This project involves a computational and spectroscopic investigation of how the extent of fluorination and interactions with noble gas matrix environments affect hydrogen-bonding and proton transfer in various fluoropyridine-hydrogen bromide complexes (e.g., pentafluoropyridine-HBr). Using DFT calculations (ωB97X-D/aug-cc-pVTZ), we obtained structures, binding energies, and H-Br frequencies, and conducted bonding analyses (AIM and NPA). Theoretically, we explored condensed-phase effects using an implicit solvation model (PCM) at a range of dielectric constants from 1.2-20. Experimentally, the complexes were examined via low-temperature infrared spectroscopy in various inert-gas matrices (solid Ne, Ar, and N2). Shifts of the HBr stretching band offer insight into hydrogen-bond strength and extent of proton transfer across the series of complexes and across the media. Two clear trends appear in these data; hydrogen-bond strength and the extent of proton transfer increases as base strength increases (extent of fluorination near the basic nitrogen decreases) and as the dielectric constant increases. A wide range of interaction strengths occur across the series of complexes in the gas phase, from relatively weak hydrogen bonds (e.g., pentafluoropyridine-HBr: ΔE = –3.8 kcal/mol, RN-H = 2.122 Å) to proton-transfer structures (pyridine-HBr ΔE = –11.6 kcal/mol and 4-Fpyridine-HBr ΔE = –10.3 kcal/mol). For the weaker hydrogen-bonded systems in bulk media, increasing the dielectric constant induces a slight contraction of the N-H bond. For complexes of intermediate hydrogen-bond strength (3Fpyridine-HBr, 35DFpyridne-HBr, 345TFpyridine-HBr), an increase in the dielectric constant induces a shift to a proton-transfer structure. Bonding analyses reveal that the extent of negative charge on Br increases as with hydrogen-bond strength, and it jumps to ~ –0.8 when proton transfer ensues. In AIM analysis, values of electron density and density Laplacian at the bond critical point signal shifts in the electron density that accompany the increase in hydrogen-bond strength and onset of proton transfer.

Fluoropyridine–HBr complexes: Hydrogen bonding and proton transfer in the gas-phase and low-dielectric media

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