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WIREs Comput Mol Sci
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First‐principles prediction of acidities in the gas and solution phase

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Abstract This paper provides an overview of contemporary computational protocols toward accurate prediction of acidities in the gas and aqueous phase. The performance of various density functional theory (DFT) methods and ab initio composite procedures, such as the G3MP2(+) method, for the prediction of gas‐phase acidities of a range of neutral and cationic acids is presented. Various methods for solution pKa predictions are also reviewed where the emphasis is on thermodynamic cycle‐based methods that combine ab initio or experimental gas‐phase energies with solvation free energies from continuum solvent models. The prediction of accurate solvation free energies, especially for ionic species, represents the bottleneck for accurate pKa prediction via the direct or absolute method. The success and limitations of alternative thermodynamic cycles are discussed and some of the difficulties and future challenges associated with the applications of these methods on more complicated molecules are also highlighted. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 649–660 DOI: 10.1002/wcms.43 This article is categorized under: Structure and Mechanism > Molecular Structures

Prototropic equilibria for fluorescein.

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Cluster‐continuum method for calculation of solvation free energies.

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The correlation between experimental and calculated (direct method using CPCM–UAKS model) aqueous acidities of some neutral and cationic carbon acids at 298 K. Dotted line refers to line of unit gradient. For the neutral acids, the errors are systematic where using acetamide as the reference resulted in an MAD of 0.8 units. For the cationic acids, the errors are sensitive to structural variations and the proton exchange method is unlikely to succeed if any one of the acids was used as a reference.

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The proton exchange method of pKa calculation.

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The direct or absolute method for pKa calculation.

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Performance of various DFT and molecular orbital theory methods on the prediction of experimental gas‐phase acidites for 47 neutral acids and 17 cationic acids. Calculations other than CBS‐QB3 (which are based on the recommended B3‐LYP/CBSB7 geometries and thermal corrections) are based on B3LYP/6–31+G(d) optimized geometries and thermal corrections (scaled).

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A schematic illustration for an ONIOM type approach to the calculation of G3 quality reaction energy (ΔE). The reaction energy involving the molecular core is calculated as accurately as possible (e.g., G3 or similar), and the full system is calculated at a lower level of theory such as MP2/DFT/MM via an isodesmic reaction so that the errors due to the inadequate treatment of electron correlation should mostly cancel. The overall reaction energy is then obtained as the sum of the two terms.

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