Solvation of carbanions in organic solvents: A test of the polarizable continuum model

The solvation of carbanions in the solvents N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) has been analyzed on the basis of experimental and t heoretical data. Experimental solvation energies are obtained from present and previously reported electrochemical measurements of reduction potential s of the corresponding radicals. Theoretical solvation energies are obtaine d from quantum chemical calculations using the polarizable continuum model (PCM). It is found that the solvation energy is relatively independent of m olecular size and structure for the saturated carbanions. This indicates th at the negative charge is strongly localized to the anionic carbon. The con jugated carbanions have considerably lower absolute solvation energies (\ D eltaG degrees (sol)\) than the saturated carbanions. This is a consequence of the strong delocalization of the negative charge in the former group. Th e propargyl anion is also found to have a surprisingly low absolute solvati on energy. However, high-level quantum chemical calculations show that the electronic structure has large contributions from two different resonance s tructures, CH=CCH2- and -CH=C=CH2, which results in a significant charge de localization. There is good agreement between calculated and experimental s olvation energies for both the conjugated and nonconjugated primary anions. However, the PCM method consistently underestimates the absolute solvation energies of the secondary and tertiary carbanions. This is attributed to a n insufficient treatment of first-layer solvation effects in the method. Ac cording to the experimental measurements, the absolute solvation energies a re on average 2-3 kcal mol(-1) lower in THF than in DMF. The theoretical da ta indicate a considerably larger solvent effect, 7-10 kcal mol(-1). The di screpancy between theory and experiment may partly be attributed to the use of a supporting electrolyte in the measurements, but the main cause seems to be that the short-range interaction tendencies of the solvent cannot be Fully characterized by its dielectric constant.