Probing the Potential Energy Profile of the I + (H 2 O) 3 → HI + (H 2 O) 2 OH 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 + (HO) → HI + (HO)OH were preliminarily examined by the DFT-MPW1K method. Related to previous predictions for the F/Cl/Br + (HO) reactions, three pathways for the I + (HO) 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 + (HO) → HI + (HO)OH reaction is predicted to be endothermic by 47.0 kcal mol. The submerged transition state is predicted to lie 43.7 kcal mol above the separated reactants. The I···(HO) entrance complex lies below the separated reactants by 4.1 kcal mol, and spin-orbit coupling has a significant impact on this dissociation energy. The HI···(HO)OH exit complex is bound by 4.3 kcal mol in relation to the separated products. Compared with simpler I + (HO) and I + HO reactions, the I + (HO) 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 + (HO) reaction. The iodine reaction I + (HO) is connected with the analogous valence isoelectronic bromine/chlorine reactions Br/Cl + (HO) but much different from the F + (HO) reaction. Significant difference with other halogen systems, especially for barrier heights, are seen for the iodine systems.