th  - 1987 - 51st ENC Conference

MK9
A 2D-EXCHANGE SEPARATED LOCAL FIELD (EXSLF) EXPERIMENT
K. Takegoshi* and C.A. McDowell, Department of C~emistry,
University of British Columbia, 2036 Main Mall, Vancouver, British
Columbia, Canada, V6T I¥6.
Rate constants for the flip-flop spin exchange process
between a IN spin coupled with a '~C spin and the surrounding 'H
spins were determined in the absence perturbing rf fields, and also
under the influence of both 'H and '*C rf fields. To determine the
flip-flop rate in the absence of the rf fields, we developed a new
technique, the Exchange Separated Local Field (EXSLF) experiment.
The EXSLF experiment was applied to the CH group in a single crystal
of dimedone, for two different orientations relative to the static
magnetic field. The inverse of the flip-flop exchange rates
determined by the EXSLF experiments are 276 and 391 ~s for the
orientations Bo//b and Bo//C*, respectively.
The flip-flop exchange motion under the influence of both 'H
and '~C rf fields was also studied by examining the transient
oscillation phenomena occuring during a spin-locking
cross-polarization experiment. The values obtained for the inverse
of the rate constants for the flip-flop exchange motion were 450 and
540 ,s for Bo//b and Bo//C*, respectively. The observed slow
flip-flop rate shows that the IH-'3C coupled spin system in a single
crystal of dimedone is isolated from the surrounding 'H spins.