Quantum-Chemical Predictions of Redox Potentials of Organic Anions in Dimethyl Sulfoxide and Reevaluation of Bond Dissociation Enthalpies Measured by the Electrochemical Methods

Abstract

A first-principle theoretical protocol was developed that could predict the absolute pK(a) values of over 250 structurally unrelated compounds in DMSO with a precision of 1.4 pK(a) units. On this basis we developed the first theoretical protocol that could predict the standard redox potentials of over 250 structurally unrelated organic anions in DMSO with a precision of 0.11 V. Using the two new protocols we systematically reevaluated the bond dissociation enthalpies (BDEs) measured previously by the electrochemical methods. It was confirmed that for most compounds the empirical equation (BDE = 1.37 pK(HA) + 23.1E(o) + constant) was valid. The constant in this equation was determined to be 74.0 kcal/mol, compared to 73.3 kcal/mol previously reported. Nevertheless, for a few compounds the empirical equation could not be used because the solvation energy changed dramatically during the bond cleavage, which resulted from the extraordinary change of dipole moment during the reaction. In addition, we found 40 compounds (mostly oximes and amides) for which the experimental values were questionable by over 5 kcal/mol. Further analyses revealed that all these questionable BDEs could be explained by one of the three following reasons: (1) the experimental pK(a) value is questionable; (2) the experimental redox potential is questionable; (3) the solvent effect cannot be neglected. Thus, by developing practical theoretical methods and utilizing them to solve realistic problems, we hope to demonstrate that ab initio theoretical methods can now be developed to make not only reliable, but also useful, predictions for solution-phase organic chemistry.