Intermolecular forces such as halogen bonding (XB) require a fundamental, experimental understanding before they can be utilized in biological, chemical, and physical applications. XB is a highly directional, non-covalent intermolecular interaction between a molecule (XB donor) presenting a halogen with an electron-poor region and a Lewis-base molecule (XB acceptor). A systematic, experimental and theoretical study of solution-phase XB strength as a function of molecular structure for both XB donor and acceptor molecules is presented. The impact of specific structural features is assessed using 19F and 1H NMR titrations to determine association constants (Ka), density functional theory calculations for interaction energies and bond lengths, as well as 19F–1H HOESY NMR measurements of intermolecular cross-relaxation between interacting XB donor-acceptor adducts. The panels of molecules are specifically selected to identify and optimize structural requirements of the XB donors—including specific halogen atoms and electron-withdrawing groups—and XB acceptors—such as electronegativity and steric effects—that promote strong XB interactions. The impact of solvent on XB interactions is also explored. Such fundamental understanding of structure-optimized XB is applied to an application-based molecular detection scheme: alkanethiolate-protected gold nanoparticles functionalized with XB donors undergo XB-induced aggregation events with targeted XB-acceptor analytes. Aggregation is characterized by UV-Vis, TEM, and DOSY NMR.

Using NMR titrations, 19F–1H HOESY NMR, and DFT computational modeling to assess halogen-bonding strength as a function of molecular structure and exploring halogen-bonding in molecular-detection nanoparticles

ACS Spring 2022

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