Search for a strong, virtually "no-shift" hydrogen bond: a cage molecule with an exceptional OH⋅⋅⋅F interaction

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Spectroscopy

Background:

• Hydrogen bonds typically involve significant shifts in vibrational frequencies.
• The nature of interactions between OH donors and covalent F acceptors is not well-understood.
• Rigid molecular frameworks can influence non-covalent interactions.

Purpose of the Study:

• To synthesize and characterize a molecule featuring a strong hydrogen bond between an OH donor and a covalent F acceptor.
• To investigate the properties of a "no-shift" hydrogen bond within a rigid, caged system.
• To elucidate the structural and electronic factors governing this unusual interaction.

Main Methods:

• Organic synthesis of a novel caged molecule.
• Infrared (IR) spectroscopy to analyze OH stretching frequencies.
• Nuclear Magnetic Resonance (NMR) spectroscopy, including coupling constants, isotopic chemical shift perturbations, and proton exchange rate measurements.

Main Results:

• Successful synthesis of a molecule exhibiting a strong hydrogen bond between an OH group and a covalent fluorine atom.
• IR spectroscopy revealed minimal to no shift in the OH stretch, characteristic of a "no-shift" hydrogen bond.
• NMR studies provided consistent evidence supporting the presence and strength of the hydrogen bond.

Conclusions:

• The rigid framework of the caged system is crucial for stabilizing the strong OH-F hydrogen bond.
• "No-shift" hydrogen bonds, while controversial, can be robust and detectable through combined spectroscopic methods.
• This study provides a unique example and detailed characterization of a strong, yet spectroscopically subtle, hydrogen bond.