| Abstract
| - This work considers calculation of pKa for a series of related alcohols, carboxylic acids, and ammonium ionsspanning a wide range of acidities, using quantum mechanical treatment of solute electronic structure inconjunction with a dielectric continuum model for solvation of each bare solute. The electronic structuremethods used are of sufficiently high quality to give very good agreement with experimental gas phase acidities.Dielectric continuum theory of solvation is used in a recently developed form that accurately takes accountof solute charge density penetrating outside the solvent cavity that nominally encloses it. The cavity surfaceis defined by a single parameter characterizing an electronic isodensity contour, and contours are examinedat and near the value 0.001 e/that has previously led to a good account of solvation effects on propertiesof neutral solutes in various solvents. In water, the pKa values calculated for alcohols and carboxylic acidsare generally much higher than experiment, while for ammonium ions they are comparable to experiment.Good results in water can be obtained from linear correlations that describe the effects of different substituentsin solutes sharing the same acidic functional group, but different correlations apply for different acidic functionalgroups. For the polar nonprotic solvents DMSO and MeCN, pKa values close to experimental results areobtained, and very good linear correlations are found that simultaneously describe well all the solutes considered.It is argued this indicates that dielectric continuum theory properly accounts for long-range bulk solventeffects on pKa without the need for special parameterization of the cavity. To achieve good pKa results inwater, further account must be taken of specific short-range effects such as hydrogen bonding. Rather thandistort the cavity from the physical solute−solvent interface region in order to artificially force dielectriccontinuum theory to serve this purpose, as is commonly done through detailed parameterization schemes, itis recommended that other complementary approaches more appropriate for describing short-range interactionsshould be sought to complete the treatment of solvation effects in water.
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