1H NMR study of water molecules confined in nanochannels of mordenite.

Behavior of water molecules entrapped in nanochannels of zeolite mordenite has been investigated by 1 H NMR technique. The 1 H spectra and spin-lattice relaxation times in the laboratory and rotating frames, T 1 and T 1ρ , respectively, as well as the dipolar relaxation time T 1 D have been measured in the temperature range from 96 to 351 K. Diffusion of water molecules along the channels was observed above ~200 K. While in bulk liquid the dipolar ordered state of nuclear spins is not formed owing to complete motional average of dipolar interactions, we show that such a state is observed for mobile molecules confined in a restricted geometry. At temperatures below ~140 K the relaxation was found to be mainly caused by interaction of 1 H nuclear spins with paramagnetic impurities. Complete lost of the fine structure of 1 H spectra above ~320 K is attributed to isotropic molecular reorientation or/and proton exchange. We show that the dipolar relaxation in mordenite is responsive to slow 180° reorientations of water molecules. The correlation times of nuclear and electron spin fluctuations were determined. [ABSTRACT FROM AUTHOR]