Chapter 16. Q-space Singlet NMR

Long-lived singlet spin states can be used to characterise the structures of microscopic pores in a liquid environment using NMR q-space diffusion imaging. The longer lifetimes of singlet states compared to conventional NMR spin coherences allow the extension of the measurements to larger pores. Two different pulsed gradient NMR q-space diffusion methods for singlet states can be utilised depending on the type of probe molecule and the dimension of the porous system. The first method, which incorporates a spin-locking pulse, is specifically designed for molecules with two spins that are weakly coupled, while the second method, that includes a series of spin-echo trains, is intended for molecules with nearly equivalent spins that are strongly coupled. These advances involving singlet states have allowed the investigation of macroscopic pores (e.g., capillaries and rectangular pores) with dimensions up to 2 mm to be studied by q-space NMR. This chapter covers the theory of q-space diffusion imaging using singlet states together with recent experimental examples from the literature.