Tuning the intermolecular proton bond in the H5 ‘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, νsp, 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.