Natural Bond Orbital (NBO) theory can help students learn how intramolecular donor-acceptor orbitals interact and can enable understanding of the physical reasoning behind chemical reactions such as SN2 backside attack or E1 elimination. NBO analysis can also help students understand the interactions important to the conformation of pharmaceutical compounds. Trigoxazonane (a natural product isolated from Trigonella foenum-graecum root exudate) contains a 9-membered heterocyclic core. The lowest energy conformations of trigoxazonane are due to orbital interactions such as the anomeric and gauche effects. In addition to favorable donor-acceptor interactions in the molecule, the transannular strain associated with analogous macrocycles comprised of all carbon is alleviated by various CH-X interactions. The three lowest energy conformers of trigoxazonane have been optimized using Density Functional Theory (DFT) and an NBO analysis has been performed to quantitate the magnitude of these orbital interactions. In addition, quantum chemical NMR predictions have been used to highlight the effects of these interactions on experimentally obtainable information.  The results presented herein provide the framework for a short teaching unit highlighting the importance of qualitative orbital analysis in organic chemistry for undergraduates.

Natural bond orbital analysis: The happiest molecule

248th National Meeting (2014)

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