protonsharing

Proton sharing is a chemical concept describing a proton that is distributed between two atoms, typically oxygen or nitrogen, within a hydrogen-bonded or proton-transfer system. In such environments, the proton can be localized closer to one atom or delocalized across the two, forming a bridge-like O–H–O arrangement when the hydrogen bond is particularly strong. When the proton is effectively equally shared, the bond is described as a symmetric, or strongly delocalized, proton bond. Quantum effects, including zero-point energy and tunneling, play a significant role in stabilizing shared protons and enabling rapid transfer.

In water and aqueous systems, two archetypal motifs illustrate proton sharing: the Zundel cation, H5O2+, in which a central proton is nearly equally shared between two adjacent water oxygens, and the Eigen cation, H9O4+, where a hydronium core remains hydrogen-bonded to surrounding water molecules with substantial proton delocalization. In other hydrogen-bonded networks, such as carboxylic acid dimers or protonated amines, similar sharing behavior can be observed, especially under strong hydrogen-bonding conditions or high pressure.

Proton sharing is central to proton transport mechanisms, notably the Grotthuss mechanism, in which rapid succession of proton-sharing steps moves charge through a network. It is also relevant to biological systems, including proton channels and ATP synthase, and to technologies like fuel cells that rely on efficient proton conduction. Experimental probes include infrared and Raman spectroscopy, neutron scattering, and advanced computational methods (ab initio molecular dynamics and quantum chemical calculations) that map the potential energy landscape and reveal the degree of proton delocalization. The term thus describes both a structural feature of certain hydrogen bonds and a dynamic process underlying proton mobility in chemical and biological systems.