STRUCTURE OF DIPOLE CHAINS IN AN MFI TYPE MOLECULAR-SIEVE

Several experimental methods demonstrate that adsorbed p-nitroaniline (pNA) forms a highly ordered guest structure in the pores of the MFI t ype molecular sieve Sb-Silicalite-1. The pNA molecules were located in the intersections of the two interconnected pore systems using single -crystal X-ray structure analysis. The molecular axes of the pNA molec ules were found to deviate uniformly by 11 degrees from the direction of the straight channels. The symmetry of the loaded molecular sieve c rystals is altered by the pNA adsorption which is connected with the l oss of inversion symmetry characterizing the unloaded crystals. The mo lecular orientation of the occluded pNA was confirmed by polarized Ram an spectroscopy. Additionally, the Raman spectra inedicate the absence of intermolecular hydrogen bonding. Optical SHG (second harmonic gene ration) studies confirm the loss of inversion symmetry which is caused by the formation of oriented dipole chains with a preferred direction over macroscopic regions in the crystal. This sorbate structure is ex plained by a special uptake mechanism of the pNA molecules entering th e molecular sieve pores. The pNA dipole chains in the MFI type framewo rk match the lattice; they are weakly interconnecting and they are col lectively aligned into the same direction.