Improved pKa Prediction of Substituted Alcohols, Phenols, and Hydroperoxides in Aqueous Medium Using Density Functional Theory and a Cluster-Continuum Solvation Model.

Abstract

Acid dissociation constants (pKa's) are key physicochemical properties that are needed to understand the structure and reactivity of molecules in solution. Theoretical pKa's have been calculated for a set of 72 organic compounds with -OH and -OOH groups (48 with known experimental pKa's). This test set includes 17 aliphatic alcohols, 25 substituted phenols, and 30 hydroperoxides. Calculations in aqueous medium have been carried out with SMD implicit solvation and three hybrid DFT functionals (B3LYP, ωB97XD, and M06-2X) with two basis sets (6-31+G(d,p) and 6-311++G(d,p)). The effect of explicit water molecules on calculated pKa's was assessed by including up to three water molecules. pKa's calculated with only SMD implicit solvation are found to have average errors greater than 6 pKa units. Including one explicit water reduces the error by about 3 pKa units, but the error is still far from chemical accuracy. With B3LYP/6-311++G(d,p) and three explicit water molecules in SMD solvation, the mean signed error and standard deviation are only -0.02 ± 0.55; a linear fit with zero intercept has a slope of 1.005 and R2 = 0.97. Thus, this level of theory can be used to calculate pKa's directly without the need for linear correlations or thermodynamic cycles. Estimated pKa values are reported for 24 hydroperoxides that have not yet been determined experimentally.