The Structure of the Hydrogen Ion (Haq+) in Water

Abstract

The hydrogen ion in water, H_aq⁺, is a unique H₁₃O₆⁺ entity that defines the boundary of positive-charge delocalization. Its central unit is neither a C_3v H₃O⁺ Eigen-type ion nor a typical H₅O₂⁺ Zundel-type ion. IR spectroscopy indicates that the H₁₃O₆⁺ ion has an unexpectedly long central O···O separation (≫2.43 Å), showing that in comparison with the gas and solid phases, the environment of liquid water is uniquely proficient in delocalizing positive charge. These results will change the description of H_aq⁺ in textbooks of chemistry, and a more extensive delocalization of positive charge may need to be incorporated into descriptions of mechanisms of aqueous proton transport.