Ion microsolvation
is a basic, yet fundamental, process of ionic
solutions underlying many relevant phenomena in either biological
or nanotechnological applications, such as solvent reorganization
energy, ion transport, catalytic activity, and so on. As a consequence,
it is a topic of extensive investigations by various experimental
techniques, ranging from X-ray diffraction to NMR relaxation and from
calorimetry to vibrational spectroscopy, and theoretical approaches,
especially those based on molecular dynamics (MD) simulations. The
conventional microscopic view of ion solvation is usually provided
by a “static” cluster model representing the first ion–solvent
coordination shell. Despite the merits of such a simple model, however,
ion coordination in solution should be better regarded as a complex
population of dynamically interchanging molecular configurations.
Such a more comprehensive view is more subtle to characterize and
often elusive to standard approaches. In this work, we report on an
effective computational strategy aiming at providing a detailed picture
of solvent coordination and exchange around aqua ions, thus including
the main structural, thermodynamic, and dynamic properties of ion
microsolvation, such as the most probable first-shell complex structures,
the corresponding free energies, the interchanging energy barriers,
and the solvent-exchange rates. Assuming the solvent coordination
number as an effective reaction coordinate and combining MD simulations
with enhanced sampling and master-equation approaches, we propose
a stochastic model suitable for properly describing, at the same time,
the thermodynamics and kinetics of ion–water coordination.
The model is successfully tested toward various divalent ions (Ca
2+
, Zn
2+
, Hg
2+
, and Cd
2+
)
in aqueous solution, considering also the case of a high ionic concentration.
Results show a very good agreement with those issuing from brute-force
MD simulations, when available, and support the reliable prediction
of rare ion–water complexes and slow water exchange rates not
easily accessible to usual computational methods.