Abstract
AbstractGas phase standard state (298.15 K, 1 atm) calculations were conducted at the B3LYP/6-311++G(d,p) density functional and the Gaussian-4 (G4) composite method levels of theory to estimate the acidity of various carbon acids. Excellent agreement was found between the G4 gas phase enthalpies and free energies of carbon acid dissociation and corresponding experimental values. The B3LYP/6-311++G(d,p) level of theory displayed a significantly lower accuracy for gas phase acidity prediction, but with similar qualitative trends as the G4 calculations. Good correlations using both theoretical methods were found with corresponding carbon acid aqueous pK~a~ values. However, the pK~a~ prediction accuracy of carbon acids based on correlations with theoretical gas phase acidities appears lower than previously reported. Some carbon acid moieties may be outliers in general aqueous pK~a~ versus gas phase acidity quantitative structure-property relationships (QSPRs), warranting further investigative efforts on broader training sets.
Highlights
The gas and aqueous phase pKa values of carbon Bronsted acids are of considerable theoretical and applied interest.[1,2,3] The pKa values of carbon acids can vary widely depending on the electron withdrawing or releasing ability of substituents attached to the reaction center.[4,5] Because of the difficulty and expense in directly measuring carbon acid pKa values, several theoretical approaches have been pursued to better predict corresponding acidity constants in the gas and aqueous phases.[6,7,8,9,10]
A number of the carbon acids investigated by Charif et al [10] have experimental ΔacidG°(g) values in the NIST database [11], these authors did not compare their B3LYP/6-311+ +G(d,p) estimates to the experimental reports
We note that Charif et al [10] reported a B3LYP/6-311++G(d,p) ΔacidG°(g) of 1408.4 kJ/mol for diethyl malonate, whereas the NIST database [11] cites an experimental value of 1432.0±8.4 kJ/mol
Summary
The gas and aqueous phase pKa values of carbon Bronsted acids are of considerable theoretical and applied interest.[1,2,3] The pKa values of carbon acids can vary widely (up to >60 units) depending on the electron withdrawing or releasing ability of substituents attached to the reaction center.[4,5] Because of the difficulty and expense in directly measuring carbon acid pKa values, several theoretical approaches have been pursued to better predict corresponding acidity constants in the gas and aqueous phases.[6,7,8,9,10]. Charif et al [10] have employed 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 a range of 21 carbon acids, and subsequently correlated the B3LYP/6-311++G(d,p) ΔacidG°(g) values to experimental aqueous pKa measurements.
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