Quantum Mechanical Behavior of Hydrogen Bonds Enables Supramolecular Structure in a Weak Acid-Base Monoprotic Complex

The unconventional supramolecular chemistry between perfluoro-tert-butanol (PFTB), as an acid, and 1-methylimidazole (MIM), as a base, is presented. Supramolecular chemistry occurs in MIM-PFTB mixtures with a base-to-acid molar ratio of 1:2, or higher, and coincides with the formation of strong hydrogen bonds (SHBs) in which the acidic hydrogen atoms are quantum mechanically delocalized. Evidence for the SHB and the hydrogen atom sharing is obtained from IR and ¹H NMR spectroscopies and X-ray crystallography. First-principles simulations incorporating both electronic and nuclear quantum effects verify the presence of the SHBs and demonstrate that the broad IR absorption band centered at 2400 cm^-1 and the large downfield ¹H NMR of the complexes are a consequence of the hydrogen atom sharing between the acid and the base. The supramolecular behavior reported for PFTB-MIM has not been previously observed in other monoprotic acid-base mixtures forming either conventional or SHBs. Hence, MIM-PFTB mixtures depart from the behavior typically exhibited by other liquid mixtures, demonstrating that electronic and nuclear quantum effects play an important role in driving the unconventional supramolecular chemistry observed in MIM-PFTB samples.