Characterization of the primary hydration shell of the hydroxide ion with H₂ tagging vibrational spectroscopy of the OH^-(H₂O)_n=2,3 and OD^-(D₂O)_n=2,3clusters

We report the isotope-dependent vibrational predissociation spectra of the H₂-tagged OH^- (H₂O)_n=2,3 clusters, from which we determine the strongly coordination-dependent energies of the fundamentals due to the OH groups bound to the ion and the intramolecular bending modes of the water molecules. The HOH bending fundamental is completely missing in the delocalized OH^- (H₂O) binary complex but is recovered upon adding the second water molecule, thereby establishing that the dihydrate behaves as a hydroxide ion solvated by two essentially intact water molecules. The energies of the observed OH stretches are in good agreement with the values predicted by Takahashi and co-workers [Phys. Chem. Chem. Phys. 17, 25505 (2015); 15, 114 (2013)] with a theoretical model that treats the strong anharmonicities at play in this system with explicit coupling between the bound OH groups and the O-O stretching modes on an extended potential energy surface. We highlight a surprising similarity between the spectral signatures of OH^- (H₂O)₃ and the excess proton analogue, H₃O⁺ (H₂O)₃, both of which correspond to completed hydration shells around the proton defect. We discuss the origin of the extreme solvatochromicity displayed by both OH^- and H⁺ in the context of the anomalously large "proton polarizabilities" of the H₅O₂ ⁺ and H₃O₂ ^- binary complexes.