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  • 1
    Electronic Resource
    Electronic Resource
    Extending the polarizable continuum model to effective ab initio pair potentials in multicomponent solutions: A test on calcium–water and calcium–ammonia potentials (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 5448-5459 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The use of the polarizable continuum model to develop ab initio effective pair potentials is extended to multicomponent solutions. The methodology takes into account nonadditivity effects on pair interactions computing wave functions perturbed by the solvent. Ca2+–water and Ca2+–ammonia potentials suitable for aqueous ammonia solutions are presented. These effective ab initio pair potentials present smaller binding energies with respect to strictly ab initio two-body potentials. The reduction is higher in Ca2+–ammonia (28%) than in Ca2+–water (22%) and brings to a small gap the difference between the binding energies of the two ligands with Ca2+ when solvent effects are considered. As a first test, metal-ligand clusters of different size and composition have been studied. The comparison with restricted Hartree–Fock ab initio calculations shows good agreement for the largest clusters considered. Results confirm that the presented methodology, based on the polarizable continuum model, describes in a proper way the interactions in the condensed phase, where the ion completes its coordination sphere. The cluster results also show that ammonia can displace water in the first ion coordination with a tendency to change the coordination number from 8 to 9 when the ion is fully surrounded by the former, the ninth ammonia molecule being positioned in an intermediate situation between the first and the second coordination shells. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1453956
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  • 2
    Electronic Resource
    Electronic Resource
    Preferential solvation of Ca2+ in aqueous solutions containing ammonia: A molecular dynamics study (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 5460-5470 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ca2+ aqueous solutions containing different proportions of ammonia have been studied by means of molecular dynamics simulations. Previously developed ab initio effective pair potentials, in the framework of the polarizable continuum model, and only tested at a cluster computation level, have been employed to describe ion–ligand interactions. Structural and dynamic changes present in the neighborhood of the ion as a function of the ammonia concentration have been followed. Results show a preferential solvation for ammonia, even at very low concentrations. For the pure aqueous solution, calcium ion is coordinated by eight water molecules, while the presence of ammonia favors an equilibrium between an octa and enna-coordinated situation when this ligand becomes predominant, confirming the prediction of cluster calculations. However, the increase in the coordination number is followed by an intrinsic loss of stability for the identifiable solvated structures because of the larger tendency of ammonia to participate in solvent exchange phenomena. Solvent exchange events show, for the most simple case (water–water exchange), a marked mechanistic variety. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1453957
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  • 3
    Electronic Resource
    Electronic Resource
    Free energy and entropy for inserting cavities in water: Comparison of Monte Carlo simulation and scaled particle theory results (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 6353-6365 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The process of inserting cavities in water is studied with the aim of a better description of some of the terms necessary in continuum quantum mechanical models. Free-energy changes for the formation of soft and hard spherical cavities in TIP4P water have been computed by Monte Carlo (MC) simulation with statistical perturbation theory, up to a radius of 6 Å. The free-energy change for the formation of a hard sphere, ΔGcav, is obtained combining the ΔGsol of a soft repulsive sphere with the ΔG corresponding to the process of transforming the soft sphere into a hard one. Two definitions of hard-sphere repulsive potentials have been considered, one only based on the distance of oxygens from the center of the cavity, while the other also excludes hydrogens from the same region. Differences in free energies are significant. The cubic polynomial expression ΔGcav, obtained by extrapolating the exact scaled particle theory (SPT) expression for very small excluding cavities, gives results in agreement with MC, with effective "hard-sphere" diameter for water larger than 2.77 Å. The SPT prediction is compared with other treatments based on surface tension. It is shown that a properly chosen surface and an "effective" surface tension of water lead to a good agreement with MC ΔGcav without curvature or microscopic corrections. The "effective" surface tension of water turns out to be very close to the experimental value. Some different simple ways to extend SPT expression to nonspherical cavities have been compared, for a limited number of nonspherical convex cavities modeled as n interlocking spheres, meant to mimic n-alkanes in the all-staggered conformation. Entropy changes for soft cavities have been computed with two methods, i.e., combining free energy and enthalpy computations and by finite difference methods. Discrepancies between SPT predictions and MC results are significant. The calculated probability distributions of relevant angles of first hydration shell waters are consistent with orientations where no O–H or O–lone pair vector points towards the cavity. Their variation when the cavity size increases is mostly quantitative and only the broadening of the bands observed for the largest cavities might indicate the early stage of the transition to hydration patterns peculiar to an infinite hydrophobic surface. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474296
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  • 4
    Electronic Resource
    Electronic Resource
    Dispersion and repulsion contributions to the solvation energy: Refinements to a simple computational model in the continuum approximation (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 12 (1991), S. 784-791 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A computational method for the evaluation of dispersion and repulsion contributions to the solvation energy is here presented in a formulation which makes use of a continuous distribution of the solvent, without introducing additional assumptions (e.g., local isotropy in the solvent distribution). The analysis is addressed to compare the relative importance of the various components of the dispersion energy (n = 6, 8, 10) and of the repulsion term, to compare several molecular indicators (molecular surface and volume, number of electrons) which may be put in relation to the dispersion-repulsion energy, and to define simplified computational strategies. The numerical examples refer to saturated hydrocarbons in water, treated with the homogeneous approximation of the distribution function which for this type of solution appears to be acceptable.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540120703
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