Microhydration of Contact Ion Pairs in M²⁺OH^–(H₂O)_n=1–5 (M = Mg, Ca) Clusters: Spectral Manifestations of a Mobile Proton Defect in the First Hydration Shell

Abstract

Vibrational predissociation spectra of D2-"tagged" Mg(2+)OH(-)(H2O)n=1-6 and Ca(2+)OH(-)(H2O)n=1-5 clusters are reported to explore how the M(2+)OH(-) contact ion pairs respond to stepwise formation of the first hydration shell. In both cases, the hydroxide stretching frequency is found to red-shift strongly starting with addition of the third water molecule, quickly becoming indistinguishable from nonbonded OH groups associated with solvent water molecules by n = 5. A remarkably broad feature centered around 3200 cm(-1) and spanning up to ∼1000 cm(-1) appears for the n ≥ 4 clusters that we assign to a single-donor ionic hydrogen bond between a proximal first solvent shell water molecule and the embedded hydroxide ion. The extreme broadening is rationalized with a theoretical model that evaluates the range of local OH stretching frequencies predicted for the heavy particle configurations available in the zero-point vibrational wave function describing the low-frequency modes. The implication of this treatment is that extreme broadening in the vibrational spectrum need not arise from thermal fluctuations in the ion ensemble, but can rather reflect combination bands based on the OH stretching fundamental that involve many quanta of low-frequency modes whose displacements strongly modulate the OH stretching frequency.

Charge transfer in MOH(H2O)+ (M = Mn, Fe, Co, Ni, Cu, Zn) complexes revealed by vibrational spectroscopy of mass-selected ions

“…This series of transition metals bridge the gap between the Mg and Ca complexes studied by Johnson et al 17 and the Cu complex studied previously in our group. 11 The evolution of the vibrational frequencies of the hydroxide and water ligands as a function of the metal center sheds light on the gradual charge transfer behavior of this series of metals.…”

“…The experimental values for the hydroxide frequency of the Mg and Ca complexes are obtained from ref. 17. The red line is a linear fit of the data points excluding the Zn complex.…”

Snapshots of Proton Accommodation at a Microscopic Water Surface: Understanding the Vibrational Spectral Signatures of the Charge Defect in Cryogenically Cooled H+(H2O)n=2–28 Clusters

“…At the low temperatures relevant for the present experiments, the dominant quantum effect is zero-point motion, which is known to couple soft modes to strongly H-bonded stretches. 99,100 We believe that this is a major factor responsible for the large widths of the features associated with the hydronium OH stretch vibrations in the cold clusters. Important future directions will involve a more quantitative explanation for the spectral breadth as well as determination of the temperature dependence of the spectral patterns.…”

“…99,100 A more detailed analysis on the origins of the spectral breadth is clearly warranted but is beyond the scope of this article. VI.…”

Effects of electronic structure on the hydration of PbNO3+ and SrNO3+ ion pairs

“…[46][47][48] Stace and co-workers showed that many complexes of Pb(II) with organic ligands are stable in the gas phase, 49,50 but that gaseous clusters of Pb 2+ (H 2 O) n are unstable to hydrolysis for n o 11 and consequently are not present in electrospray ionization (ESI) mass spectra. 52 IRPD studies of ion pair hydrates have so far been limited to two reports on the first hydration shell of metal hydroxides, 53,54 and a report on [MgNO 3 ] + (H 2 O) [1][2][3][4] . 52 IRPD studies of ion pair hydrates have so far been limited to two reports on the first hydration shell of metal hydroxides, 53,54 and a report on [MgNO 3 ] + (H 2 O) [1][2][3][4] .…”