The H3O+ Cation: Molecular Structure of an Oxonium-Macrocycle Polyether Complex

Article abstract of DOI:10.1021/ja00381a007

The oxonium ion has been isolated as a discrete entity by inclusion into a macrocyclic cavity.The structure of the complex formed by a tetracarboxylic 18-crown-6 ligand with H3O⁺ has been determined by X-ray crystallography (P2₁2₁2₁; a = 10.526 (2); b = 14.325 (2), and c= 15.324 (2) Angstroem).The structure was solved by direct methods and refined to R = 0.048 for 1957 (I <*> 2?(I)) reflections.Hydrogen atoms were located on a Fourier difference map.The overall shape of the ligand is similar to that found in related complexes, as two carboxylic acid groups extend on both sides of the pseudoplanar macrocycle.The H3O⁺ cation is anchored in the center of the cavity by three OH⁺...O hydrogen bonds (2.67, 2.73, and 2.74 Angstroem) whereas the chloride counterion is H bonded to the three carboxylic acid groups 5.5 Angstroem away from the cation.The pyramidal geometry found for the oxonium cation in the present molecular complex shows this conformation to be the most stable in an ion-solvating environment.