H5O2

H5O2+, commonly known as the Zundel cation, is a protonated water dimer consisting of two water molecules sharing a single proton. The structure is often described as a symmetric O–H–O bridge, where the excess proton is delocalized along the two water moieties, giving a compact, strongly hydrogen-bonded core that contains five hydrogens in total.

In chemical physics and physical chemistry, H5O2+ serves as a fundamental model for proton transfer in hydrogen-bond networks. It represents the Zundel motif, a key intermediate in the rearrangement of protons between water molecules. The cation is particularly important for understanding the Grotthuss mechanism of proton conductivity in aqueous systems, where protons move through networks of hydrogen bonds via rapid shuttling and delocalization.

Detection and study of H5O2+ are primarily achieved in gas-phase clusters, mass spectrometry experiments, and infrared spectroscopy, often complemented by high-level quantum chemical calculations. In bulk water, the cation is transient and exists as one of several hydrated proton structures that interconvert rapidly; its presence helps explain how protons propagate with high mobility despite the strong hydrogen-bond network.

Significance: H5O2+ is a benchmark system for investigating proton localization and delocalization, hydrogen-bond dynamics, and quantum effects in hydrated proton chemistry. It is frequently discussed alongside other hydrated-proton motifs, such as the Eigen cation (H9O4+) in studies of proton hydration and transfer.