13C, 6Li and 7Li NMR T1 and T1ρ study of ion-pair dynamics and structure of lithium fluorenide

✍ Scribed by Ingmar Sethson; Bertil Eliasson; Ulf Edlund

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 640 KB
• Volume: 29
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260291007

✦ Synopsis

Abstract

The rate of interconversion between tight and loose ion pairs of lithium fluorenide in 2‐methyltetrahydrofuran was determined from ^7^Li relaxation in the rotating frame. The rate constant for the tight to loose ion‐pair interconversion at −60°C is 2.3 × 10^4^ s^−1^ and for the reverse process 2.6 × 10^3^ s^−1^. The ^13^C relaxation times (T~1~) of the fluorenyl anion, adjusted for viscosity changes, do not change significantly with the ion‐pair structure. The electric quadrupole relaxation contribution to the ^7^Li T~1~ was derived from ^6^Li and ^7^Li T~1~ measurements. Lower limits of ^7^Li quadrupole splitting constants (QSCs) were obtained from the ^7^Li quadrupolar and the ^13^C 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 ^13^C T~1~ at high temperature for the diethyl ether solution may be caused by aggregation.