Gas-Phase Vibrational Spectra of [C₂H₅O₂]⁺Isomers: Protonated Glycolaldehyde, Acetic Acid, and Methyl Formate

Methyl formate, acetic acid, and glycolaldehyde comprise an isomeric family that is abundant in the interstellar medium. Their astronomically observed relative abundances are commonly used to gain insights into the potential formation pathways of these complex organic molecules (iCOMs). Several of the proposed synthesis routes involve their protonated analogs. However, to date, only limited spectroscopic data are available on any of the protonated isomers, hampering their astronomical detection that might shed light on specific formation pathways. Here, we present the first broadband vibrational study of protonated methyl formate, acetic acid, and glycolaldehyde (m/z 61, [C₂H₅O₂]⁺). The protonated species were formed in a storage ion source by electron impact ionization of the respective vapors (the dimer in the case of glycolaldehyde) and subsequent (self-)protonation reactions. The vibrational spectra of the mass-selected ions were recorded in the range of 640–1800 cm^–1using Ne-tagging infrared predissociation spectroscopy in a cryogenic 22-pole ion trap instrument coupled to the infrared free-electron lasers at the FELIX Laboratory. The experimental spectra are compared to calculated vibrational frequencies obtained at the B2PLYP-D3 level of theory using different basis sets and methods, and the influence of anharmonicity on the spectra is discussed. Protonated forms of both structural conformers of methyl formate (syn and anti) were observed and assigned, with protonation on the carbonyl oxygen in the trans (lowest-energy conformer) and cis orientation, respectively. Dominantly, the lowest-energy conformer EZ of carbonyl-protonated acetic acid was observed when using acetic acid as precursor. When using the glycolaldehyde dimer as a precursor, the lowest-energy conformers of both protonated glycolaldehyde and acetic acid were observed.