Optical and NMR investigations on alkali ion pairs of carbanions and nitranions

Abstract

Abstract The ion pair formation of some cyclic conjugated carbanions and nitranions has been studied by absorption and fluorescence spectroscopy. From the spectra, recorded as a function of temperature, solvent and counter ion it becomes clear that the absorption spectra of contact ion pairs are always shifted to high energy with respect to the spectra of solvent separated ion pairs, but in the fluorescence spectra shifts in both directions have been found. The observed phenomena can be explained satisfactorily by theoretical π-electron charge distributions in the ground and first excited states. Also with 1H and 13C NMR spectroscopy valuable information concerning the ion pairing has been obtained. An apparent discrepancy between the structure of the contact ion pair of the carbanion indenyl inferred from calculations and from 1H NMR is resolved by a theoretical examination of the effects of the cationic field on the proton chemical shifts. The 13C NMR data support the interpretation of the temperature dependence of the 1H chemical shifts in terms of a “direct” and an “indirect” effect of the cation. In order to explain the 1H chemical shifts of some anions with 16 π-electrons the occurrence of paramagnetic ring currents has to be postulated. 7Li NMR data give evidence for the structure of the contact ion pairs, σ- or π-complex. The influence of ion pair formation on the reaction rate of the symmetrical proton exchange reaction between a carbanion and its parent molecule has been studied from the exchange line broadening for the system indene/indenyl in various solvents.