im ersten Teil der Ausarbeitung werden physikalisch motivierte Verfahren präsentiert, welche die korrekte Ermittlung des Schwingungszustands von Interesse innerhalb konfigurationsselektiver VCI-Algorithmen, insbesondere bei der Verwendung von Koordinaten und Basisfunktionen ohne Symmetrie-Adaptierung, sicherstellen. Im zweiten Teil der Arbeit liegt der Fokus auf Laufzeitoptimierungen des betrachteten konfigurationsselektiven VCI-Algorithmus. Es werden vier verschiedene physikalisch motivierte Methoden und Techniken vorgestellt, welche die Gesamtrechenzeit um ungefähr eine Größenordnung verkürzen, jedoch keine Einbußen hinsichtlich der erzielten Genauigkeit zur Folge haben. Die Kombination beider Teile führt zu einer wesentlichen Erweiterung des Anwendungsbereichs des VCI-Verfahrens, was anhand einer aufwendigen Anwendungsrechnung für das Alkinylthiocyanat HCCNCS demonstriert wird.Item Open Access Investigation of chemical reactivity by machine-learning techniques(2022) Zaverkin, Viktor; Kästner, Johannes (Prof. Dr.)Item Open Access Development and application of embedded methods to strongly and weakly correlated systems(2022) Vitale, Eugenio; Alavi, Ali (Prof. Dr.)Coupled cluster (CC) theory is a popular and reliable tool in quantum chemistry due to its improvable hierarchy of methods able to rapidly converge to the full configuration interaction (FCI) limit in weakly correlated systems. Although it represents one of the most efficient single reference methods to treat many-body correlations with high accuracy and reliable outcomes, it yields qualitatively erroneous results when applied to strongly correlated systems. Within this thesis, the Distinguishable Cluster (DC) method (i.e., a small modification of CC amplitude equations able to qualitatively describe strongly correlated systems), is combined with FCI Quantum Monte Carlo (FCIQMC) in order to present a new tailored approach, the tailored DC (TDC), which is more accurate than the corresponding tailored CC and the pure DC. To demonstrate this, the method is first benchmarked with a variety of test cases and then further evaluated with computation of spin-state splittings in a few Fe(II) complexes. The systematic improvability of the TDC method is shown as the active space is increased. In the last part of the thesis, a further embedding scheme to treat strong correlation effects is evaluated. Specifically, the development and application of a screened Coulomb formalism is discussed. This simple approach inspired by Random Phase approximation (RPA) shows to be extremely efficient in the dissociation of one- and two-dimensional hydrogen systems.