The goal of this project is to explore the strength of hydrogen-bond interactions and extent of proton transfer in a series of fluoropyridine-hydrogen fluoride complexes. We are specifically interested in the impacts of fluorine substitution and low-dielectric media. Using wB97X-D/aug-cc-pVTZ computations, we obtained equilibrium structures, binding energies, H–F vibrational frequencies, atomic charges (Natural Population Analysis), and Atoms in Molecules parameters. These complexes exhibit strong hydrogen bonds (–13.2 to –7.0 kcal/mol), and there are clear trends in the structural and energetic data. Specifically, the complexes weaken as fluoridation of the pyridine increases, substitution at the 2- and 6- positions has a greater effect. In turn the N-H distances increase, the H-F bonds shorten and the extent of negative charge on the fluorine decreases. There is no significant proton transfer observed in the gas-phase compounds. The interactions strengthen in dielectric media. N-H distances decrease, H-F bonds lengthen and the extent of negative charge on the fluorines increases. Even with these stronger interactions, there is still no significant degree of proton transfer. The Atoms in Molecules analysis depict shifts in the electron density that parallel these structural and energetic changes. We are currently mapping N-F potential curves to further illustrate the trends in structural analysis and energetic properties in the gas phase as well as dielectric media. In addition, the data and trends for this set of compounds will be compared to our recent work focusing on fluoropyridines with HBr and HCl.

Exploring medium effects on fluoropyridine-HF complexes

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