ISSN:
0749-1581
Keywords:
13C NMR
;
7Li NMR
;
Rotating frame relaxation
;
Ion-pair exchange
;
Ion-pair structure
;
Quadrupole splitting constant
;
Lithium fluorenide
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The rate of interconversion between tight and loose ion pairs of lithium fluorenide in 2-methyltetrahydrofuran was determined from 7Li relaxation in the rotating frame. The rate constant for the tight to loose ion-pair interconversion at -60°C is 2.3 × 104 s-1 and for the reverse process 2.6 × 103 s-1. The 13C relaxation times (T1) of the fluorenyl anion, adjusted for viscosity changes, do not change significantly with the ion-pair structure. The electric quadrupole relaxation contribution to the 7Li T1 was derived from 6Li and 7Li T1 measurements. Lower limits of 7Li quadrupole splitting constants (QSCs) were obtained from the 7Li quadrupolar and the 13C dipole-dipole relaxation times. The QSC values are in the range 37-200 kHz and appear to reflect changes in the ion-pair structure, with a low value corresponding to a solvent-separated ion pair. The higher value for the contact ion pair is in agreement with a structure where the anion disrupts the symmetrical coordination of solvent molecules around the cation. A decreased QSC and a shortened 13C T1 at high temperature for the diethyl ether solution may be caused by aggregation.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260291007
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