Studies of Contact and Solvent-Separated Ion Pairs of Carbanions. II. Conductivities and Thermodynamics of Dissociation of Fluorenyllithium, -sodium, and -cesium

Hogen‐Esch, Thieo E.; Smid, Johannes

doi:10.1021/ja00954a025

Cited by 181 publications

(96 citation statements)

References 2 publications

(4 reference statements)

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“…As a negative sign means an increase of the ir-electron charge density and a positive sign a decrease we must conclude that the formation of contact ion pairs is accompanied by a displacement of negative charge from the six-to the five-membered ring. These data corroborate the conclusions from optical and 'H NMR measurements, •that in the contact ion pair the cation (6) of indenyl-lithium and -sodium in the solvent DME. 6 is given in ppm from TMS.…”

supporting

confidence: 89%

Optical and NMR Investigations on Alkali Ion Pairs of Carbanions and Nitranions

Velthorst

1979

Ions and Ion Pairs and Their Role in Chemical Reactions

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-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 71-electron charge distributions in the ground and first excited states. Also with 'H and '3C 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 'H NMR is resolved by a theoretical examination of the effects of the cationic field on the proton chemical shifts. The '3C NMR data support the interpretation of the temperature dependence of the 'H chemical shifts in terms of a "direct" and an "indirect" effect of the cation. In order to explain the 'H chemical shifts of some anions with 16 r-electrons the occurrence of paramagnetic ring currents has to be postulated. 7Li NMR data give evidence for the structure of the contact ion pairs, a-or r-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.

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supporting

confidence: 89%

Optical and NMR Investigations on Alkali Ion Pairs of Carbanions and Nitranions

Velthorst

1979

Ions and Ion Pairs and Their Role in Chemical Reactions

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“…The red shift observed in the The reaction was first order in both reactants spectra of R3SiM compounds with the increase and the overall second order kinetics were rein the radius of gegenion is consistent with the tained to at least 90% of the total reaction. These results reported for various anionic species kinetics results were further supported by the (8,(14)(15)(16)(17)(18).…”

Section: Discussionsupporting

confidence: 64%

The Reactions of Organometallic Compounds Involving Silicon. Reactions of Methyldiphenyl-, Triphenylsilyl-sodium, -potassium, -rubidium, and -caesium with 9-Methylfluorene in Tetrahydrofuran

Hamid¹

1973

Can. J. Chem.

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The reactions of triphenylsilyl-sodium (Ph,SiNa), diphenylmethylsilyl-sodium (Ph2MeSiNa), triphenylsilyl-potassium (Ph3SiK), triphenylsilyl-rubidium (Ph,SiRb), and triphenylsilyl-caesium (Ph,SiCs) with 9-methylfluorene in tetrahydrofuran (THF) were studied using the stop-flow technique at low temperatures. The rate constant k (1 mol-'s-') depends on the nature of the cation; at -5O0C k is 11.5 for Ph3SiNa, 29.5 for PhlMeSiNa, 26.8 for Ph3SiK, 35.6 for Ph3SiRb, and 47.9 for Ph3SiCs. It seems that the organosilylalkali-metal compounds (R3SiM) exist in the form of contact ion-pair and therefore the reactivity of an ion-pair is determined by the Coulombic binding energy of the pair. The therrnodynamic constants of activation for these reactions were calculated for -50 "C and compared with the corresponding values of triphenylsilyl-lithium Ph,SiLi reaction with 9-methylfluorene in THF. The plots of log k, AS*, and AH* against l/(r, + 2) and AS* against AH* were drawn to show that Li+ due to its existence as solvent separated ion-pair behaves differently to its N a + , K + , Rb+, and Cs+ counterparts.The electronic spectra of R,SiM, 9-methyl-fluoren-9-yl-sodium, -potassium, -rubidium and -caesium were determined at temperatures varying from 20 to -80 "C. A red shift was observed in spectra of R3SiNa at low temperatures; this was attributed to the increase in dipole moment which occursduring the electronic transition. The spectra of 9-methylfluoren-9-yl-sodium, -potassium, -rubidium, and -caesium were compared with their lithium counterpart to explain the effects of gegenion on t h~ solvation of these species.On a ttudit les rtactions dans le tttrahydrofuranne du triphtnylsilyl-sodium (Ph,SiNa), du diphenylmtthylsilyl-sodium (Ph2MeSiNa), du triphtnylsilyl-potassium (Ph,SiK), du triphenylsilyl-rubidium (Ph,SiRb) et du triphtnylsilyl-caesium (Ph3SiCs) avec le m6thyl-9 fluorhe en utilisant la technique d'interruption de debit pour des basses temptratures. La constante de vitesse K (1 mol-' s-') varie selon la nature du cation: a -50 "C, K est Cgal a 11.5 pour Ph,SiNa, 29.5 pour PhzMeSiNa, 26.8 pour Ph3SiK, 35.6 pour Ph3SiRb et 47.9 pour Ph3SiCs. I1 semble que les composts du type organosilylalcalino-metal (R3SiM) existent sous la forme de paire d'ions intirnes et que par constquent la rtactivite de cette paire d'ions est deterrnide par l'tnergie d'interactions coulombiennes de la paire. Les constantes therrnodynamiques d'activation pour ces rtactions ont Cte calcultes a -50 "C et compartes avec les valeurs correspondantes de la reaction du triphtnylsilyl-lithium (Ph3SiLi) avec le rntthyl-9 fluorene dans le T H F .Les courbes du log k, AS* et AH* en fonction de l/(r, + 2) et de AS* en fonction de AH* ont Ct C faites afin de montrer que le Li+, a cause de son existence sous forme d'une paire d'ions solvatCs, possede un comportement difftrent de celui des autres entitts Na+, K + , Rb+, et Cs+.Les spectres Clectroniques des R3SiM, methyl-9 fluorenyl-9-sodium, -potassium, -rubidium et -caesium ont Ctt determines pour des tempt...

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“…Investigations on carbanions with delocalized electrons are an interesting field of application 255,256) . Their spectra in solvents of low permittivity suggest equilibria between tight and solvent-separated ion pairs and higher aggregates.…”

Section: Absorption Spectroscopymentioning

confidence: 99%