Intramolecular proton bonding in aliphatic dicarboxylate anions: dynamic conformational landscapes and spectral signatures

Carboxylate moieties are central to organic chemistry and a main driving force of biomolecular recognition. Their diffuse anionic structure is prone to build proton-mediated supramolecular bonds with a marked delocalization of charge, a chemical motif that is key to processes of proton storage and transfer. We investigate intramolecular proton bonding interactions in benchmark aliphatic dicarboxylate anions of the form HOOC(CH₂)_n-2COO^- (n=4-8), hence succinate, glutarate, adipate, pimelate and suberate. Infrared ion spectroscopy is employed to expose the vibrational fingerprints of the mass-selected anions isolated at room temperature. Ab initio Molecular Dynamics calculations are applied to rationalize the fluxional character of the shared proton and its impact on the cyclic structure adopted by the anion. Our findings indicate that anions with shorter alkyl chains are constrained to symmetrically shared protons in anti-anti configurations of the carboxylate moieties. Longer chain lengths increase the conformational flexibility of the alkyl backbone and stabilize anti-syn configurations with asymmetric proton sharing. As a result, the vibrational spectrum evolves towards progressively more differentiated carboxylic and carboxylate stretching modes. In all systems, the dynamic character of the proton bond can be recognized through a characteristic broad O-H stretching band that narrows down and blue shifts as proton delocalization is reduced in the larger anions.