10,508 research outputs found
Quasi-Chemical and Structural Analysis of Polarizable Anion Hydration
Quasi-chemical theory is utilized to analyze the roles of solute polarization
and size in determining the structure and thermodynamics of bulk anion
hydration for the Hofmeister series Cl, Br, and I. Excellent
agreement with experiment is obtained for whole salt hydration free energies
using the polarizable AMOEBA force field. The quasi-chemical approach exactly
partitions the solvation free energy into inner-shell, outer-shell packing, and
outer-shell long-ranged contributions by means of a hard-sphere condition.
Small conditioning radii, even well inside the first maximum of the
ion-water(oxygen) radial distribution function, result in Gaussian behavior for
the long-ranged contribution that dominates the ion hydration free energy. The
spatial partitioning allows for a mean-field treatment of the long-ranged
contribution, leading to a natural division into first-order electrostatic,
induction, and van der Waals terms. The induction piece exhibits the strongest
ion polarizability dependence, while the larger-magnitude first-order
electrostatic piece yields an opposing but weaker polarizability dependence. In
addition, a structural analysis is performed to examine the solvation
anisotropy around the anions. As opposed to the hydration free energies, the
solvation anisotropy depends more on ion polarizability than on ion size:
increased polarizability leads to increased anisotropy. The water dipole
moments near the ion are similar in magnitude to bulk water, while the ion
dipole moments are found to be significantly larger than those observed in
quantum mechanical studies. Possible impacts of the observed over-polarization
of the ions on simulated anion surface segregation are discussed.Comment: slight revision, in press at J. Chem. Phy
Ab initio investigation of intermolecular interactions in solid benzene
A computational strategy for the evaluation of the crystal lattice constants
and cohesive energy of the weakly bound molecular solids is proposed. The
strategy is based on the high level ab initio coupled-cluster determination of
the pairwise additive contribution to the interaction energy. The
zero-point-energy correction and non-additive contributions to the interaction
energy are treated using density functional methods. The experimental crystal
lattice constants of the solid benzene are reproduced, and the value of 480
meV/molecule is calculated for its cohesive energy
T Cells Use Rafts for Survival
T cell homeostasis must be tightly controlled. In this issue of Immunity, Cho et al. (2010) describe results that begin to define the roles of the T cell receptor, self-peptide-MHC ligands, cytokines, and membrane rafts in this dynamic process
Book Reviews
Reviews of the following books: Lobstering and The Maine Coast by Kenneth R. Martin and Nathan R. Lipfert; Beothuk Bark Canoes: An Analysis and Comparative Study by Ingeborg C.L. Marshall; Records of Meduncook Plantation and Friendship Maine, 1763-1899 edited by Melville B. Cook
Analysis of Clumps in Molecular Cloud Models: Mass Spectrum, Shapes, Alignment and Rotation
Observations reveal concentrations of molecular line emission on the sky,
called ``clumps,'' in dense, star-forming molecular clouds. These clumps are
believed to be the eventual sites of star formation. We study the
three-dimensional analogs of clumps using a set of self-consistent,
time-dependent numerical models of molecular clouds. The models follow the
decay of initially supersonic turbulence in an isothermal, self-gravitating,
magnetized fluid. We find the following. (1) Clumps are intrinsically triaxial.
This explains the observed deficit of clumps with a projected axis ratio near
unity, and the apparent prolateness of clumps. (2) Simulated clump axes are not
strongly aligned with the mean magnetic field within clumps, nor with the
large-scale mean fields. This is in agreement with observations. (3) The clump
mass spectrum has a high-mass slope that is consistent with the Salpeter value.
There is a low-mass break in the slope at \sim 0.5 \msun, although this may
depend on model parameters including numerical resolution. (4) The typical
specific spin angular momentum of clumps is . This is larger than the median specific angular momentum of binary
stars. Scaling arguments suggest that higher resolution simulations may soon be
able to resolve the scales at which the angular momentum of binary stars is
determined.Comment: 14 pages, 13 figures, to appear in 2003 July 20 Ap
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