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=2,3 clusters

Abstract

We report the isotope-dependent vibrational predissociation spectra of the H-tagged OH ⋅ (HO) 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 ⋅ (HO) 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 ⋅ (HO) and the excess proton analogue, HO ⋅ (HO), 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 HO and HO binary complexes.

Infrared spectroscopy of O˙− and OH− in water clusters: evidence for fast interconversion between O˙− and OH˙OH−

“…2,3 To approach the structure of the primary hydration shell of ions, infrared multiple photon dissociation (IRMPD) spectroscopy has been successfully applied. [4][5][6][7][8][9][10][11] The hydroxide ion (OH À ) is an essential ionic species in aqueous chemistry and its microscopic hydration has been studied in numerous reactivity 12,13 and spectroscopy 7,[14][15][16][17][18][19][20] experiments, mainly elucidating the structure of clusters. Infrared (IR) spectra exhibit broad absorption features that were assigned to rapid fluctuation of isomeric structures.…”

Structural Motifs in Cold Ternary Ion Complexes of Hydroxyl-Functionalized Ionic Liquids: Isolating the Role of Cation–Cation Interactions

“…We note that the asymmetric doublet in the HEMIm + spectrum near 3650 cm –1 arises from attachment of the N 2 tag (* in Figure c) to the free OH group, as discussed further in the SI (Figure S1). The red-shifted bands (A 1 –A 4 ) in the (2,1) spectrum occur in the region corresponding to fundamentals of OH groups that strongly interact with anionic domains. ,− The picture that emerges from qualitative behavior of the ring CH and OH stretching bands is therefore that at least one of these two groups (available from the two cations) is strongly interacting with the anion. Moreover, the plethora of transitions in the OH stretching region raises the specter that different isomers may contribute to the pattern (as opposed to reflecting strong anharmonic coupling, which is also known to result in soft mode progressions built on OH stretching fundamentals).…”

Disentangling the Complex Vibrational Spectrum of the Protonated Water Trimer, H+(H2O)3, with Two-Color IR-IR Photodissociation of the Bare Ion and Anharmonic VSCF/VCI Theory

“…Given this relatively straightforward assignment scheme, however, the observation of a strong doublet (b 8a and b 8b ) in the 3D isotopologue is unexpected (Figure 2f), as anharmonic fundamentals in this energy range typically follow an approximate scaling of 1/1.23 upon deuteration without a significant change in the overall band contour. 39,40 Such mass-dependent spectral complexities can arise, however, when modes couple as they become close in energy (e.g., Fermi resonances 41 ). To understand the behavior of the 3D-D 2 spectrum, we extended these calculations to 3D using the same PES and dipole surfaces as those used for 3H, with the result presented in Figure 2d.…”