Mechanisms of ion migration in ceramic oxides

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Dft Study Of Geometry And Energetics Of Transition Metal Systems

This dissertation focuses on computational study of the geometry and energetics small molecules and nanoclusters involving transition metals (TM). These clusters may be used for various industrial applications including catalysis and photonics. Specifically, in this work we have studied hydrides and carbides of 3d-transition metal systems (Sc through Cu), small nickel and gold clusters. Qualitatively correct description of the bond dissociation is ensured by allowing the spatial and spin symmetry to break. We have tested applicability of new exchange-correlation functional and alternative theoretical descriptions (spin-contamination correction in broken symmetry DFT and ensemble Kohn-Sham (EKS)) as well. We studies TM hydrides and carbides systems to understand the importance of underlying phenomenon of bond breaking in catalytic processes. We have tested several exchange-correlation functionals including explicit dependence on kinetic energy density for the description of hydrides (both neutral and cationic) and carbides formed by 3d-transition metals. We find M05-2x and BMK dissociation energies are in better agreement with experiment (where available) than those obtained with high level wavefunction theory methods, published previously. This agreement with experiment deteriorates quickly for other functionals when the fraction of the Hartree-Fock exchange in DFT functional is decreased. Higher fraction of HF exchange is also essential in EKS formalism, but it does not help when spin-adapted unrestricted approach is employed. We analyze the electron spin densities using Natural Bond Orbital population analysis and find that simple description of 3d electrons as non-bonding in character is rarely correct. Unrestricted formalism results in appreciable spin-contamination for some of the systems at equilibrium, which motivated us to investigate it further in details. In order to correct the spin contamination effect on the energies, we propose a new scheme to correct for spin contamination arising in broken-symmetry DFT approach. Unlike conventional schemes, our spin correction is introduced for each spin-polarized electron pair individually and therefore is expected to yield more accurate energy values. We derive an expression to extract the energy of the pure singlet state from the energy of the broken-symmetry DFT description of the low spin state and the energies of the high spin states (pentuplet and two spin-contaminated triplets in the case of two spin-polarized electron pairs). We validate our spin-contamination correction approach by a simple example of H2 and applied to more complex MnH system. Ensemble KS formalism is also applied to investigate the dissociation of C2 molecule. We find that high fraction of HF exchange is essential to reproduce the results of EKS treatment with exact exchange-correlation functional. We analyze the geometry and energetics of small nickel clusters (Ni2-Ni5) for several lowest energy isomers. We also study all possible spin states of small nickel cluster isomers and report observed trends in energetics. Finally we determine the geometry and energetics of ten lowest energy isomers of four small gold clusters (Au2, Au4, Au6, and Au8). We have also investigated the influence of cluster geometry, ligation, solvation and relativistic effects on electronic structure of these gold clusters. The effect of one-by-one ligand attachment in vacuum and solvent environment is also studied. Performance of five DFT functionals are tested as well; Local Spin Density Approximation (SVWN5), Generalized Gradient Approximation (PBE), kinetic energy density-dependent functional (TPSS), hybrid DFT (B3LYP), and CAM-B3LYP which accounts for long-range exchange effects believed to be important in the analysis of metal bonding in gold complexes and clusters. Our results exhibit the ligand induced stability enhancement of otherwise less stable isomers of Au4, Au6 and Au8. Ligands are found to play a crucial role in determining the 2D to 3D transition realized in small gold clusters. In order to select an appropriate theory level to use in this study, we investigate the effect of attachment of four different ligands (NH3, NMe3, PH3, PMe3) on cluster geometry and energetics of Au2 and Au4 in vacuum and in solution. Our results benchmark the applicability of DFT functional model and polarization functions in the basis set for calculations of ligated gold cluster systems. We employ five different basis sets with increasing amount of polarization and diffuse functions; LANL2DZ, LANL2DZ-P, def2-SVP, def2-TZVP, and def2-QZVP. We obtain NMe3 = NH3 \u3e PH3 \u3e PMe3 order of ligand binding energies and observe shallow potential energy surfaces in all molecules. Our results suggest appropriate quantum-chemical methodologies to model small noble metal clusters in realistic ligand environment to provide reliable theoretical analysis in order to complement experiments

Evaluation and Development of Quantum Chemical Methodologies for Noncovalent Interactions and Supramolecular Thermochemistry

This thesis focuses on the application and development of electronic structure methods for noncovalent interactions in general and the evaluation of multilevel methodologies for an accurate description of supramolecular thermochemistry in particular. Noncovalent interactions are omnipresent in systems of various domains of science, such as supramolecular chemistry, structural biology, and surface science. Within supramolecular chemistry, host-guest complexes are of particular importance due to their diverse applicability in various fields like molecular recognition or self-assembly. The binding situation in a supramolecular complex is often unknown and sampling many different conformations is desired. Therefore, the first part of this thesis is concerned with cost-efficient density functional theory (DFT) and Hartree-Fock (HF) based electronic structure methods for noncovalent interactions, which are about a factor of 50 to 100 faster than calculations in a large basis set. The main errors in a DFT or HF calculation with small atomic orbital basis sets are the missing London dispersion and the basis set superposition error (BSSE). An exemplary benchmark study shows that modern correction strategies clearly outperform plain DFT or HF for energies and geometries of small dimers, large supramolecular complexes, and molecular crystals. Further, the development and evaluation of a minimal basis set Hartree--Fock method with three atom-pairwise corrections for London dispersion, BSSE, and basis set incompleteness (HF-3c) is presented. With nine global parameters, the empiricism of HF-3c is moderate, the method is self-interaction error free, and noiseless analytical frequencies can be obtained. HF-3c provides accurate geometries of organic supramolecular systems and small proteins, and good noncovalent interaction energies. The mean absolute deviations (MADs) for the S22 set of small noncovalently bound dimers and the S12L set of supramolecular host-guest association energies are 0.6 and 4.4 kcal mol-1, respectively. This is excellent compared to dispersion corrected DFT methods whose MADs are in the range of 0.3-0.5 and 2-5 kcal mol-1, respectively. The second part focuses on the application and evaluation of multilevel methodologies for an accurate description of Gibbs free energies of association (Δ Ga) for supramolecular host-guest complexes in solution. First, state-of-the-art dispersion corrected DFT (DFT-D3ATM) is used together with a large quadruple-zeta (QZ) basis set to obtain association energies in the gas phase. A semiempirical method is utilized to compute the thermostatistical corrections from energy to free energy and last, a continuum solvation model is employed. The general procedure is illustrated with a case study on eight typical complexes. The SAMPL4 blind test challenge provides a unique opportunity to test this methodology in a realistic setting. Relative Δ Ga in water are predicted for a cucurbit[7]uril host and 14 guest molecules containing ammonia groups. The HF-3c method was employed to sample possible binding conformations and the final Δ Ga were calculated on the PW6B95-D3ATM/QZ level with HF-3c thermal corrections and COSMO-RS solvation contributions. Compared to other methods theses predictions rank in the top three of all statistical measurements. The MAD and RMSD are only 2.0 and 2.6 kcal mol-1, respectively. Further, the S30L benchmark set is proposed as an extension of the S12L set for association (free) energies of host-guest complexes. Larger systems with up to 200 atoms, more divers interaction motifs, and higher charges are represented by experimentally measured complexes with Δ Ga values in the range from -0.7 to -24.7 kcal mol-1. In order to obtain a theoretical best estimate for Δ Ga different dispersion corrected density functionals, semiempirical methods, and continuum solvation models are tested. The best method combination is similar to the one used for the SAMPL4 bind test and yields an MAD with respect to experiment of only 2.4 kcal mol-1. Inclusion of counterions for the charged systems (S30L-CI) were found to improve the results overall. Synergy between theory and experiment is demonstrated in the last part of this thesis with the application of quantum chemical methods to two specific chemical problems related to supramolecular chemistry. Experimentally, it was found that titanocene(III) catalysts can be stabilized by chloride additives and the calculations reveal that the stabilities of these adducts are determined by the extent of hydrogen bonding between the catalyst and the ammonium cation. 1,1'-Binaphthol based ligands can be used to obtain enantiomerically pure double- and triple-stranded helicates with transition-metal ions in a completely diastereoselective self-assembly process. Electronic circular dichroism spectra of precursors for paracyclophane based ligands have been investigated computationally in order to identify their absolute configuration

Formación y comportamiento de complejos de β-ciclodextrina con pesticidas y alcoholes : Estudio computacional: métodos de mecánica molecular y semiempíricos

En general, el propósito de este trabajo fue obtener conocimiento acerca de los complejos de β-ciclodextrina, estudiando la influencia de su presencia sobre las propiedades del huésped. Específicamente, en el caso de los plaguicidas este estudio ayudaría a evitar los efectos perjudiciales de los mismos, y además en el caso de alcoholes cíclicos predecir las diferencias estructurales de estos compuestos. Y en ambos, por último, contribuye a las investigaciones de los complejos originados por medio de macromoléculas.Facultad de Ciencias Exacta