published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

En este trabajo abarcamos el estudio de la estructura electrónica de una serie de fulerenos denominados fulerenos endoédricos, así como también su reactividad. En este trabajo de tesis previo, se define lo que se conoce como la regla del gap (LUMO-3)-(LUMO-4), la cual indica que aquellas cajas fulerénicas vacías cuyo gap entre el tercer y cuarto orbitales desocupados sea mayor grande tendrán la capacidad de aceptar la transferencia de seis electrones desde un clúster metálico. En el capítulo 3, la regla del gap (LUMO-3)-(LUMO-4) ha sido extendida para aquellas cajas fulerénicas de hasta 100 átomos de carbono. También se ha analizado la reactividad exoédrica del M3N@D5h-C80 en una reacción de cicloadición de Diels-Alder con 1, 3-butadieno sobre los diferentes tipos de enlaces de la caja D5h-C80. En el capítulo 4 estudiamos la estructura electrónica de los fulerenos endoédricos con carburos metálicos (M2C2@C82). Se han estudiado seis isómeros de Sc2C2@C82, confirmando que su estructura electrónica se puede explicar de manera sencilla con el modelo iónico (clúster)4+@(caja)4–, adoptando el carburo metalico diferentes orientaciones dentro de la caja de C82 . En el capítulo 5 hemos estudiado la estructura y propiedades electrónicas de una otras dos famílias de fulerenos endoédricos: la de los óxideos de escandio y la de los sulfuros de escandio. Por último hemos estudiado las propiedades electroquímicas de una serie de fulerenos endoédricos con diferentes clústeres (capítulo 6). Los orbitales frontera y las propiedades redox de Sc3N@C80 son diferentes cuando son comparados con fulerenos endoédricos de metales más electro positivos (Gd e Y). / We have extended the application of the general rule for the stabilization of nitride EMFs proposed by Campanera et al. to IPR carbon cages of dimensions up to C100. This simple rule, based on the formal transfer of six electrons from the cluster to the carbon cage, can be seen as an empirical rule that is able to predict the most abundant cage isomer for all the nitride EMFs known to date. Some EMF with M2 clusters, however, seem to escape this simple rule. We have proposed six large carbon cages (from C92 to C100) with sizeable (LUMO-4)–(LUMO-3) gaps and achievable energies as candidates for encapsulating metal nitride units or M2 clusters on condition that the formal six-electron transfer and other factors are accomplished. We have confirmed that the ionic model based on the formal transfer of four electrons from the encapsulated M2C2 carbide to the carbon cage is valid for the M2C2@C82 family. We have observed that the internal metal- carbide cluster is able to rotate inside the carbon cage. Using the aforesaid ionic model we have understood the higher stability of M2C2@C82 (C3v:8) when compared to carbide endohedrals with other IPR C82 isomers. The electrochemical properties for a series of EMFs have been studied theoretically. Analogously to the neutral M3N@Ih-C80 (M = Sc and Y) systems, rotation of the M3N unit inside the fullerene cage is predicted for the neutral, oxidized and reduced states of all the nitride EMFs with IPR cages studied through this work (from Sc3N@D5h-C80 to La3N@C96).

This thesis addresses the electronic structure of molecules and solids using resonant X-ray emission and photoemission spectroscopy. The use of monochromatic synchrotron radiation and the improved performance of the instrumentation have opened up the possibility of detailed analyses of the response of the electronic systems under interaction with X-rays. The experimental studies are accompanied by numerical ab initio calculations in the formalism of resonant inelastic scattering. The energy selectivity has made it possible for the first time to study how the chemical bonds in a molecule break up during resonant inelastic X-ray scattering. In the conjugated polymer systems, the element selectivity of the X-ray emission process made it possible to probe the different atomic elements separately. The X-ray emission technique proved to be useful for extracting isomeric information, and for measuring the change in the valence levels at different degrees of doping. In this thesis, spectral satellite features in transition metals were thoroughly investigated for various excitation energies around a core-level threshold. By measuring the relative spectral intensity of the satellites it was possible to extract information on the partial core-level widths. Using the nickel metal system as an example, it was shown that it is possible to probe the different core-excited states close toshake-up thresholds by measuring the relative spectral intensity variation of the Auger emission.Resonant photoemission measurements showed unambiguous evidence of interference effects. Theseeffects were also thoroughly probed using angle-dependent measurements. The combination of X-rayemission and absorption were useful for studying buried layers and interfaces due to the appreciable penetration depth of soft X-rays. X-ray scattering was further found to be useful for studying low-energy excited states of rare earth metallic compounds and transition metal oxides.

Massey and Hemmerich proposed that the different reactivities displayed by different flavoenzymes could be achieved as a result of dominance of different flavin ring resonance structures in different binding sites. Thus, the FMN cofactor would engage in different reactions when it had different electronic structures. To test this proposal and understand how different protein sites could produce different flavin electronic structures, we are developing solid-state NMR as a means of characterizing the electronic state of the flavin ring, via the 15N chemical shift tensors of the ring N atoms. These provide information on the frontier orbitals. We propose that the 15N chemical shift tensors of flavins engaged in different hydrogen bonds will differ from one another. Tetraphenylacetyl riboflavin (TPARF) is soluble in benzene to over 250 mM, so, this flavin alone and in complexes with binding partners provides a system for studying the effects of formation of specific hydrogen bonds. For N5, the redoxactive N atom, one of the chemical shift principle values (CSPVs) changed 10 ppm upon formation of a hydrogen bonded complex, and the results could be replicated computationally. Thus our DFT-derived frontier orbitals are validated by spectroscopy and can be used to understand reactivity. Indeed, our calculations indicate that the electron density in the diazabutadiene system diminishes upon H-bond complex formation, consistent with the observed 100 mV increase in reduction midpoint potential. Thus, the current studies of TPARF and its complexes provide a useful baseline for further SSNMR studies aimed at understanding flavin reactivity in enzymes.

A method of generating and testing pseudopotentials will be presented. This required the development of PPTester, a custom software program to analyze and quantify various parameters. These methods were first used to study bulk Si and verify the installation and performance of SIESTA. Plots, which agreed with published results, for band gap and charge density were generated.Next, pseudopotentials for Cd and Se were constructed and tested. Two separate Cd potentials were evaluated. Electronic structure calculations for two, four and six atom small cadmium selenide (CdSe) colloidal quantum dots were performed. The changes in geometry of initial versus relaxed atomic positions of these systems were evaluated. Output values of the electronic structure calculation, for example Fermi energy, were analyzed. / Department of Physics and Astronomy

The first part of this thesis is a study of thermally activated conduction, Hall effect, and Far-Infrared absorption in n-InP. Accurate measurements of threshold energies are deduced from the temperature dependence of these effects, after correction for Fermi level variations, and it is shown that the threshold for electrical conduction is higher than for Hall effect, or cyclotron resonance. An explanation is given in terms of the long range impurity potential fluctuations. The second part of this thesis presents measurements of the thermopower in heterostructures for the GaInAs - InP, GaInAs - InAlAs, and GaAs - GaAlAs systems, with and without magnetic field, for temperature varying from 2K to 10K. In high magnetic fields, strong oscillations of the thermopower are seen, with the same phase as the Shubnikov - de Haas oscillations in the resistivity, in accordance with theory. The magnitude and temperature dependence of the thermopower are mostly as predicted for the superlattice but those of the heterojunctions disagree with the predictions. A discussion of the cause of this disagreement is given. In the final part, the value of the relative energy shift between different valleys of the conduction band of a thin film of PbTe grown on BaF₂ is obtained. This will be used to obtain the deformation potential of PbTe.

Thesis (Ph.D.)--McMaster University (Canada), 1995. / Source: Dissertation Abstracts International, Volume: 56-12, Section: B, page: 6709. Adviser: A. P. Hitchcock.

Thesis (Ph. D.)--University of Washington, 2008. / Vita. Includes bibliographical references (leaves 111-119).

Thesis--University of Oulu, 1987. / Includes bibliographical references (p. 43-46).

Thesis (M. Phil.)--University of Hong Kong, 1998. / Includes bibliographical references.