Preferential solvation and dynamics of Li⁺ in aqueous ammonia solution: An ONIOM-XS MD simulation study

A more sophisticated quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) technique based on the ONIOM-XS method, called the ONIOM-XS MD, has been applied for studying the characteristics of Li⁺ in an aqueous ammonia solution. As compared to the conventional QM/MM MD study, which predicts a clear water preference with the arrangement of the Li⁺[(H₂O)₄][(H₂O)₄] type, the ONIOM-XS MD simulation clearly reveals that this ion can order both water and ammonia molecules to form the preferred Li⁺[(H₂O)₃NH₃][(H₂O)₁₁(NH₃)₃] complex. Of particular interest, it is observed that the "structure-making" ability of Li⁺ is not too strong and that the first solvation shell of Li⁺ is somewhat flexible, in which other different 4-fold coordinated species, such as Li⁺(H₂O)₄ and Li⁺(H₂O)₂(NH₃)₂, can frequently be formed. In addition, it is found that the second solvation shell of Li⁺ is less structured, implying a small influence of Li⁺ in ordering the solvent molecules in this shell.