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 ¹H NMR technique. The ¹H spectra and spin-lattice relaxation times in the laboratory and rotating frames, T₁ and T_1ρ, respectively, as well as the dipolar relaxation time T_1D 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 ¹H nuclear spins with paramagnetic impurities. Complete lost of the fine structure of ¹H 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.