Based on a systematic investigation of trajectories of ab initio quantum mechanical/molecular mechanical simulations of numerous cations in water a standardized procedure for the evaluation of mean ligand residence times is proposed. For the characterization of reactivity and structure-breaking/structure-forming properties of the ions a measure is derived from the mean residence times calculated with different time limits. It is shown that ab initio simulations can provide much insight into ultrafast dynamics that are presently not easily accessible by experiment.

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Articles you may be interested inDynamics of ligand exchange mechanism at Cu(II) in water: An ab initio quantum mechanical charge field molecular dynamics study with extended quantum mechanical region Structural and dynamical properties of Co(III) in aqueous solution: Ab initio quantum mechanical/molecular mechanical molecular dynamics simulation Copper͑II͒ was used as a model system to investigate the relevance of including the full second hydration shell in ab initio treatment while describing hydrated ions in hybrid quantum mechanical/ molecular mechanical molecular dynamics ͑QM/MM MD͒ simulations. Three different simulation techniques were applied ͑Hartree-Fock, B3LYP, and resolution of the identity density functional theory͒ to find a good compromise between accuracy and simulation speed. To discuss details of the hydration structure radial distribution functions, coordination number distributions and various angular distributions have been used. Dynamical properties such as vibrational motions of water molecules and ion-oxygen stretching motions were investigated using approximative normal coordinate analyses. QM/MM MD simulations offer a detailed time picture of the dynamic Jahn-Teller effect of Cu 2ϩ showing short-term as well as long-term distortions to occur within Ͻ200 fs and 2-3 ps. The results prove that for transition metal ions such as Cu 2ϩ the inclusion of the second shell into the ab initio treated region can be of decisive importance for obtaining accurate results and that such simulations can offer new insights into chemical dynamics on the picosecond scale.

A classical molecular dynamics simulation including three-body corrections was compared with combined ab initio quantum mechanics/molecular mechanics molecular dynamics simulations (QM/MM–MD), which were carried out at Hartree–Fock (HF) and density functional theory (DFT) level for Ca2+ in water. In the QM approach the region of primary interest—the first hydration sphere of the calcium ion—was treated by Born–Oppenheimer quantum mechanics, while the rest of the system was described by classical pair potentials. Coordination numbers of 7.1, 7.6, and 8.1 were found in the classical, the HF, and the DFT simulation, respectively, using the same double-ζ basis set in both QM methods. The CPU time for one DFT step was about 50% above the time for a HF step, but due to a smaller number of steps needed for equilibration in the DFT case, there was no significant difference in the overall simulation time.

Structural and dynamical properties of the hydrated Cs + ion have been investigated by performing ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations at different quantum mechanical levels (HF, B3LYP and BP86). The first shell coordination number was found to be ∼8 in the HF and ∼9 in the B3LYP and BP86 case and several other structural parameters such as angular distribution functions, radial distribution functions, and tilt-and θ-angle distributions allowed to fully characterize the hydration structure of the Cs + ion. Velocity autocorrelation functions were used to calculate librational and vibrational motions, ion-ligand motions, as well as reorientation times. The strong "structure breaking" effect of Cs + can be interpreted on the basis of different dynamical parameters such as accelerated water reorientation, mean ligand residence time, and the number of ligand exchange processes.

The CuII hydration shell structure has been studied by means of classical molecular dynamics (MD) simulations including three-body corrections and hybrid quantum-mechanical/molecular-mechanical (QM/MM) molecular dynamics (MD) simulations at the Hartree-Fock level. The copper(II) ion is found to be six-fold coordinated and [Cu(H2O)6]2+ exhibits a distorted octahedral structure. The QM/MM MD approach reproduces correctly the experimentally observed Jahn-Teller effect but exhibits faster inversions (< 200 fs) and a more complex behaviour than expected from experimental data. The dynamic Jahn-Teller effect causes the high lability of [Cu(H2O)6]2+ with a ligand-exchange rate constant some orders or magnitude higher than its neighbouring ions NiII and ZnII. Nevertheless, no first-shell water exchange occurred during a 30-ps simulation. The structure of the hydrated ion is discussed in terms of radial distribution functions, coordination numbers, and various angular distributions and the dynamical properties as librational and vibrational motions and reorientational times were evaluated, which lead to detailed information about the first hydration shell. Second-shell water-exchange processes could be observed within the simulation time scale and yielded a mean ligand residence time of approximelty 20 ps.