Structure and dynamics of the hydration shells of the Zn2+ ion from ab initio molecular dynamics and combined ab initio and classical molecular dynamics simulations

Abstract

Results of ab initio molecular dynamics ͑AIMD͒ simulations ͑density functional theory+ PBE96͒ of the dynamics of waters in the hydration shells surrounding the Zn 2+ ion ͑T Ϸ 300 K, Ϸ 1 gm/ cm 3 ͒ are compared to simulations using a combined quantum and classical molecular dynamics ͓AIMD/molecular mechanical ͑MM͔͒ approach. Both classes of simulations were performed with 64 solvating water molecules ͑ϳ15 ps͒ and used the same methods in the electronic structure calculation ͑plane-wave basis set, time steps, effective mass, etc.͒. In the AIMD/MM calculation, only six waters of hydration were included in the quantum mechanical ͑QM͒ region. The remaining 58 waters were treated with a published flexible water-water interaction potential. No reparametrization of the water-water potential was attempted. Additional AIMD/MM simulations were performed with 256 water molecules. The hydration structures predicted from the AIMD and AIMD/MM simulations are found to agree in detail with each other and with the structural results from x-ray data despite the very limited QM region in the AIMD/MM simulation. To further evaluate the agreement of these parameter-free simulations, predicted extended x-ray absorption fine structure ͑EXAFS͒ spectra were compared directly to the recently obtained EXAFS data and they agree in remarkable detail with the experimental observations. The first hydration shell contains six water molecules in a highly symmetric octahedral structure is ͑maximally located at 2.13-2.15 Å versus 2.072 Å EXAFS experiment͒. The widths of the peak of the simulated EXAFS spectra agree well with the data ͑8.4 Å 2 versus 8.9 Å 2 in experiment͒. Analysis of the H-bond structure of the hydration region shows that the second hydration shell is trigonally bound to the first shell water with a high degree of agreement between the AIMD and AIMD/MM calculations. Beyond the second shell, the bonding pattern returns to the tetrahedral structure of bulk water. The AIMD/MM results emphasize the importance of a quantum description of the first hydration shell to correctly describe the hydration region. In these calculations the full d 10 electronic structure of the valence shell of the Zn 2+ ion is retained. The simulations show substantial and complex charge relocation on both the Zn 2+ ion and the first hydration shell. The dipole moment of the waters in the first hydration shell is 3.4 D ͑3.3 D AIMD/MM͒ versus 2.73 D bulk. Little polarization is found for the waters in the second hydration shell ͑2.8 D͒. No exchanges were seen between the first and the second hydrations shells; however, many water transfers between the second hydration shell and the bulk were observed. For 64 waters, the AIMD and AIMD/MM simulations give nearly identical results for exchange dynamics. However, in the larger particle simulations ͑256 waters͒ there is a significant reduction in the second shell to bulk exchanges.

Zinc complexation in chloride-rich hydrothermal fluids (25–600 °C): A thermodynamic model derived from ab initio molecular dynamics

“…2a); the calculations reveal steady state Zn-O distances of 2.13 Å and a Debye-Waller factor of 0.017 Å 2 . This is similar to the results of the ab initio MD simulation of Cauët et al (2010) which yielded Zn-O distances of 2.13-2.15 Å and r 2 8.4-9.4 Â 10 À3 Å 2 at 300 K. Observation of the dynamic Zn-Cl distances confirms that Cl À did not complex to Zn 2+ during the simulation (Fig. S1a).…”

Cation substitution in synthetic meridianiite (MgSO4·11H2O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates

“…into the 11-hydrate structure is all the more remarkable given the substantial volumetric strain imposed by this large cation . Ab initio calculations on hydrated clusters of various divalent metal cations (Inada et al 2002;Remsungnen and Rode 2003;Armunanto et al 2003;Mohammed et al 2005;Rode et al 2005;Egorov et al 2006;Cauët et al 2010;Moin et al 2010) display an intriguing trend in the coordination number of the second hydration shells: Co 2? (15.9); Mn 2?…”

Ab Initio Molecular Dynamics Study on the Interactions between Carboxylate Ions and Metal Ions in Water

“…In that way, the electrostatic interactions may affect the behaviour of the complex and may, for example, reduce the probability for the magnesium ion to associate with a second carboxylate ion, since it is screened with the most negative charge by the surrounding water molecules. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 In Table 3, the number of exchanges of water molecules [34][35][36][37][38] in the first solvation shell has been reported for separation distances from 2 Å to 6 Å. For the divalent ions, most exchanges are found for the bigger ions, Ba 2+ and Sr 2+ , compared with the small ones (Mg 2+ and Ca 2+ ).…”