The stability of [Zn(NH3)4]2+ in water: A quantum mechanical/molecular mechanical molecular dynamics study

Abstract

To investigate the structural and dynamical properties of the tetraamminezinc(II) complex (Zn-tetraamine) in aqueous solution, ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulation was performed for 50 ps at the Hartree–Fock (HF) level of theory. A predominant 4-coordinate solvation structure with a maximum probability of the Zn–N distance at ∼2.1 Å was observed, which seems to be involved in the associative mode of water exchange reactions to produce a short-lived, 5-coordinated trigonal bipyramidal structure. Several sets of structural and dynamical parameters such as radial distribution functions (RDF), coordination number distributions (CND), angular distributions (ADF), ligands’ mean residence times (MRT) and ion-ligand stretching frequencies have been evaluated in order to get an in depth knowledge of the physical and chemical properties of the tetraamminezinc(II) complex in aqueous solution. A comparative study of the tetraamminezinc(II) complex with previously published mono-, di- and triamminezinc(II) complexes has been also performed, which yielded significant insights into the complex properties as a function of an increasing number of first-shell ammonia ligands.