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. 2 H NMR and 13 C NMR revealed that the motion of pNA was a 180 flip-flop around the C 2 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 AE 17 kJ mol À1 . 13 C NMR confirmed the presence of different adsorbed states of pNA when the adsorbed amount was more than four molecules. Furthermore, 2 H 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.

Dynamics of p-Nitroaniline Molecules in Micoporous Aluminophosphate AlPO4-5 Studied by Solid-State NMR

“…Strong intermolecular hydrogen bonds are formed in crystalline pNA- d solid. , The AlPO 4 -5/pNA- d sample of 5.0 mass % shows low-frequency shifts in C(1)−NH 2 and C(3,5)−H and high-frequency shift in C(4)−NO 2 , compared with crystalline pNA- d . Similar shifts are observed in ZSM-5/pNA, where hydrogen bonding interaction between pNA molecules is decreased by the inclusion. , Therefore, the above shifts observed in the sample of 5.0 mass % indicate that inclusion in the nanospace causes weakening of the hydrogen bonds. The intensities of the spinning sidebands reflect the magnitude of the chemical shift anisotropy when comparison is made between the spectra measured at the same spinning rate.…”

“…The molecular size of acetonitrile- d 3 is smaller than that of pNA- d , and the basicity of the former is smaller than that of the latter. The pNA- d molecules undergo 180° flip-flop motion in the channel of ZSM-5 . This motion is slower than that in the channel of AlPO 4 -5.…”

“…The symmetry of the channel cross section also makes these motions unfavorable. A two-site jump, or 180° flip-flop, is also excluded because this motion results in much different line shapes …”

“…Solid-state NMR spectroscopy is advantageous for investigating dynamics and chemical environments of guest molecules. 13 C NMR can observe the organic guest molecules selectively and can probe the dynamics and chemical environments of the guest molecules. − 2 H NMR offers powerful approaches to investigate dynamics of molecules whose rate is of the order of 100 kHz. − Previously, we have investigated the dynamics of benzene, cyclohexane, and n -hexane in KL zeolite, pNA in ZSM-5 zeolite, − acetonitrile and n -hexane in AlPO 4 -5, and pNA in mesoporous silica FSM-16 …”

“…10-13 2 H NMR offers powerful approaches to investigate dynamics of molecules whose rate is of the order of 100 kHz. [14][15][16] Previously, we have investigated the dynamics of benzene, cyclohexane, and n-hexane in KL zeolite, 17 pNA in ZSM-5 zeolite, [18][19][20] acetonitrile and n-hexane in AlPO 4 -5, 21 and pNA in mesoporous silica FSM-16. 22 In the present work, we have studied the motion of pNA molecules in the micropores of AlPO 4 -5 by means of solidstate NMR.…”