Interpretation of ab initio Calculations of Cerium Compounds and Predictive Power of Density Functional Theory Calculations for Iodine Catalysis

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

Emergence of comparable covalency in isostructural cerium(IV)- and uranium(IV)-carbon multiple bonds

We report comparable levels of covalency in cerium- and uranium-carbon multiple bonds in the isostructural carbene complexes [M(BIPMTMS)(ODipp)2] [M = Ce (1), U (2), Th (3); BIPMTMS = C(PPh2NSiMe3)2; Dipp = C6H3-2,6-Pri2] whereas for M = Th the M=C bond interaction is much more ionic. On the basis of single crystal X-ray diffraction, NMR, IR, EPR, and XANES spectroscopies, and SQUID magnetometry complexes 1-3 are confirmed formally as bona fide metal(IV) complexes. In order to avoid the deficiencies of orbital-based theoretical analysis approaches we probed the bonding of 1-3 via analysis of RASSCF- and CASSCF-derived densities that explicitly treats the orbital energy near-degeneracy and overlap contributions to covalency. For these complexes similar levels of covalency are found for cerium(IV) and uranium(IV), whereas thorium(IV) is found to be more ionic, and this trend is independently found in all computational methods employed. The computationally determined trends in covalency of Ce ~ U > Th are also reproduced in experimental exchange reactions of 1-3 with MCI4 salts where 1 and 2 do not exchange with ThCl4, but 3 does exchange with MCl4 (M = Ce, U) and 1 and 2 react with UCl4 and CeCl4, respectively, to establish equilibria. This study therefore provides complementary theoretical and experimental evidence that contrasts to the accepted description that generally lanthanide-ligand bonding in non-zero oxidation state complexes is overwhelmingly ionic but that of uranium is more covalent

Assigning the Cerium Oxidation State for CH₂CeF₂ and OCeF₂ Based on Multireference Wave Function Analysis

The geometric and electronic structure of the recently experimentally studied molecules ZCeF₂ (Z = CH₂, O) was investigated by density functional theory (DFT) and wave function-based ab initio methods. Special attention was paid to the Ce–Z metal–ligand bonding, especially to the nature of the interaction between the Ce 4f and the Z 2p orbitals and the possible multiconfigurational character arising from it, as well as to the assignment of an oxidation state of Ce reflecting the electronic structure. Complete active space self-consistent field (CASSCF) calculations were performed, followed by orbital rotations in the active orbital space. The methylene compound CH₂CeF₂ has an open-shell singlet ground state, which is characterized by a two-configurational wave function in the basis of the strongly mixed natural CASSCF orbitals. The system can also be described in a very compact way by the dominant Ce 4f¹ C 2p¹ configuration, if nearly pure Ce 4f and C 2p orbitals are used. In the basis of these localized orbitals, the molecule is almost monoconfigurational and should be best described as a Ce­(III) system. The singlet ground state of the oxygen OCeF₂ complex is of closed-shell character when a monoconfigurational wave function with very strongly mixed Ce 4f and O 2p CASSCF natural orbitals is used for the description. The transformation to orbitals localized on the cerium and oxygen atoms leads to a multiconfigurational wave function and reveals characteristics of a mixed valent Ce­(IV)/Ce­(III) compound. Additionally, the interactions of the localized active orbitals were analyzed by evaluating the expectation values of the charge fluctuation operator and the local spin operator. The Ce 4f and C 2p orbital interaction of the CH₂CeF₂ compound is weakly covalent and resembles the interaction of the H 1s orbitals in a stretched hydrogen dimer. In contrast, the interaction of the localized active orbitals for OCeF₂ shows ionic character. Calculated vibrational Ce–C and Ce–O stretching frequencies at the DFT, CASSCF, second-order Rayleigh–Schrödinger perturbation theory (RS2C), multireference configuration interaction (MRCI), as well as single, doubles, and perturbative triples coupled cluster (CCSD­(T)) level are reported and compared to experimental infrared absorption data in a Ne and Ar matrix