03 Fakultät Chemie
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Item Open Access Highly active cooperative Lewis acid : ammonium salt catalyst for the enantioselective hydroboration of ketones(2021) Titze, Marvin; Heitkämper, Juliane; Junge, Thorsten; Kästner, Johannes; Peters, RenéEnantiopure secondary alcohols are fundamental high‐value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5-3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working.Item Open Access Field evaporation and atom probe tomography of pure water tips(2020) Schwarz, T. M.; Weikum, E. M.; Meng, K.; Hadjixenophontos, E.; Dietrich, C. A.; Kästner, J.; Stender, P.; Schmitz, G.Measuring biological samples by atom probe tomography (APT) in their natural environment, i.e. aqueous solution, would take this analytical method, which is currently well established for metals, semi-conductive materials and non-metals, to a new level. It would give information about the 3D chemical structure of biological systems, which could enable unprecedented insights into biological systems and processes, such as virus protein interactions. For this future aim, we present as a first essential step the APT analysis of pure water (Milli-Q) which is the main component of biological systems. After Cryo-preparation, nanometric water tips are field evaporated with assistance by short laser pulses. The obtained data sets of several tens of millions of atoms reveal a complex evaporation behavior. Understanding the field evaporation process of water is fundamental for the measurement of more complex biological systems. For the identification of the individual signals in the mass spectrum, DFT calculations were performed to prove the stability of the detected molecules.Item Open Access Computational investigation of catalytic reaction mechanisms(2024) Heitkämper, Juliane; Kästner, Johannes (Prof. Dr.)Item Open Access Spectroscopic characterization of diazophosphane - a candidate for astrophysical observations(2023) Tschöpe, Martin; Rauhut, GuntramQuite recently, diazophosphane, HP-N≡N, was synthesized for the first time. This was accomplished by a reaction of PH3 with N2 under UV irradiation at 193 nm. As these two molecules have been observed in different astrophysical environments, as for example, in the circumstellar medium and, in particular, in the AGB star envelope IRC+10216, the question arises whether HPN2 can be found as well. So far there is only the aforementioned experimental work, but neither rotational nor rovibrational data are available. Hence, the lack of accurate line lists, etc. to identify diazophosphane is the subject of this work, including a detailed analysis of the rotational, vibrational, and rovibrational properties for this molecule. Our calculations rely on multidimensional potential energy surfaces obtained from explicitly correlated coupled-cluster theory. The (ro)vibrational calculations are based on related configuration interaction theories avoiding the need for any model Hamiltonians. The rotational spectrum is studied between T = 10 and 300 K. In contrast, the partition functions for HPN2 and DPN2 are given and compared for temperatures up to 800 K. In addition, more than 70 vibrational transitions are calculated and analyzed with respect to resonances. All these vibrational states are considered within the subsequent rovibrational calculations. This allows for a detailed investigation of the infrared spectrum up to 2700 cm-1 including rovibrational couplings and hot bands. The results of this study serve as a reference and allow, for the first time, for the identification of diazophosphane, for example, in one of the astrophysical environments mentioned above.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 Linear scaling high-spin open-shell local correlation methods(2011) Liu, Yu; Werner, Hans-Joachim (Prof. Dr.)There are two kinds of popular methods in quantum chemistry: those based on wave function theory, which are usually named ab initio methods, and density functional theory (DFT). DFT is applicable to systems with 100-300 atoms, but it is also well known that its accuracy can not be systematically improved. DFT can sometimes provide results with errors smaller than 1 kcal/mol, but it can also lead to errors of more than 10 kcal/mol. Consequently, DFT is a powerful method, but not always a reliable one. On the other hand, the accuracy of ab initio methods can be systematically improved by increasing the level of the treatment of electron correlation effects. Therefore, if enough calculation resources are available, ab initio methods can always provide reliable results for electronic structure calculations. However, it is well know that ab initio methods consume huge amounts of computational resources and their cost increases too fast with the system size. One of the main reasons for the steep scaling of ab initio methods is caused by the use of canonical molecular orbitals (MOs), which are generally extended over the whole system. Delocalized MOs not only prevent the omission of small correlation effects of distant electrons, but also lead to the circumstance that for correlating each particular electron pair the number of virtual orbitals needed increases unphysically fast. By localizing molecular orbitals and using projected atomic orbitals as virtual orbitals, a set of local correlation methods have been developed in the last 15 years. Using these methods, high level correlated treatments can be performed for much larger systems. For example, using the local singles and doubles coupled cluster (LCCSD(T)) program in Molpro together with the density fitting technique, it is possible to treat the system with about 150 atoms with basis sets of triplezeta plus polarization quality. However, so far most of the low scaling correlated methods were only implemented for closed shell cases, especially, the high accuracy one, LCCSD(T). Given the importance of the open-shell systems in chemistry, it is fundamental to develop efficient high-level low scaling open-shell local correlation methods. In this work, a set of newly developed open-shell linear scaling or low scaling correlation methods is presented. They are the local restricted second-order Møller-Plesset perturbation theory (LRMP2), local unrestricted singles and doubles coupled cluster (LUCCSD), and local unrestricted CCSD(T) with perturbative triples (LUCCSD(T)) methods. In all above methods, restricted open-shell Hartree-Fock (RHF) orbitals are used as the reference. Two different localization schemes are compared and discussed. In the first case localization is performed separately in the closed-shell and open-shell orbital spaces. In the second case localization is performed separately for the alpha and beta spin orbitals. The excitations are restricted to domains, and only strong pairs are treated at the highest level. Local density fitting approximations are used to compute integrals. Provided that the orbitals can be well localized, this leads to linear scaling of the computational effort with molecular size and extends the applicability of the local RMP2 and UCCSD methods to systems with 100-150 correlated orbitals and 2000-4000 basis functions. The scaling of the methods is demonstrated, and then the accuracy is tested. The methods are tested for computing radical stabilization energies, vertical ionization potentials, and molecular electron affinities. The accuracy is found to be comparable to the corresponding canonical methods. Finally, two applications using the new methods have been carried out and the results are discussed.Item Open Access Analytical energy gradients for open-shell local second-order Møller-Plesset perturbation theory and applications to large molecules(2018) Dornbach, Mark; Werner, Hans-Joachim (Prof. Dr.)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 Computersimulationen und spektroskopische Untersuchungen von Clustern und Doppelprotonentransferprozessen mit strukturlosen Übergangszuständen(2005) Schweiger, Stefan; Rauhut, Guntram (PD Dr.)Innerhalb dieser Arbeit wurden am Beispiel mehrerer substituierter Pyrazol-Guanidin-Cluster Doppelprotonentransferreaktionen im Grenzbereich zwischen konzertiertem und schrittweisem Verlauf betrachtet. Dabei ergaben die Berechnungen eindimensionaler Reaktionspfade bzw. mehrdimensionaler Reaktionsflächen auf der Potentialhyperfläche, dass bei einigen dieser Reaktionen ein eindeutiger Übergangszustand einem Gebiet konstanter Energie weicht und somit strukturlos wird. Aufgrund der konstanten Energie im Übergangsbereich wurde für diese Reaktionen der Begriff Plateaureaktion eingeführt. Ziel dieser Arbeit war es, Plateaureaktionen sowohl mit Hilfe von theoretischen als auch experimentellen Methoden zu untersuchen, den Mechanismus besser zu verstehen sowie Unterschiede im Vergleich zu herkömmlichen Reaktionen zu finden. Dazu standen mehrere theoretische und experimentelle Methoden zur Verfügung: So wurden Reaktionspfade, sogenannte IRCs, für insgesamt 5 verschieden substituierte Pyrazol-Guanidin-Systeme berechnet. Durch geeignete Substituentenwahl gelang es, Beispiele für konzertiert und schrittweise verlaufende Reaktionen sowie Plateaureaktionen zu finden. Die Dynamik einiger dieser Reaktionen wurde mit Hilfe eines im Rahmen dieser Arbeit neu geschriebenen Programmes untersucht. Es basiert auf der klassischen Reaction Path Hamiltonian Theorie (RPH) und erlaubt eine Betrachtung der Reaktionen im 6N-12-dimensionalen Phasenraum. Unter Verwendung der klassischen Trajektorien konnten klassisch-exakte Reaktionsgeschwindigkeitskonstanten sowie Verweilzeiten der einzelnen Systeme im Plateaubereich berechnet werden. Auf der Quantenmechanik basierende Tunneleffekte, die bei Protonentransferreaktionen i.A. eine wichtige Rolle spielen, konnten mit Hilfe eines eigens geschriebenen Tunnelprogramms berücksichtigt werden. Unter Verwendung eines konventionellen doppelfokussierten Massenspektrometers sollten einige der theoretisch beschriebenen Pyrazolcluster massenspektrometrisch nachgewiesen und ihr Zerfall im feldfreien Raum mit der MIKE-Technik untersucht werden. Des weiteren wurden REMPI-Untersuchungen an einigen Pyrazol-Clustern durchgeführt. Bei dieser Methode der Mehrphotonenionisation wird das Ionensignal in Abhängigkeit der Anregungswellenlänge massenselektiv registriert. Als Ergebnis erhält man ein Absorptionsspektrum (REMPI-Spektrum) der Moleküle und Cluster in der Gasphase.Item Open Access Investigation of chemical reactivity by machine-learning techniques(2022) Zaverkin, Viktor; Kästner, Johannes (Prof. Dr.)