Although various organometallic carbene complexes have found use in industry or research, they still lack some fundamental footing in theory. These complexes have found significant use in catalysis. This is especially true for Schrock carbene complexes in olefination reactions. A few such titanium-based olefination catalyst examples are the Tebbe reagent, Petasis reagent and the Ziegler-Natta catalyst. The nature of organometallic carbene bonding is still not well defined theoretically. Schrock carbenes are expected to have a covalent bonding nature, but multiconfigurational studies have shown this to not be the best description of the bonding. Furthermore, expected Schrock carbenes have been shown to be Fischer carbenes due to the electronic structure. This work investigates the nature of the carbene bond in titanium Schrock carbene complexes by utilising DFT and further application of MO, NBO, QTAIM and FALDI methods. This allows for a modernised description of the nature of this bond as well as the identification of an important long-range ligand-ligand interaction that has not been reported on previously. The research aims to define the nature of titanium Schrock carbene bonding on a theoretical basis by the use of integrated cross-sections on the electron and orbital densities to determine the σ- and π-character of the interaction. These cross-sections provided the means to determine the major components of the bonding interaction. This is further investigated by defining FALDI fragment-based delocalisation indices which revealed the presence of long-range ligand-ligand interactions. The FALDI fragment approach also provided the means to quantify the inter-fragment delocalisation along with intra-fragment localisation and delocalisation which would prove useful for further investigation into the characteristics or various chemical interactions. The fragment-based description should prove to be more intuitive to the chemist than diatomic interactions between atoms where a chemical bond or interaction is not classically expected. This study was followed by a decomposition of the molecular orbitals into localised and delocalised components from atomic contributions which provides a novel approach to determining the bond order in compounds. This provided a quantitative means to describe which atoms contribute to the formation of each molecular orbital as well as providing a measure of the degree to which these atoms are contributing localised as well as delocalised electrons to the molecular orbital. / Dissertation (MSc)--University of Pretoria, 2020. / DST-CSIR Interbursary Support Program / Chemistry / MSc / Unrestricted

This licentiate thesis deals with quantum chemistry methods for large systems. In particular, the thesis focuses on the efficient construction of the Coulomb and exchange matrices which are important parts of the Fock matrix in Hartree--Fock calculations.The methods described are also applicable in Kohn--Sham Density FunctionalTheory calculations, where the Coulomb and exchange matrices areparts of the Kohn--Sham matrix. Screening techniques for reducing the computational complexity of bot Coulomb and exchange computations are discussed, as well as the fast multipole method, used for efficient computation of the Coulomb matrix. The thesis also discusses how sparsity in the matrices occurring in Hartree--Fock and Kohn--Sham Density Functional Theory calculations can be used to achieve more efficient storage of matrices as well as more efficient operations on them. As an example of a possible type of application, the thesis includes a theoretical study of Heisenberg exchange constants, using unrestricted Kohn--Sham Density Functional Theory calculations. / QC 20101123

In this work, biradical boron containing systems with various structures are investigated to reveal the dependency of the biradical character on the ligated carbene (NHC, CAAC) and the related steric demands of the substituents. / In dieser Arbeit werden biradikalische Bor-Systeme mit unterschiedlichen Strukturen untersucht, um die Abhängigkeit des biradikalischen Charakter von der Carben-Einheit, die am Bor (NHC, CAAC) gebunden ist, und den sterischen Anspruch der Substituenten zu analysieren.

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Nonlinear optics is a field with new applications being regularly discovered, which leads to a growing interest in computing these properties. In this work, we attempt to determine new methods of computationally determining the properties of biologically relevant systems. We do so through testing a novel finite-field method to compute these properties. To facilitate the computation of molecular energies required for finite-field calculations, we tested a hypergeometric resummation scheme. Together, these projects form a strong step into being able to compute the nonlinear optical properties for larger systems of biological relevance. / Thesis / Master of Science (MSc)

This thesis reports a state-of-the-art theoretical study of photophysical properties of organic charge-transfer aromatic molecules. These molecules are building blocks of molecular functional materials used in modern photonics technology and play essential roles in chemistry and biology in general. A good understanding of these systems is thus important. The theoretical results for permanent dipole moments of some substituted benzenes have been obtained using the coupled cluster singles and doubles (CCSD) method. The performance of density functional theory (DFT) for the geometry and electronic properties has been compared with that of traditional ab initio methods, such as Hartree-Fock, second-order Möller Plesset perturbation theory (MP2), CCSD and CCSD(T). Limitations of the DFT methods for charge transfer molecules have been demonstrated. The multi-configuration self-consistent field (MCSCF) method has been applied to understand properties of the triplet states of benzene derivatives by studying their phosphorescence with the inclusion of contributions from vibronic coupling. It has also been employed to calculate the photophysics of the thioxanthone molecule containing three benzene rings in combination with the CASPT2 method, resolving a long-standing problem concerning the possible stable conformations of the molecule. With knowledge of the building blocks a series of porphyrin derivatives with exceptionally large two-photon absorption cross sections were designed, and proposed for use in bioimaging applications. The static and dynamic properties of a few zinc and platinum organometallic compounds, being possible candidates for optical limiting devices, have also investigated. / QC 20101011

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Electrochemical analysis using mercury electrodes has been a prominent methodology since the beginning of voltammetry, however, due to its toxic nature their use is being slowly phased out in favour of environmentally friendlier alternatives. In this thesis, carbon electrodes feature heavily as a means to provide a cheaper and non-toxic approach to voltammmetric sensing. Pulse techniques have been used for the sensitive detection of commonly studied analytes, including antimony and iron. A simple fabrication of a vibrating electrode is presented by modifying a commercially available vibrating toothbrush with platinum foil as a means to enhance mass transport for electroanalytical work. Finally, pH determination at carbon electrodes is investigated. Carbon electrodes have been proposed as a simple sensor for pH determination by exploiting the pH sensitive nature of surface quinone groups intrinsic within carbon edge-plane like sites. Using this approach, both EPPG and GC electrodes are suggested as new materials for pH determination and can be used in aqueous solutions over the pH range 1.0 to 13.0, as a cheap and simple alternative to the classic glass electrode.

The behavior of colloidal liquid crystals in confinement is addressed on the single particle level using laser scanning confocal microscopy. We seek to disentangle how equilibrium director fields are controlled by the complex interplay between confinement, elasticity and surface anchoring. First, we study the nematic phase confined to wedge structured channels. Varying the wedge opening angle leads to a splay to bend transition mediated by a defect in the bulk of the wedge. Our results are in quantitative agreement with lattice Boltzmann simulations, and we show that comparison between experiments and simulation yields a new method to obtain the splay-to-bend elasticity ratios of colloidal and biological liquid crystals. Next, we extend our study of the wedge structured channels to the cholesteric phase, and measure a splay to twist transition with increasing wedge angle. We directly visualise the 3D nature of the twisted state, and explain how the transition is intricately determined by the anchoring strength and the splay, bend, and twist elasticities. Next, we investigate the effect of rectangular confinement on the nematic phase. The rectangle aspect ratio is systematically varied and we observe five distinct director fields. Comparison with computations of the Frank-Oseen energies yields the extrapolation length, which we find to be of the order of the rod length. Next, we confine the nematic phase to annular geometries of varying dimensions, and observe the novel director fields that are adopted. We approach a level of confinement which is of the order of the particle size. Interpreting our observations with Monte Carlo simulations, which take into account the finite size of the particles, illuminates the applicability of continuum theories down to microscopic lengthscales. We finish with a study of the isotropic-nematic interface in bulk and confinement. We show that parallel anchoring occurs at the interface, and measure the width of the interface to be of the order of the rod length.

Uniformly dimpled colloidal particles are studied using laser scanning confocal microscopy and optical tweezing, alongside real-space image analysis. In particular, a comparison is made between the behaviour of these particles with that of isotropic spherical particles of the same size and polydispersity. We begin by detailing the synthetic techniques as well as the methods used for both experiments and simulations in this work. We then probe the structures and ordering exhibited by the dimpled particles and their spherical counterparts at a single featureless wall. Only the first layer at the wall is observed, and pronounced differences in both the translational and orientational order between the two types of particle is found. Furthermore, we show that the presence of the dimple leads to disordered structures that develop over time. Next, we reduce the dimensionality of the system to quasi-two and study the depletion induced interactions in a monolayer of colloidal particles at a single featureless wall. Using both confocal microscopy and Monte Carlo simulations, we illustrate the selectivity of the depletion interaction with regard to particle shape and polymer size. A level of complexity is then added to the problem by introducing a second colloidal system of small spherical particles. The resulting binary mixture, still with additional non-adsorbing polymer and in a monolayer at a wall, allows us to investigate so-called “lock and key” binding. We show that the inclusion of a lock particle cavity, whose shape and size is complementary to those of the key particle, significantly favours binding behaviour, which is further improved by using a depletant consisting of a small polymer and charge-screening salt. Finally, the depletion induced force between lock and key particles at contact is directly measured using optical tweezers. The dependence of the force due to depletion upon the overlap volume between the particles is illustrated, and we find that the strongest force is produced when the key is held within the cavity of the dimpled lock, demonstrating semiquantitative agreement with theoretical predictions.