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

Vibrational predissociation spectra of D2-tagged Mg2+OH-(H2O)(n=1-6) and Ca2+OH-(H2O)(n=1-5) clusters are reported to explore how the M2+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 similar 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.