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

- An ONIOM-XS MD simulation of Li+ in aqueous ammonia solution been performed.
- Li+ forms a favorable Li+[(H2O)3NH3][(H2O)11(NH3)3] complex.
- The first solvation shell of Li+ is somewhat flexible.
- The “structure-making” ability of Li+ is not too strong.
- The second solvation shell of Li+ is less structured.

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.