Proton solvation in protic and aprotic solvents

Abstract

Protonation pattern strongly affects the properties of molecular systems. To determine protonation equilibria, proton solvation free energy, which is a central quantity in solution chemistry, needs to be known. In this study, proton affinities (PAs), electrostatic energies of solvation, and pKA values were computed in protic and aprotic solvents. The proton solvation energy in acetonitrile (MeCN), methanol (MeOH), water, and dimethyl sulfoxide (DMSO) was determined from computed and measured pKA values for a specially selected set of organic compounds. pKA values were computed with high accuracy using a combination of quantum chemical and electrostatic approaches. Quantum chemical density functional theory computations were performed evaluating PA in the gas-phase. The electrostatic contributions of solvation were computed solving the Poisson equation. The computations yield proton solvation free energies with high accuracy, which are in MeCN, MeOH, water, and DMSO -255.1, -265.9, -266.3, and -266.4 kcal/mol, respectively, where the value for water is close to the consensus value of -265.9 kcal/mol. The pKA values of MeCN, MeOH, and DMSO in water correlates well with the corresponding proton solvation energies in these liquids, indicating that the solvated proton was attached to a single solvent molecule.

Absolute proton hydration free energy, surface potential of water, and redox potential of the hydrogen electrode from first principles: QM/MM MD free-energy simulations of sodium and potassium hydration

“…Clearly, the formulation of a consistent single-ion solvation thermodynamics also involves analogous determinations pertaining to other solvents, 151,322 relevant in particular in the context of transfer free energies, 323,324 and charge-asymmetric solvation effects. 17,325 Furthermore, not only the values of these quantities are of relevance, but also that of their P-or/and T -derivatives, related to the partial molar enthalpies, entropies, compressibilities, and expansivities.…”

Erratum: Proton solvation in protic and aprotic solvents [J. Comput. Chem. 2015, 37, 1082–1091]

“…The value of pK A -MSD comp-exp remains practically the same. [1] Hence, pK A values seem to be robust against moderate deviations (less than 20%) of the dielectric constant used for the computation.…”

“…As final result no change in the value of proton solvation in DMSO was obtained remaining at 2266.4 kcal/mol. [1] The individual computed pK A values exhibit minor changes (see Table 1) with a root mean square deviation (pK A -RMSD) of 0.42 and a mean signed deviation (pK A -MSD new-old ) of 20.2 between the new and old pK A values. The pK A -RMSD between computed and measured pK A values is with 0.65 (without outliers) slightly smaller than before, where it was 0.75.…”

“…Considering the three sulfur compounds separately, the optimized sulfur radius is 2.18 Å , which is the same value as before. [1] However, when including The computed values are obtained with the dielectric constant 36.7 (old) and 46.7 (new). The numbers in brackets refer to the numbering in Fig.…”

“…1 of the referred article. [1] Computed values considered to be outliers are marked in bold digits. In the last but one line RMSD values between measured and computed pK A values are listed.…”