Chemistry Reference
In-Depth Information
2 Positively Charged Ions in Water: Quantum Calculations
Water as a natural solvent plays a central role in many biological, physical, and
chemical processes. Owing to its unique capability of building a hydrogen bonded
network, the liquid state properties remain difficult to describe in computer simula-
tions or theoretical calculations. Although many of its important features have been
uncovered over the past years by experimental and theoretical research efforts,
several problems remain unsolved and their solution is crucial for addressing the
current demand in biological, chemical and industrial applications. One problem
of central importance is the behavior of water as a solvent of positive ions [
12
-
19
].
On a large scale, thermodynamic properties characterize ionic hydration, expressed,
for example, by the free energy of solvation. These thermodynamical quantities are
experimentally accessible and thus are of interest to theoretical studies whose
methodology can reach these large scales, e.g., classical simulations. However,
structural properties of the solvation shell, at the level of the single molecule, are
characterized by a local microscopic scale (e.g., electronic) and go deep into the
very specific chemistry of the ion-water interaction; at the same time the overall
structure and stability of the first solvation shells are strongly influenced by the rest
of the system (bulk). This interplay of local and global scales is not yet fully
understood; this section concerns this latter aspect.
2.1 Theoretical Framework
Chemical intuition would suggest that the dominant ingredient in the solvation
process is the direct ion-water interaction [
20
]. In fact one would expect that water
molecules in the first solvation shell of a positive ion are not screened by the
presence of other molecules and thus are highly polarized due to the direct interac-
tion of the positive charge of the ion with the lone pair electrons of the oxygen.
However, recent experimental [
20
,
21
] and theoretical work [
22
-
30
] suggested
that this effect is not crucial for the structure of liquid water around the ion. In
reality the problem is rather complex and, as anticipated, a detailed understanding
of the process requires, at theoretical level, a multiscale analysis that clarifies the
interplay between the molecular scale in the vicinity of the solute (local scale) and
the bulk scale (global scale) of the solvent. In the following we will present a
scale analysis, in the sense mentioned above, of ab initio Density Functional (DFT),
Car-Parrinello
1
simulations of several positive ions in water, namely Li
þ
,Na
þ
,K
þ
,
Mg
2
þ
, and Ca
2
þ
. This scale analysis is based on the calculation of the molecular
dipole moment of water; such a quantity represents a physical
indicator of
1
The results discussed here were produced with the version 3.9.2 of the CPMD code. CPMD,
Copyright IBM Corp. 1990-2004, Copyright MPI f
€
ur Festk
€
orperforschung Stuttgart 1997-2001/
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