Structure of concentrated HCl solutions

A pentagonal ring is suggested as the basic structural unit of HCl(H2O)(6) and (HCl)(2)(H2O)(6) in solution. Modeled after the X-ray structure of a caged H13O6+ compound, it contains bridged H5O2+ and Cl- ions, each with a coordination number around four. In the most concentrated HCl solutions, one of the ligands solvating Cl- is HCl, giving rise to a bichloride moiety. The proposed structure explains qualitatively the stoichiometry, ion mobility, (IR, Raman) spectroscopy and (X-ray, neutron) diffraction data of concentrated HCl solutions and provides a semiquantitative fit to the X-ray radial distribution function. The ring structure is a likely candidate for the structure of protonated HCl clusters in the gas phase and for the contact ion-pair formed following HCl dissociation in solution.