The rotation of NO3− as a probe of molecular ion - water interactions

Abstract

The hydration dynamics of aqueous nitrate, NO3−(aq), is studied by 2D-IR spectroscopy, UV-IR- and UV-UV transient absorption spectroscopy. The experimental results are compared to Car-Parinello molecular dynamics (MD) simulations. The 2D-IR measurements and MD simulations of the non-degenerate asymmetric stretch vibrations of nitrate reveal an intermodal energy exchange occurring on a 0.2 ps time scale related to hydrogen bond fluctuations. The transient absorption measurements find that the nitrate ions rotate in 2 ps. The MD simulations indicate that the ion rotation is associated with the formation of new hydrogen bonds. The 2 ps rotation time thus indicates that the hydration shell of aqueous nitrate is rather labile.