Factors influencing Al(3+)-dimer speciation and stability from density functional theory calculations

Area of Science:

• Computational Chemistry
• Aqueous Solution Chemistry
• Inorganic Chemistry

Background:

• Aluminum (Al(3+)) speciation in aqueous solutions is complex and pH-dependent.
• Understanding the formation and stability of Al-dimer complexes is crucial for predicting solution composition.

Purpose of the Study:

• To investigate the structure, reactivity, and stability of aqueous Al-dimer complexes.
• To elucidate the role of water and hydroxide coordination on Al-dimer properties.
• To evaluate the influence of intramolecular hydrogen bonds and bridging structures on dimer stability.

Main Methods:

• Density Functional Theory (DFT) using B3LYP/6-311++G(d,p).
• Explicit treatment of cation-ligand interactions and continuum solvation (IEF-PCM).
• Analysis of geometric, electronic, and thermodynamic properties (Gibbs free energy of formation).

Main Results:

• Cooperativity mechanism observed: decreased Al-water bond stability with increasing hydroxide coordination.
• Dimer stability depends on ligand number; bridging structure (singly, doubly, triply bridged) does not correlate with stability.
• Intramolecular hydrogen bonds (H3O, H4O2, H2O bridges) significantly influence dimer stability.
• Predicted equilibrium mole fractions align with experimental observations after refining OH/Al ratio cutoff.

Conclusions:

• A wide variety of aqueous Al-dimer species exist, influenced by cooperativity and intramolecular hydrogen bonding.
• Singly and doubly bridged dimers are dominant in the pH range of ~4-7.
• The computational model accurately predicts Al speciation trends with increasing pH, particularly after refinement.