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Description
Hydrogen bond interactions are essential for structure formation and the dynamics in most biological systems. The influence of structural constraints on the properties of hydrogen bonds can be studied by investigating small isolated model systems, in our case proton bound molecule complexes. A useful tool for probing them is vibrational spectroscopy. Because of their cryogenic temperature, an ideal environment for vibrational spectroscopy is provided in helium nanodroplets, greatly reducing thermal broadening of vibrational bands.
It is known that in many cases, proton-bound carboxylate homo-dimers exhibit a motif in which the proton is equally shared between the carboxylate groups. This can no longer occur for trimers or tetramers, where a more complex hydrogen bond interaction pattern is expected. Since the formic acid dimer has been proven to be an important model system, the investigation of the formic acid trimer was initiated to shine light on the structural peculiarity of the proton bound trimers. Here we report the first gas-phase infrared spectra of the formic acid trimer anion in the 400-2000 cm-1 spectral range, obtained by means of cold-ion vibrational action spectroscopy of ions embedded in helium nanodroplets. Our results indicate that the prevalent structure of the anion in the gas phase may resemble a structure that combines the structural motifs of the two lowest energy conformers found previously for the proton-bound formate dimer.
