Infrared Spectroscopy of the Microhydrated Nitrate Ions NO3-(H2O)1-6

We present infrared photodissociation spectra of the microhydrated nitrate ions NO3-(H2O)(1-6), measured from 600 to 1800 cm(-1). The assignment of the spectra is aided by comparison with calculated B3LYP/aug-cc-pVDZ harmonic frequencies, as well as with higher-level calculations. The IR spectra are dominated by the antisymmetric stretching mode of NO3-, Which is doubly degenerate in the bare ion but splits into its two components for most microhydrated ions studied here due to asymmetric solvation of the nitrate core. However, for NO3-(H2O)(3), the spectrum reveals no lifting of this degeneracy, indicating an ion with a highly symmetric solvation shell. The first three water molecules bind in a bidentate fashion to the terminal oxygen atoms of the nitrate ion, keeping the planar symmetry. The onset of extensive water-water hydrogen bonding is observed starting with four water molecules and persists in the larger clusters.