Evolution of the linker in microhydrated hydrogen dinitrate anions: From H⁺ to H₅ O₂

Hydrogen dinitrate anion, HNO₃ (NO₃^- ), is a proton-bound dimer with a very strong hydrogen bond. By employing ab initio molecular dynamics (AIMD) method, we studied the effects of the proton transfer and the rotation of the nitrates on the vibrational profiles of HNO₃ (NO₃^- )(H₂ O)_n (n = 0-2). The AIMD results indicate that the structure of the n = 0 cluster is very flexible, even though its hydrogen bond is quite strong. Significant rotations around the hydrogen bond and frequent transfers of proton from HNO₃ to NO₃^- are observed in AIMD simulations. Dynamic changes are therefore an important factor in understanding the broadening of vibrational features. For n = 1, the extent of structural fluctuation increases further, as H₂ O could move around the anion while the HNO₃ (NO₃^- ) core also goes through structural changes. Its vibrational spectrum can be understood as a mixture of many isomers visited during AIMD simulations. By n = 2, the structure is stabilized around one isomer, with the linker between the two nitrates being H₅ O₂⁺ , rather than H⁺ . Due to strong hydrogen bonds between nitrates and water molecules, this H₅ O₂⁺ takes the extraordinary structure with the H⁺ localized on one H₂ O, rather than being shared. While this novel structure is stable during AIMD simulations, the dynamic fluctuations in hydrogen bond distances still produce significant broadening in its vibrational profile.