Computational Studies of The Effects of Substitutions on Silatranes and Ring Strain Energies, as Well as Determination of Aromaticity and Complexation in Polypnictogen Analogues of The Cyclooctatetraenyl Dianion
Gilbert, Thomas M.
Ph.D. (Doctor of Philosophy)
Department of Chemistry and Biochemistry
The work in this dissertation computationally examines four areas involving variations on archetype molecules and how they affect reactivity and physical properties. Studies in Chapters 2 and 3 assess substituent effects on cyclic systems, while those in Chapters 4 and 5 explore both partial and complete substitution of ring component atoms as well as the chemical implications of the substitution.The effect of substituents on molecular reactivity is a critical concept that affects every branch of chemistry. Previous work within our group showed that 1-hydridosilatrane was an effective and selective reducing agent for a variety of compounds. Fully 3,3,7,7,10,10-substituted silatranes have been previously synthesized, but not the 3,3,7,7,10,10-substituted 1-hydridosilatrane. Chapter 2 details the effects of 3,3,7,7,10,10-substitution of 1-hydridosilatranes and the mechanistic implications of that substitution. Silatrane cages were found to be resilient to substituent electronic effects, and a transition state was located for the reduction of acetone using 3,3,7,7,10,10-hexamethyl-1-hydridosilatrane. Ring strain energy (RSE) is a purely conceptual idea in chemistry, unable to be measured experimentally but estimated computationally. Observable consequences of RSE include the ease of ring-opening reactions and the overall reactivity of cyclic structures. Attempts to estimate ii RSE using homodesmotic reactions leave many intermolecular interactions unaccounted for, and because of that produce unreliable results for highly substituted rings. In Chapter 3, application of the semi-homodesmotic method (SHM) to fully substituted three-membered heterocycles containing nitrogen and phosphorus resulted in chemically reasonable RSEs for perfluorinated and permethylated rings. SHM RSEs predict that the perfluorinated rings are expected to be far more reactive than their unsubstituted counterparts, while the permethylated rings are expected to be roughly equivalent to the parent. Much like RSE, aromaticity is a concept that cannot be directly measured but can be analyzed computationally. Aromaticity is a stabilizing effect via the delocalization of electrons across a cyclic structure. The Hückel rules of aromaticity classify planar cyclic structures with 4n+2 π-electrons as aromatic, and a well-known example of that is the cyclooctatetraenyl dianion (c-C8H82– = COT2-). In Chapter 4 we detail the aromaticities of partially inorganic (c-(CH)4Pn42-) and all-pnictogen (c-Pn82-) analogues to COT2-. A majority of the rings were found to be aromatic as determined by nucleus independent chemical shift calculations. One use for COT2- is in the formation of sandwich-style actinide complexes (8-COT)2An, also known as actinocenes. Chapter 5 of this work investigates the complexation behavior of selected pnictogen-containing COT2- analogues. Certain (c-(CH)4Pn4)2An and (c-Pn8)2An complexes were found to be stationary points on their potential energy surfaces, implying the possibility of their experimental preparation. Geometric distortions were seen in the rings of the (c-Pn8)2An complexes when An was electron-poor f0 Th4+ or f2 U4+, but not when An was f4 Pu4+. All-pnictogen sandwich complexes appear to be synthetically possible, with a low energy requirement for ring-exchange reactions.
Pixler, Alexander Stuart, "Computational Studies of The Effects of Substitutions on Silatranes and Ring Strain Energies, as Well as Determination of Aromaticity and Complexation in Polypnictogen Analogues of The Cyclooctatetraenyl Dianion" (2023). Graduate Research Theses & Dissertations. 7175.
Northern Illinois University
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