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⁺, NO₂⁺, and protonated formaldehyde, H₂COH⁺, 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 H₂O ligands bound to stable ion cores, but as the cluster size increases, there is a sudden onset for intracluster rearrangements, e.g. NO₂⁺+ H₂O → H₃O⁺ + HNO₃ 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 acidcatalyzed hydrolysis of amides.