Unveiling Methoxyacetone's Conformational Landscape

Recently, Igor Reva and his team at the University of Coimbra in Portugal explored the conformational space of methoxyacetone (MA) and learned more about its structural dynamics and behavior at varying energy states. Their findings were published in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (1).

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Reva and his team’s investigation of the mechanistic interpretation of MA's photochemical behavior provided crucial insights into its reactivity under UV light, unraveling pathways toward the formation of key photoproducts.

The research revealed insights into MA's molecular arrangements. The study was conducted using computational methods at the MP2/6–311++G(d,p) and DFT(B3LYP)/6–311++G(d,p) levels (1). The team predicted four distinct conformers of MA, primarily arising from internal rotations around specific dihedral angles (1).

Out of the four conformers that were predicted, the Trans-trans conformer (abbreviated as Tt), was the most stable configuration, whereas the gauche conformers, abbreviated as Tg and Cg, exhibited higher energies in the 3– kJ/mol range (1). As a result, Tg and Cg accounted for a large percentage of the room temperature–gas phase equilibrium (1).

The research team also looked at the signatures of the conformers, finding that some of them remained undetectable, even when exposed to UV radiation. It suggested that during deposition, a conformational cooling process occurred (1).

The researchers also looked at the infrared (IR) spectroscopy of the MA monomers that were isolated in cryogenic argon matrices, which helped confirm the presence of the Tt conformer in the samples (1). Furthermore, the team successfully assigned the IR spectrum of the dominant Tt form using advanced anharmonic DFT computations, enriching the understanding of its spectroscopic characteristics (1).

There was one interesting observation the researchers noted in the study when they exposed MA to UV irradiation in the 300–260 nm range. The process unveiled a photolysis mechanism, indicative of the Norrish type II mechanism (1). This led to the observation of a cage-confined intermediate photoproduct—an enol acetone and formaldehyde dimer—preceding the formation of carbon monoxide as the predominant photoproduct, a result of the Norrish type I photoreaction (1).

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Reference

(1) Sharma, A.; Gupta, V. K.; Reva, I. Methoxyacetone Revisited. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2024, 308, 123651. DOI: 10.1016/j.saa.2023.123651