Resonance Raman and ab initio studies of the electronic transitions of aqueous azide anion

Resonance Raman spectra and absolute cross sections have been measured for the azide anion (N-3(-)) in dilute aqueous solution at excitation wavelengt hs of 246, 228, 223, 218, and 208 nm, on resonance with the longest-wavelen gth UV absorption bands. The spectra are dominated by the fundamental of th e 1343 cm(-1) symmetric stretch, with much lower intensities in the first o vertone of the symmetric stretch and the overtone of the bending mode at 12 75 cm(-1). The weak overtones and generally low resonance Raman cross secti ons suggest unusually small changes in the N-N bond lengths relative to tho se expected for valence transitions of small molecules, and/or particularly strong coupling of the electronic transitions to solvent degrees of freedo m leading to rapid effective electronic dephasing. Ab initio calculations h ave been performed on complexes of N3- with three and four water molecules at the single CI level using the 6-311++g** basis with additional diffuse f unctions on the N atoms. These calculations predict that the low-lying dipo le-allowed electronic excitations have large contributions from rather diff use upper orbitals, and the forces on the atoms upon vertical excitation ar e mainly along azide-water and water-water intermolecular coordinates rathe r than internal vibrations of the azide. The experimental and computational results together are most consistent with significant charge-transfer-to-s olvent precursor character of the resonant electronic state(s).