Dynamics of p-nitroaniline molecules in siliceous ZSM-5 studied by solid-state NMR

Abstract

Sites and dynamics of p-nitroaniline (pNA) molecules in the micropores of siliceous ZSM-5 zeolite have been investigated by means of solid-state NMR. The adsorbed amounts of pNA studied were about 1.5, 4 and 6 molecules per unit cell of ZSM-5. ²H NMR and ¹³C NMR revealed that the motion of pNA was a 180° flip-flop around the C₂ axis of the molecule when the adsorbed amount was less than four molecules per unit cell. The rate of the motion was about 50 kHz at 297 K, and the apparent activation energy was 57 ± 17 kJ mol⁻¹. ¹³C NMR confirmed the presence of different adsorbed states of pNA when the adsorbed amount was more than four molecules. Furthermore, ²H NMR revealed that the flip-flop motion of pNA was largely suppressed. These results indicate that the motion of pNA in the micropores of ZSM-5 depended on the adsorbed amount of pNA, and that the interaction between the guest molecules influenced the dynamics of the guest molecules.