Unveiling Superacidity in Alcohol-BF₃ Complexes Using a Vibrational Probe

Area of Science:

• Chemistry
• Physical Chemistry
• Supramolecular Chemistry

Background:

• Boron trifluoride (BF3) is a potent Lewis acid forming complexes with Lewis bases like water and alcohols.
• These BF3-alcohol complexes exhibit strong Brønsted acidity, some reaching the superacidity range.
• While their catalytic applications are known, their hydrogen bonding properties remain understudied.

Purpose of the Study:

• To systematically investigate hydrogen bonding in BF3-alcohol complexes.
• To quantify the acidity of these complexes using a vibrational probe.
• To explore the potential of these complexes as next-generation reagents.

Main Methods:

• Utilized deuterated acetonitrile as a vibrational probe to assess hydrogen bonding.
• Measured the blue shift in the CN vibrational frequency of acetonitrile.
• Employed computational methods to confirm acidity and analyze electronic structure changes.

Main Results:

• Observed a linear blue shift in CN frequency correlating with alcohol acidity.
• BF3 complexation significantly enhanced the blue shift, indicating superacidic properties.
• The hexafluoroisopropanol (HFIP)-BF3 complex exhibited higher acidity than triflic acid.

Conclusions:

• Nitrile frequency serves as a sensitive probe for hydrogen bonding and acidity, even in the superacidity range.
• BF3-alcohol complexes represent a tunable class of superacidic systems.
• This research provides a method for quantifying superacidic environments and designing novel reagents.