The NMR spectra of the carbanions of xanthene and thioxanthene. Evidence for paratropism

Vos, H. W.; Bakker, Y. W.; MacLean, C.; Velthorst, N.H.

doi:10.1002/mrc.1270060413

Cited by 5 publications

(12 citation statements)

References 23 publications

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“…The anions_with a five-membered ring are completely DIATROPIC; in other words XH and TxH are less "aromatic" than the homocyclic carbanions under study (29). The partially paratropic character of XH and TxH can be inferred from the proton chemical shifts (31). These are drawn schematically for a series of carbanions in Fig have nearly the same values.…”

Section: Alkali Ion Pairs Of Carbanions and Nitranionsmentioning

confidence: 91%

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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Section: Alkali Ion Pairs Of Carbanions and Nitranionsmentioning

confidence: 91%

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 ring-current model implies that in polycyclic systems the total electronic motions should break down into currents flowing around each individual ring. Several authors have considered the feasibility of such a breakdown, both from a theoretical [7,12,14,21,22] and from an experimental [2,3] …”

Section: Ring Currents and Geometric Factorsmentioning

confidence: 99%

“…The chemical shifts are presented in table 1. The large differences between shifts of corresponding protons have been attributed to ring-current effects ; in the series TxH-, XH-, Dba-and DbcH-an increasing paramagnetic ring current in the central ring has been postulated [2,3]. In this paper a theoretical study of the proton chemical shifts of the anions is given.…”

Section: Introductionmentioning

confidence: 97%

“…spectra of a series of tricyclic conjugated anions have been published [1][2][3], i.e. of the conjugate bases of fluorene (F1H2) , 9,10-dihydroanthracene (AH~), xanthene (XH2) , thioxanthene (TxH~), dibenzo-[a,d]cycloheptene (DbcH2) and dibenz[b,f]azepine (iminostilbene, DbaH) (figure 1).…”

Section: Introductionmentioning

confidence: 99%

“…Other contributions to a n are ignored, e.g. those arising from the magnetic anisotropies and lone-pair dipoles of the heteroatoms ; these effects are small, as can be inferred from calculations [2,3] and from the variation of (a 1-a,). It is assumed furthermore that the shifts of F1H-in 1,2-dimethoxyethane (DME) are comparable with those of the other systems in liquid NHa, Downloaded by [University of California, San Diego] at 12:48 29 June 2016 (The numbering of DbcH-and Dba-is not standard, and also differs from that used in reference [3]).…”

Section: Introductionmentioning

confidence: 99%

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Semi-empirical calculations of chemical shifts in partially paratropic anions

1978

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A theoretical study is presented of the ring-current and excess-charge effects on the chemical shifts of the conjugate bases of fluorene, 9,10-dihydroanthracene, xanthene, thioxanthene, dibenzo[a,d]cycloheptene and dibenz-[b,f]azepine. Several approaches (SCF methods, coupled and uncoupled perturbation theories and ring-current models) have been compared.The use of coupled perturbation theory is necessary for those systems which exhibit paramagnetic ring currents. The chemical shifts are satisfactorily described if the PPP method and the so-called LA I approximation are invoked, and the various contributions are parametrized properly.

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Heavier alkali metals annual survey covering the year 1974

Kaiser

1975

Journal of Organometallic Chemistry

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The NMR spectra of the carbanions of xanthene and thioxanthene. Evidence for paratropism

Cited by 5 publications

References 23 publications

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

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

Semi-empirical calculations of chemical shifts in partially paratropic anions

Heavier alkali metals annual survey covering the year 1974

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