Water-Induced\nDissociative Mechanism of Carboxylate\nand Divalent Calcium Ions Revealed by IR Laser Spectroscopy

The dissociation\nof carboxylate and divalent calcium ions is investigated\nat the molecular level in microsolvation experiments by gradually\nincreasing the number of water molecules around the ions. IR photodissociation\n(IRPD) laser spectroscopy of H<sub>2</sub>-tagged (Ca<sup>2+</sup>, AcO<sup>–</sup>)(H<sub>2</sub>O)<sub><i>n</i>=8–21</sub> clusters in the ν(CO<sub>2</sub><sup>–</sup>) spectral\nrange combined with RI-B97-D3-BJ-abc/TZVPPD frequency calculations\nis used to identify the type of ion pairs involved in this process.\nThese results reveal that the ion dissociation follows a multistep\nmechanism involving in particular pseudobridged monodentate contact\nion pairs (CIPs), which are found to be the first intermediate species\nformed from bidentate CIPs along the ion dissociation path. Altogether,\nstructural assignments suggest a sequence of simple reactions in the\nfirst coordination shell of the carboxylate group, leading us to propose\ntwo possible dissociation paths. The appearance threshold of monodentate\nstructures is measured at <i>n</i> = 10, with that of solvent-shared\nion pairs (SIPs) being potentially at <i>n</i> = 18. By\nshowing in detail how solvation progressively takes over from the\nionic interaction in shaping these supramolecular structures, this\nstudy can serve as a reference for solving ion-pairing/dissociation\nproblems.