13 C, 6 Li and 7 Li NMR T 1 and T 1ρ study of ion‐pair dynamics and structure of lithium fluorenide

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.