The alkali cation effect on the ¹H NMR chemical shifts of the indenyl carbanion

Abstract

The complex character of the temperature dependence of the 'H N M R chemical shifts of indenyl-lithium and -sodium in dimethoxyethane is explained. It is shown that the cation causes a polarisation of the C-H bonds and thus inflnenees the proton shifts of the anion, both directly by i t s electric field along the bonds (the direct effect) and induectly via its effect on tEe Ir-electron distribution (the indirect effect). The indirect effect is inferred from uC NMR chemical shift data. By subtracting the temperature dependent contribution of the indirect effect from the experimental 'H data, the direct effect is visualised. It appears that information on ion pairs obtained by 'H and "C NMR on the one hand and optical spectroscopy on the other hand is complementary. Apparently, aggregation of ion pairs does not seriously affect the chemical shift data.

The fluorescence of ion pairs of the fluorenyl carbanion

“…2 shows the relative positions of the ¤ Electronic Supplementary Information available. See http : // www.rsc.org/suppdata/cp/b1/b103730g/ energy levels adapted from the scheme suggested by Vos et al for the indenyl carbanion.9 With the Ñuorenyl anion and the alkali metal cations, the blue-shift is greatest with lithium and an approximately linear relationship is observed between the frequency of the strongest absorption band of the tight pairs and where is the cation radius.2 1/r c r c Studies by 13C NMR spectroscopy have conÐrmed the existence of two kinds of ion pairs10 as has 6Li and 1H heteronuclear Overhauser e †ect spectroscopy.11 Similar studies on the closely related indenyl anion which led to entirely analogous conclusions provide supporting evidence.12, 13 Generally, the electrostatic work that has to be done to increase the interionic separation to the extent required to convert the tight pair into its loose counterpart is more than o †set by the release of energy arising from the increased solvation of the cation that thereby becomes possible. Overall, the conversion is exothermic and involves a decrease in entropy as a consequence of the localisation of solvent molecules on the cation (not included in Fig.…”

Reduction and oxidation of annulenes

“…It is clear from an inspection of the data in Table III that the ion-pairing-induced changes of XH chemical shifts are much larger for the paratropic dianions 222--252-than for the diatropic dianion 32-, the diatropic monoanion cyclononatetraenyl161 or the aromatic monoanions of the indenyl and fluorenyl type. 276,[307][308][309] Protons attached to annulene ions should experience the influence of a counterion in three ways: (a) via the (direct) electric field effect along the C-H axis, (b) via the induced modification of the local x-charge densities, and (c) by the perturbed electronic structure and magnetic anisotropy of the entire x-system.162…”

A Comparative 1H‐ and 13C‐NMR Study of the Dianion and Dication of Biphenylene

“…Since only a minor deshielding ( < 0.3 ppm) can be expected due to the Li' effect in 12- [28], we can conclude that 12+ and 12-sustain ring currents of comparable magnitude, in agreement with theory [25] and an other experimental result [23]. Here, the diatropic ring currents for the dianion and dication of 1,6;9,14-bismethano[l6]annulene 2, By using simple, cyclic ionic species to derive the proportionality constant in Eqn.…”

“…where Ap, is the charge-density change at position, i, R, the distance vector between i and the proton of interest, and 0, the angle between R, and the C-H bond, we now derive Ad(lH), values that lead to charge-density-corrected chemical shifts and ring-current contributions relative to the parent hydrocarbon whichconsidering the different experimental conditions used for the generation of the ions and the fact that the electric field effect of Li' on the 'H chemical shifts of 12-was omittedare similar for both systems (Table 3 ) . Since only a minor deshielding ( < 0.3 ppm) can be expected due to the Li' effect in 12- [28], we can conclude that 12+ and 12-sustain ring currents of comparable magnitude, in agreement with theory [25] and an other experimental result [23]. Here, the diatropic ring currents for the dianion and dication of 1,6;9,14-bismethano[l6]annulene 2, By using simple, cyclic ionic species to derive the proportionality constant in Eqn.…”