Mg(II) and Ca(II) Microsolvation by Ammonia: Born-Oppenheimer Molecular Dynamics Studies

Area of Science:

• Computational chemistry
• Physical chemistry
• Materials science

Background:

• Understanding metal cation solvation is crucial for chemical processes.
• Ammonia's unique properties influence solvation shells differently than water.
• Previous studies focused on aqueous solvation, necessitating ammonia-specific investigations.

Purpose of the Study:

• To investigate the structural and energetic characteristics of Mg²⁺ and Ca²⁺ in ammonia microsolvation.
• To determine how the number of ammonia molecules affects cation solvation patterns and binding energies.
• To compare the solvation behavior of Mg²⁺ and Ca²⁺ in ammonia.

Main Methods:

• Born-Oppenheimer molecular dynamics simulations at 300 K.
• Hybrid density functional theory (DFT) calculations for electronic structure.
• Analysis of binding energies, cation-ammonia distances, and coordination numbers.
• Calculation of vibrational and EXAFS spectra.

Main Results:

• Mg²⁺ solvation shows increasing Mg-N distances with ammonia number until the first shell is full (6 NH₃), then slight decreases.
• Ca²⁺ forms a first solvation shell with 8 ammonia molecules in larger clusters, differing from the [Ca(NH₃)₆]²⁺ structure.
• Ammonia evaporation at 300 K was observed for Mg²⁺ clusters (n ≥ 10) but not for Ca²⁺ clusters.
• Vibrational and EXAFS spectra were obtained for comparison with experimental data.

Conclusions:

• Mg²⁺ and Ca²⁺ exhibit distinct solvation behaviors in ammonia, influenced by the number of ammonia molecules.
• The study provides a detailed molecular-level understanding of these dications in ammonia, complementing aqueous solvation studies.
• The calculated spectra offer valuable benchmarks for experimental validation in liquid ammonia systems.