Tuning the intermolecular proton bond in the H₅O2+ 'Zundel ion' scaffold

The Zundel ion, H₂O·H⁺·H₂O, provides a versatile scaffold with which to explore the quantum structure of the intermolecular proton bond (IPB). This information is encoded in the vibrational frequencies adopted by the shared proton, ν_sp, which are observed to follow a remarkably similar trend as the exterior OH groups are sequentially solvated or are replaced by methyl substituents. In effect, solvents H-bonding to exterior OH groups act to increase the proton affinity of the water to which they are bound in a roughly additive fashion. We discuss this behavior in the context of the extreme sensitivity of IPBs to their solvation environments.