Size-Dependent Onset of Nitric Acid Dissociation in Cs⁺·(HNO₃)(H₂O)_n=0–11 Clusters at 20 K

We report the water-mediated charge separation of nitric acid upon incorporation into size-selected Cs⁺·(HNO₃)­(H₂O)_n=0–11 clusters at 20 K. Dramatic spectral changes are observed in the n = 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 n = 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 NO₃^– anion that is directly complexed to the Cs⁺ cation. Theoretical analysis of the n = 10 spectrum indicates that the dissociated ions adopt a solvent-separated ion-pair configuration such that the Cs⁺ and H₃O⁺ cations flank the NO₃^– anion in a microhydrated salt bridge. This charge separation motif is evidently assisted by the electrostatic stabilization of the product NO₃^–/H₃O⁺ ion pair by the proximal metal ion.