Diffusion–diffraction using singlet spin states and various NMR coherences in a J-coupled AX spin system

Abstract

The diffusion–diffraction decay profile of singlet states and various NMR coherences in a J-coupled AX spin system was studied using capillaries with internal diameters ranging from 20 to 450 μm. A pulsed gradient diffusion sequence based on the singlet state NMR diffusion sequence was used for generating longitudinal spin-pair order, zero-quantum and double-quantum coherences. The degree of definition in diffusion–diffraction profiles was observed to be dependent on the type of coherence and the sequence employed. Singlet states, longitudinal spin-pair order and zero-quantum coherences produced diffusion–diffraction profiles that were similar and had more pronounced features than those obtained from transverse and longitudinal magnetisations using standard spin-echo (i.e., PGSE) and stimulated echo (i.e., PGSTE) based pulsed gradient sequences. Although PGSTE provides a higher signal-to-noise ratio at shorter diffusion times, the singlet state sequence affords better diffusion–diffraction profiles and an enhanced signal-to-noise ratio at longer diffusion times. These developments allow pores with macroscopic dimensions to be probed, which was previously only possible using gas-phase NMR diffusion measurements.