The silicalite(MFI)/p-nitroaniline system.: I.: Location of the sorbed molecules at medium and high pore-fillings.: A study by X-ray powder diffraction, 29Si mas NMR and energy minimization calculations

Structural investigation of the silicalite(MFI)/np-nitroaniline system by X-ray powder diffraction, establishes the existence of two single phased host/guest complexes corresponding to n=4 and 8 at medium and high pore-fillings respectively. Predictive studies by molecular mechanics simulations indicate that the pNAN molecules are located at the channel-intersections (for n=4) or in the elliptic channel-sections of the MFI topology (for n=8). Si-29 solid-state mas NMR measurements show that the actual MFI framework symmetry of the silicalite.4pNAN complex is acentric Pn2(1)a. Interpretation of the XRD pattern corresponding to the saturated silicalite.8pNAN complex by Rietveld type structure refinements shows that two independent pNAN molecules are located on sites II (zig-zag channel sections) and sites III (straight channel sections). The van der Waals type interactions between the sorbed molecules take place at the channel-intersections (sites I). These sites are not occupied by extra-framework species. This is in agreement with the commensurability criterion already observed in several MFI/sorbate systems.