Dynamics of p-nitroaniline molecules in FSM-type mesoporous silicas studied by solid-state NMR

Dynamics of p-nitroaniline (pNA) in FSM-type mesoporous silicas was investigated by means of H-2 and C-13 solid-state NMR. Deuterated pNA (pNA-d) was loaded to FSM-16 (FSM) and modified FSM samples which were prepared by reactions of FSM with trimethylethoxysilane (C1FSM), n-octyltriethoxysilane (C8FSM) and 3-aminopropyltriethoxysilane (NH2FSM). C-13 MAS NMR spectra of the samples at room temperature showed four sharp signals assigned to pNA-d, indicating a fast motion of pNA-d at room temperature. Temperature dependence of H-2 NMR spectra exhibited the presence of two modes of motions, the isotropic motion and another motion such as a wobbling one (Motion I). Analysis of spin-lattice relaxation times (T-1) for 2 H spins demonstrated that the apparent activation energies of Motion I were 20 kJmol(-1) for FSM and 31-32 kJmol(-1) for C1FSM and NH2FSM. On the other hand, temperature dependence of T, of Motion I in C8FSM was complicated presumably because of the motion of alkyl chains of the reacted silylating agent. These results indicated that the dynamics of pNA molecules was varied with the surface properties of the mesopores. (C) 2003 Elsevier Inc. All rights reserved.