Structure of Dipole Chains in an MFI Type Molecular Sieve

Abstract

Several experimental methods demonstrate that adsorbed p-nitroaniline (pNA) forms a highly ordered guest structure in the pores of the MFI type 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 molecules were found to deviate uniformly by 11° from the direction of the straight channels. The symmetry of the loaded molecular sieve crystals is altered by the pNA adsorption which is connected with the loss of inversion symmetry characterizing the unloaded crystals. The molecular orientation of the occluded pNA was confirmed by polarized Raman spectroscopy. Additionally, the Raman spectra inedicate the absence of intermolecular hydrogen bonding. Optical SHG (second harmonic generation) 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 explained by a special uptake mechanism of the pNA molecules entering the molecular sieve pores. The pNA dipole chains in the MFI type framework match the lattice; they are weakly interconnecting and they are collectively aligned into the same direction.