Symmetry breaking effects in NO3-: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate-water complexes

Ground-state structures and vibrational frequencies are calculated for comp lexes of the nitrate anion with one and two water molecules at the ab initi o Hartree-Fock level with a basis set including diffuse and polarization fu nctions. Two local minimum geometries are found for each complex. Calculati ons of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lo west-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtain ed previously from empirical simulations of the experimental resonance Rama n intensities of NO3- in dilute aqueous solution. The calculations on two-w ater clusters agree well with the experimental excited-state geometry chang es along the totally symmetric N-O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenera te in the isolated D-3h ion) and the resonance Raman intensity in this mode , suggesting that bulk solvent polarization enhances the asymmetry of the l ocal environment for NO3- in water. Comparison of the ground-state vibratio nal frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures sugges ts that the rms difference among the three N-O bond lengths for nitrate ani on in water probably exceeds 0.01 A. (C) 2001 American Institute of Physics .