Date of Award
Doctor of Philosophy (PhD)
Mark R. Hoffmann
Computational chemistry has grown into a large field and is continuing to grow every year in both number and variety of applications. This dissertation will give a few such applications relevant to cleaner energy production from coal, catalytic degradation of renewable agricultural and forest waste into valuable chemicals, and extending the reach of electronic structure methods to systems of biological and macromolecular interest. The first two studies presented in this dissertation are concerned with the remediation of trace elements released into the environment through the combustion of coal for power production. In flue gases, arsenic and antimony exists most often as oxides. Despite the prevalence and importance of remediating these oxides, critical information on the thermodynamics of plausible intermediates and transition states in reaction pathways have been missing prior to these studies. Several of the intermediates, and essentially all transition states, were found to be electronically multiconfigurational for the arsenic oxides. In this work, the electronic structures of several oxides of arsenic, AsxOy, where x = 1, 2 and y = 1-5, were investigated using the second-order generalized van Vleck variant of multireference perturbation theory (GVVPT2), using the cc-pVTZ basis set, with comparison to multi-reference configuration interaction (MRCISD) and the linked completely renormalized coupled cluster through perturbative triple excitations (CR-CCSD(T)L or CR-CC(2,3)) when relevant. Calculated oxidation reaction energies for the formation of AsO2 and AsO3 from AsO were predicted to be energetically favorable and formation energies of the lowest energy compounds containing two metalloid atoms, called dimers for brevity, from the monomers were also predicted to be energetically favorable. The energetics of the monomers, five isomers of As2O3 and eleven isomers of As2O5 were characterized using a composite methodology along with the key transition states between the isomers. Geometry optimizations as well as harmonic vibrational frequencies of AsxOy were obtained at the B3LYP/6-311G* level of theory and gave satisfactory agreement with experimental data when available. It was discovered that several isomers of As2O3 and As2O5 have comparable energies and relatively low barrier heights. Therefore, we expect these isomers to be chemically relevant.
The antimony oxides were also found to be electronically multiconfigurational. The electronic structures of several antimony oxides, SbxOy, where x = 1, 2 and y = 1-5, were investigated using GVVPT2 and the SBD-aug-cc-pVTZ basis set. The oxidation reaction energies of elemental antimony toward the formation of SbO and SbO2 was found to be energetically favorable, while the further oxidation of those species to SbO3 was found to be unfavorable. It was found that the accretion of the monomers into Sb2O3 was highly energetically favorable at both the B3LYP/SBD-aug-cc-pVTZ and GVVPT2/SBD-aug-cc-pVTZ//B3LYP/SBD-aug-cc-pVTZ levels of theory. However, while the reaction of SbO2 and SbO3 toward Sb2O5 was found to be favorable, it was found to be unfavorable for Sb2O5 to form from the oxidation of Sb2O3.The energetics of the monomers, three isomers of Sb2O3 and four isomers of Sb2O5 were characterized using the same composite methodology as the arsenic oxides. Geometry optimizations and harmonic vibrational frequencies of all antimony oxides were obtained at the B3LYP/SBD-aug-cc-pVTZ level of theory. Several of the dimeric antimony oxide isomer structures were found to be quite similar to the arsenic dimers and are also expected to be chemically relevant.
The third study is pertinent to the catalytic degradation of lignin, one of the most renewable carbon sources on Earth. Unfortunately, it has only seen limited industrial use due to its chemical stability and complex structure; thus lignin is typically disposed of as non-commercialized waste product. However, if a viable path could be found for the decomposition of lignin, the by-products could be used to replace high-value petrochemicals. Catalytic decomposition by amorphous silica-alumina (ASA) based catalysts may be a viable path, but the mechanisms and the effects of metal doping are not strictly known. In this work, DFT calculations with the B3LYP hybrid functional, as implemented in the NWChem software package, are used to elucidate this information. A cluster model of an amorphous silica-alumina catalyst has been studied and a monomer and Î²-O-4 linked dimer have been adsorbed to the surface. They were both found to have a favorable interaction with the surface and, for the dimer, a favorable cleavage of the Î²-O-4 bond. The desorption energy of the cleaved monomers was also shown to be favorable when compared to the free dimer, suggesting ASA is a viable catalyst.
The fourth study, the Cu2O22+ core, has both biological and computational relevance and has received much attention over the years. This is due to its importance in biological systems and to the computational difficulties associated with modeling its relevant isomers. The complexity for computation arises, inter alia, to both a varying degree of biradical character as it isomerizes, as well as a rapidly changing degree of dynamic and static electron correlation effects along the isomerization coordinate. In this work, the two dominant isomers, bis(Î¼-oxo) and Î¼-Î·2:Î·2 peroxo, along with four points along the reaction coordinate, were considered. MCSCF and GVVPT2 were used with a variety of active spaces. The starting active spaces were developed using a recently established approach, in which a valence picture is used rather than the typical Hartree-Fock description. The active spaces were then modified based on numerical considerations in an iterative fashion. Finally, a stable active space of 13 electrons in 12 orbitals (13e,12o) was found and used to obtain results which were then compared to the more expensive methods of complete active space second-order perturbation theory (CASPT2) using an active space of (16e, 14o), restricted active space second-order perturbation theory (RASPT2) using an active space of (24e,28o) which considered up to quadruple excitations, and linked completely renormalized coupled cluster through perturbative triple and quadruple excitations (CR-CCSD(TQ)L or CR-CC(2,4)). GVVPT2 was found to agree well with the CASPT2 results. It was also determined that a larger active space than (13e,12o) will likely be required for it to approach the relative energies of RASPT2(24e,28o)//4 or CR-CCSD(TQ)L.
Hicks, Jason M., "Computational Studies Of Oxides Relevant To Clean Energy, Catalytic Processing Of Renewables, And Biological Systems" (2018). Theses and Dissertations. 2232.