Photodetachment and theoretical study of free and water-solvated nitrate anions, NO3-(H2O)n (n=0-6)

We investigated free and water-solvated gas phase nitrate anions, NO3-(H2O)(n) (n=0-6), by photodetachment photoelectron spectroscopy and theoretical calculations. We obtained the electronic structure and electron binding energies of the free and solvated NO3- at three detachment photon energies, 4.661, 6.424, and 7.866 eV. The ground and two low-lying electronic excited states of the NO3 radical (X (2)A(2)('),A E-2('),B E-2(')) were observed at the 6.424 and 7.866 eV photon energies. The photoelectron spectra of the solvated nitrate complexes are similar to that of the bare NO3-, except that they become broadened and diffuse due to the solvation. The spectrum of NO3-(H3O)(3) showed a resolved vibrational progression of the N-O symmetric stretching (1000 cm(-1)), suggesting the cluster possesses a high symmetry. NO3- and NO3 were calculated at various levels of theory. Based on the good agreement between density functional theory calculations and experiment for NO3- and NO3, we carried out systematic calculations for NO3-(H2O)(n) (n=1-6) using primarily density function theory methods. The calculations indicate that NO3-(H2O)(n) (n=1-3) are all planar, with the first three H2O forming the first solvation shell around NO3-, giving rise to a highly symmetric C-3h NO3-(H2O)(3). The next three waters form a second solvation shell without direct contact with NO3-. The C-3h NO3-(H2O)(3) solvation structure was observed to be rather robust and largely preserved in the larger clusters. (C) 2002 American Institute of Physics.