Al⁺ Activates Acetone to Form Pinacolate

Area of Science:

• Physical Chemistry
• Inorganic Chemistry
• Spectroscopy

Background:

• Gas-phase interactions of metal cations with organic molecules are crucial for understanding chemical processes.
• Aluminum cations (Al+) are model systems for studying metal-ligand bonding and reactivity.
• Acetone is a common organic solvent with a reactive carbonyl group.

Purpose of the Study:

• To investigate the structural and electronic properties of aluminum cation-acetone complexes in the gas phase.
• To elucidate the interaction mechanism between Al+ and acetone molecules using vibrational spectroscopy.
• To identify the formation of pinacolate structures in aluminum-acetone clusters.

Main Methods:

• Photodissociation vibrational spectroscopy was employed to study Al+(acetone)n clusters (n=2-5).
• Density Functional Theory (DFT) calculations were performed to predict and assign vibrational spectra.
• Comparison of experimental and calculated spectra was used to determine complex structures.

Main Results:

• Observed red shift in the C=O stretch and blue shift in the CCC stretch of acetone upon complexation with Al+.
• Shifts in vibrational frequencies decreased with increasing cluster size (n).
• Experimental evidence for pinacolate formation (a C-C coupled product) was found for Al+(acetone)5, indicated by a peak at 1185 cm-1.

Conclusions:

• The study reveals the structural evolution of Al+(acetone)n complexes with increasing ligand number.
• DFT calculations successfully predicted the observed spectral changes and favored pinacolate structures for n>=3.
• Experimental observation of pinacolate formation in Al+(acetone)5 confirms the proposed reductive coupling mechanism involving Al+ oxidation.