Temperature dependence of structure and dynamics of the hydrated Ca2+ ion according to ab initio quantum mechanical charge field and classical molecular dynamics

Simulations using ab initio quantum mechanical charge field molecular dynamics (QMCF MD) and classical molecular dynamics using two‐body and three‐body potentials were performed to investigate the hydration of the Ca²⁺ ion at different temperatures. Results from the simulations demonstrate significant effects of temperature on solution dynamics and the corresponding composition and structure of hydrated Ca²⁺. Substantial increase in ligand exchange events was observed in going from 273.15 K to 368.15 K, resulting in a redistribution of coordination numbers to lower values. The effect of temperature is also visible in a red‐shift of the ion‐oxygen stretching frequencies, reflecting weakened ligand binding. Even the moderate increase from ambient to body temperature leads to significant changes in the properties of Ca²⁺ in aqueous environment. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Characterization of the structure and dynamics of an aqueous Hg solution by an ab initio molecular dynamics study

Quantum mechanical charge field (QMCF) MD simulation has been performed to investigate the structure and dynamics of Hg²⁺ hydrate. The first-shell hexacoordinated [Hg(H₂O)₆]²⁺ complex with an average Hg²⁺-O distance of 2.40 Å is dominantly found, which corresponds to the neutron diffraction and extended X-ray absorption fine structure (EXAFS) experiments. Other species, in particular the 7-fold coordinated complexes, can be formed transiently, according to the water exchange processes with an associative interchange (I_a) mechanism. The second hydration shell exhibits a Hg²⁺-O distance of 4.6 Å with a coordination number of ~ 14. The mean residence times (MRTs) of first- and second-shell waters clearly indicate a strong “structure-forming” ability of Hg²⁺ in aqueous solution.     

An ab initio quantum mechanical charge field molecular dynamics simulation of a dilute aqueous HCl solution

An ab initio quantum mechanical charge field (QMCF) molecular dynamics simulation has been performed to study the structural and dynamical properties of a dilute aqueous HCl solution. The solute molecule HCl and its surrounding water molecules were treated at Hartree‐Fock level in conjunction with Dunning double‐ζ plus polarization function basis sets. The simulation predicts an average H? Cl bond distance of 1.28 Å, which is in good agreement with the experimental value. The H_HCl···O_w and Cl_HCl···H_w distances of 1.84 and 3.51 Å were found for the first hydration shell. At the hydrogen site of HCl, a single water molecule is the most preferred coordination, whereas an average coordination number of 12 water molecules of the full first shell was observed for the chloride site. The hydrogen bonding at the hydrogen site of HCl is weakened by proton transfer reactions and an associated lability of ligand binding. Two proton transfer processes were observed in the QMCF MD simulation, demonstrating acid dissociation of HCl. A weak structure‐making/breaking effect of HCl in water is recognized from the mean residence times of 2.1 and 0.8 ps for ligands in the neighborhood of Cl and H sites of HCl, respectively. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Structure and dynamics of methanol in water: a quantum mechanical charge field molecular dynamics study

An ab initio quantum mechanical charge field molecular dynamics simulation was carried out for one methanol molecule in water to analyze the structure and dynamics of hydrophobic and hydrophilic groups. It is found that water molecules around the methyl group form a cage-like structure whereas the hydroxyl group acts as both hydrogen bond donor and acceptor, thus forming several hydrogen bonds with water molecules. The dynamic analyses correlate well with the structural data, evaluated by means of radial distribution functions, angular distribution functions, and coordination number distributions. The overall ligand mean residence time, τ identifies the methanol molecule as structure maker. The relative dynamics data of hydrogen bonds between hydroxyl of methanol and water molecules prove the existence of both strong and weak hydrogen bonds. The results obtained from the simulation are in excellent agreement with the experimental results for dilute solution of CH(3)OH in water. The overall hydration shell of methanol consists in average of 18 water molecules out of which three are hydrogen bonded.     

Hydrated germanium (II): Irregular structural and dynamical properties revealed by a quantum mechanical charge field molecular dynamics study

Structural and dynamical properties of Ge (II) in aqueous solution have been investigated using the novel ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) formalism. The first and second hydration shells were treated by ab initio quantum mechanics at restricted Hartree–Fock (RHF) level using the cc‐pVDZ‐PP basis set for Ge (II) and Dunning double‐ζ plus polarization basis sets for O and H. Besides ligand exchange processes and mean ligand residence times to observe dynamics, tilt‐ and theta‐angle distributions along with an advanced structural parameter, namely radial and angular distribution functions (RAD) for different regions were also evaluated. The combined radial and angular distribution depicted through surface plot and contour map is presented to provide a detailed insight into the density distribution of water molecules around the Ge²⁺ ion. A strongly distorted hydration structure with two trigonal pyramidal substructures within the first hydration shell is observed, which demonstrates the lone‐pair influence and provides a new basis for the interpretation of the catalytic and pharmacological properties of germanium coordination compounds. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Cu2+ in liquid ammonia—The impact of solvent flexibility and electron correlation in ab initio quantum mechanical charge field molecular dynamics

The impact of solvent flexibility and electron correlation on the simulation results of Cu²⁺ in liquid ammonia has been investigated via an ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation approach. To achieve this, three different simulation systems were considered in this study, namely Cu²⁺ in rigid and flexible ammonia at Hartree-Fock (HF) level of theory, as well as resolution of identity second order Møller-Plesset (MP2) perturbation theory in the rigid body case. In all cases, a stable octahedral [Cu(NH₃)₆]²⁺ complex subject to dynamic Jahn-Teller distortions without the occurrence of ligand exchange was observed. The Cu²⁺ − NH₃ distance in the first shell agrees well with the experimental and other theoretical data. In all three cases, the structural data shows that the rigid-body ammonia model in conjunction with the HF level of theory provides accurate data for the first solvation shell, while at the same time, the computational demand and thus the achievable simulation time are much more beneficial. The vibrational analysis of the Cu²⁺ − NH₃ interaction yields similar force constants in the three investigated systems indicating that there is no distinct difference on the dynamical properties of the first solvation shell. In addition to the QMCF MD simulations, a number of natural bond orbital (NBO) analyses were carried out, confirming the strong electrostatic character of the Cu²⁺ − NH₃ interaction.     

Quantum mechanical charge field molecular dynamics simulation of the TiO2+ ion in aqueous solution

Structural and dynamical properties of the TiO(2+) ion in aqueous solution have been investigated by using the new ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) formalism, which does not require any other potential functions except those for solvent-solvent interactions. Both first and second hydration shell have been treated at Hartree-Fock (HF) quantum mechanical level. A Ti-O bond distance of 1.5 A was observed for the [Ti=O](2+) ion. The first hydration shell of the ion shows a varying coordination number ranging from 5 to 7, five being the dominant one and representing one axial and four equatorial water molecules directly coordinated to Ti, which are located at 2.3 A and 2.1 A, respectively. The flexibility in the coordination number reflects the fast exchange processes, which occur only at the oxo atom, where water ligands are weakly bound through hydrogen bonds. Considering the first shell hydration, the composition of the TiO(2+) hydrate can be characterized as [(H(2)O)(0.7)(H(2)O)(4) (eq)(H(2)O)(ax)](2+). The second shell consists in average of 12 water molecules located at a mean distance of 4.4 A. Several other structural parameters such as radial and angular distribution functions and coordination number distributions were analyzed to fully characterize the hydration structure of the TiO(2+) ion in aqueous solution. For the dynamics of the TiO(2+) ion, different sets of dynamical parameters such as Ti=O, Ti-O(eq), and Ti-O(ax) stretching frequencies and ligands' mean residence times were evaluated. During the simulation time of 15 ps, 3 water exchange processes in the first shell were observed at the oxo atom, corresponding to a mean residence time of 3.6 ps. The ligands' mean residence time for the second shell was determined as 3.5 ps.     

Structure and dynamics of phosphate ion in aqueous solution: an ab initio QMCF MD study

A simulation of phosphate in aqueous solution was carried out employing the new QMCF MD approach which offers the possibility to investigate composite systems with the accuracy of a QMMM method but without the time consuming creation of solute-solvent potential functions. The data of the simulations give a clear picture of the hydration shells of the phosphate anion. The first shell consists of 13 water molecules and each oxygen of the phosphate forms in average three hydrogens bonds to different solvent molecules. Several structural parameters such as radial distribution functions and coordination number distributions allow to fully characterize the embedding of the highly charged phosphate ion in the solvent water. The dynamics of the hydration structure of phosphate are described by mean residence times of the solvent molecules in the first hydration shell and the water exchange rate.     

Ab Initio quantum mechanical charge field study of hydrated bicarbonate ion: Structural and dynamical properties

The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate the bicarbonate ion, HCO₃^?, in aqueous solution. The difference in coordination numbers obtained by summation over atoms (6.6) and for the solvent‐accessible surface (5.4) indicates the sharing of some water molecules between the individual atomic hydration shells. It also proved the importance to consider the hydration of the chemically different atoms individually for the evaluation of structural and dynamical properties of the ion. The orientation of water molecules in the hydration shell was visualized by the θ–tilt surface plot. The mean residence time in the surroundings of the HCO₃^? ion classify it generally as a structure‐breaking ion, but the analysis of the individual ion‐water hydrogen bonds revealed a more complex behavior of the different coordination sites. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

A hybrid forces Quantum Mechanical Charge Field Molecular Dynamics of Cs+ ion in aqueous ammonia: A solvation lability, “structure breaking” phenomenon and hydrogen bond properties

The dynamics and structure of Cs⁺ solvation in an aqueous ammonia solution have been investigated using the Quantum Mechanical Charge Field Molecular Dynamics (QMCF-MD) simulation method. The system contained 18.6% ammonia in aqueous solution at 298.15 K. The QM region was set to a radius of 7.0 Å to include the first and second solvation shells. The Hartree-Fock (HF) level was applied to calculate the ion-ligand and ligand–ligand interactions in the QM region using the LANL2DZ-ECP basis set for the ion and DZP-Dunning for the ligands. The radial distribution revealed only one solvation shell, indicating a labile solvation structure. The various coordination number of ligands suggests an instant exchange between them. The mean residence times of 1.29 and 0.9 ps were less than in the pure water and liquid ammonia system, indicating higher mobility ligands of the aqueous ammonia system. The preferential factor 0.55 supports the hypothesis that the Cs⁺ ion prefers to be dissolved in water. As the ligands moves further away from the ion, the observed number of hydrogen bonds increases, revealing the “structure-breaking” property of the Cs⁺ ion. The minimum angle ligand-ion-ligand tends to be higher near the ion, confirming this phenomenon.