The geometric and electronic structures of I3− and I5− from effective-potential calculations

The (ab initio) effective-potential theory developed by Ewig et al. is applied to the structures of the polyiodide ions, I₃^? and I₅^?. The bare ions I₃^? and I₅^? are found by optimization of the geometry to be symmetric and linear. The counterion environment, however, greatly influences the equilibrium structure. A symmetric, flexible counterion environment produces only a slightly altered symmetric, linear equilibrium structure for the I₃^? anion ; whereas an asymmetric, rigid counterion frame leads to unequal bond lengths and bending of the anion. For the I₅^? anion, the potential energy calculated for bending of the central I—I—I angle, α, is very small so that a slight interaction with the lattice will readily lead to the experimental bent structure with α = 94°. At this value of α, the two outer angles are found to be equal and close to the experimental value. The electronic structures of the I₃^? and I₅^? ions are also discussed.