A 2D-exchange separated local field (EXSLF) experiment: An application to a 13C–1H isolated spin system in the solid state

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Abstract
To understand the spin dynamics involved in double resonance NMR experiments, rate constants for the flip–flop spin exchange process between a 1H spin coupled with a 13C spin and the surrounding 1H spins were determined in the absence of perturbing rf fields, and also under the influence of both 1H and 13C 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 of the crystal relative to the static magnetic field. The inverse of the flip–flop exchange rates determined by the EXSLF experiments are 276 and 391 μs for the orientations B0//b and B0//c*, respectively. The flip–flop exchange motion under the influence of both 1H and 13C rf fields was also studied by examining the transient oscillation phenomena occurring 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 μs for B0//b and B0//c*, respectively. The observed slow flip–flop rates show that the 1H–13C coupled spin system in a single crystal of dimedone is isolated from the surrounding 1H spins.