The use of static electronic structure calculations to
compute
solution-phase pK
as offers a great advantage
in that a macroscopic bulk property could be computed via microscopic
computations involving very few molecules. There are various sources
of errors in the quantum chemical calculations though. Overcoming
these errors to accurately compute pK
as of a plethora of acids is an active area of research in physical
chemistry pursued by both computational as well as experimental chemists.
We recently developed the pK-Yay method in our attempt
to accurately compute aqueous pK
as of
strong and weak acids. The method is fully black-box, computationally
inexpensive, and is very easy for even a nonexpert to use. However,
the method was thus far tested on very few molecules (only 16 in all).
Herein, in order to assess the future applicability of pK-Yay, we study the effect of multiple conformers, the presence of
tautomers under equilibrium, and the impact of a wide variety of functional
groups (derivatives of acetic acid with substituents at various positions,
dicarboxylic acids, aromatic carboxylic acids, amines and amides,
phenols and thiols, and fluorine bearing organic acids). Starting
with more than 1000 conformers and tautomers, this study establishes
that overall errors of ∼ 1.0 pK
a units are routinely obtained for a majority of the molecules. Larger
errors are noted in cases where multiple charges, intramolecular hydrogen
bonding, and several ionizable functional groups are simultaneously
present. An important conclusion to emerge from this work is that,
the computed pK
as are insensitive (difference
<0.5) to whether we consider multiple conformers/tautomers or only
choose the most stable conformer/tautomer. Further, pK-Yay captures the stereoelectronic effects arising due to differing
axial vs equatorial pattern, and is useful to predict the dominant
acid–base equilibrium in a system featuring several equilibria.
Overall, pK-Yay may be employed in several chemical
applications featuring organic molecules and biomonomers.