Dynamic Processes in Annulenes

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The quantum chemical study proposed here focuses on the mechanisms of the dynamic processes in [4n]- and [4n+2]annulenes which include conformational changes, valence isomerizations, bond shifting, and configuration changes. By employing a combination of DFT, coupled cluster theory, and multi-configurational quantum mechanical methods, detailed mechanisms for these processes will be determined. The computed energy barriers will be compared with measured ones. Investigation of the scope of twist-coupled bond shifting mechanism for configuration change in annulenes will enable a detailed comparison of Mobius, closed-shell vs. planar, open-shell bond shifting in systems such as [12]- and [16]annulene. This will provide information regarding factors influencing the barriers for these processes. In addition, non-planar bond shifting in [4n+2]annulenes (e.g. [10]- and [14]-) and [4n]annulenes (e.g. [12]-) will be examined. This bond-shifting transition states will be studied in both closed-shell systems and singlet diradicals. This work will establish twist-coupled bond shifting as a fundamental mechanism in annulene chemistry and confirm that Mobius aromaticity exists in parent annulenes. It will also clarify the diradical nature and degree of antiaromatic character of planar, bond-equalized forms of larger [4n]annulenes and will enable the identification of new Mobius aromatic species as well as new annulene isomers that are viable synthetic targets.Broader Impacts. Because this project draws on fundamental concepts that students learn in sophomore organic chemistry-aromaticity, molecular orbital theory, the Huckel rule, and pericyclic reactions-it represents an excellent opportunity to engage students in research. Given the student population at the University of San Francisco, there is particular potential for training members of under-represented groups in science, especially women and Hispanic students. Students will gain experience in a wide variety of computational techniques and, in addition, they will learn how to approach a research project, from searching the literature and formulating questions, to presenting and publishing the work. The majority of undergraduates who have worked in the Castro-Karney research group have gone on to (or are applying to) pursue advanced degrees at other institutions. The project outlined here will enhance cyberinfrastructure by providing training for students who will be future experts in the application of quantum mechanics to the solution of chemical problems.