NH ⁺ - F hydrogen bonding in a fluorinated "Proton Sponge" derivative: Integration of solution, solid-state, gas-phase, and computational studies

We report detailed studies on the characterization of an intramolecular NH - F hydrogen bond formed within a fluorinated "proton sponge" derivative. An ammonium ion, generated from 8-fluoro-N,N-dimethylnaphthalen-1- amine, serves as a charged hydrogen bond donor to a covalently bound fluorine appropriately positioned on the naphthalene skeleton. Potentiometric titrations of various N,N-dimethylnaphthalen-1-amines demonstrate a significant increase in basicity when hydrogen bonding is possible. X-ray crystallography reveals that NH - F hydrogen bonding in protonated 8-fluoro-N,N-di-methylnaphthalen-1-amine is heavily influenced by ion pairing in the solid state; bifurcated and trifurcated hydrogen bonds are formed depending on the counterion utilized. Compelling evidence of hydrogen bonding in the 8-fluoro-N,N-dimethylnaphthyl-1- ammonium cation is provided by gas-phase cryogenic vibrational photodissociation spectroscopy. Solution-phase infrared spectroscopy provides complementary results, and the frequencies of the N - H stretching mode in both phases are in excellent agreement with the computed vibrational spectra. NMR analysis of protonated 8-fluoro-N,N-dimethylnaphthalen-1-amine demonstrates significant H - F coupling between the N - H hydrogen and fluorine that cannot be attributed to long-range, through-bond interactions; the couplings correlate favorably with calculated values. The results obtained from these experiments are congruent with the formation of an NH - F hydrogen bond upon protonation of 8-fluoro-N,N-dimethylnaphthalen-1-amine.