Structure and dynamics of the [Zn(NH3)(H2O)5]2+ complex in aqueous solution obtained by an ab initio QM/MM molecular dynamics study

Abstract

A model complex of the hexahydrated zinc(II) cation with one water substituted by ammonia in aqueous solution has been studied by hybrid ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) at the double-ζ Hartree–Fock (HF) quantum mechanical level. The first solvation shell, consisting of 5 + 1 ligand(s) at mean distances of 2.2 and 2.1 Å, respectively, from the Zn(II) ion, was found to remain stable with respect to exchange processes within the simulation time. The labile second shell consists on average of ∼19 water molecules. For structural elucidation of the pentaaquozinc(II) amine complex in aqueous solution several data sets such as radial distribution functions (RDF), coordination number distributions (CND) and different angular distributions (ADF, tilt and θ angle) were employed. Dynamics were characterised by the ligands’ mean residence time (MRT), ion–ligand stretching frequencies and the vibrational and librational motions of water ligands. The labile second shell’s MRT value decreases upon introduction of one NH₃ ligand to 7.2 ps from the 10.5 ps observed for the hexaaquozinc(II) complex.