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Cryogenic Infrared Action Spectroscopy Fingerprints the Hydrogen Bonding Network in Gas-Phase Coumarin Cations.

We report cryogenic vibrational spectra of gas-phase cations of two common hydroxycoumarins, scopoletin and esculetin, as well as their glycosidic derivatives, scopolin and esculin. The study allows direct observation of the intramolecular interactions between the hydroxyl groups of these molecules. We use cryogenic messenger-tagging IR action spectroscopy to detect vibrational bands in the 3100-3800 cm-1 spectral range and discuss the corresponding structural characteristics and hydrogen bonding networks that they imply. The experimental data are supported by a thorough computational evaluation, including investigation of the conformational space. Through comparison of the calculated conformers with the experimental results, we identify the main types of OH oscillators and infer how protonation and sodiation affect the structural arrangement of these molecules. The results presented here provide direct evidence of how slight structural differences sensitively affect the hydrogen bonding network in coumarin derivatives.