Rossini E, Bochevarov AD, Knapp EW. Empirical Conversion of p
K a Values between Different Solvents and Interpretation of the Parameters: Application to Water, Acetonitrile, Dimethyl Sulfoxide, and Methanol.
ACS OMEGA 2018;
3:1653-1662. [PMID:
31458485 PMCID:
PMC6641400 DOI:
10.1021/acsomega.7b01895]
[Citation(s) in RCA: 71] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/30/2017] [Accepted: 01/26/2018] [Indexed: 05/08/2023]
Abstract
An empirical conversion method (ECM) that transforms pK a values of arbitrary organic compounds from one solvent to the other is introduced. We demonstrate the method's usefulness and performance on pK a conversions involving water and organic solvents acetonitrile (MeCN), dimethyl sulfoxide (Me2SO), and methanol (MeOH). We focus on the pK a conversion from the known reference value in water to the other three organic solvents, although such a conversion can also be performed between any pair of the considered solvents. The ECM works with an additive parameter that is specific to a solvent and a molecular family (essentially characterized by a functional group that is titrated). We formally show that the method can be formulated with a single additive parameter, and that the extra multiplicative parameter used in other works is not required. The values of the additive parameter are determined from known pK a data, and their interpretation is provided on the basis of physicochemical concepts. The data set of known pK a values is augmented with pK a values computed with the recently introduced electrostatic transform method, whose validity is demonstrated. For a validation of our method, we consider pK a conversions for two data sets of titratable compounds. The first data set involves 81 relatively small molecules belonging to 19 different molecular families, with the pK a data available in all four considered solvents. The second data set involves 76 titratable molecules from 5 additional molecular families. These molecules are typically larger, and their experimental pK a values are available only in Me2SO and water. The validation tests show that the agreement between the experimental pK a data and the ECM predictions is generally good, with absolute errors often on the order of 0.5 pH units. The presence of a few outliers is rationalized, and observed trends with respect to molecular families are discussed.
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