Ground-State Structures of Hydrated Calcium Ion Clusters From Comprehensive Genetic Algorithm Search

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Materials Science

Background:

• Hydrated metal ions are crucial in various chemical and biological processes.
• Understanding the solvation structure of calcium ions (Ca²⁺) is key to explaining their behavior in aqueous solutions.

Purpose of the Study:

• To determine the lowest-energy structures of hydrated calcium ion clusters, Ca²⁺(H₂O)_n, for n = 10-18.
• To analyze the coordination environment and interactions within these clusters as a function of cluster size.

Main Methods:

• Utilized a comprehensive genetic algorithm (CGA) to explore the potential energy surface.
• Analyzed cluster structures, coordination numbers, bond distances, interaction energies, and charge transfer using Natural Bond Orbital (NBO) analysis.

Main Results:

• Identified the first-shell coordination number of Ca²⁺, which is typically six but can increase to seven or eight in larger clusters (n=12-18).
• Observed an increase in second-shell water molecules and total hydrogen bonds with increasing cluster size.
• Found that Ca²⁺-water interactions weaken while water-water interactions strengthen as clusters grow.

Conclusions:

• The primary interaction between Ca²⁺ and water is dominated by direct interactions with first-shell molecules.
• Charge transfer from oxygen lone pairs to the Ca²⁺ empty orbital is the main driving force for Ca²⁺-water binding.
• The solvation shell structure dynamically adapts, allowing outer-shell packing even when the inner shell is not fully occupied.