Solvation effects on the N-O and O-H stretching modes in hydrated NO₃^-(H₂O)_n clusters

NO3-(H2O)n clusters are a molecular model used to understand the solvation interaction between water and nitrate, an important anion in nature, industrial processes and biology. We demonstrate by ab initio molecular dynamics simulations that among the many isomeric structures at each cluster size with n = 1-6, thermal stability is an important consideration. The vibrational profile at a particular size, probed previously by infrared multiple photon dissociation (IRMPD) spectroscopy, can be accounted for by the isomers, which are both energetically and dynamically stable. Conversion and broadening due to the fluctuation of hydrogen bonds are important not only for the O-H stretching modes but also for the N-O stretching modes. Distinct patterns for the O-H stretching modes are predicted for the various solvation motifs. We also predict a surface structure for NO3-(H2O)n as n increases beyond 6, which can be verified by an early onset of strong libration bands for H2O in the IRMPD spectra and a flattening of the vertical detachment energy in the photoelectron spectra.