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DOI: 10.1039/A907913K (Paper) Reference Section for: Phys. Chem. Chem. Phys., 1999, 1, 5643-5647

Abinitio quantum mechanical calculations of p Kas of isolated molecules and molecules undergoing chemical reactions: p Ka of acetic acid during α-proton abstraction

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Mikael Peräkylä

Abstract

Aqueous-phase proton affinities (PA(aq)) were calculated for 14 carboxylic acids, 15 amines, and 16 alcohols using abinitio quantum mechanically calculated gas phase proton affinities (PA(gas), MP2/6-31+G**//HF/6-31+G**) and relative solvation energies of the acid and its conjugate base (ΔΔGsolv, IPCM-HF/6-31+G**) calculated with the isodensity surface-polarised continuum model (PA(aq)=PA(gas)+ΔΔGsolv). After empirical linear scaling the correlation coefficients (r2) between the calculated and experimental pKas were 0.94–0.98 for the three groups of molecules. Carboxylic acids, amines and alcohols have different correlation equations indicating systematic errors in the calculated energies for molecules with different acidic groups. The linear equation for carboxylic acids was used to estimate the change in the pKa of the carboxyl group of acetic acid from the calculated 4.8 (pKa(exp)=4.6) of the reactant, acetic acid, to 13.3 of the product, enolate of carboxylic acid, during α-proton abstraction by methoxide. It was shown that the calculated PA(aq) values can be used to estimate pKas of isolated molecules and molecules undergoing chemical reactions.

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