No abstract

The hydrated sulfate ion has been characterized in aqueous solution in structural and dynamic aspects using ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation and large angle X-ray scattering (LAXS) methods. The LAXS data show an average coordination number of the sulfate ion of up to 12 water molecules bound through hydrogen bonding, while the QMCF MD simulation displays a wide range of coordination numbers between 8 and 14 with an average value of approximately 11. The Os...Ow distance cannot be distinguished from the Ow...Ow distance in the LAXS experiment; the weighted mean O...O distance is 2.880(10) A. In the simulation, the Os...Ow and Ow...Ow distances are found to be very similar, namely, 2.86 and 2.84 A, respectively. The S-Os bond and S...Ow distance have been determined by the LAXS experiment as 1.495(6) and 3.61(2) A, respectively, indicating an average nearly tetrahedral S-Os...Ow angle. The approximately 5% deviations of simulation distances (1.47 and 3.82 A) from the experimental ones can probably be ascribed to the neglect of correlation energy in the quantum mechanical method. The mean residence time of water ligands at O atoms, 2.57 ps, is longer than that in pure water, 1.7 ps, characterizing the sulfate ion as a weak structure maker.

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.

Proton transfer reactions and dynamics in hydrated complexes formed from CH(3)OH, H(3)O(+) and H(2)O were studied using theoretical methods. The investigations began with searching for equilibrium structures at low hydration levels using the DFT method, from which active H-bonds in the gas phase and continuum aqueous solution were characterized and analyzed. Based on the asymmetric stretching coordinates (Deltad(DA)), four H-bond complexes were identified as potential transition states, in which the most active unit is represented by an excess proton nearly equally shared between CH(3)OH and H(2)O. These cannot be definitive due to the lack of asymmetric O-H stretching frequencies (nu(OH)) which are spectral signatures of transferring protons. Born-Oppenheimer molecular dynamics (BOMD) simulations revealed that, when the thermal energy fluctuations and dynamics were included in the model calculations, the spectral signatures at nu(OH) approximately 1000 cm(-1) appeared. In continuum aqueous solution, the H-bond complex with incomplete water coordination at charged species turned out to be the only active transition state. Based on the assumption that the thermal energy fluctuations and dynamics could temporarily break the H-bonds linking the transition state complex and water molecules in the second hydration shell, elementary reactions of proton transfer were proposed. The present study showed that, due to the coupling among various vibrational modes, the discussions on proton transfer reactions cannot be made based solely on static proton transfer potentials. Inclusion of thermal energy fluctuations and dynamics in the model calculations, as in the case of BOMD simulations, together with systematic IR spectral analyses, have been proved to be the most appropriate theoretical approaches.

The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate the bisulfate ion, HSO4-, in aqueous solution. The averaged geometry of bisulfate ion supports the separation of six normal modes of the O*-SO3 unit with C3v symmetry from three modes of the OH group in the evaluation of vibrational spectra obtained from the velocity autocorrelation functions (VACFs) with subsequent normal coordinate analyses. The calculated frequencies are in good agreement with the observations in Raman and IR experiments. The difference of the averaged coordination number obtained for the whole molecule (8.0) and the summation over coordinating sites (10.9) indicates some water molecules to be located in the overlapping volumes of individual hydration spheres. The averaged number of hydrogen bonds (H-bonds) during the simulation period (5.8) indicates that some water molecules are situated in the molecular hydration shell with an unsuitable orientation to form a hydrogen bond with the ion. The mean residence time in the surroundings of the bisulfate ion classify it generally as a weak structure-making ion, but the analysis of the individual sites reveals a more complex behavior of them, in particular a strong interaction with a water molecule at the hydrogen site.