Ab initio calculations of the ground electronic states of polyiodide anions

Equilibrium bond lengths, harmonic frequencies, electron affinities, and dissociation energies (as appropriate) were determined for I, I^-,I₂,I₂^-, and I₃^- by CCSD(T) and four DFT methods (BLYP, BPW91, B3LYP, and B3PW91) using a basis set consisting of a relativistic effective core potential and a triple-zeta plus 2df polarization functions (ECP-TZ(2df)) for the valence electrons. Comparison of the DFT results with the CCSD(T) results and available experimental information indicates that the B3PW91 approach does particularly well describing the bonding in these species. B3PW91/ECP-TZ(2df) calculations of I₅^- in linear (D_∞h) and bent (C_2v,) geometries indicate that the linear structure is a low-energy transition state lying only about 0.1-0.2 eV above the C_2v global minimum-energy structure. Harmonic frequencies and infrared and Raman intensities are calculated for both structures and compared to available experimental information.