Comment on "Correlation of Aqueous pKa Values of Carbon Acids with Theoretical Descriptors: A DFT Study"

Authors: Sierra Rayne

In their article, Charif et al. [J. Mol. Struct. THEOCHEM 818 (2007) 1] used the B3LYP/6-311++G(d,p) density functional level of theory to estimate gas phase standard state (298.15 K, 1 atm) free energies of acid dissociation (ΔacidG°(g)) for 21 carbon acids. These authors then examined correlations between their B3LYP/6-311++G(d,p) ΔacidG°(g) values and corresponding experimental aqueous pKa measurements. Large errors are evident between experimental values and the B3LYP/6-311++G(d,p) calculated ΔacidG°(g) for propanedioic acid, diethyl ester, dimedone, isopropylidene malonate, barbituric acid, and toluene from this study. The findings call into question the generality of the correlation between ΔacidG°(g) and aqueous experimental pKa values for carbon acids proposed by Charif et al., and also highlight the need for additional studies to investigate what other carbon acid moieties may be outliers. In the present case, either the experimental aqueous pKa of toluene in the literature is incorrect, or the quantitative structure-property relationship proposed in Charif et al. is subject to large outliers that greatly diminish its broad applicability.

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[v1] 2013-03-20 14:07:39

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