Electronic structure and proton transfer in ground-state hexafluoroacetylacetone

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Crystallography

Background:

• Intramolecular hydrogen bonding in enol tautomers is a subject of ongoing research.
• Hexafluoroacetylacetone (HFAA) has shown ambiguous hydrogen bonding characteristics.

Purpose of the Study:

• To resolve controversies regarding intramolecular hydrogen bonding in HFAA's enol tautomer.
• To provide a synergistic experimental and theoretical investigation of HFAA.

Main Methods:

• Cryogenic (93K) X-ray diffraction on zone-melt grown HFAA crystals.
• Quantum chemical calculations including HF, DFT (B3LYP), MPn, and CCSD(T) methods.
• Geometry optimizations and transition state analyses using CCSD(T)/aug-cc-pVDZ.

Main Results:

• X-ray diffraction revealed distinguishable O(1)-H and H...O(2) bonds with an interoxygen distance of 2.680 ± 0.003 Å.
• Computational methods predicted an asymmetric equilibrium structure (C(s)) with a 2.628 Å O...O separation.
• A symmetric transition state (C(2v)) for proton transfer was identified with a barrier of 21.29 kJ/mol.

Conclusions:

• The study confirms distinct hydrogen bonds in HFAA, supporting experimental findings.
• Computational results align with crystallographic data, offering a detailed molecular description.
• HFAA exhibits weaker hydrogen bonding compared to acetylacetone.