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First-Principles Calculation of pKa Values for Organic Acids in Nonaqueous Solution

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journal contribution
posted on 2009-04-03, 00:00 authored by Feizhi Ding, Jeremy M. Smith, Haobin Wang
Electronic structure theory, mainly the density functional theory (DFT), is applied to calculate the pKa values for a variety of organic acids in several nonaqueous solvents: namely DMSO, MeCN, and THF. Following the supermolecule approach the solute molecule, together with a few solvent molecules in close proximity, is treated explicitly by the electronic structure theory, and the remaining solvent environment is approximated by using a standard dielectric continuum model. It is found that in most cases including only one explicit solvent molecule gives satisfactory results for pKa estimations. Next, the equilibrium position free energy difference is calculated between a reference acid−base pair whose pKa is known experimentally and the acid−base pair whose pKa is to be determined theoretically. This bypasses the step of treating the solvated proton that most of the current theories have difficulty with and, to a large extent, induces favorable error cancelations in the final theoretical results. Accurate theoretical predictions of pKa values are thus obtained at a moderate level of theory (MP2 single point on B3LYP/6-31+G(d) optimized geometry) for a series of organic acids spanning a wide range of acidities in DMSO, MeCN, and THF. Furthermore, the correlation between the pKa values of these acids in different solutions is investigated theoretically, and excellent agreement is found with the experimental results.

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