『Abstract
Silicic acid and the hexa-aqua of Al3+ are fundamental
model aqueous species of chemical importance in nature. in order
to investigate their hydroxyl dissociation mechanisms, Car-Parrinello
molecular dynamics (CPMD) simulations were carried out, which
allow treating the solutes and solvents on the same footing. The
method of constraint was employed to trigger the reactions by
taking coordination number as the reaction coordinate and the
thermodynamic integration was used to obtain the free-energy profiles.
The approximate transition states were located and the reactant
and product states were also characterized. The free-energy changes
of dissociation are found about 15.0 kcal/mol and 7.7 kcal/mol
for silicic acid and Al-aqua, respectively. From the simulation
results, the first pKas were calculated by using two approaches,
which are based on the pristine thermodynamic relation and the
RDF (radial distribution function)-free energy relation, respectively.
Because of more uncertainties involved in the RDF way, it is suggested
that the pristine way should be favored, which shows an error
margin of 1 pKa unit. This study provides an encouraging basis
for applying the present methodology to predict acidity constants
of those groups that are difficult to measure experimentally.』
1. Introduction
2. Methodology
2.1. Car-Parrinello MD
2.2. Method of constraint
2.3. pKa calculation
3. Results and discussions
3.1. Reactant states
3.2. Mean force, free energy and pKa
3.3. Approximate transition states and product states
4. Summary
Acknowledgment
References