Theoretical Study of the Solvent Effect on Triiodide Ion in Solutions

摘要

The free-energy surfaces of I_3~- in aqueous, methanol, and acetonitrile solutions as well as in the gas phase are examined in the ground state by means of the ab initio RISM-MCSCF (reference interaction site model-multiconfigurational self-consistent field) method. The X~1∑_g~+ state in the gas phase has a highly symmetrical D_(∞h) geometry as its stable structure. In solution phases, the electronic structure of I_3~- is strongly affected by the surrounding solvent molecules and the energy profiles are drastically changed. Especially in aqueous solution, the ground-state free-energy surface around the gas-phase equilibrium geometry becomes virtually flat, indicating an increased population of asymmetrical structures due to the solvent effect. It is suggested that this broken symmetry can explain the appearance of transitions in the IR and Raman spectra, which are symmetry-forbidden in the gas phase.