posted on 2021-03-29, 13:33authored bySayoni Mitra, Nan Yang, Laura M. McCaslin, R. Benny Gerber, Mark A. Johnson
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.