Spectroscopic Studies of Intracluster Chemistry

Infrared predissociation spectroscopy of ion-solvent clusters has allowed us to examine the effects of sequential hydration on the reactivity of cations such as NO^+, NO_2^+, and protonated formaldehyde, H_2COH^+, stable gas phase ions which are known to undergo rapid reactions in aqueous solution. Our experiments demonstrate that these ions undergo hydration reactions at critical cluster sizes. The smaller clusters have spectra characteristic of H_2O ligans bound to stable ion cores, but as the cluster size increases, there is a sudden onset for intracluster rearrangements, e.g. NO_2^+ + H_2O yields H_3O^+ + HNO_3 which occurs upon hydration with four water molecules. With this approach, we can probe microscopic aspects of solvent effects on chemical reactions including the hydration of carbonyls and acid-catalyzed hydrolysis of amides.