Ion Pairing in HCl-Water Clusters: From Electronic Structure Investigations to Multiconfigurational Force-Field...

Area of Science:

• Chemical Physics
• Computational Chemistry
• Physical Chemistry

Background:

• In bulk solution, hydrogen chloride (HCl) dissociates into chloride ions (Cl-) and protons.
• However, in the gas phase and small water clusters, HCl retains its covalent character.

Purpose of the Study:

• To investigate the solvation environments of HCl-(H2O)n clusters (n=2-22).
• To understand the transition from covalent HCl to ionic states (contact and solvent-separated ion pairs).

Main Methods:

• Electronic structure calculations using MP2 level of theory.
• Ab initio metadynamics simulations at the DFT level.
• Development and validation of a multiconfigurational force field (MS-EVB3.2) incorporating covalent states.

Main Results:

• Identified distinct solvation environments across different cluster sizes.
• Observed the persistence of covalent HCl in clusters up to 21 water molecules.
• Developed a force field capable of modeling both covalent and ionic HCl states.

Conclusions:

• The transition from covalent to ionic HCl in water clusters is size-dependent.
• The developed force field accurately models the complex interactions in HCl-water systems.
• Many-body interactions involving Cl-, water, and excess protons were effectively modeled.