Improved pK_a Prediction of Substituted Alcohols, Phenols, and Hydroperoxides in Aqueous Medium Using Density...

Acid dissociation constants (pK_a's) are key physicochemical properties that are needed to understand the structure and reactivity of molecules in solution. Theoretical pK_a's have been calculated for a set of 72 organic compounds with -OH and -OOH groups (48 with known experimental pK_a'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 pK_a's was assessed by including up to three water molecules. pK_a's calculated with only SMD implicit solvation are found to have average errors greater than 6 pK_a units. Including one explicit water reduces the error by about 3 pK_a 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 pK_a's directly without the need for linear correlations or thermodynamic cycles. Estimated pK_a values are reported for 24 hydroperoxides that have not yet been determined experimentally.