A MCSCF study of homoaromaticity and the role of ion pairing in the stabilization of carbanions

Abstract

The bicyclo[3.2.1]octa-3,6-dien-2-yl anion (I), the anion I lithium cation complex, the allyl anion (X), the allylradical, the allyl anion lithium cation complex, the ethene molecule, and the ethene lithium cation complex have been studiedby means of multiconfigurational SCF (MCSCF) and analytical gradients. The calculations have been confined to minimaland split-valence basis sets. The large distance between the C2 olefinic bridge and the C3 carbanionic bridge of anion I andthe short C6-C7 bond distance imply bishomoaromaticity to be negligible. According to these results, homoaromaticity isnot responsible for the observed stability in many potentially homoaromatic carbanions. The stability of anion I in the gasphase is instead explained in terms of a simple electrostatic model, where the quadrupole moment in the C2 olefinic bridgestabilizes the charge in the C, carbanionic bridge. This model agrees quantitatively with experiment. Calculations on theanion I lithium cation complex showed that in solution an additional attractive interaction between the C2 olefinic bridge andthe lithium cation can contribute. This additional interaction is estimated to stabilize the carbanion-lithium ion pair in thegas phase by about 16 kcal/mol. However, the stabilizing interactions of anion I in solution (quadrupole-charge andcounterion-anion interactions) will be reduced by solvent shielding. The relative ratios of the different stabilizing interactionsare therefore difficult to estimate. The geometrical findings of this paper have been verified by a recent X-ray experiment.