%0 Journal Article
%A Jackson, Virgil
E.
%A Felmy, Andrew R.
%A Dixon, David A.
%D 2015
%T Prediction
of the pKa’s
of Aqueous Metal Ion +2 Complexes
%U https://acs.figshare.com/articles/journal_contribution/Prediction_of_the_p_i_K_i_sub_a_sub_s_of_Aqueous_Metal_Ion_2_Complexes/2182432
%R 10.1021/jp5118272.s001
%2 https://ndownloader.figshare.com/files/3816355
%K MP 2 level
%K M øller perturbation theory
%K solvation shell
%K CPCM
%K 2O
%K solvation shell sizes
%K prediction
%K 1 pKa unit
%K COSMO
%K CCSD
%K CN
%K SCRF
%K calculation
%K cluster
%K coordination number
%K complex
%K model
%X Aqueous metal ions play an important
role in many areas of chemistry.
The acidities of [Be(H2O)4]2+, [M(H2O)6]2+, M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+,
and [M(H2O)n]2+,
M = Ca2+ and Sr2+, n = 7 and
8, complexes have been predicted using density functional theory,
second-order Møller–Plesset perturbation theory (MP2),
and coupled cluster CCSD(T) theory in the gas phase. pKa’s in aqueous solution were predicted by using
self-consistent reaction field (SCRF) calculations with different
solvation models. The most common binding motif of the majority of
the metal +2 complexes is coordination number (CN) 6, with each hexaaquo
cluster having reasonably high symmetry for the best arrangement of
the water molecules in the first solvation shell. Be2+ is
tetracoordinated, but a second solvation shell of 8 waters is needed
to predict the pKa. The Ca2+ and Sr2+ aquo clusters have a coordination number of
7 or 8 as found in terms of the energy of the reaction M(H2O)72+ + H2O → M(H2O)82+ and the pKa values. The calculated geometries are in reasonable agreement with
experiment. The SCRF calculations with the conductor-like screening
model (COSMO), and the conductor polarized continuum model (CPCM)
using COSMO-RS radii, consistently agree best with experiment at the
MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels of theory. The CCSD(T)
level provides the most accurate pKa’s,
and the MP2 level also provides reliable predictions. Our predictions
were used to elucidate the properties of metal +2 ion complexes. The
pKa predictions provide confirmation of
the size of the first solvation shell sizes. The calculations show
that it is still difficult to predict pKa’s using this cluster/implicit solvent approach to better
than 1 pKa unit.
%I ACS Publications