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When Counting Electrons Matter: Tuning Noncovalent Interactions with Aromaticity

Prof. Judy I-Chia Wu
Prof. Judy I-Chia Wu
Department of Chemistry
University of Houston
Chemistry Building, Room 400
Departmental Colloquium

Aromaticity and hydrogen bonding are traditionally considered to be largely separate ideas in chemistry. We find however, that just as a change in aromaticity can enhance chemical reactivity (e.g., consider the effect of an aromatic transition state on a cycloaddition reaction)—a change in aromaticity can perturb the strengths of hydrogen bonding interactions (and even more exotic types of noncovalent interactions). Hydrogen-bonding interactions that polarize π-electrons to increase cyclic [4n+2] π-electron delocalizations (i.e., enhance aromaticity) in heterocycles are stronger than expected, while those that decrease cyclic [4n+2] π-electron delocalization (i.e., reduce aromaticity) are weakened. Following Baird’s Rule for excited-state aromaticity (a reverse of the Hückel rule), hydrogen bonding interactions at specific ππ* excited states display the opposite trend. Anion-π interactions—even without a π-acidic ring—can be especially favorable when they achieve 3D “six-interstitial electron” aromaticity. I will talk about the interpretive merit, predictive power, and probable impacts of these special connections for applications in organic and supramolecular chemistry (e.g., in enzyme catalysis, self-assembly, host-guest recognition)

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