Browsing by Author "Ziegler, Tom"
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- ItemOpen AccessA Computational Analysis of Supported and Unsupported Group 6 Transition Metal-Metal Bonds Based on the Natural Orbitals for Chemical Valence (NOCV) and the Extended Transition State (ETS) Techniques(2013-02-14) Ndambuki, Sylvester Wambua; Ziegler, TomA computational study has been carried out on the nature of the metal-metal bond of group 6 transition metal complexes based on the extended transition state (ETS) and natural orbitals for chemical valence (NOCV) methods. The analysis of the unsupported triple (M2L6) and quadruple (M2L2L´2; M=Cr,Mo,W; L,L´=π-acceptor/σ-donor ligands) metal-metal bonds demonstrated that the M-M bond strength follows the trend Cr<
- ItemOpen AccessA computational study of h2s adsorption on transition and noble metal surfaces(2009) Vartak, Swati; Ziegler, Tom
- ItemOpen AccessA density functional study on four-centre transition states and intermediates in organometallic chemistry(1993) Folga, Elzbieta; Ziegler, Tom
- ItemOpen AccessA theoretical investigation of the structure and function of MAO (methylaluminoxane)(2002) Zurek, Eva; Ziegler, Tom
- ItemOpen AccessA theoretical study of ethylene polymerization catalysis incorporating counterion effects(2004) Vanka, Kumar; Ziegler, Tom
- ItemOpen AccessA theoretical study of methane functionalization by homogeneous transition metal catalysts(2003) Hristov, Iordan Hristov; Ziegler, Tom
- ItemOpen AccessA theoretical study on the mechanism of the original shilov reaction(2008) Zhu, Hongjuan; Ziegler, Tom
- ItemOpen AccessApplication of density functional theory to the vibrational characterization of transitional metal complexes(1995) Bérces, Attila; Ziegler, Tom
- ItemOpen AccessApplications of thermodynamic integration: studying chemical reactions(2008) Jechow, Jason; Ziegler, TomThe present work examines the theoretical foundation of thermodynamic integration (TI) and applies it to study chemical reactions. First, a hypotetical chemical reaction is examined to demonstrate that the average force along a constraint in a molecular dynamics simulation is the negative of the derivative of the free energy. Next, the benzyl-isocyanide isomerization and the butene-plus-singlet-oxygen reaction were investigated using TI. For the benzyl-isocyanide isomerization, activation parameters calculated using TI compared well to experiment. Also, thermodynamic properties specific to the phenyl rotation were calculated. For the butene-plus-singletoxygen reaction, one methyl group is deuterated. With the linear constraint active in the molecular dynamics, free energy along the path leading to deuterium abstraction was calculated. There exists a barrier for the free energy profile while no such barrier exists for the potential energy profile. Two other reactions are proposed for future research: hydroboration rearrangement and the Staudinger reaction.
- ItemEmbargoCalculation of multiplet and bond energies by the HFS- method(1978) Ziegler, Tom; Rauk, Arvi
- ItemOpen AccessCircular dichroism of transition metal complexes within time-dependent density functional theory(2010) Fan, Jing; Ziegler, Tom
- ItemOpen AccessDensity Functional Theory Studies on the Active Site of the Multicopper Oxidases(2013-12-04) Zhekova, Hristina Rumenova; Ziegler, TomMulticopper oxidases (MCO) feature a complex active site which contains a T1 copper center and a trinuclear center responsible for the binding and reduction of oxygen to water. In this thesis we apply novel methods based on density functional theory (DFT) for studies of the copper centers found in MCO. The absorption, circular dichroism, magnetic circular dichroism (MCD) and electron paramagnetic resonance parameters of models of the T1 copper site were calculated and compared to experimentally measured values. The observed deviations of the calculated excitation energies were discussed as a result of deficiencies in the used computational method (time dependent DFT). The spin-spin coupling interactions in some binuclear copper models which resemble the T3 site of the trinuclear copper center, as well as two synthetic models of the native intermediate (NI) of MCO were studied successfully with the second order spin-flip constricted variational DFT (SF-CV(2)-DFT). In addition, the MCD spectrum of one of the NI models was simulated in the presence of zero-field splitting and alternative assignments have been suggested for some of the spectral bands. Finally, the orientation of the oxygen moiety in the peroxide intermediate (PI) of MCO relative to the trinuclear copper center has been studied with SF-CV(2)-DFT, conventional DFT and hybrid quantum mechanics/molecular mechanics. It was determined that the formation of a fully bridged structure arises due to coordination unsaturation of the three copper atoms, rather than the influence of the second coordination sphere. Studies with the extended transition state - natural orbitals for chemical valence method of the spin density redistribution in the trinuclear center upon oxygen binding revealed that all copper atoms participate in the reduction of O2 and are partially oxidized in the PI.
- ItemOpen AccessExploring the activation of olefin polymerisation catalysts with density functional theory(2000) Vanka, Kumar; Ziegler, Tom
- ItemOpen AccessLocal approximations and nonlocal corrections in density functional theory: applications to spectroscopy and reaction mechanism studies(1992) Fan, Liangyou; Ziegler, Tom
- ItemOpen AccessMagnetic circular dichronism of porphyrins and derivatives: a td-dft study(2007) Gonzalez Peralta, Alejandro; Ziegler, Tom
- ItemOpen AccessNonrelativistic and scalar relativistic nuclear spin-spin couplings in density functional theory(1999) Khandogin, Yang; Ziegler, Tom
- ItemOpen AccessPolyene Cyclization by a Double Intramolecular Heck Reaction. A DFT Study(American Chemical Society, 2004) Balcells, David; Maseras, Feliu; Keay, Brian A.; Ziegler, Tom
- ItemOpen AccessRedox reactions, epr and mcd parameters of molybdenum-containing enzymes: a density functional theory study(2010) Hernandez-Marin, Elizabeth; Ziegler, Tom
- ItemOpen AccessRelativity and magnetic properties: a density functional study(1996) Schreckenbach, Hans Georg; Ziegler, TomThis thesis has two major, related subjects, relativity and magnetic properties. All the investigations are based on density functional theory (DFf). An algorithm has been developed and implemented that allows the determination of relativistic energy gradients. This implementation enables automated geometry optimization at the relativistic level. The scheme has been applied to the calculation of M-CO bond lengths and first bond dissociation energies in binary transition metal carbonyls. A unique program system has been developed that allows the calculation of NMR shieldings and EPR g-tensors. The program is based on the use of "gauge including atomic orbitals" (GIAO). Expressions have been derived for the extension of the program to include the frozen core approximation, a scheme in which only the valence electrons are treated explicitly, and scalar relativistic effects. These expressions have been implemented into the DFf-GIAO program. The program has been applied to the chemical shift in systems ranging from small first row compounds to the metal chemical shift in transition metal carbonyls M(CO)6. This represents the first calculation of heavy element shifts that is based on a relativistic first principle quantum mechanical method. The calculated metal shifts of M( CO)6, taken relative to [MO4] 2-, are -1,846, -1804, and -3615 ppm for M = Cr, Mo, W, respectively. The corresponding experimental values are -1,795, -1,857, and -3,505 ppm. The inclusion of relativity is crucial for a proper description of ligand and metal shifts in 5 d complexes. Various aspects of NMR shielding calculations are discussed in this thesis. They comprise basis set requirements, exchange-correlation functionals, the importance of core orbitals, relativistic effects, the relation of the GIAO method to the common-gauge scheme, among others. Certain limitations of the method and possible future directions have been considered as well. The DFf-GIAO NMR program has been extended to include the calculation of the EPR g-tensor. This extension is the first GIAO and the only first principle DFT program for the g-tensor. The program is validated by calculations of g-tensors for a comprehensive set of small radicals.
- ItemOpen AccessThe calculation and interpretation of NMR chemical shifts in compounds of transition metals and heavy elements(1997) Ruiz-Morales, Yosadara; Ziegler, Tom