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Title: Quantum Chemical Calculation of pKas of Environmentally Relevant Functional Groups: Carboxylic Acids, Amines, and Thiols in Aqueous Solution

Journal Article · · Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

Developing accurate quantum chemical approaches for calculating pKas is of broad interest. Useful accuracy can be obtained by using density functional theory (DFT) in combination with a polarizable continuum solvent model. However, some classes of molecules present problems for this approach, yielding errors greater than 5 pK units. Various methods have been developed to improve the accuracy of the combined strategy. These methods perform well but either do not generalize or introduce additional degrees of freedom, increasing the computational cost. The Solvation Model based on Density (SMD) has emerged as one of the most commonly used continuum solvent models. Nevertheless, for some classes of organic compounds, e.g., thiols, the pKas calculated with the original SMD model show errors of 6–10 pK units, and we traced these errors to inaccuracies in the solvation free energies of the anions. To improve the accuracy of pKas calculated with DFT and the SMD model, we developed a scaled solvent-accessible surface approach for constructing the solute–solvent boundary. By using a “direct” approach, in which all quantities are computed in the presence of the continuum solvent, the use of thermodynamic cycles is avoided. Furthermore, no explicit water molecules are required. Three benchmark data sets of experimentally measured pKa values, including 28 carboxylic acids, 10 aliphatic amines, and 45 thiols, were used to assess the optimized SMD model, which we call SMD with a scaled solvent-accessible surface (SMDsSAS). Of the methods tested, the M06-2X density functional approximation, 6-31+G(d,p) basis set, and SMDsSAS solvent model provided the most accurate pKas for each set, yielding mean unsigned errors of 0.9, 0.4, and 0.5 pK units, respectively, for carboxylic acids, aliphatic amines, and thiols. As a result, this approach is therefore useful for efficiently calculating the pKas of environmentally relevant functional groups.

Research Organization:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Biological and Environmental Research (BER)
Grant/Contract Number:
AC05-00OR22725
OSTI ID:
1459306
Journal Information:
Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory, Vol. 122, Issue 17; ISSN 1089-5639
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 47 works
Citation information provided by
Web of Science

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Cited By (5)

Absolute and relative pKa predictions via a DFT approach applied to the SAMPL6 blind challenge journal August 2018
An explicit-solvent hybrid QM and MM approach for predicting pKa of small molecules in SAMPL6 challenge journal October 2018
Theoretical modeling of p K a 's of thiol compounds in aqueous solution journal January 2019
The AQUA‐MER databases and aqueous speciation server: A web resource for multiscale modeling of mercury speciation journal October 2019
A mechanism of alkali metal carbonates catalysing the synthesis of β-hydroxyethyl sulfide with mercaptan and ethylene carbonate journal January 2019

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