Probing the Potential Energy Profile of the I + (H2O)3 → HI + (H2O)2OH Forward and Reverse Reactions: High Level CCSD(T) Studies with Spin-Orbit Coupling Included

Three different pathways for the atomic iodine plus water trimer reaction I + (H2O)(3) -> HI + (H2O)(2)OH were preliminarily examined by the DFT-MPW1K method. Related to previous predictions for the F/Cl/Br + (H2O)(3) reactions, three pathways for the I + (H2O)(3) reaction are linked in terms of geometry and energetics. To legitimize the results, the "gold standard" CCSD(T) method was employed to investigate the lowest-lying pathway with the correlation-consistent polarized valence basis set up to cc-pVQZ(-PP). According to the CCSD(T)/cc-pVQZ(-PP)//CCSD(T)/cc-pVTZ(-PP) results, the I + (H2O)(3) -> HI + (H2O)(2)OH reaction is predicted to be endothermic by 47.0 kcal mol(-1). The submerged transition state is predicted to lie 43.7 kcal mol(-1) above the separated reactants. The I center dot center dot center dot(H2O)(3) entrance complex lies below the separated reactants by 4.1 kcal mol(-1), and spin-orbit coupling has a significant impact on this dissociation energy. The HI center dot center dot center dot(H2O)(2)OH exit complex is bound by 4.3 kcal mol(-1) in relation to the separated products. Compared with simpler I + (H2O)(2) and I + H2O reactions, the I + (H2O)(3) reaction is energetically between them in general. It is speculated that the reaction between the iodine atom and the larger water clusters may be energetically analogous to the I + (H2O)(3) reaction. The iodine reaction I + (H2O)(3) is connected with the analogous valence isoelectronic bromine/chlorine reactions Br/Cl + (H2O)(3) but much different from the F + (H2O)(3) reaction. Significant difference with other halogen systems, especially for barrier heights, are seen for the iodine systems.