Exploring unvisited regions to investigate solution properties: The backyard of H3O+ and its aggregates

Interactions between bare and tri-solvated (Eigen) hydronium with water and methanol is investigated with ab initio methods. Unexplored configurations on the oxygen side of H3O+ are covered to improve our understanding of the solvation energetics. Results indicate that the interaction between the charged and polar species, with oxygen atoms oriented toward H3O+ oxygen (‘back to back’ or ‘O-side’ to ‘O-side’ interaction), is similar to or stronger than a hydrogen bond. The repulsion along the O–O directrix onset in the range 2.2–2.7 Å, indicating H2O short-range O–O contacts with (H3O+) as possible despite water coordination.