Theoretical and Computational Study of the Role of Hyperconjugation in Chemical Systems

Moumbogno Tchodimo, Falonne Colbie
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Orbital interactions, including hyperconjugation (HC), have been used to great extent to explain or support structural and conformational phenomena in organic molecules. In many instances, these interactions can be quantified and further understood using computational methods. With hyperconjugation, experimental evaluation is complicated by the difficulty of isolating hyperconjugative effects from other contributing factors such as steric, normal conjugation and resonance. In this dissertation, we describe the role played by hyperconjugation in stabilizing several organic molecules and ions through computational means. To address this goal, three different approaches were pursued. In the first approach, an analysis of HC in benzene versus benzenium and toluene versus toluenium was evaluated by examining bong lengths, dipole moments, and other molecular properties using self-consistent field (SCF), coupled-cluster theory (CCSD(T)), and configuration interaction (CISD) calculations, as well as density functional theory (DFT). The different properties examined demonstrate that HC is a major contributor to the stability in the benzenium and toluenium molecules. In the second approach, the impact of HC on the stability of butenolide molecules was studied. Here, the results of DFT calculations on αbutenolide and β-butenolide showed that the preference for α-butenolide is due to the presence of stronger HC in the α -isomer. In the third approach, quantum mechanical methods for the calibration of HC were developed that reveal the characteristic behavior of HC in substituent-aromatic carbonium ion molecules are a combination of Mulliken overlap populations, electric field gradients, and the delocalized dipole polarizabilities.

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Benzene, Benzenium, Butenolide, Gaussian, Toluene, Toluenium