Spectroscopic Determination of the OH ⁻ Solvation Shell in the OH ⁻ ·(H ₂ O) _n Clusters

Abstract

There has been long-standing uncertainty about the number of water molecules in the primary coordination environment of the OH- and F- ions in aqueous chemistry. We report the vibrational spectra of the OH-.(H2O)n and F-.(H2O)n clusters and interpret the pattern of OH stretching fundamentals with ab initio calculations. The spectra of the cold complexes are obtained by first attaching weakly bound argon atoms to the clusters and then monitoring the photoinduced evaporation of these atoms when an infrared laser is tuned to a vibrational resonance. The small clusters (n </= 3) display an isolated, sharp feature near the free OH stretching vibration, the signature of open solvation morphologies where each water molecule binds independently to the ion. Pronounced changes in the spectra are observed at n = 4 in the hydroxide ion and at n = 5 in the fluoride ion. In both cases, new features appear in the region typically associated with interwater hydrogen bonding. This behavior establishes that the primary hydration shells occur at n = 3 and 4 in hydroxide and fluoride, respectively.

Nature of the Aqueous Hydroxide Ion Probed by X-ray Absorption Spectroscopy

“…[10][11][12][13][14][15] Argon vibrational predissociation spectroscopy of OH -(H 2 O) m and H + (H 2 O) m clusters has elucidated some aspects of the fundamental ion-water interactions. 18,19 EUV photoelectron spectroscopy of aqueous HCl and NaOH solutions has demonstrated how the solvation environment affects the electron binding energies of the solvated ions and, complementarily, how the presence of the ions influences the water electron binding energies. 20 Herein, we report the first measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous potassium hydroxide solutions.…”

“…For example, the mobility of hydroxide and hydrated protons in liquid water has long been recognized as being anomalously high, 1 although considerable debate continues as to the underlying causes. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Ab initio molecular dynamics (MD) methods have become particularly useful in the elucidation of the molecular-scale proton and OH -transport mechanisms, which have been shown to depend strongly on both the local solvation environment of the ions and the hydrogen bonding fluctuations of the water molecules. [2][3][4][5][6][7][8] However, despite these important computational advances, disagreement remains with regard to the proper description of OH -mobility in liquid water.…”

Structure and mechanism of proton transport through the transmembrane tetrameric M2 protein bundle of the influenza A virus

“…Studies of water clusters have shed light not only on their fundamental properties (1)(2)(3)(4) but also on the mechanism employed by various nano-bio systems to fine-tune water and proton transport (5)(6)(7)(8)(9). A relevant example from biology is the M2 protein of the influenza A virus (10,11), which is the target of the influenza drugs amantadine and rimantadine (12)(13)(14)(15)(16)(17).…”

Untitled

“…The nearest aqueous oxygen atom to the hydroxide proton appears to average about 0.25 nm, almost twice the distance of the hydroxide ions accepting hydrogen bonds (~0.14 nm), well outside the normal hydrogen-bond signature distance of 0.15-0.21 nm (Botti et al 2004b) and at a distance often considered as showing the absence of a bond (Khan, 2000). The O-H stretch vibration behaves as the free hydroxyl group in small gasphase clusters (Robertson et al 2003) and both concentrated and more dilute hydroxide solutions (Corridoni et al 2007). In confirmation, Fourier transform infrared (FTIR) spectroscopy of HDO isotopically diluted in H 2 O finds the free hydroxide O-H stretch at higher frequency indicative of very weak or absent hydrogen bonding (Smiechowski and Stangret, 2007).…”

“…Certainly the Raman spectra of hydroxide solutions changes when the solution is diluted below OH -:H 2 O 1:20 (Corridoni et al 2007). Also, HO -(••HOH) 4 was found to be energetically unfavorable using quasi-chemical theory (Asthagiri et al 2003) and spectroscopic studies indicate the 4th H 2 O in HO -(••HOH) 4 to be preferably hydrogen bonded to the other three forming a second shell (Robertson et al 2003).…”