With the support of a RUI award from the National Science Foundation's program in Chemical Structure, Dynamics and Mechanisms, Professors William Karney and Claire Castro at the University of San Francisco will work with undergraduate students to study mechanisms of hydrocarbon rearrangements using quantum chemical methods. The hydrocarbons that will be studied, dehydroannulenes and polycyclic aromatic hydrocarbons (PAHs), have high carbon-to-hydrogen ratios, making them useful for understanding properties of carbon-rich molecules. Such molecules are appealing because of their potential as precursors to the synthesis of carbon nanotubes. This research unites fundamental organic chemistry concepts with modern computational methods to elucidate processes from the rearrangements in dehydroannulenes at ambient and sub-ambient temperature to the very high temperature (> 800°C) rearrangements of phenylenes and related systems. Previous work by the PIs invoked novel topologies (Möbius, two-twist Hückel), coupled with pi-bond shifting, to solve longstanding mechanistic problems in annulene dynamic processes. The PIs and their students will now focus on the application of this mechanism to cis-trans isomerization in dehydroannulenes to determine: (1) the nature and topology of the bond-shifting steps required, and (2) whether cumulenes act as intermediates or transition states in such reactions. These studies will clarify the scope of bond shifting as a route for configuration change, as well as shed light on intramolecular dehydro Diels-Alder reactions. The mechanistic insights generated will be helpful to experimentalists using such reactions in their syntheses. Further, mechanisms for benzene ring contractions in phenylenes and other systems will be probed to identify the most important factors influencing reactivity. This work will determine whether the lowest energetic pathways involve carbenes or diradicals and will clarify whether or not key steps are concerted or stepwise. Potential energy surface mapping for these reactions will provide useful information to researchers engaged in the synthesis of bowl-shaped PAHs, fullerenes, and carbon nanotubes.
In addition to the scientific findings, this project will provide research opportunities and training for undergraduates, expose them to current topics in organic chemistry, and give them practical experience in modern computational methods as well as help them develop the ability to frame research questions. Students will present their results at regional and national chemistry research conferences. Active participation in this research program will prepare USF students for graduate study in chemistry or for employment in the chemical industry. The PIs' research team consistently reflects the diversity of The University of San Francisco as well as that of the Bay Area, and half of their research students have been women. Many former group members have gone into the chemistry workforce or have matriculated into graduate programs in chemistry. The PIs will continue to train students from underrepresented groups in Chemistry. Finally, the improved computational resources enabled by this award enhance the university's cyberinfrastructure and will be available to other research groups in the department.
