NMR IMAGING OF ANISOTROPIC SOLID-STATE CHEMICAL-REACTIONS USING MULTIPLE-PULSE LINE-NARROWING TECHNIQUES AND H-1 T1-WEIGHTING

The reactions of substituted benzoic acid crystals and powders with ammonia gas have been monitored using solid-state H-1 NMR imaging techniques. The reactions are inherently (crystal) or by design (powder) spatially anisotropic: For a crystal of 4-bromobenzoic acid, the expected reaction anisotropy was not seen, either optically or with H-1 NMR imaging, most likely due to poor crystal quality or an unexpected crystal morphology. Nevertheless, some anisotropy in the reaction was observed and the extent of reaction was obtained from the NMR images. For a deep bed of powdered toluic acid, an anisotropic reaction profile is found. The reaction of the more exposed top layers of the bed is rapid whereas there is a delay in the reaction of the bottom layers associated with the rate of diffusion of ammonia into the bed. The apparent reaction rate constant, k = 5 (2) X 10(-4) mol-1 m3 s-1, and the effective diffusivity, D(e) = 1.0 (4) X 10(-5) m2 s-1, were obtained from a fit to a simultaneous diffusion with reaction model for a slab. This work is the first application of H-1 NMR imaging, using multiple-pulse line-narrowing techniques, for monitoring a chemical reaction. For this work, the special advantage of line narrowing is that the images are weighted not by the H-1 T2 but rather by the T1 relaxation time. Thus, for the materials studied herein, selection of an appropriate relaxation time in the NMR experiment enables observation of either the reaction product or the total sample.