THEORETICAL-STUDY OF HYDROXIDE ION-WATER CLUSTERS

The optimal structures, harmonic vibrational frequencies, and incremen tal association enthalpies for the OH-(H2O)(n)(n = 1-3) clusters have been computed at the MP2/aug-cc-p VDZ level of theory, the first ones reported at the correlated level for the n = 2 and 3 clusters. The inc remental association enthalpies at 298 K were estimated at -27.0, -20. 1, and -16.9 kcal/mol, respectively, within the error bars of recent e xperimental measurements for the n = 1, 2, and 3 clusters. Two almost isoenergetic isomers were identified for the n = 3 cluster. Hydrogen b onding between water molecules was found in the n = 3 but not the n = 2 cluster, the structure of which is determined by the strong hydroxid e ion-water interaction. Hydration of the hydroxide ion results in a d ecrease in its bond length with an accompanying increase in the analog ous frequency that eventually ''scrambles'' with the one of the ''free '' (non-hydrogen bonded) stretches in water. The most active infrared (IR) modes correspond to OH stretches which are hydrogen bonded to the hydroxide ion in the range 2500-3000 cm(-1) in agreement with experim ental infrared multiple internal reflectance (IR-MIR) measurements of aqueous hydroxide.