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Structure of Concentrated HCl Solutions Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew UniVersity,
 

Summary: Structure of Concentrated HCl Solutions
Noam Agmon
Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew UniVersity,
Jerusalem 91904, Israel
ReceiVed: March 5, 1997; In Final Form: October 14, 1997X
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.
I. Introduction
Hydrochloric acid dissolves in liquid water up to a 1:3 mole
ratio.1 Similar stoichiometric restrictions do not hold in HCl
crystals.2 The crystal structure of the monohydrate,3 dihydrate,4
trihydrate,5 and hexahydrate6 have all been characterized by

  

Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem

 

Collections: Chemistry