**2.5. Surface immobilized linkers and catalysts and chiral stationary phases**

HR-MAS NMR spectroscopy has also been extensively used in investigating the mobility of linkers and catalysts attached to other materials, including 1H and 31P HR-MAS NMR studies of phosphine, bisphosphinoamine linkers and corresponding metal catalysts.[63-66] These investigations demonstrate very nicely that the surface mobility as probed by HR-MAS is strongly dependent on the swelling solvent employed in the studies.[67] The grafting of organotin catalysts to polystyrene for the tranesterification reaction between ethyl acetate and *n*-octanol or the ring-opening polymerization of -caprolactone has been followed by both 1D and 2D 1H, 13C and 119Sn HR-MAS NMR.[68-70] While the catalytic activity of the attached tin was unchanged following numerous cycles, the NMR revealed there was actually a change in the liquid-solid interface with a reduction in the mobility of the undecyltin trichloride catalyst at the resin surface. For experiments with rapid catalytic turnover rate there was a reduction in performance. This reduction in activity was reversible with solvent extraction, and was confirmed by HR-MAS to involve the change of the Sn environment back to the original state.[69] The use of 119Sn HR-MAS NMR has also been demonstrated to provide a quantitative measure of the tin loading in supporting catalysts.[71] This HR-MAS NMR study was also the first to incorporate the ERETIC (*e*lectronic *re*ference *t*o access *i*n vivo *c*oncentration) method.

HR-MAS NMR techniques have been used to characterize the surfaces of silica and polystyrene particles modified with chiral agents that allow identification of different enantiomers. In one study the NMR was able to probe regiochemistry of the surface chemistry, along with monitoring the stability against decomposition of the polystyrene resins.[72] Another investigation used 2D transfer NOESY NMR experiments to evaluate the stereoselective binding of molecules to the chiral stationary phase based on the negative cross-peaks associated with the transfer NOE effect.[73]
