Universität zu Köln

Mooßen, Oliver (2018). Interpretation of ab initio Calculations of Cerium Compounds and Predictive Power of Density Functional Theory Calculations for Iodine Catalysis. PhD thesis, Universität zu Köln.

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Abstract

This thesis is divided into two parts, the investigation of the electronic structure of cerium complexes paying special attention on the relevance of the cerium 4f orbitals in order to assign the oxidation state of cerium and the quality of density functional theory (DFT) computations and their consistency with experimental data in order to investigate the reliability of such calculations and their predictive credibility for reactions. In the first part, the electronic structure of the ground state of several cerium complexes, Ce(C8H8)2, Ce(C8H6)2, Cp2CeZ ( Z = CH2, CH-, NH, O, F+) as well as CH2CeF2 and OCeF2 were investigated. Using CASSCF computations including orbital rotations of the active orbitals, the underlying reason for the different interpretations of the cerium oxidation state (Ce(III) and Ce(IV)) of cerocene was found. By orbital rotation nearly pure cerium 4f and ligand pi orbitals were obtained for cerocene. The CASSCF wavefunction based on these localized orbitals was analyzed and a leading f1pi3 was obtained. Therefore, cerocene was classified as a Ce(III) compound. This result is in agreement to spectroscopic XANES data. Using the same computational technique, the electronic structure of all other cerium compounds was investigated. Similar to cerocene, nearly pure Ce 4f and ligand orbitals were obtained for Ce(C8H6)2, Cp2CeCH2, Cp2CeCH- and CH2CeF2 resulting in a leading f1pi1 or f1p1 configuration. Therefore these systems were classified as Ce(III) compounds. In contrast the complexes Cp2CeNH, Cp2CeO and OCeF2 should be described as mixed valent Ce(III)/Ce(IV) compounds, whereas the Cp2CeF+ complex can be categorized as a Ce(IV) compound. It can be shown that the most compact wavefunction, which correctly describes the influence of the Ce 4f orbitals can be obtained for all molecules, except cerocene, at the CASSCF(2,2) level. These compact wavefunctions based on localized orbitals were used to investigate the nature of the orbital interactions of the active orbitals. The results revealed that the 4f-pi orbital interaction of Ce(C8H6)2 as well as the 4f-p orbital interaction of CH2CeCp2, CH-CeCp2 and CH2CeF2 of the Ce-CH2 or Ce-CH bonds can be classified as covalent interactions. The mixed valent systems revealed an increased ionic character of the active orbital interactions for the Ce-NH and Ce-O bonds, whereas the Ce-F bond can be clearly described as ionic. These results are in a good agreement to the assigned oxidation states. In the second part of this thesis, the quality and reliability of DFT computations of reactions compared to experimental results was investigated. The energies of the starting materials, the products as well as the transition states of several iodine catalyzed reactions were computed using various DFT methods. The results revealed that experimental outcomes (reaction time and product yields) can not be computed reliably for the whole set of investigated reactions. Additionally, it revealed that modern and older functionals possess the same predictive credibility. Nevertheless it was shown that all experimental outcomes of the reactions between methyl acrylate and aniline derivatives were reproduced by DFT methods. Therefore a reliable reaction prediction using DFT methods is not generally performable, but based on experimental results, DFT computations can predict reaction trends of very similar systems correctly. These correct predictions were obtained by all used functionals, which emphasizes that for a specific application modern and older functionals might possess the same quality.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Mooßen, Oliver omoossen@aol.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-82730
Date:	15 March 2018
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Theoretical Chemistry
Subjects:	Natural sciences and mathematics Chemistry and allied sciences
Uncontrolled Keywords:	Keywords Language oxidation state, cerium, electronic structure, f-elements, density functional theory, iodine catalysis English
Date of oral exam:	16 May 2018
Referee:	Name Academic Title Dolg, Michael Prof. Dr. Hanrath, Michael Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/8273