Len Herald V. Lim

Revisiting the hydration of Pb(II): a QMCF MD approach.

A quantum mechanical charge field (QMCF) molecular dynamics (MD) study of Pb(II) in an aqueous medium was carried out in order to gain insight into its solvation behavior, for both structural and dynamic aspects. Applying the advanced methodology and different basis sets, some new aspects concerning the solvation of Pb(II) have been revealed. One of the most interesting outcomes of the current simulation is the variation of first shell coordination number from 7 to 9 in the Pb(H2O)n(2+) complex with Pb(H2O)8(2+) as a major species. Moreover, a far more dynamic and labile hydration shell was found compared to previous QM/MM MD simulation with only the first hydration shell treated by quantum mechanics, which reported a very rigid first hydration shell with a fixed coordination number of 9. The current simulation results are in much better agreement with the properties reported from the recent thermodynamic studies than the previous QM/MM MD study.

The Hydration Structure of Sn(II) : An ab initio Quantum Mechanical Charge Field Molecular Dynamics Study

The structural properties of the hydrated Sn(2+) ion have been investigated using ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulations at double-xi HF quantum mechanical level. The results from the work significantly extend previous study using QM/MM MD simulation and are in good agreement with X-Ray and EXAFS diffraction experiments. The data indicate a set of characteristics for the first hydration shell uncommon among metal ions. Although frequent ligand exchange prevents the formation of a well defined structure, more detailed analyses reveal an asymmetric distribution of ligands around Sn(II). An average of eight water molecules coordinate with the Sn(2+) ion and are distributed at proximal and distal positions that are distinguishable from the second hydration shell and manifest dissimilar degrees of lability.

Sulfur dioxide in water: structure and dynamics studied by an ab initio quantum mechanical charge field molecular dynamics simulation.

An ab initio Quantum Mechanical Charge Field Molecular Dynamics Simulation (QMCF MD) was performed to investigate structure and dynamics behavior of hydrated sulfur dioxide (SO(2)) at the Hartree-Fock level of theory employing Dunning DZP basis sets for solute and solvent molecules. The intramolecular structural characteristics of SO(2), such as S═O bond lengths and O═S═O bond angle, are in good agreement with the data available from a number of different experiments. The structural features of the hydrated SO(2) were primarily evaluated in the form of S-O(wat) and O(SO(2))-H(wat) radial distribution functions (RDFs) which gave mean distances of 2.9 and 2.2 Å, respectively. The dynamical behavior characterizes the solute molecule to have structure making properties in aqueous solution or water aerosols, where the hydrated SO(2) can easily get oxidized to form a number of sulfur(VI) species, which are believed to play an important role in the atmospheric processes.

Structural and dynamic aspects of hydration of HAsO4(-2): an ab initio QMCF MD simulation.

An ab initio quantum mechanical charge field simulation has been carried out in order to obtain molecular level insight into the hydration behavior of HAsO4(-2), one of the major biologically active components of As(V) oxoanion in neutral to slightly alkaline aqueous medium. Moreover, a geometrical definition of hydrogen bonding has been used to probe and characterize both solute-solvent and solvent-solvent hydrogen bonding present in the system. The asymmetry of the anion induced by the protonation of one of the oxygens of the arsenate anion causes rather irregular hydration structure. The nonprotonated oxygen atoms preferably form relatively stable hydrogen bonds with two to three water molecules in their vicinity, while the protonated oxygen forms one or two hydrogen bonds, weaker than water-water hydrogen bonds. The two types of As-O distances obtained from the simulation (1.68 and 1.78 A for the protonated and nonprotonated oxygens, respectively) are in good agreement with the experimental data. The two types of As-O stretching frequencies obtained from the simulation (855 and 660 cm(-1) reproduce well the experimental ATR-FTIR results (859 and 680-700 cm(-1)).

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.

Structural and Dynamical Aspects of the Unsymmetric Hydration of Sb(III): An ab initio Quantum Mechanical Charge Field Molecular Dynamics Simulation

An ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation was performed to investigate the behavior of the Sb(3+) ion in aqueous solution. The simulation reveals a significant influence of the residual valence shell electron density on the solvation structure and dynamics of Sb(3+). A strong hemidirectional behavior of the ligand binding pattern is observed for the first hydration shell extending up to the second hydration layer. The apparent domain partitioned structural behavior was probed by solvent reorientational kinetics and three-body distribution functions. The three-dimensional hydration space was conveniently segmented such that domains having different properties were properly resolved. The approach afforded a fair isolation of localized solvent structural and dynamical motifs that Sb(3+) seems to induce to a remarkable degree. Most intriguing is the apparent impact of the lone pair electrons on the second hydration shell, which offers insight into the mechanistic aspects of hydrogen bonding networks in water. Such electronic effects observed in the hydration of Sb(3+) can only be studied by applying a suitable quantum mechanical treatment including first and second hydration shell as provided by the QMCF ansatz.

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.