Manipulation of phase and amplitude modulation of spin magnetization in magic angle spinning nuclear magnetic resonance in the presence of molecular diffusion

Abstract

Nuclear magnetic resonance (NMR) experiments with a spinning sample [magic angle sample spinning (MASS)] are used to remove the line broadening in composite systems, where the susceptibility contrast of its constituents gives rise to an inhomogeneous field that causes a line broadening and obscures chemical information. The NMR signal in these experiments has a phase and an amplitude part. In the absence of diffusion, i.e., in the MASS spectra of solids, the amplitude of the signal from an isochromat is a constant independent of position and time and the phase is a periodic function of the rotor frequency νr. In fluids, the amplitude of a spin packet is a function of its position and time. The amplitude modulation and relaxation in diffusive MASS encodes the dynamics of motion and the landscape (geometry of pores and field gradients) probed by the motion. Here we use spin manipulation—total suppression of sidebands (TOSS)—to suppress the effects of phase with the goal of isolating the amplitude term. By the TOSS sequence the phase factor at time t for a spin packet at an azimuthal angle φ is made to depend on φ only as a function of ωrt−φ, which suppresses the sidebands in solids upon an integration over φ. Due to molecular diffusion, the amplitude part depends on φ, and, thus, diffusive TOSS cannot suppress the sidebands. The residual sidebands carry the information of dynamics and pore and magnetic field geometry, in addition, by reducing the size of the sidebands, TOSS is of course, also useful in identifying various fluid components in situ. The diffusive MASS gives a measure of the spread in local fields and diffusive TOSS gives a measure of the spread in local gradients.