Proton two-dimensional exchange nuclear magnetic resonance study of a p-xylene-d₆ zeolite inclusion compound

Abstract

Proton two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiments and dipolar spin-lattice relaxation rate, T _1D, measurements were performed using magic-angle spinning (MAS) on a zeolite Na-ZSM-5/p-xylene α - α′-d ₆ inclusion compound. The value of T _1D was found to be 3·2 ms, which is much longer than the T _1D values reported for rigid solids when the sample is spun perpendicular to the static magnetic field. This long T _1D can be interpreted as arising from the weak intermolecular ¹H-¹H dipolar interactions, because of the significant separation of the molecules in the cavities of the host. A satisfactory theoretical treatment which describes the 2D spectra, was developed. At short mixing times the 2D spinning-sideband intensities observed depended on the flip-angles of the second and third pulses. We show that the off-diagonal 2D spinning sidebands could be suppressed by the appropriate choice of the flip-angles of the second and the third pulses, since in the present sample the included molecules are close to being isolated two-spin systems. When the mixing time was increased, the 2D spinning-sideband intensities became less dependent on the pulse flip-angles. This experimental feature was interpreted to occur because of the decay of the dipolar order. This was confirmed by computer calculations and simulations of the 2D spinning-sideband intensities.