An ab-initio SCF-MO study of the solvent effect in α-substituted carbanions

Abstract

This study reports an investigation at an ab-initio SCF-MO level of a supermolecule which involves a carbanion ( −CH 2-F), a counterion (Li +) and a certain number of solvent molecules. It was found that only the solvation of the counterion is very large, while that of the carbanion is quite small. Thus we have considered solvent molecules only around the counterion; in particular, three water molecules and two methanol molecules, to simulate the effect of an ether, the solvent most commonly used in reactions involving carbanions. We have found that for this supermolecule two distinct minima exist; the one with the lowest energy shows the feature of a tight ion pair and the other those of a solvent-separated ion pair. It was also found that the geometry of the carbanion is almost the same in both ion pairs, and more similar to that of the isolated carbanion than to that of the H 2CFLi species.