03 Fakultät Chemie

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/4

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Institute: search.filters.UBSInstitut.Institut für Theoretische ChemieAuthor: search.filters.author.Rauhut, Guntram

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Now showing 1 - 6 of 6
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    Spectroscopic characterization of diazophosphane - a candidate for astrophysical observations
    (2023) Tschöpe, Martin; Rauhut, Guntram
    Quite 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.
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    Advances in vibrational configuration interaction theory : part 1: efficient calculation of vibrational angular momentum terms
    (2021) Mathea, Tina; Rauhut, Guntram
    Finite basis vibrational configuration interaction theory (VCI) is a highly accurate method for the variational calculation of state energies and related properties, but suffers from fast growing computational costs in dependence of the size of the correlation space. In this series of papers, concepts and techniques will be presented, which diminish the computational demands and thus broaden the applicability of this method to larger molecules or more complex situations. This first part focuses on a highly efficient implementation of the vibrational angular momentum (VAM) terms as occurring in the Watson Hamiltonian and the prediagonalization of initial subspaces within an iterative configuration selective VCI implementation. Working equations and benchmark calculations are provided, the latter demonstrating the increased performance of the new algorithm.
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    Quantum chemical rovibrational analysis of aminoborane and its isotopologues
    (2022) Schneider, Moritz; Rauhut, Guntram
    Aminoborane, 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.
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    Quantenmechanische Berechnungen molekularer Schwingungsspektren : Methoden und Anwendungen
    (2000) Rauhut, Guntram; Werner, Hans-Joachim (Prof. Dr.)
    Die Berechnung molekularer Schwingungsspektren mit quantenchemischen Methoden ist eine Routineanwendung. Aufgrund hoher Rechenzeiten wird in der Regel die harmonische Näherung zugrunde gelegt und die erhaltenen Frequenzen werden anschließend mit einem empirischen Faktor justiert. Eine Alternative besteht darin, nicht die Frequenzen, sondern die Kraftkonstanten zu skalieren. Damit kann man den unterschiedlichen Eigenschaften verschiedener Koordinatentypen Rechnung zu tragen, wodurch man genauere Ergebnisse erzielen kann. Im Rahmen diseser Arbeit wurden Beispiele aus der organischen Chemie gewählt, in denen berechnete Schwingungsspektren chemische Fragestellungen beantworteten. Das sind im Detail die Isomerentrennung polychlorierter Dibenzodioxine, die Strukturaufklärung des Azulens sowie die Reaktionsaufklärung in Umlagerungsreaktionen von Benzofuroxanen. Als Grundvoraussetzung dafür wurden übertragbare Faktoren für die Skalierung der Kraftkonstantenmatrizen bestimmt. Neben diesen Anwendungen werden neue ab-initio-Verfahren vorgestellt, die eine bessere Berücksichtigung von Elektronenkorrelationseffekten in Kraftkonstanten zum Ziel haben. Dabei wurden zwei Aspekte betrachtet: Zum einen wurde durch besondere Transformationstechniken und Näherungsverfahren versucht, bestehende Korrelationsmethoden effizienter zu machen, und zum anderen wurden prinzipiell neue Verfahren entwickelt. Alle Arbeiten zur Methodenentwicklung betreffen die lokalen Korrelationsmethoden von Pulay und Saebø. Für das lokale MP2-Verfahren wird eine neue Integraltransformation für die internen Austauschintegrale vorgestellt und die Berücksichtigung von Dispersionsenergien über ein neuronales Netzwerk. Im Rahmen der Neuentwicklung von ab-initio-Methoden wurden lokale QCISD-Energiegradienten entwickelt, deren Herleitung mit der konventionellen QCISD-Gradiententheorie verglichen wird. Es werden exemplarische Rechnungen an molekularen Strukturen sowie Schwingungsspektren vorgestellt.
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    Advances in vibrational configuration interaction theory : part 2: fast screening of the correlation space
    (2021) Mathea, Tina; Petrenko, Taras; Rauhut, Guntram
    For larger molecules, the computational demands of configuration selective vibrational configuration interaction theory (cs‐VCI) are usually dominated by the configuration selection process, which commonly is based on second order vibrational Møller‐Plesset perturbation (VMP2) theory. Here we present two techniques, which lead to substantial accelerations of such calculations while retaining the desired high accuracy of the final results. The first one introduces the concept of configuration classes, which allows for a highly efficient exploitation of the analogs of the Slater‐Condon rules in vibrational structure calculations with large correlation spaces. The second approach uses a VMP2 like vector for augmenting the targeted vibrational wavefunction within the selection of configurations and thus avoids any intermediate diagonalization steps. The underlying theory is outlined and benchmark calculations are provided for highly correlated vibrational states of several molecules.
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    High-level rovibrational calculations on ketenimine
    (2021) Tschöpe, Martin; Schröder, Benjamin; Erfort, Sebastian; Rauhut, Guntram
    From an astrochemical point of view ketenimine (CH2CNH) is a complex organic molecule (COM) and therefore likely to be a building block for biologically relevant molecules. Since it has been detected in the star-forming region Sagittarius B2(N), it is of high relevance in this field. Although experimental data are available for certain bands, for some energy ranges such as above 1200 cm-1 reliable data virtually do not exist. In addition, high-level ab initio calculations are neither reported for ketenimine nor for one of its deuterated isotopologues. In this paper, we provide for the first time data from accurate quantum chemical calculations and a thorough analysis of the full rovibrational spectrum. Based on high-level potential energy surfaces obtained from explicitly correlated coupled-cluster calculations including up to 4-mode coupling terms, the (ro)vibrational spectrum of ketenimine has been studied in detail by variational calculations relying on rovibrational configuration interaction (RVCI) theory. Strong Fermi resonances were found for all isotopologues. Rovibrational infrared intensities have been obtained from dipole moment surfaces determined from the distinguishable cluster approximation. A comparison of the spectra of the CH2CNH molecule with experimental data validates our results, but also reveals new insight about the system, which shows very strong Coriolis coupling effects.