Ab initio and diatomics in molecule potentials for I2−, I2, I3−, and I3

Abstract

The electronic structure of the I3− molecular anion and its photoproducts I2−, I2, and I3 were studied. Ab initio calculations were carried out using the multireference configuration interaction (MRCI) method for the valence electrons together with a relativistic effective core potential. The ab initio wave functions were also used to compute some spin–orbit coupling matrix elements, as well as approximate valence bond wave functions, used as guidelines in the construction of a 108-state diatomics in molecule (DIM) description of the electronic structure of I3−. In the DIM model, spin–orbit coupling was introduced as a sum of atomic operators. For I2− the ab initio and the DIM ground-state potentials show excellent agreement with the experimental results. The results for I2 are also in very good agreement with experimental data. For I3−, the MRCI calculations give a very good description of the spectroscopic constants and agree with the vertical excitation energies, provided spin–orbit coupling is included. The DIM description fails both quantitively by leading to erroneous spectroscopic constants, and qualitatively by not even reproducing the MRCI ordering of the excited-states. The failure of the DIM is attributed to the omission of ionic states. The overall qualitative picture of the excited-state potentials shows a maze of dense avoided crossings which means that all energetically allowed photoproducts will be present in the experiment. The ground electronic state of I3 was calculated to be a collinear and centrosymmetric Πu,3/22. The collinear state is stabilized by spin–orbit coupling relative to a bent configuration. Calculated vertical transition energies from the ground to low-lying excited states of the radical are in excellent agreement with the experimental data. The spin–orbit assignment of these states is provided.