Solvation structure and dynamics of Na+ in liquid ammonia studied by ONIOM-XS MD simulations

- ONIOM-XS MD simulation of Na+ in liquid ammonia has been performed.
- Na+ can order the surrounding ammonia molecules to form its specific solvation complexes.
- Na+ forms a preferred Na+(NH3)5 complex with a distorted square pyramidal geometry.
- The second solvation shell of Na+ is also recognizable.

The molecular dynamics (MD) technique based on the ONIOM-XS method, known as the ONIOM-XS MD, has been applied to investigate the solvation structure and dynamics of Na+ in liquid ammonia. Regarding the ONIOM-XS MD results, it is observed that Na+ is able to order the surrounding ammonia molecules to form its specific first and second solvation shells with the average coordination numbers of 5.1 and 11.2, respectively. The first solvation shell of Na+ is rather well-defined, forming a preferred 5-fold coordinated complex with a distorted square pyramidal geometry. In this respect, the most preferential Na+(NH3)5 species could convert back and forth to the lower probability Na+(NH3)6 and Na+(NH3)4 configurations. The second solvation shell of Na+ is detectable, in which a number of ammonia molecules, ranging from 7 to 14, are involved in this layer and they are arranged according to recognizable influence of the ion.

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