Polarization and charge transfer in the hydration of chloride ions
Zhen Zhao,
1,a͒
David M. Rogers,
1,b͒
and Thomas L. Beck
2,c͒
1
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, USA
2
Department of Chemistry and Department of Physics, University of Cincinnati,
Cincinnati, Ohio 45221-0172, USA
͑Received 9 October 2009; accepted 11 December 2009; published online 4 January 2010͒
A theoretical study of the structural and electronic properties of the chloride ion and water molecules
in the first hydration shell is presented. The calculations are performed on an ensemble of
configurations obtained from molecular dynamics simulations of a single chloride ion in bulk water.
The simulations utilize the polarizable AMOEBA force field for trajectory generation and MP2-level
calculations are performed to examine the electronic structure properties of the ions and surrounding
waters in the external field of more distant waters. The ChelpG method is employed to explore the
effective charges and dipoles on the chloride ions and first-shell waters. The quantum theory of
atoms in molecules ͑QTAIM͒ is further utilized to examine charge transfer from the anion to
surrounding water molecules. The clusters extracted from the AMOEBA simulations exhibit high
probabilities of anisotropic solvation for chloride ions in bulk water. From the QTAIM analysis, 0.2
elementary charges are transferred from the ion to the first-shell water molecules. The default
AMOEBA model overestimates the average dipole moment magnitude of the ion compared to the
quantum mechanical value. The average magnitude of the dipole moment of the water molecules in
the first shell treated at the MP2-level, with the more distant waters handled with an AMOEBA
effective charge model, is 2.67 D. This value is close to the AMOEBA result for first-shell waters
͑2.72 D͒ and is slightly reduced from the bulk AMOEBA value ͑2.78 D͒. The magnitude of the
dipole moment of the water molecules in the first solvation shell is most strongly affected by the
local water-water interactions and hydrogen bonds with the second solvation shell, rather than by
interactions with the ion. © 2010 American Institute of Physics. ͓doi:10.1063/1.3283900͔
I. INTRODUCTION
Fundamental studies of the thermodynamic and struc-
tural properties of ions in water and near proteins are impor-
tant for understanding a wide range of chemical and biologi-
cal phenomena. For example, in the central nervous system,
ion-coupled membrane transporters regulate signaling by uti-
lizing the electrochemical potential of specific ions to pump
organic substrates and amino acids across the cell
membrane.
1–3
As another example, chloride transporters ex-
change two chloride ions for one proton during the transport
cycle.
4–6
To gain more insight into the mechanism, specific-
ity, and function of these transporters, the binding properties
of specific ions to the transporters and the ion hydration pro-
cess in bulk water need to be characterized.
The hydration of atomic and molecular ions has been the
focus of intensive research for over 100 years. More recently,
specific ion effects have resurfaced in diverse fields,
7
includ-
ing hydration free energies, ion activities, surface tension
increments, bubble interactions, colloid interactions, biologi-
cal membrane multilayer swelling,
8
and polymer phase
equilibria,
9
to name several examples. Such specificity re-
quires theoretical treatments that go beyond simple dielectric
models. Specific interactions at the molecular level are in-
volved and that specificity can lead to dramatic changes in
bulk properties when one ion is substituted with another.
8
It
has been argued that ion specificity is a central problem in
connecting physical science to biological systems, and that
the connection has not been fully made so far.
10
Calculation of the solvation and binding properties of an
ion in water and near proteins for a classical model is now
routine on modern workstations. The accuracy of these cal-
culations depends sensitively on the classical model potential
used, however. In the same spirit as Doren, Wood, and
co-workers,
11–14
we are interested in incorporating quantum
mechanical calculations in the study of the thermodynamics
of ions in water and near proteins. The basic idea in Refs.
11–14 is to perform classical simulations, obtain the free
energy from those classical simulations, and then use the
classical configurations coupled with statistical mechanical
perturbation theories to correct those free energies toward the
quantum result. This method is less expensive than a direct
ab initio molecular dynamics ͑AIMD͒ quantum
simulation
15–18
since it uses a classical simulation to generate
configurations for thermal averages. It is also more accurate
than a classical simulation since it can, in principle, represent
the electronic properties near the ions from first principles.
As a first step, here we study the aqueous solvation
structures of the chloride ion in bulk water by utilizing clas-
sical polarizable AMOEBA force field simulations and then
by performing detailed quantum mechanical calculations on
a͒
Present address: Department of Chemistry, Penn State University, Univer-
sity Park, PA 16802.
b͒
Present address: Sandia National Laboratories, MS 1314, P.O. Box 5800,
Albuquerque, NM 87185.
c͒
Electronic mail: thomas.beck@uc.edu.
THE JOURNAL OF CHEMICAL PHYSICS 132, 014502 ͑2010͒
0021-9606/2010/132͑1͒/014502/10/$30.00 © 2010 American Institute of Physics132, 014502-1