Size-Dependent Onset of Nitric Acid Dissociation in Cs·(HNO)(HO) Clusters at 20 K.

We report the water-mediated charge separation of nitric acid upon incorporation into size-selected Cs·(HNO)(HO) clusters at 20 K. Dramatic spectral changes are observed in the = 7-9 range that are traced to the formation of many isomeric structures associated with intermediate transfer of the acidic proton to the water network. This transfer is complete by = 10, which exhibits much simpler vibrational band patterns consistent with those expected for a tricoordinated hydronium ion (the Eigen motif) along with the NO stretching bands predicted for a hydrated NO anion that is directly complexed to the Cs cation. Theoretical analysis of the = 10 spectrum indicates that the dissociated ions adopt a solvent-separated ion-pair configuration such that the Cs and HO cations flank the NO anion in a microhydrated salt bridge. This charge separation motif is evidently assisted by the electrostatic stabilization of the product NO/HO ion pair by the proximal metal ion.