Quantum Chemical Approach for Calculating Stability Constants of Mercury Complexes

Stability
constants are central to the multiscale modeling of the
thermodynamic speciation, cycling, and transport of mercury (Hg) and
other contaminants in aquatic environments. However, for Hg, experimental
values for many relevant complexes are not available, and for others
can span ranges in excess of 10 log units. The missing data and large
uncertainties lead to significant knowledge gaps in predictions of
thermodynamic speciation. As an alternative to experimental measurements,
thermodynamic quantities can be calculated with quantum chemical methods.
Among these, density functional theory (DFT) with a polarizable continuum
solvent combines accuracy with practicability. Here, we present an
accurate and quick approach in which we use DFT with continuum solvation
to calculate stability constants of Hg complexes with inorganic and
low molecular-weight organic ligands in aqueous solution. Specifically,
we use the M06/[SDD]­6-31+G­(d,p) level of theory in combination with
a modified version of the SMD solvent model in which the solute radii
are reoptimized with a scaled solvent-accessible surface approach.
For the set of 37 Hg complexes used for optimization, which contain
environmentally relevant functional groups and have reliable experimental
stability constants, we obtain a mean unsigned error of 1.4 log units.
Testing the method on an independent set of 12 Hg complexes reproduces
the experimental stability constants to a mean unsigned error of 1.6
log units. This approach is a substantial step toward generally applicable
rapid stability constant derivation for a wide range of Hg complexes,
including those present in dissolved organic matter.