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Molecular Orbital Theory Calculations of Aqueous Solvation Effects on Chemical Equilibria

Cramer, C. J.; Truhlar, D. G.
J. Am. Chem. Soc. 1991, 113, 8552.

The effects of solvation on tautomeric equilibria and acid-base proton transfer reactions are computed via the AM1-SM1 formalism and compared to experiment. AM1-SM1 is a general, parametrized model within the AM1 semiempirical formalism which treats the solvent as a bulk continuum with a generalized Born model with dielectric screening for the polarization energy, and surface tension terms, based on the solvent-accessible surface area, for cavity and dispersion effects. The model accurately predicts the changes in the free energy of reaction upon proceeding from the gas phase to aqueous solution with an rms error of 4.2 kcal/mol for forty-five separate proton transfer reactions involving substrates ranging in basicity from the cyclopentadienyl anion to ammonia. In addition, the solvation induced changes in a variety of tautomeric equilibria are also well reproduced by the model. These include hydroxyketone/lactol, hydroxypyridine/pyridone, enol/ketone and amide-rotameric equilibria. The SM1 treatment of solvation effects is at least as accurate as the AM1 prediction of electronic energies.

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