CONCLUSION

1. 경성대 Kyung sung Univ. Song Hi Lee, Department of Chemistry, Kyungsung University, Busan, Korea 608-736, shlee@ks.ac.kr Jayendran C. Rasaiah, Department of Chemistry, University of Maine, Orono, ME 04469, Jay_Rasaiah@umit.maine.edu ABSTRACT Table 1. Relaxation times (τ ) and activation energies Ea. * The two Ea for the OH- ion are determined from the 3 lower (30~50oC) and 3 higher (0~20oC) relaxation times . Hydrogen (H+) and hydroxide (OH-) ions in aqueous solution have anomalously large diffusion coefficients, and the mobility of the H+ ion is nearly twice that of the OH- ion. We describe molecular dynamics (MD) simulations of a dissociating model for liquid water based on scaling the interatomic potential for water developed by Ojamäe-Shavitt-Singer from ab-initio studies at the MP2 level. We use the scaled model to study proton transfer that occurs in the transport of hydrogen and hydroxide ions in acidic and basic solutions containing 215 water molecules. The model supports the Eigen-Zundel-Eigen mechanism of proton transfer in acidic solutions and the transient hyper-coordination of the hydroxide ion in weakly basic solutions at room temperature. The free energy barriers for proton transport are low indicating significant proton delocalization accompanying proton transfer in acidic and basic solutions. The reorientation dynamics of the hydroxide ion suggests changes in the proportions of hyper-coordinated species with temperature. The mobilities of the hydrogen and hydroxide ions and their temperature dependence between 0 and 50o C are in excellent agreement with experiment and the reasons for the large difference in the mobilities of the two ions are discussed. The model and methods described provide a novel approach to studies of liquid water, proton transfer and acid-base reactions in aqueous solutions, channels and interfaces. O* of OH- H* of OH- H* of H+ Fig.5 (a) Free energy profile. (b) Jump distances. RESULTS Proton Transfer using sOSS2 Water Model ●H3O+ + Cl- + 215 water molecules Proton transfer and the mobilities of the H+ and OH- ions from studies of a dissociating model for water MODEL • OSS2 model by Ojamäe, Shavitt & Singer – JCP 109, 5547 (1998) • ●A dissociable water model – • Electrostatic potentials with self-consistency of polarizability on O, • Interaction between H+ and O2- and 3-body (H-O-H), • Obtained from ab initio MP2 calculation, Modeling for H+-transfer • reaction, Ewald sum for the long-ranged interaction. • Scaling the OSS2 model (sOSS2 model) • ● The original OSS2 model behaves like a glass under ambient condition • as shown MSD in Fig. 1 (a) (solid line). • ● Increasing temperature: MSD at T’ = 540 K (Long-dashed line) gives • the experimental D at T = 298.15 K. • This indicates the total potential Vtot is too strong for water molecules, • reducing Vtot of the system causes the same effect as increasing T. • ● Rescaling the OSS2 potential VOSS2 at constant temperature by λ : • VsOSS2 = λVOSS2 is equal to multiplying β (=1/kT) by the same factor • at constant potential. Since βVsOSS2 = β’VOSS2 where β = 1/kT and • β’= 1/kT’, β’= βλand λ = β’/β = T/T’ = 298.15/540 = 0.552. • ● Using λ = 0.552, D = 2.00 x 10-5 cm2/s at 298.15 K and 0.9970 g/cm3 • and by choosing λ=0.530 the experimental D was obtained. Fig.6 (a) Probability distribution of consecutive PT (b) as a function of T. Reorientation Dynamics of the OH- Ion See Fig.2 and Table 1. Mobilities of H+ and OH- Ions in Water Fig.4 (a) Configuration showing mechanism of single proton transfer Fig.7 Change in index numbers of (a) H+ ion and (b) OH- ion. Fig.4 (b) gO*H, gO*O, nO*H, and nO*O δ = │RO*-H* – RO^-H*│ ●OH- + Na+ + 215 water molecules (a) Fig.1 (a) MSD of OSS2 water Fig.1 (b) D of sOSS2 water Fig.8 (a) MSD of H* of H+, O* of OH- and water (b) D of H+ and OH-. CONCLUSION Classical MD simulations using a scaled dissociating model potential (sOSS2) for liquid water provide unambiguous and clear pictures of the hydrated OH- and H+ ions and a computationally fast, accurate and robust method of studying PT reactions in aqueous solutions. The model supports the Eigen-Zundel-Eigen (EZE) mechanism of proton transfer in acidic solutions and the transient hyper-coordination of the hydroxide ion in weakly basic solutions at room temperature.The proportions of three, four and hyper-coordinated solvation of the hydroxide ion change with temperature. Large numbers of successive PT events were observed, which enabled us to determine accurately the relative and absolute mobilities of hydrogen and hydroxide ions in aqueous solution, and their temperature dependence between 0 and 50oC. Differences between the mobilities of these ions are traced to differences in the solvation structures and to different time delays for solvent reorganization of hydrated H+ and OH- ions before PT occurs, thereby breaking the hole-particle symmetry of the dynamics of proton transfer in H+ and OH- ions.50,51 Nuclear quantum effects2,3,44 are neglected in our study and future work could involve optimization of scaling to include these effects. Fig.4 (c) Configuration showing mechanism of single proton transfer Fig.2. Orientation relaxation time Fig.3 (a) gOO(r) at 298 K SHL was supported by the National Research Foundation of Korea Grant funded by MEST (NRF-2010-0023062) and JCR was supported by a National Science Foundation Grant - CHE 0549187. Fig.3 (b) gOH(r) at 298 K Fig.3 (c) gHH(r) at 298 K Fig.4 (d) gO*H, gO*O, nO*H, and nO*O