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.