Structure and Mechanisms in Annulenes and Polycyclic Aromatic Hydrocarbons

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Hydrocarbons constitute a fundamental class of organic compounds. Despite the fact that they contain only two elements,carbon and hydrogen, the dazzling variety of possible (and known) hydrocarbon structures provides chemists with a means to study key properties of molecules, from the nature of chemical bonding to detailed mechanisms of complex reactions. The present computational study focuses on two subclasses of hydrocarbons: annulenes and polycyclic aromatic hydrocarbons. The high carbon-to-hydrogen ratio in these compounds makes them useful for understanding properties of carbon-rich materials, including fullerenes and carbon nanotubes. The project will employ undergraduates and provide them with training in essentials of computational chemistry. The group's previous studies on annulenes (analogues of benzene with a number indicating the number of carbons in the ring) revealed that cis-trans isomerization in [12]annulene, [14]annulene, and [16]annulene occurs by double-bond shifting via a transition state having the topology of a Möbius strip. One new project will be to determine whether isomerization can occur in other topologies, such as planar and two-twist (figure-eight) forms. In addition, the group will attempt to computationally design a stable Möbius aromatic annulene. Finally, inspired by recent experimental results on dehydro[12]annulene ([12]annulene with a triple bond), the group intends to generate detailed information on the structures, relative energies, and predicted NMR spectra of the numerous possible isomers of this system, as well as the energy barriers for possible rearrangements and conformational motions. This project will also provide valuable training for undergraduates. Because the work draws on fundamental concepts taught in organic and physical chemistry, it is particularly appealing and accessible to many students, and thus represents an excellent opportunity to engage them in research. Given the highly diverse student population at the University of San Francisco, there is particular potential for involving members of underrepresented groups in science, especially women. Students will gain experience in a wide range of computational techniques, including potential surface scans, location of transition states, and calculation of properties. In short, the project will provide training for future experts in the application of quantum mechanics to the solution of chemical problems.