Abstract
Abstract The solvation structure and dynamics of Cu2+ in 18.6 % aqueous ammonia have been investigated using an ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation approach at the Hartree–Fock (HF) level of theory applying the LANL2DZ ECP and Dunning DZP basis sets for Cu2+, ammonia and water, respectively. During a simulation time of 20 ps, only NH3 molecules are observed within the first solvation shell of Cu2+, resulting in the formation of an octahedral [Cu(NH3)6]2+ complex. While no exchange of these ligands with the second solvation shell are observed along the simulation, the monitoring of the associated N-Ntrans distances highlight the dynamics of the associated Jahn-Teller distortions, showing on average 2 elongated axial (2.19 Å) and 4 equatorial Cu–N bonds (2.39 Å). The observed structural properties are found in excellent agreement with experimental studies. In addition, an NBO analysis was carried out, confirming the strong electrostatic character of the Cu2+–NH3 interaction.
Highlights
Detailed studies focused on the solvation of transition metal ions, in particular Cu2+, are essential to obtain a detailed understanding of the broad range of physico-chemical properties being highly relevant for environmental and biochemistry as well as material sciences [1,2,3,4,5]
Decomposition of the total radial distribution functions (RDFs) into the contributions arising from the Cu–N and Cu–O interaction shows that the first shell is exclusively occupied by ammonia molecules
The distance of the Cu2+–NH3 interaction in aqueous ammonia is shorter than that observed in the aqueous case [6, 15], which was shown to be longer than in pure liquid ammonia solutions [7, 12]. These findings highlight the impact of the presence of water on the interaction between Cu2+ and NH3 ligands in aqueous ammonia, which is in line with the hard/soft Lewis acids and bases (HSAB) classification [70] of water being a hard ligand compared to NH3, which is typically attributed as being soft [71, 72]
Summary
Detailed studies focused on the solvation of transition metal ions, in particular Cu2+, are essential to obtain a detailed understanding of the broad range of physico-chemical properties being highly relevant for environmental and biochemistry as well as material sciences [1,2,3,4,5]. The QMCF MD technique has proven to be an accomplished method for studying ion solvation [29,30,31,32,33,34,35,36,37,38,39,40] and has been successfully applied to investigate the structural and dynamical properties of various systems in aqueous ammonia solution [41, 42]. A combination of the QMCF-MD simulation approach and an NBO analysis was applied to study the structural and dynamical properties of Cu2+ in 18.6 % aqueous ammonia solution, with special focus on the characterisation of the underlying Jahn-Teller effect. The results of an NBO evaluation are reported, providing detailed insight of the underlying ion-ligand interaction and the respective donor-acceptor stabilization energies
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