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.

No abstract

The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate the bicarbonate ion, HCO(3)(-), 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 theta-tilt surface plot. The mean residence time in the surroundings of the HCO(3)(-) 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.

A quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation including the first and second hydration shells in the QM region has been carried out to describe the structural and dynamical properties of Be(2+) in aqueous solution. In this methodology, the full first and second hydration shells are treated by ab initio quantum mechanics supplemented by a fluctuating electrostatic embedding technique. From the simulation, structural properties were extracted and were found to be in good agreement with previously published experimental and theoretical results. The radial distribution function (RDF) showed the maximum probability of the Be-O bond length at 1.62 A. The first tetrahedrally arranged hydration shell is highly inert with respect to ligand-exchange processes. Application of local-density-corrected three-body correlation analysis showed minor structural influence of the ion beyond the second hydration layer, contrary to the findings of a previous QM/MM MD simulation. The dynamics of the hydrate were studied in terms of ligand mean residence times (MRTs) and the power spectrum of the Be(2+)-O stretching frequency. A comparison of the "classical" QM/MM framework with the QMCF method clearly demonstrated the advantages of the latter, as ambiguities arising from the coupling of the subregions occurring in QM/MM MD simulations did not appear when the QMCF ansatz was applied.

Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism has been applied in conjunction to experimental large angle X-ray scattering to study the structure and dynamics of the hydrated sulfite ion in aqueous solution. The results show that there is a considerable effect of the lone electron-pair on sulfur concerning structure and dynamics in comparison with the sulfate ion with higher oxidation number and symmetry of the hydration shell. The S-O bond distance in the hydrated sulfite ion has been determined to 1.53(1) Å by both methods. The hydrogen bonds between the three water molecules bound to each sulfite oxygen are only slightly stronger than those in bulk water. The sulfite ion can therefore be regarded as a weak structure maker. The water exchange rate is somewhat slower for the sulfite ion than for the sulfate ion, τ(0.5) = 3.2 and 2.6 ps, respectively. An even more striking observation in the angular radial distribution (ARD) functions is that the for sulfite ion the water exchange takes place in close vicinity of the lone electron-pair directed at its sides, while in principle no water exchange did take place of the water molecules hydrogen bound to sulfite oxygens during the simulation time. This is also confirmed when detailed pathway analysis is conducted. The simulation showed that the water molecules hydrogen bound to the sulfite oxygens can move inside the hydration shell to the area outside the lone electron-pair and there be exchanged. On the other hand, for the hydrated sulfate ion in aqueous solution one can clearly see from the ARD that the distribution of exchange events is symmetrical around the entire hydration sphere.